Cambridge Natural History Mammalia/Chapter II

by Frank E. Beddard

 

CHAPTER II

STRUCTURE AND PRESENT DISTRIBUTION OF THE MAMMALIA

External Form.—It would be quite impossible for any one to confuse any other quadrupedal animal with a mammal. The body of a reptile is, as it were, slung between its limbs, like the body of an eighteenth century chariot between its four wheels; in the mammal the body is raised entirely above, and is supported by, the four limbs. The axes of these limbs too, as a general rule, are parallel with the vertical axis of the body of their possessor. There is thus a greater perfection of the relations of the limbs to the trunk from the point of view of a terrestrial creature, which has to use those limbs for rapid movement. The same perfection in these relations is to be seen, it should be observed, in such running forms among the lower Vertebrata as the Birds and the Dinosaurs, where the actual angulation of the limbs is as in the purely running Mammalia. These relations are of course absolutely lost in the aquatic Cetacea, and not marked in various burrowing creatures. The way in which the fore- and hind-limbs are angulated is considerably different in the two cases. In the latter, which are most used and, as it were, push on the anterior part of the body, the femur has its lower end directed forwards, the tibia and the fibula project backwards at the lower end, while the ankle and foot are again inclined in the same direction as the femur. With the fore-limbs there is not this regular alternation. The humerus is directed backwards, the fore-arm forwards, and the hand still more forwards. This angulation seems to facilitate movement, inasmuch as it is seen in even the Amphibia and the lower Reptiles, in which, however, the differences between the fore- and hind-limbs are less marked, indicating therefore a less specialised condition of the limbs. It is an interesting fact that the angulation of the limbs is to some extent obliterated in very bulky creatures, and almost entirely so in the elephants (see p. 217), which seem to need strong and straight pillars for the due support of their huge bodies.

The alertness and general intellectual superiority of mammals to all animals lying below them in the series (with the exception of the birds, which are in their way almost on a level with the Mammalia) are seen by their active and continuous movements. The lengthy periods of absolute motionlessness, so familiar to everybody in such a creature as the Crocodile, are unknown among the more typical Mammalia except indeed during sleep. This mental condition is clearly shown by the proportionate development of the external parts of all the organs of the higher senses. The Mammalia as a rule have well-developed, often extremely large, flaps of skin surrounding the entrance to the organ of hearing, often called "ears," but better termed "pinnae." These are provided with special muscles, and can be often moved and in many directions. The nose is always, or nearly always, very conspicuous by its naked character; by the large surface, often moist, which surrounds the nostrils; and again by the muscles, which enable this tract of the integument to be moved at will. The eyes, perhaps, are less marked in their predominance over the eyes of lower Vertebrates than are the ears and nose; but they are provided as a rule with upper and lower eyelids, as well as by a nictitating membrane as in lower Vertebrates. The apparent predominance of the senses of smell and hearing over that of sight appears to be marked in the Mammalia, and may account for their diversity of voice as well as of odour, and for the general sameness of coloration which distinguishes this group from the brilliantly-coloured birds and reptiles. The head, too, which bears these organs of special sense, is more obviously marked out from the neck and body than is the case with the duller creatures occupying the lower branches of the Vertebrate stem.

The Hair.—The Mammalia are absolutely distinguished from all other Vertebrates (or, for the matter of that, Invertebrates) by the possession of hair. To define a mammal as a Vertebrate with hair would be an entirely exclusive definition; even in the smooth Whales a few hairs at least are present, which may be reduced to as few as two bristles on the lips. The term "hair," however, is apt to be somewhat loosely applied; it has been made use of to describe, for example, the slender processes of the

Fig. 1.—A, Section of human skin. Co, Dermis; D, sebaceous glands; F, fat in dermis; G, vessels in dermis; GP, vascular papillae; H, hair; N, nerves in dermis; NP, nervous papillae; Sc, horny layer of epidermis; SD, sweat gland; SD1, duct of sweat gland; SM, Malpighian layer. B, Longitudinal section through a hair (diagrammatic). Ap, Band of muscular fibres inserted into the hair-follicle; Co, corium (dermis); F, external longitudinal; F1, internal circular, fibrous layer of follicle; Ft, fatty tissue in the dermis; GH, hyaline membrane between the root-sheath and the follicle; HBD, sebaceous gland; HP, hair-papilla with vessels in its interior; M, medullary substance (pith) of the hair; O, cuticle of root-sheath; R, cortical layer; Sc, horny layer of epidermis; Sch, Hair shaft; SM, Malpighian layer of epidermis; WS, WS1, outer and inner layers of root-sheath. (From Wiedersheim's Comparative Anatomy.)

chitinous skin of the Crustacea. It will be necessary, therefore, to enter into the microscopical structure and development of the mammalian hair. Hair is found in every mammal. The first appearance of a hair is a slight thickening of the stratum Malpighii of the epidermis, the cells taking part in this being elongated and converging slightly above and below. Dr. Maurer has called attention to the remarkable likeness between the embryonic hair when at this stage and the simple sense-organs of lower Vertebrates. Later there is formed below this a denser aggregation of the corium, which ultimately becomes the papilla of the hair. This is the apparent homologue of the first formed part of a feather, which projects as a papilla before the epidermis has undergone any modification. Hence there is from the very first a difference between feathers and hairs—a difference which must be carefully borne in mind, especially when we consider the strong superficial resemblance between hairs and the simple barbless feathers. Still later the knob of epidermic cells becomes depressed into a tubular structure, which is lined with cells also derived from the stratum Malpighii, but is filled with a continuation of the more superficial cells of the epidermis. This is the hair-follicle, and from the epidermic cells arises the hair by direct metamorphosis of those cells; there is no excretion of the hair by the cells, but the cells become the hair. From the hair-follicle also grows out a pair of sebaceous glands, which serve to keep the fully-formed hair moist.

Fig. 2.—Four diagrams of stages in the development of a hair. A, Earliest stage in one of those mammals in which the dermal papilla appears first; B, C, D, three stages in the development of the hair in the human embryo. blb, Hair-bulb; crn, horny layer of the epidermis; foll, hair-follicle; grm, hair-germ; h, hair, in D, projecting on the surface; muc, Malpighian layer of epidermis; pp, dermal papilla; seb, developing sebaceous glands; sh.1, sh.2, inner and outer root-sheaths. (After Hertwig.)

Dr. Meijerle^[1] has lately described in some detail the particular arrangement of the individual hairs among mammals; they are not by any manner of means scattered without order, but show a definite and regular arrangement, which varies with the animal. For instance, in an American Monkey (Midas), the hairs arise in threes—three hairs of equal size springing from the epidermis close together; in the Paca (Coelogenys) there are in each group three stout hairs alternating with three slender hairs. In some forms a number of hairs spring from a common point: in the Jerboa (Dipus) twelve or thirteen arise from a single hole; in Ursus arctos there is the same general plan, but there is one stout hair and four or five slender ones. There are numerous other complications and modifications, but the facts, although interesting, do not appear to throw any light upon the mutual affinities of the animals. Allied forms may have a very different arrangement, while in forms which have no near relationship the plan may be very similar, as is shown by the examples cited from Dr. Meijerle's paper. The groups of hairs, moreover, have themselves a definite placing, which the same anatomist has compared with the disposition of the bundles of hairs behind and between the scales of the Armadillo, and which has led him to the view that the ancestors of mammals were scaly creatures—a view also supported by Professor Max Weber,^[2]and not in itself unreasonable when we consider the numerous points of affinity between the primitive Mammalia and certain extinct forms of reptiles.^[3]

The hairs are greatly modified in form in different mammals and in different parts of their bodies. It is very commonly the case that a soft under-fur can be distinguished from the longer and coarser hairs, which to some extent hide the latter. Thus the "sealskin" of commerce is the under-fur of the Otaria ursina of the North. The coarser hairs may be further differentiated into bristles; these again into spines, such as those of the Hedgehog and of the Porcupine. Again, the flattening and agglutination of hairs seems to be responsible for the scales of the Manis and for the horns of the Rhinoceros. It is a matter of common knowledge that upon the head of various animals, e.g. the Domestic Cat, long and sensitive hairs are developed, which are connected with the terminations of nerves, and perform a sensory, probably tactile function. These occur on the snout, above the eyes, and in the neighbourhood of the ears. It is an interesting fact that a tuft of quite similar hairs occurs on the hand of many mammals close to the wrist, which, at least in the case of Bassaricyon, are connected with a strong branch from the arm-nerve. These tufts also occur in Lemurs, in the Cat, various Rodents and Marsupials, and are probably quite general in mammals who "feel" with their fore-limbs;—in which, in fact, the fore-limbs are not exclusively running organs. That the last remaining hairs of the Cetacea are found upon the muzzle, is perhaps significant of the importance of these sensory bristles. The entire absence of hairs is quite common in this order, although traces of them are sometimes found in the embryo. The Sirenia, too, are comparatively hairless, as are also many Ungulates. Whether the presence of blubber in the former case and the existence of a very thick skin in the latter animals are facts which have had anything to do with the disappearance of hair or not, is a matter for further inquiry.

The intimate structure of the hair varies considerably. The variations concern the form of the hair, which may be round in transverse section, or so oval as to appear quite flat when the hair is examined in its entirety. The substance of the hair is made up of a central medulla or pith with a peripheral cortex; the latter is scaled, and the scales are often imbricated and with prominent edges. The amount of the two constituents also differs, and the cortex may be reduced to a series of bands surrounding only tracts of the enclosed pith. In the hair is contained the pigment to which the colour of mammals is chiefly due. Tracts of brightly-coloured skin may exist, as in the Apes of certain genera; but such structures are not general. The pigment of the hair seems to consist of those pigmentary substances known as melanins. It is remarkable to find such a uniform cause of coloration, when we consider the great variety of feather-pigments found in birds. The variations of colour of the hair of mammals are due to the unequal distribution of these brown pigments. There are very few mammals which can be called brightly coloured. The Bats of the genus Kerivoula have been compared to large butterflies, and some of the Flying Squirrels have strongly-marked contrasts of reddish brown, white, and yellow. The same may be said of the spines of certain Porcupines. But we find in the hair no bright blues, greens, and reds such as are common among birds.

There are certain general facts about the coloration of mammals which require some notice here. Next to the usually sombre hues of these animals the general absence of secondary sexual coloration is noteworthy. In but a few cases among the Lemurs and Bats do we find any marked divergences in hues between males and females. Secondary sexual characters in mammals are, it is true, often exhibited by the great length of certain hair-tracts in the male, such as the mane of the Lion, the throat- and leg-tufts of the Barbary Sheep, and so forth; but apart from these, the secondary sexual characters of mammals are chiefly shown in size, e.g. the Gorilla, or in the presence of tusks, e.g. various Boars, or of horns, as in the Deer, etc. The coloration of mammals frequently exhibits conspicuous patterns of marking. These are in the form of longitudinal stripes, of cross-stripes, or of spots; the latter may be "solid" spots, or broken up, as in the Leopard and Jaguar, into groups of smaller spots arranged in a rosette-fashion. We never find in mammals the complicated "eyes" and other markings which occur in so many birds and in other lower Vertebrates. It is important to note that in the Mammalia whose sense of sight is quite keen there should be a practical absence of secondary sexual colours. As to the relationship of the various forms of marking that do occur, it seems clear that there has been a progression from a striped or spotted condition to uniform coloration. For we find that many Deer have spotted young; that the young Tapir of the New World is spotted, while its parents are uniform blackish brown; the strongly-marked spotting of the young Puma contrasts with the uniform brown of the adult; and the Lion cub, as every one knows, is also spotted, the adult lioness showing considerable traces of the spots.

The seasonal change in the colours of certain mammals is a subject upon which much has been written. The extreme of this is seen in those creatures, such as the Polar Hare and the Arctic Fox, which become entirely blanched in the winter, recovering their darker coat in the spring. This is, however, only an extreme case of a change which is general. Most animals get a thicker fur in winter and exchange it for a lighter one in summer. And the hues of the coat change in correspondence.

Glands of the Skin.—The great variety of integumental glands possessed by the Mammalia distinguishes them from any group of lower Vertebrates. This variability, however, only concerns the anatomical structure of the glands in question. Histologically they are all of them apparently to be referred to one of two types, the sudoriparous or sweat gland and the sebaceous gland. Simple sweat and sebaceous glands are abundant in mammals, with but a few exceptions. The structures that we are now concerned with are agglomerations of these glands. The mammary glands will be treated of in connexion with the marsupium; they are either masses of sweat glands, or of sebaceous glands whose secretion has been converted into milk.

Many Carnivora possess glands opening to the exterior, near the anus, by a large orifice. These secrete various odoriferous substances, of which the well-known "civet" is an example. Other odoriferous glands are the musk glands of the Musk-deer and of the Beaver; the suborbital gland of many Antelopes; the dorsal gland of the Peccary, which has given the name of Dicotyles to the genus on account of its resemblance in form to a navel. This gland may be seen to secrete a clear watery fluid. The Elephant has a gland situated on the temple, which is said to secrete during certain periods only, and to be a warning to leave the animal alone. Very remarkable are the foot glands of certain species of Rhinoceros; they are not universally present in those animals, and are therefore useful as specific distinctions. On the back of the root of the tail in many Dogs are similar glands. The Gentle Lemur (Hapalemur) has a peculiar gland upon the arm, about the size of an almond, which in the male underlies a patch of spiny outgrowths. In Lemur varius is a hard patch of black skin which may be the remnants of such a gland. It is thought that the callosities on the legs of Horses and Asses are remnants of glands.

One of the most complex of these structures which has been examined microscopically exists in the Marsupial Myrmecobius.^[4] On the skin of the anterior part of the chest, just in front of the sternum, is a naked patch of skin which is seen to be perforated by numerous pores. Besides the ordinary sebaceous and sweat glands there are a series of masses of glands, opening by larger orifices, which present the appearance of groups of sebaceous glands, and are of a racemose character; but the existence of muscular fibres in their coats seems to show that they should be referred rather to the sudoriparous series. Beneath the integument is a large compound tubular gland quite half an inch in diameter.

In Didelphys dimidiata there is a precisely similar glandular area and large underlying gland, the correspondence being remarkable in two Marsupials so distant in geographical position and affinities. Even among the Diprotodont genera there is something of the kind; for in Dorcopsis luctuosa and D. muelleri is a collection of four unusually large sebaceous follicles upon the throat, and in the Tree Kangaroo (Dendrolagus bennettii) there is the same collection of enlarged hair-follicles, though they are apparently somewhat reduced as compared with those of Dorcopsis. These are of course a few examples out of many.

It seems to be possible that the functions of these various glands is at least twofold. In the first place, they may serve, where predominant in one sex, to attract the sexes together. In the second place, the glands may be useful to enable a strayed animal of a gregarious species to regain the herd. It is perfectly conceivable too that in other cases the glands may be a protection, as they most undoubtedly are in the Skunk, from attacks. In connexion with the first, and more especially the second, of the possible uses of these glands, it is interesting to note that in purely terrestrial creatures, such as the Rhinoceros, the glands are situated on the feet, and would therefore taint the grass and herbage as the animal passed, and thus leave a track for the benefit of its mate. The same may be said of the rudimentary glands of Horses if they are really glands. The secretion of the "crumen" of Antelopes is sometimes deposited deliberately by Oreotragus upon surrounding objects, a proceeding which would attain the same end. One may even perhaps detect "mimicry" in the similar odours of certain animals. Prey may be lured to their destruction, or enemies frightened away. The defenceless Musk-deer may escape its foes by the suggestion of the musky odour of a crocodile. It is at any rate perfectly conceivable that the variety of odours among mammals may play a very important part in their life, and it is perhaps worthy of note that birds with highly-variegated plumage are provided only with the uropygial gland, while mammals with usually dull and similar coloration have a great variety of skin glands. Scent is no doubt a sense of higher importance in mammals than in birds. The subject is one which will bear further study.

Nails and Claws.—Except for the Cetacea (where rudiments have been found in the foetus), the extremities of the fingers and of the toes of mammals are covered by, or encased in, horny epidermic plates, known as nails, claws, and hoofs.

The variety in the shape and development of these corneous sheaths to the digits is highly characteristic of mammals as opposed to lower Vertebrates. If we take extreme cases, such as the nail of the thumb in Man, the hoof of a Horse, and the claw of a Cat, it is easy to distinguish the three kinds of phalangeal horny coverings. But the differences become extinguished as we pass from these to related types. The nail of the little finger in Man approaches the claw-like form; and the hoofs of the Lama are almost claws in the sharpness of their extremities. On the whole it may be said that claws and hoofs embrace the bone which they cover, while nails lie only upon its dorsal surface. The form of the distal phalanx which bears the nail shows, however, two kinds of modification which do not support such a classification. When those phalanges are clad with hoofs or covered by a nail they end in a rounded and flattened termination. On the other hand, when they bear a claw they are themselves sharpened at the extremity and often grooved above.

The Marsupium.—It may appear to be unnecessary at this juncture to speak of the marsupial pouch, which is so usually believed to be a characteristic of the group Marsupialia. Rudiments of this structure have, however, been recently discovered in the higher mammals, and, as Dr. Klaatsch^[5] has remarked, all researches into the "history of the mammals culminate in the question whether the placental mammals pass through a marsupial stage or not." We cannot, therefore, look upon the marsupial pouch as a matter affecting only the Marsupials, though it is true that this organ is at present functional only in them and in the Monotremata.

Fig. 3.—Echidna hystrix. A, Lower surface of brooding female; B; dissection showing a dorsal view of the pouch and mammary glands; ††, the two tufts of hair in the lateral folds of the mammary pouch from which the secretion flows, b.m, Pouch; cl, cloaca; g.m, groups of mammary glands. (From Wiedersheim's Comparative Anatomy, after W. Haacke.)

In the Marsupials the pouch shelters the young, which are born in an exceedingly imperfect state, minute, nude, and blind, with a "larval" mouth fitted only to grasp in a permanent fashion the teat, upon which they are carefully fixed by the parent. But even later the pouch is made use of as a temporary harbour of refuge: from the pouch of female Kangaroos at the Zoological Gardens may frequently be observed to protrude the tail and hind-legs of a young Kangaroo as big as a Cat, and perfectly well able to take care of itself.

In the Monotremata (in Echidna) there is a deep fold of the skin which lodges the unhatched egg, and into which the mammary glands open, one on either side. This structure is only periodically developed, and arises from two rudiments, one corresponding to each mammary area; but in the female with eggs or young there is but a single deep depression, which occupies the same region of the body as the marsupial pouch of the Marsupials.^[6] It is usually held that this structure is not of precisely the same morphological value as the pouch of the Marsupial; and the difference is expressed by terming the one (that of Echidna) the mammary pouch, and the other the marsupium. At first sight it may appear to be an unnecessary refinement to separate two structures which have so many and such obvious likenesses. It is not quite certain, however, that the difference is not even more profound than later opinions seem to indicate. The Monotremata not only have no teats, as has already been pointed out, but the mammary glands themselves are of a perfectly different nature to those of the higher mammals, including the Marsupials. There is therefore no a priori objection to the view that the accessory parts developed in connexion with the mammary glands should also be different. The teat of the higher Mammalia grows up round the area upon which the ducts of the mammary glands open; it is a fold of skin which eventually assumes the cylindrical form of the adult teat, and which includes the ducts of the milk glands. It has been suggested that the two folds of skin which form the mammary pouch of Echidna are to be looked upon as the equivalent of the commencing teat of the higher mammal.^[7] In this case it is clear that the marsupial folds of the Marsupial cannot correspond accurately with the apparently similar folds of Echidna, because there are teats as well. It is the teats which correspond to the marsupial folds of Echidna. This view is in apparent contradiction to an interesting discovery in a specimen of a Phalanger by Dr. Klaatsch.^[8] This Marsupial, like most others, has a well-developed marsupial pouch, in which the young are lodged at birth; but round two of the teats is another distinct fold on either side, the outer wall of which forms the general wall of the pouch. Dr. Klaatsch thinks that these smaller and included pouches are the equivalents of the mammary pouches of Echidna. They contain teats, but this comparison does not do away with the validity of Gegenbaur's suggestion already referred to, because the teats are (see above) secondary. If this fact be fairly to be interpreted in the sense which Dr. Klaatsch attaches to it, we have an interesting case of the growth of a new organ out of and partly replacing an old organ. In the Monotremes there is a pouch which facilitates or performs both nutritive and protective functions; in the Phalanger these two functions are carried on in separate pouches; finally, in other Marsupials, there is a return to the undifferentiated state of affairs found in the Monotremata, but with the help of a new organ not found in them.

Fig. 4.—Diagram of the development of the nipple (in vertical section). A, Indifferent stage, glandular area flat; B, elevation of the glandular area with the nipple; C, elevation of the periphery of the glandular area into the false teat, a, Periphery of the glandular area; b, glandular area; gl, glands. (From Gegenbaur.)

Though so characteristic of Marsupials, the marsupial pouch is not always developed in them. It is present in all the Kangaroos, Wallabies, and Wombats, in fact in the Diprotodonts. It is also present in a number of the carnivorous Polyprotodont Marsupials; but in Phascologale it is only present in rudiment, and in Myrmecobius it is entirely obsolete. In the American Opossums the state of the pouch is variable. "Generally absent, sometimes merely composed of two lateral folds of skin separate at each end, rarely complete," is Mr. Thomas' summary in his definition of the family Didelphyidae.^[9] Another curious feature of the pouch in the Marsupials is the variability in the position of the mouth of the pouch: in all the Diprotodonts it looks forward; but in many Polyprotodonts it looks backward. This, however, has some connexion with the habitual attitude of the possessor: in the Kangaroo, leaping along on its hind-legs, it is requisite that the pouch should open forwards; but in the dog-like Thylacine, going on all fours, the fact that the pouch opens backwards is less disadvantageous to the contained young.

The male Thylacine has a pouch which is quite or very nearly as well formed as in the female. There are also rudiments of a pouch in the male foetuses of many Marsupials, especially of those belonging to the Polyprotodont section of the order, though these rudiments are by no means confined to that subdivision. Up to so late a period as the age of four months (length 19⋅8 cm.) the male Dasyurus ursinus has a pouch.

We have now to consider the interesting series of facts relative to the permanence—in a rudimentary condition it is true—of the mammary pouch in the higher Mammalia, facts which seem to be an additional proof that they have been derived from an ancestor in which the pouch was an organ of functional importance. The first definite proof of the occurrence of a pouch in any mammal not a Marsupial or a Monotreme was made by Malkmus, who found this structure in a Sheep. It seems, however, that the structures found in the higher mammals are not always comparable to the marsupium of the Marsupials, but sometimes to the mammary pouch of the Monotreme. That the Marsupials are a side line, and not involved in the ancestry of the Eutheria, is an opinion which is at present widely held. At the same time it is reasonable to suppose that the original stock lying between the Prototheria and the Metatheria, whence the latter and the Eutheria have arisen, preserved both the mammary pouch of the lower mammal and the marsupium of the further-developed stage, as does Phalangista occasionally at the present day. Hence to find remnants of both structures in existing mammals would not so incredible. This is what Dr. Klaatsch believes to be the case. In certain Ungulates, including two species of Antelope, Dr. Klaatsch found very considerable rudiments of folds provided with unstriated muscular fibre; there were in the adult Cervicapra isabellina a pair of pouches, one on each side, and a rudiment of a second on either side; possibly this multiplication of the pouches has relation to the number of young. That there is more than one pouch makes a comparison with the mammary pouch rather than with the marsupium probable. The Ungulate teat, it must be remembered (see p. 16), is a secondary teat; hence there is no difficulty in the comparison from this point of view. A pouch containing a primary teat would of course be absolutely incomparable with a mammary pouch, because in that case the wall of the teat itself would be the pouch.

Mammals belonging to quite different Orders show traces more or less marked of a marsupium. In young Dogs the teats are borne upon an area where the skin is thinner, the covering of hair less dense than elsewhere—all points of resemblance to the inside of the pouch of a Marsupial; in addition to this there are traces of the sphincter marsupii muscle. In other Carnivora there are similar vestiges. In Lemur catta a more complete rudiment of a marsupial pouch is to be met with. In this Lemur the teats are both inguinal and pectoral; the skin in these regions is thin and but slightly hairy, and extends forwards as two bands of the same thinness and smoothness on each side of the densely hairy skin covering the sternum. This area is sharply separated from the rest of the integument by a fold which runs parallel to the longitudinal axis of the body, and can be comparable with nothing save the rudiment of the marsupial fold.

One is tempted to wonder how far the habit which certain Lemurs have of carrying their young across the abdomen with the tail wrapped round the body of the mother is a reminiscence of a marsupial pouch.

Skeleton.

The skeleton of the Mammalia consists almost solely of the endoskeleton. It is only among the Edentata that an exoskeleton of bony plates in the skin is met with. As in other Vertebrates, the skeleton is divisible into an axial portion, the skull and vertebral column, and an appendicular skeleton, that of the limbs. The bones of mammals are well ossified, and in the adult there are but few and small tracts of cartilage left.

Vertebral Column.—The vertebral column of the mammals, like that of the higher Vertebrata, consists of a number of separate and fully-ossified vertebrae.

The constitution of a vertebra upon which all the usual processes are marked is as follows:—There is first of all the body or centrum of the vertebra, a massive piece of bone shaped like a disc or a cylinder. The centra of contiguous vertebrae are separated by a certain amount of fibrous tissue forming the intervertebral disc, and the apposed surfaces of the centra are as a rule nearly flat. In this last feature, and in the important fact that the centra are ossified from three distinct centres, the anterior and posterior pieces ("epiphyses") remaining distinct for a time, even for a long time (as in the Whales), the centra in the mammals differ from those of reptiles and birds. The epiphyses are not found throughout the vertebral column of the lowly-organised Monotremata, and they do not appear to exist in the Sirenia.

Fig. 5.—Anterior surface of Human thoracic vertebra (fourth), × ⅔. az, Anterior zygapophysis; c, body or centrum; l, lamina, and p, pedicle, of the neural arch; nc, neural canal; t, transverse process. (From Flower's Osteology of the Mammalia.)	Fig. 6.—Side view of first lumbar vertebra of Dog (Canis familiaris). × ¾. a, Anapophysis; az, anterior zygapophysis; m, metapophysis; pz, posterior zygapophysis; s, spinous process; t, transverse process. (From Flower's Osteology.)

From each side of the centrum on the dorsal side arises a process of bone which meets its fellow in the middle line above, and is from there often prolonged into a spine. A canal is thus formed which lodges the spinal cord. This arch of bone is known as the neural arch, and the dorsal process of the same as the spinous process. The sides of the neural arch bear oval facets, by which successive vertebrae articulate with one another: those situated anteriorly are the anterior zygapophyses, while those on the posterior aspect of the arch are the posterior zygapophyses; these articular facets do not exist in the tail-region of many mammals, e.g. Whales.

In addition to the dorsal median spinous process of the vertebra there may be a ventral median process, arising of course from the centrum, termed the hypapophysis.

From the sides of the neural arch, or from the centrum itself, there is commonly a longer or shorter process on each side, known as the transverse process. This is sometimes formed of two distinct processes, one above the other; in such cases the upper part is called a diapophysis, the lower a parapophysis.

The neural arch may also bear other lateral processes, of which one directed forwards is the metapophysis, the other directed backwards the anapophysis.

The series of bones which constitute the vertebral column can be divided into regions. It is possible to recognise cervical, dorsal, lumbar, sacral, and caudal vertebrae. In the case of animals with only rudimentary hind-limbs, such as the Whales, there is no recognisable sacral region. The neck or cervical vertebrae are nearly always seven in number. The well-known exceptions are the Manatee, where there are six, and certain Sloths, where there are six, eight, or nine. These rare exceptions only accentuate the very remarkable constancy in number, which is very distinctive of the mammals as compared with lower Vertebrata. There are of course abnormalities, the last cervical, and sometimes the last two, assuming the characters of the ensuing dorsals, by developing a more or less complete rib. There are also recorded examples of Bradypus, in which the number of cervicals is increased to ten. The characteristics, then, of the cervical vertebrae are, in the first place, that they do not normally bear free ribs, and that there is a break as a rule between the last cervical and the first dorsal on this account. In birds, for example, the cervicals, differing in number in different families and genera, gradually approach the dorsals by the gradually lengthening ribs. The transverse processes of the vertebrae are commonly perforated by a canal for the vertebral artery, and are bifid at their extremities. In some Ungulates these vertebrae, moreover, approximate to the vertebrae of lower Vertebrata in the fact that there are ball and socket joints between the centra, instead of only the fibrous discs of the remaining vertebrae.

The first two vertebrae of the series are always very different from those which follow. The first is termed the atlas, and articulates with the skull. The most remarkable fact about this bone (shared, however, by lower Vertebrates) is that its centrum is detached from it and attached to the next vertebra, in connexion with which it will be referred to immediately. The whole bone thus gets a ring-like form, and the salient processes of other vertebrae are but little developed, with the exception of the transverse processes, which are wide and wing-like. In many Marsupials, such as the Wombat and Kangaroo, the arch of the atlas is open below, there being no centre of ossification. In others, such as Thylacinus, there is a distinct nodule of bone in this situation not concrescent with the rest of the arch.

Fig. 7.—Human atlas (young), showing development. × ¾. as, Articular surface for occiput; g, groove for first spinal nerve and vertebral artery; i a, inferior arch; t, transverse process. (From Flower's Osteology.)	Fig. 8.—Inferior surface of atlas of Dog. × ½. sn, Foramen for first spinal nerve; v, vertebrarterial canal. (From Flower's Osteology.)

Fig. 9.—Atlas of Kangaroo.... (From Parker and Haswell's Zoology.)

The second vertebra, which is known as the axis or epistropheus, is a compound structure, the anterior "odontoid process," which fits into the ring of the atlas, being in reality the detached centrum of that vertebra.^[10] It is a curious fact about that process that it has independently become spoon-shaped in two divisions of Ungulates; that it has become so seems to be shown by the fact that in the earlier types of both it has the simple peg-like form, which is the prevailing form. The cervical vertebrae are occasionally wholly (Right Whales) or partially (many Whales, Jerboa, certain Edentates) welded into a combined mass. Indications of this have even been recorded in the human subject.

Fig. 10.—Side view of axis of Dog. × ⅔. o, Odontoid process; pz, posterior zygapophysis; s, spinous process; t, transverse process; v, vertebrarterial canal. (From Flower's Osteology.)	Fig. 11.—Anterior surface of axis of Red Deer. × ⅔. o, Odontoid process; pz, posterior zygapophysis; sn, foramen for second spinal nerve. (From Flower's Osteology.)

The dorsal vertebrae vary greatly in number: nine (Hyperoodon) seems to be the lowest number existing normally; while there may be as many as nineteen, as in Centetes, or twenty-two, as in Hyrax. These vertebrae are to be defined by the fact that they carry ribs, and the first one or two lumbars are often "converted into" dorsals by the appearance of a small supernumerary rib. The spinous processes of these vertebrae are commonly long, and sometimes very long. It is only among the Glyptodons that any of these vertebrae are fused together into a mass.

The lumbar vertebrae, which follow the dorsal, vary greatly in number. There are as few as two in the whale Neobalaena, as many as seventeen in Tursiops; this group, the Cetacea, contains the extremes. Nine lumbars are found in the Lemurs Indris and Loris. As a rule the number of lumbars is to some extent dependent upon that of the dorsals. It often happens that the number of thoraco-lumbar vertebrae is constant for a given group. Thus the Artiodactyles have nineteen of these vertebrae, and the Perissodactyles as a rule twenty-three. A greater number of dorsals implies a smaller number of lumbars, and of course vice versa. The existence of a sacral region formed of a number of vertebrae fused together and supported by the pelvic girdle is characteristic of the mammals, but is not found in the Cetacea and the Sirenia, where functional hind-limbs are wanting. Strictly speaking, the sacrum is limited to the two or three vertebrae whose expanded transverse processes meet the ilia. But to these are or may be added a variable number of vertebrae withdrawn from both the lumbar and the caudal series, which unite with each other to form the massive piece of bone which constitutes the sacrum of the adult.

Fig. 12.—Lepus cuniculus. Innominate bones and sacrum, ventral aspect. acet, Acetabulum; il, ilium; isch, ischium; obt, obturator foramen; pub, pubis; sacr, sacrum; sy, symphysis. (From Parker and Haswell's Zoology.)	Fig. 13.—Anterior surface of fourth caudal vertebra of Porpoise (Phocoena communis), × ½. h, Chevron bone; m, metapophysis; s, spinous process; t, transverse process. (From Flower's Osteology.)

The caudal vertebrae complete the series. They begin in as fully developed a condition as the lumbars, with well-marked transverse processes, etc.; but they end as no more than centra, from which sometimes tiny outgrowths represent in a rudimentary way the neural arches, etc. Very often the caudal vertebrae are furnished with ventral, generally V-shaped, appendages, the chevron bones or intercentra.^[11] These are particularly conspicuous in the Whales and in the Edentates. In the former group the occurrence of the first intercentrum serves to mark the separation of the caudal from the lumbar series. The number of caudals varies from three in Man—and those quite rudimentary—to nearly fifty in Manis macrura and Microgale longicaudata.

Fig. 14.—Lateral view of skull of a Dog. C.occ, Occipital condyle; F, frontal; F.inf, infra-orbital foramen; Jg, jugal; Jm, premaxilla; L, lachrymal; M, maxilla; Maud, external auditory meatus; Md, mandible; N, nasal; P, parietal; Pal, palatine; Pjt, process of squamosal; Pt, pterygoid; Sph, alisphenoid; Sq, squamosal; Sq.occ, supraoccipital; T, tympanic. (From Wiedersheim's Comparative Anatomy.)

The Skull.—The skull in the Mammalia differs from that of the lower Vertebrata in a number of important features, which will be enumerated in the following brief sketch of its structure. In the first place, the skull is a more consolidated whole than in reptiles; the number of elements entering into its formation is less, and they are on the whole more firmly welded together than in Vertebrates standing below the Mammalia in the series. Thus in the cranial region the post- and pre-frontals, the post-orbitals and the supra-orbitals have disappeared, though now and again we are reminded of their occurrence in the ancestors of the Mammalia by a separate ossification corresponding to some of the bones. Nowhere is this consolidation seen with greater clearness than in the lower jaw. That bone, or rather each half of it, is in mammals formed of one bone, the dentary (to which occasionally, as it appears, a separate mento-Meckelian ossification may be added). The angular, splenial, and all the other elements of the reptilian jaw have vanished, though the numerous points from which the mammalian dentary ossifies is a reminiscence of a former state of affairs; and here again an occasional continuance of the separation is preserved, as the case observed by Professor Albrecht of a separate supra-angular bone in a Rorqual attests. Among other reptilian bones that are not to be found in the mammalian skull are the basipterygoids, quadrato-jugal, and supratemporal. A few of these bones, however, though no longer traceable in the adult skull save in cases of what we term abnormalities, do find their representatives in the foetal skull. Professor Parker, for example, has described a supra-orbital in the embryo Hedgehog; a supratemporal also appears to be occasionally independent.

In the mode of the articulation of the lower jaw to the skull the Mammalia apparently, perhaps really, differ from other Vertebrates. In the Amphibia and Reptilia, with which groups alone any comparisons are profitable, the lower jaw articulates by means of a quadrate bone, which may be movably or firmly attached to the skull. In the mammals the articulation of the lower jaw is with the squamosal. The nature of this articulation is one of the most debated points in comparative anatomy. Seeing that Professor Kingsley^[12] in the most recent contribution to the subject quotes no less than fifty-two different views, many of which are more or less convergent, it will be obvious that in a work like the present the matter cannot be treated exhaustively. As, however, Professor Kingsley justly says that "no single bone occupies a more important position in the discussion of the origin of the Mammalia than does the quadrate," and with equal justice adds that "upon the answer given as to its fate in this group depends, in large measure, the broader problem of the phylogeny of the Mammalia," it becomes, or indeed has long been, a matter which cannot be ignored in any work dealing with the mammals. A simple view, due to the late Dr. Baur and to Professor Dollo, commends itself at first sight as meeting the case. The last-named author holds, or held, that in all the higher Vertebrates it is at least on a priori grounds likely that two such characteristically vertebrate features as the lower jaw and the chain of bones bringing the outer world into communication with the internal organ of hearing would be homologous throughout the series. He believed, therefore, that the entire chain of ossicula auditus in the mammal is equal to the columella of the reptile, since their relations are the same to the tympanum on the one hand and to

Fig. 15.—Head of a Human embryo of the fourth month. Dissected to show the auditory ossicles, tympanic ring, and Meckel's cartilage, with the hyoid and thyroid apparatus. All these parts are delineated on a larger scale than the rest of the skull. an, Tympanic ring; b.hy, basihyal element; hy, so-called hyoid bone; in, incus; md, bony mandible; ml, malleus; st, stapes; tp, tympanum; tr, trachea; I. (mk), first skeletal (mandibular) arch (Meckel's cartilage); II. second skeletal (hyoid) arch; III. third (first branchial) arch; IV. V. fourth and fifth arches (thyroid cartilage). (From Wiedersheim's Structure of Man.)

the foramen ovale on the other; and that the lower jaw articulates in the same way in both. It follows, therefore, that the glenoid part of the squamosal must be the quadrate which has become ankylosed with it after the fashion of concentration in the mammalian skull that has already been referred to. The fact that occasionally the glenoid part of the squamosal is a separate bone^[13] appeared to confirm this way of looking at the matter. But the hall-mark of truth is not always simplicity; indeed the converse appears to be frequently the case. And on the whole this view does not commend itself to zoologists at present. For it must be borne in mind that the lower jaw of the mammal is not the precise equivalent of that of the reptiles. Apart from the membrane bones, which may be collectively the equivalents of the dentary of the mammal, there is the cartilaginous articular bone to be considered, which forms the connexion between the rest of the jaw and the quadrate in reptiles. Even in the Anomodontia, whose relations to the Mammalia are considered elsewhere, there is this bone. But in these reptiles the articular bone articulates not only with the quadrate, but also to a large extent with the squamosal, the quadrate shrinking in size and developing processes which give to it very much the look of either the incus or the malleus of the mammalian ear. In fact it seems on the whole to fit in with the views of the majority, as well as with a fair interpretation of the facts of embryology, to consider that the chain of ear bones in the mammal is not the equivalent of the columella of the reptile, but that the stapes of the mammal is the columella, and that the articulare is represented by the malleus and the quadrate by the incus. It is very interesting to note this entire change of function in the bones in question. Bones which in the reptile serve as a means of attachment of the lower jaw to the skull are used in the mammal to convey the waves of sound from the tympanum of the ear to the internal organ of hearing.

Another important and diagnostic feature in the mammalian skull is that the first vertebra of the vertebral column always articulates with two separate occipital condyles, which are borne by the exoccipital bones and formed mainly though not entirely by them. Certain Anomodontia form the nearest approach to the mammals in this particular. The two condyles of Amphibia are purely exoccipital in origin.

In the Mammalia, unlike what is found in lower Vertebrates (but here again the Anomodontia form at least a partial exception), the jugal arch does not connect the face with the quadrate, for, as already said, that bone does not exist, in the Sauropsidan form, in mammals. This arch passes from the squamosal to the maxillary, and has but one separate bone in addition to those two, viz. the jugal or malar.

Fig. 16.—Under surface of the cranium of a Dog. × ½. apf, Anterior palatine foramen; as, posterior opening of alisphenoid canal; AS, alisphenoid; BO, basioccipital; BS, basisphenoid; cf, condylar foramen; eam, external auditory meatus; Ex.O, exoccipital; flm, foramen lacerum medium; flp, foramen lacerum posterius; fm, foramen magnum; fo, foramen ovale; fr, foramen rotundum; Fr, frontal; gf, glenoid fossa; gp, post-glenoid process; Ma, malar; Mx, maxilla; oc, occipital condyle; op, optic foramen; Per, mastoid portion of periotic; pgf, post-glenoid fossa; Pl, palatine; PMx, premaxilla; pp, paroccipital process; ppf, posterior palatine foramen; PS, presphenoid; Pt, pterygoid; sf, sphenoidal fissure or foramen lacerum anterius; sm, stylomastoid foramen; SO, supraoccipital; Sq, zygomatic process of squamosal; Ty, tympanic bulla; Vo, vomer. (From Flower's Osteology.)

In connexion with the elaboration of the chain of auditory ossicles it is very usual for mammals to possess a thin inflated bone, sometimes partly or entirely formed out of the tympanic bone, and known as the tympanic bulla. Whether this structure is thin and inflated or thick and depressed in form it is characteristic of the mammals, and does not occur below them in the series. But it is not present in all mammals. It is absent, for example, in the Monotremes. When it is present it is sometimes formed from other bones, as, for instance, from the alisphenoids. The tympanic ring has been held to be the equivalent of the quadrate. It is more probably the quadrato-jugal.^[14]

Ribs.—All mammals are furnished with ribs, of which the number of pairs differs considerably from group to group, or it may be even from species to species. The ribs are attached as a rule by two heads, of which one, the capitulum, arises as a rule between two centra of successive vertebrae. The other, the tuberculum, springs from the transverse process. Only in the Monotremes are there ribs with but one, the capitular, head. In the posterior part of the series the two heads often gradually coalesce, so that there comes to be but one, the capitular, head. The Whales also, at least the Whalebone Whales, are exceptional in possessing but one head to the ribs, which is the capitular. The first rib joins the sternum below, and a variable number after this have the same attachment. There are always a number of ribs, sometimes called floating ribs, which have no sternal attachment.

Fig. 17.—A, First thoracic skeletal segment for comparison with B, fifth cervical vertebra (Man), b.v. Body of vertebra; c, first thoracic rib; c′, cervical rib (which has become united with the transverse process, tr), the two enclosing the costo-transverse foramen (f.c.t); st, sternum; zy, articular process of the arch (zygapophysis). (From Wiedersheim's Structure of Man.)

In the Whalebone Whales it is the first rib alone which is so attached. As a rule, to which the Whales mentioned are again an exception, the rib is divided into at least two regions—the vertebral portion which is always ossified, and the sternal moiety which is usually cartilaginous. This is, however, often very short in the first rib. They are, however, ossified in the Armadillos and in some other animals. Between the vertebral and sternal portions an intermediate tract is separated off and ossified in the Monotremata. The ribs of existing mammals belong only to the dorsal region of the vertebral column, but there are traces of lumbar ribs and also of cervical ribs. In the Monotremata, indeed, these latter are persistently free for a very long period, and in some cases never become ankylosed with their vertebrae. But it should be noted that in this group there is no approximation to the state of affairs which exists in many lower Vertebrates, where there is a gradual transition between the ribs of the cervical and those of the dorsal region of the vertebral column; for that of the seventh ribs in Monotremes is smaller than those which precede it.

Fig. 18.—Sternum and sternal ribs of the Common Mole (Talpa europaea), with the clavicles (cl) and humeri (H); M, manubrium sterni. Nat. size. (From Flower's Osteology.)	Fig. 19.—Sternum of the Pig (Sus scrofa). × ¼. ms, Mesosternum; ps, presternum; xs, xiphisternum. (From Flower's Osteology.)

The Sternum.—All the Mammalia so far as is known possess a sternum. This is the bone, or series of bones (sternebrae), which lies upon the ventral surface of the chest, and to which the ribs are attached below. The development of the sternum has been shown to take place from the fusion of the ribs below into two lateral bands, one on each side; the approximation of these bands forms the single and unpaired sternum of most mammals. Very considerable traces, however, of the paired state of the sternal bones often exist; thus in the Sperm Whale the first piece of the sternum is divided into two by a longitudinal division, and the second piece is longitudinally grooved. The development of the sternum out of the fused ends of ribs is shown in a more complete condition in some species of Manis than in many other mammals. Thus in M. tricuspis the last ribs of those which are attached to the sternum are completely fused together into a single piece on each side.^[15] As a general rule the last ribs which come into relation with the sternum do so only in an imperfect way, being simply firmly attached at their sides to, but not fused with, the last ribs which are definitely articulated with the sternum. Contrary to what is found in lower Vertebrates, the sternum of the Mammalia consists of a series of pieces, as many as eight or nine or even sixteen in Choloepus, of which the first is called the manubrium sterni, and the last the ensiform cartilage, xiphisternum, or xiphoid process. The latter often remains largely cartilaginous throughout life; in fact this is generally but not universally the case with that part of the breastbone. The most extraordinary modification of the xiphoid process is seen in the African species of the genus Manis, where it diverges into two long cartilages, which run back to the pelvis and then, curving round, run forwards and fuse together in the middle line anteriorly. These processes serve for the attachment of certain tongue-muscles. They were looked upon by Professor Parker as the equivalents of the "abdominal ribs" of reptiles elsewhere non-existent among mammals. This view is not, however, usually held. The manubrium sterni is often keeled in the middle line below; this is so with the Bats, which thus approach the birds, and probably for the same reason, i.e. the need of an enlarged origin for the pectoral muscle, which is concerned in the movements of flight. In many forms this part of the sternum is much broader than the pieces which follow; this is so with the Viscacha. In the Pig the precise reverse is seen, the manubrium being narrower than the rest of the sternal bonelets. It will be noticed, however, that in this and similar cases there are no clavicles. Ribs are attached between the successive pieces of the sternum. When the sternum is reduced, as it is in the Cetacea and in the Sirenia, it is the intermediate part of the series of bones which becomes abbreviated or vanishes. The Sperm Whale has only a manubrium sterni and a following piece belonging to the mesosternum. It is fair to say that the xiphoid process and the rest of the sternum have disappeared, since among the Toothed Whales a progressive shortening of the sternum can be seen. In the Whalebone Whales the sternum is still further reduced; the manubrium is alone left, and to it are attached but a single pair of ribs. In Balaena, however, a rudimentary piece, apparently comparable to a xiphoid process, has been detected.

Fig. 20.—Sternum of Rudolphi's Whale (Balaenoptera borealis), showing its relation to the inferior extremities of the first pair of ribs. × 1⁄10. (From Flower's Osteology.)	Fig. 21.—Sternum of a young Dugong (Halicore indicus). × ¼. From a specimen in the Leyden Museum, ps, Presternum; xs, xiphisternum. (From Flower's Osteology).

From the instances which have been described, as well as from the mode of development of the sternum and from the number of free ribs, i.e. ribs which are not attached to it, it would seem that the sternum has undergone a considerable reduction in its size. This reduction may be possibly accounted for by the need for respiratory activity, which is clearly increased by a less-marked fixity of the walls of the thoracic cavity. In the case of the Whales one can hardly help coming to that conclusion. The arrangement in the Monotremata does not, however, point in the same direction; for these animals are precisely like the higher Mammalia in the reduction of the sternum and of the number of ribs which reach it.

The Episternum.—The Mammalia are as a rule to be distinguished from lower Vertebrates by the absence of an episternum, or interclavicle as it is also called. In the Monotremata, however, there is a large T-shaped bone which does not overlie the sternum as in reptiles, but is anterior to it. The relations of this bone to the clavicles seem to leave no doubt that it is the equivalent of the Lacertilian interclavicle or episternum. The Monotremata are not, however, the only mammals in which this structure is to be seen. The Mole in the embryonic condition is provided with pieces of bone which overlie the manubrium sterni

Fig. 22.—Shoulder girdle of Ornithorhynchus. c¹, c², c³, First, second, third ribs; cl, clavicle; e.c, epicoracoid; es′ and es″, interclavicle (episternum); m.c, metacoracoid; m.s, manubrium sterni; sc, scapula; st, sternebra. (From Wiedersheim's Structure of Man.)

Fig. 23.—Episternum of an embryo Mole. (After A. Götte.) cl, Clavicle; es′, central portion of the episternum; es″, lateral portion of the same; r.c, costal ribs; st, sternum. (The figure was constructed from two consecutive horizontal sections.) (From Wiedersheim's Structure of Man.)

and are attached to the clavicles, and are no doubt to be regarded as the same structure. Probably in many mammals the manubrium will be found to be partly made up of corresponding rudiments. In any case, vestiges of an episternum in the shape of two minute ossicles have been discovered in Man, lying in front of the manubrium. They have been termed ossa suprasternalia. In Man and in the Mole the paired nature of the episternum is clearly apparent. It has been suggested that this structure in its entirety belongs to the clavicles, just as the sternum belongs to the ribs; i.e. that it formed out of the approximated and fused ends of the clavicles. Dr. Mivart^[16] figured a good many years since a pair of ossicles in Mycetes, lying in one case between the ends of the clavicles and the manubrium sterni, and in another example anterior to the ventral ends of the clavicles. Gegenbaur has figured a pair of similar bones in the Hamster.^[17] It is possible that these are to be referred to the same category. It has also been suggested that these supposed episternal rudiments are the vestiges of a pair of cervical ribs.

Fig. 24.—Episternal vestiges in Man. cl, Clavicle, sawn through; es, "episternum" (sternoclavicular cartilage); l′, interclavicular ligament; l″, costoclavicular ligament; m.s, manubrium sterni; o.s, ossa suprasternalia; r.c, first rib; st, sternum. (From Wiedersheim's Structure of Man.)

The Pectoral Girdle.—The skeleton by which the fore-limb is connected with the trunk is known as the Pectoral Girdle. The main part of this girdle is formed by the large scapula, or blade-bone as it is often termed. The coracoidal elements will be dealt with later. The scapula is not firmly connected with the backbone; it is attached merely by muscles, thus presenting a great difference from the corresponding pelvic girdle. The reason for this difference is not easy to understand. On the one hand it may be pointed out that in all running animals at any rate there is a greater need for the fixation in a particularly firm way of the hind-limbs; but, again, in the climbing creatures both limbs would, one might suppose, be bettered by a firm fixation. It must be remembered, however, that in the latter case the same result is at least partly brought about by a well-developed clavicle, which fixes the girdle to the sternum and so to the vertebral column by means of the ribs.

Broadly speaking, too, the fore-limbs require a greater freedom and variety of movement than the hind-limbs, which are supports for or serve to push along the rapidly-moving body. Stronger fixation is therefore a greater necessity posteriorly than anteriorly. In any case, whatever the explanation, this important difference exists.

Fig. 25.—Right scapula of Dog (Canis familiaris). × ¼. a, Acromion; af, prescapular fossa; c, coracoid; cb, coracoid or anterior border; css, indicates the position of the coraco-scapular suture, obliterated in adult animals by the complete ankylosis of the two bones; gb, glenoid or posterior border; gc, glenoid cavity; pf, postscapular fossa; s, spine; ss, suprascapular border. (From Flower's Osteology.)

Fig. 26.—Right scapula of Red Deer (Cervus elaphus). × ¼. a, Acromion; af, anterior or prescapular fossa; c, coracoid; gc, glenoid cavity; pf, postscapular fossa; ss, partially ossified suprascapular border. (From Flower's Osteology.)

The shoulder-blade of mammals is as a rule a much-flattened bone with a ridge on the outer surface known as the spine; this ridge ends in a freely-projecting process, the acromion, from which a branch often arises known as the metacromion. This gives a bifurcate appearance to the end of the ridge. The spine is less developed and the scapula is narrower in such animals as the Dog and the Deer which simply run, and whose fore-limbs therefore are not endowed with the complexity of movement seen, for instance, in the Apes.

It has been pointed out that the area which lies in front of

Fig. 27.—Right scapula of Dolphin (Tursiops tursio). × ¼. a, Acromion; af, prescapular fossa; c, coracoid; gc, glenoid cavity; pf, postscapular fossa. (From Flower's Osteology.)

Fig. 28.—Side view of right half of shoulder girdle of a young Echidna (Echidna hystrix). × ⅔. a, Acromion; c, coracoid; cb, coracoid border; cl, clavicle; css, coraco-scapular suture; ec, epicoracoid; gb, glenoid border; gc, glenoid cavity; ic, interclavicle; pf, postscapular fossa; ps, presternum; s, spine; ss, suprascapular epiphysis; ssf, subscapular fossa. (From Flower's Osteology.)

the spine, the prescapular lamina, is most extensively developed in such animals as perform complex movements with the fore-limbs. The Sea Lion and the Great Anteater are cited by Professor G. B. Howes as examples of this preponderance of the anterior portion of the scapula over that which lies behind the spine. The general shape of the scapula varies considerably among the different orders of mammals; but it always presents the characters mentioned, which are nowhere seen among the Sauropsida except among certain Anomodonts, which will be duly referred to (see p. 90). The most conspicuous divergences from the normal are to be found in the Cetacea and the Monotremata. In the former the acromion is approximated so nearly to the anterior border of the blade-bone that the prescapular fossa is reduced to a very small area; and in Platanista the acromion actually coincides with the anterior border, so that that fossa actually disappears. In the Whales, too, the scapula is as a rule very broad, especially above; it has frequently a fan-like contour. In the Monotremata the acromion also coincides with the anterior border of the scapula; but the sameness of appearance which it thus presents (in this feature) to the Cetacean scapula is apparently not due to real resemblance. What has happened in the Monotremata is, that the prescapular fossa is so enormously expanded that it occupies the whole of the inner side of the blade-bone, while the subscapular fossa which, so to speak, should occupy that situation, has been thus pushed round to the front, where it is divided from the postscapular fossa by a slight ridge only.

The clavicle is a bone which varies much in mammals. It is sometimes indeed, as in the Ungulata, entirely absent; in other forms it shows varying degrees of retrocession in importance; it is only in climbing, burrowing, digging, and flying mammals that it is really well developed.

Fig. 29.—Shoulder girdle, with upper end of sternum (inner surface) of Shrew (Sorex), after Parker, × 7. a, Acromion; c, coracoid; cl, clavicle; ec, partially ossified "epicoracoid" of Parker, or rudiment of the sternal extremity of the coracoid; ma, metacromial process; mss, ossified "mesoscapular segment"; ost, omosternum; pc, rudiment of precoracoid (Parker); ps, presternum; sr¹, first sternal rib; sr², second sternal rib. (From Flower's Osteology.)

In the higher Mammalia the coracoid^[18] is present, but does not reach the sternum as in the Monotremata. It is known to human anatomists as the coracoid process of the scapula. It has been found, however, by Professor Howes^[19] and others, that this process really consists of two separate centres of ossification, forming two separate bonelets, which in the adult become firmly ankylosed to each other and to the scapula. These two separate bones have been met with in the embryo of Lepus, Sciurus, and the young of various other mammals belonging to very diverse orders, such as Edentates and Primates. The separation even occasionally persists in the adult. The question is, What is the relation of these bonelets to the coracoid of the Monotremata and to the corresponding regions of reptiles? Professor Howes terms the lower patch of bone the metacoracoid and the upper the epicoracoid; the former is alone concerned with the glenoid cavity. It must therefore, one would suppose, correspond to the "coracoid" of the Monotremata, while the upper piece of bone is the epicoracoid process of that mammal. The Mammalia, therefore, higher as well as lower, differ from the reptiles in that the coracoid is formed of two bones, the exceptions being, among some other extinct forms, certain of the Anomodontia, a group which it will be recollected is the nearest of all reptiles to the mammals.

Fig. 30.—Distal extremity of the humerus to show Epicondylar Foramina. A, In Hatteria; B, in a Lizard (Lacerta ocellata); C, in the Domestic Cat; D, in Man. c.e, External condyle; c.i, internal condyle. In A the two foramina are developed (at i, the entepicondylar; at ii, the ectepicondylar). The only canal (†) present in the Lizard (B) is on the external ulnar side, in the cartilaginous distal extremity. In Man (D) an entepicondylar process (pr) is sometimes developed and continued as a fibrous band. (From Wiedersheim's Anatomy of Man.)

The Fore-limb.—The humerus is of varying length among mammals. A feature which it sometimes shares with the humerus of lower forms is the presence of an entepicondylar foramen, a defect of ossification situated above the inner condyle of that bone which transmits a nerve. The same foramen and an additional ectepicondylar foramen are found in the ancient reptilian type Hatteria (Sphenodon); it occurs also in the Anomodont reptiles. It is as a rule only the lower forms among mammals which show this foramen; thus it is present in the Mole and absent in the Horse. The fact that it is occasionally met with in Man is an additional proof of the, in many respects, ancient structure of the highest type of Primate.

The radius and the ulna, which together constitute the fore-arm, are both present in a large number of mammals, but the ulna tends to vanish in the purely walking and digitigrade Ungulates, being present, however, in the more ancient forms of these Ungulates. In Man and in many other mammals the radius can be moved from its normal position and crossed over the ulna; this movement of pronation has been permanently fixed in the Elephant, where the bones are crossed but cannot be altered in position by the contractions of any muscles. Other types agree with the Elephant in this fixation of the two bones.

Fig. 31.—Bones of fore-arm and manus of Mole (Talpa europaea). × 2. c, Cuneiform; ce, centrale; l, lunar; m, magnum; p, pisiform; R, radius; rs, radial sesamoid (falciform); s, scaphoid; td, trapezoid; tm, trapezium; U, ulna; u, unciform; I-V, the digits. (From Flower's Osteology.)

The bones of the wrist show great variation among mammals. The greatest number present are to be seen in such a type as the Mole. Here we have a proximal row, consisting of the scaphoid, lunar, cuneiform, and pisiform, which are arranged in their proper order, beginning with that on the radial side of the limb, that side which bears the first digit. A second row articulates proximally with these bonelets and distally with the metacarpals; the bones composing it are, mentioning them in the same order, trapezium, trapezoid, centrale, magnum, unciform.

The centrale does not, however, really belong to the distal carpal row, and is as a rule situated in the middle of the carpus away from articulation with the metacarpals. It is a bone which is not commonly present in the mammalian hand, but is present in various lower forms, such as the Beaver and Hyrax. It also occurs in such high types as the majority of Monkeys; it is to be found in the Human foetal carpus. Many extinct forms possessed a separate centrale. Its importance in the formation of the interlocking condition of the Ungulate foot is referred to later, on p. 196. The only mammal which appears to have the proper five bones in the distal row of the carpus corresponding to the five metacarpals is Hyperoodon, where this state of affairs at least occasionally occurs. The final bone of that series, the unciform, seems to represent two bones fused. Very often the carpus is reduced by the fusion of certain of the carpal bones; thus among the Carnivora it is usual for the scaphoid and the lunar to be fused. It is interestingly significant that these bones retain their distinctness in the ancestral Creodonts. In many Ungulates the trapezium vanishes. The reduction of the toes in fact implies a reduction of the separate elements of the carpus.

As to the digits of the mammalian hand, the greatest number is five, the various supplementary bonelets known as prepollex and postminimus being, it is now generally held, merely supplementary ossifications not representing the rudiments of pre-existing fingers. They may, however, bear claws.^[20] The number of phalanges which follow upon the metacarpals is almost constantly three in the mammals, excepting for the thumb, which has only two. This is highly characteristic of the group as opposed to reptiles and birds, and the increase in the number of these bones in the Whales and to a very faint degree in the Sirenia is a special reduplication, which will be mentioned when those animals are treated of.

The Pelvic Girdle.—The pelvic girdle or hip girdle is the combined set of bones which are attached on the one hand to the sacrum and on the other articulate with the hind-limb. Four distinct elements are to be recognised in each "os innominatum," the name given to the conjoined bones of each half of the entire pelvis. These are:—the ilium, which articulates with the sacrum; the ischium, which is posterior; the pubis, which is anterior; and finally, a small element, the cotyloid, which lies within the acetabular cavity where the femur articulates. The epipubes of the Monotreme and the Marsupial are dealt with elsewhere (see p. 116) as they are peculiar to those groups.

Professor Huxley pointed out many years since that while the Eutherian Mammalia differ from the reptiles in the fact that the axis of the ilium lies at a less angle with that of the sacrum, Ornithorhynchus comes nearest to the reptile in the fact that this axis is nearly at right angles to that of the sacrum. It is particularly interesting to find that this peculiarity of Ornithorhynchus is only acquired later in life, and that the pelvis of the foetus conforms in these angles to the adults of other mammalian groups. In any case, the backward rotation of the pelvis is a mammalian characteristic, and it is most nearly approached among reptiles by the extinct Anomodontia, whose affinities to mammals will be dealt with on a later page (p. 90). Another peculiarity of the mammalian pelvis appears to be the cotyloid bone already referred to. In the Rabbit this bone completely shuts out the pubis from any share in the acetabular cavity; later it ankyloses with that bone. In Ornithorhynchus the cotyloid or os acetabuli is a larger element of the girdle than is the pubis. In other mammals, therefore, it seems to be a rudimentary structure. But it seems to be a bone peculiar to and thus distinctive of the mammals as compared with other vertebrates. The acetabular cavity is perforated in Echidna as in birds; but in certain Rodents the same region is very thin and only closed by membrane, as in Circolabes villosus.

The number and the arrangement of the bones in the hind-limb correspond exactly to those of the fore-limb. The femur, which corresponds to the humerus, shows some diversities of form. The neck, which follows upon the almost globular head, the surface of articulation to the acetabular cavity of the pelvis, has two roughened areas or tuberosities for the insertions of muscles. A third such area, known as the third trochanter, is present or absent as the case may be, and its presence or absence is of systematic import. As a general rule the thigh-bones of the ancient types of mammals are smoother and less roughened by the presence of these three trochanters than in their modern representatives. The radius and the ulna are represented in the hind-leg by the tibia and the fibula. These bones are not crossed, and do not allow of rotation as is the case with the radius and the ulna. In Ungulate animals there is the same tendency to the shortening and rudimentary character of the fibula that occurs in the case of the ulna, but it is more marked. It has been shown in tracing the history of fossil Ungulates that the hind-limbs in their degree of degeneration are as a rule ahead of the fore-limbs. This is natural when we reflect that the hind-limbs must have preceded the fore-limbs in their thorough adaptation to the cursorial mode of progression. In the Mammalia the ankle-joint is always what is termed cruro-tarsal, i.e. between the ends of the limb-bones and the proximal row of tarsals; not in the middle of the tarsus as in some Sauropsida (reptiles and birds). The bones of the ankle are much like those of the hand; but there are never more than two bones in the proximal row, which are the astragalus and the calcaneum. The former is perhaps to be looked upon as the equivalent of the cuneiform and lunar together. But the views as to the homologies of the tarsal bones differ widely. Below these is the navicular, regarded as a centrale. The distal row of the tarsus has four bones, three cuneiforms and a cuboid. Reduction is effected by the soldering together of two cuneiforms as in the Horse, by the fusion of the navicular and cuboid as in the Deer. No mammal has more than five toes, and the number tends to become reduced in cursorial animals (Rodents, Ungulates, Kangaroos).

Fig. 32.—Anterior aspect of right femur of Rhinoceros (Rhinoceros indicus). × ½. h, Head; t, great trochanter; t′, third trochanter. (From Flower's Osteology.)

Teeth.—The teeth of the Mammalia^[21] differ from those of other vertebrated animals in a number of important points. These, however, entirely concern the form of the adult teeth, their position in the mouth, and the succession of the series of teeth. Developmentally and histologically there are no fundamental divergences from the teeth of vertebrates lower in the scale.

In mammals, as for example in the Dog, the teeth consist of three kinds of tissue—the enamel, the dentine, and the cement. The enamel is derived from the epidermis of the mouth cavity, and the two remaining constituents from the underlying dermis. The teeth originate quite independently of the jaws, with which they are later so intimately connected; the independence of origin being one of the facts upon which the current theory of the nature of teeth is founded. It has been pointed out that the scales of the Elasmobranch fishes consist of a cap of enamel upon a base of dentine, the former being derived from the epidermis and modelled upon a papilla of the dermis whose cells secrete the dentine. The fact that similar structures arise within the mouth (i.e. the teeth) is explicable when it is remembered that the mouth itself is a late invagination from the outside of the body, and that therefore the retention by its tissues of the capacity to produce such structures is not remarkable.

Fig. 33.—Diagrammatic sections of various forms of teeth. I, Incisor or tusk of Elephant, with pulp cavity persistently open at base; II, Human incisor during development, with root imperfectly formed, and pulp cavity widely open at base; III, completely formed Human incisor, with pulp cavity opening by a contracted aperture at base of root; IV, Human molar with broad crown and two roots; V, molar of the Ox, with the enamel covering the crown deeply folded, and the depressions filled up with cement; the surface is worn by use, otherwise the enamel coating would be continuous at the top of the ridges. In all the figures the enamel is black, the pulp white; the dentine represented by horizontal lines, and the cement by dots. (After Flower and Lydekker.)

The relations of the three constituents of the tooth in its simplest form is shown in the accompanying diagram, where the intimate structure of the enamel, dentine, and cement (or crusta petrosa as it is sometimes called) is not indicated. The latter has the closest resemblance to bone. The dentine is traversed by fine canals which run parallel to each other and anastomose here and there. The enamel is formed of long prismatic fibres, and is excessively hard in structure, containing less animal matter than the other tooth tissues. To this fact is frequently due the complicated patterns upon the grinding teeth of Ungulates, which are produced by the wearing away of the dentine and the cement, and the resistance of the enamel.

The centre of the tooth papilla remains soft and forms the pulp of the tooth, which is continuous with the underlying tissues of the gum by a fine canal or a wide cavity as the case may be. In teeth which persistently grow throughout the lifetime of the animal, as for example the incisors of the Rodents, there is a wide intercommunication between the cavity of the tooth and the tissues of the gum; only a narrow canal exists in, for instance, the teeth of Man, and in fact in the vast majority of cases. The three constituents of the typical teeth are not, however, found in all mammals; the layer which is sometimes wanting is the enamel. This is the case with most Edentates; but the interesting discovery has been made (by Tomes) that in the Armadillo there is a downgrowth of the epidermis similar to that which forms the enamel in other mammals, a rudimentary "enamel organ."

Teeth are present in nearly all the Mammalia; and where they are absent there is frequently some evidence to show that the loss is a recent one. The Whalebone Whales, the Monotremata, Manis, and the American Anteaters among the Edentata are devoid of teeth in the adult state. In several of these instances, however, more or less rudimentary teeth have been found, which either never cut the gums or else become lost early in life. The latter is the case with Ornithorhynchus, where there are teeth up to maturity (see p. 113). Kükenthal has found germs of teeth in Whales, and Röse in the Oriental Manis. The loss of the teeth in these cases seems to have some relation to the nature of the food. In ant-eating mammals, as in the Anteaters and Echidna, the ants are licked up by the long and viscid tongue, and require no mastication. Yet it must be remembered that Orycteropus is also an anteater, like the Marsupial Myrmecobius, both of which genera have teeth.

The first of the essential peculiarities of the mammalian teeth as compared with those of other vertebrates concerns the position of the teeth in the mouth. There is no undoubted mammal extinct or living in which the teeth are attached to any bones other than the dentary, the maxilla, and the premaxilla. There are no vomerine, palatine, or pterygoid teeth, such as are met with in Amphibia and Reptilia.

The other peculiarities of the mammalian teeth, though true of the great majority of cases, are none of them absolutely universal.

But it is necessary to go into the subject at some length on account of the great importance which has been laid upon the teeth in deciding questions of relationship; moreover, largely no doubt on account of their hardness and imperishability, our knowledge of certain extinct forms of Mammalia is entirely based upon a few scattered teeth; while of some others, notably of the Triassic and Jurassic genera, there is not a great deal of evidence except that which is furnished by the teeth. Indeed the important place which odontography holds in comparative anatomy is from many points of view to be regretted, though inevitable. "In hardly any other system of organs of vertebrated animals," remarks Dr. Leche, "is there so much danger of confounding the results of convergence of development with true homologies, for scarcely any other set of organs is less conservative and more completely subservient to the lightest impulse from without." Affinities as indicated by the teeth are sometimes in direct contradiction to those afforded by other organs; or, as in the case of the simple Toothed Whales, no evidence of any kind is forthcoming. Dr. Leche has pointed out that, judged merely from its teeth, Arctictis would be referred to the Raccoons, though it is really a Viverrid; while Bassariscus, which Sir W. Flower showed to be a Raccoon, is in its teeth a Viverrid. Mr. Bateson has been obliged to hamper the subject with another difficulty.

In dealing with the variations of teeth,^[22] Mr. Bateson has brought together an immense number of facts, which tend to prove that the variability of these structures is much greater than had been previously recognised; that this variability is often symmetrical; and that in some animals, as in "Canis cancrivorus, a South American fox, the majority showed some abnormality." When we learn from Mr. Bateson that "of Felis fontanieri, an aberrant leopard, two skulls only are known, both showing dental abnormalities," it seems dangerous to rear too lofty a superstructure upon a single fossil jaw. It must be noted too that, contrary to the prevailing superstition, it is not domestic animals which show the greatest amount of tooth variation. As to special homologies between tooth and tooth, with which we shall deal on a later page, Mr. Bateson has urged almost insuperable difficulties.

Fig. 34.—Skull of Dasyurus (lateral view). al.sph, Alisphenoid; ang, angular process of mandible; fr, frontal; ju, jugal; lcr, lachrymal; max, maxilla; nas, nasal; oc.cond, occipital condyle; par, parietal; par.oc, paroccipital process; p.max, premaxilla; s.oc, supraoccipital; sq, squamosal; sq′, zygomatic process of squamosal. (From Parker and Haswell's Zoology.)

Fig. 35.—Upper and lower teeth of one side of the mouth of a Dolphin (Lagenorhynchus), illustrating the homodont type of dentition in a mammal. (After Flower and Lydekker.)

The teeth of the Mammalia are almost without exception "heterodont," i.e. they show differences of structure in different parts of the mouth. As a general rule, teeth can be grouped into cutting incisors, sharp conical canines, and molars, with a surface which is in the majority of cases suited for grinding. In this they contrast with the majority of the lower vertebrates, where the teeth are "homodont" (or, better, homoeodont), i.e. all more or less similar and not fitted by change of form to perform different duties. But there are exceptions on both sides. In the Toothed Whales the teeth are homodont, as they are in the frog and in most reptiles; on the other hand, some of the remarkable reptiles belonging to Professor Huxley's order of the Anomodontia have distinct canines, and show other differentiations in their teeth.

A second characteristic of the mammalian dentition is the limited number of the teeth, which rarely exceeds fifty-four. Here again the Toothed Whales are an exception, the number of their teeth being as great as in many reptiles. In the Mammalia the number of the teeth is fixed (excepting of course for abnormalities), while in reptiles there is frequently no precise normal. Two regions may be distinguished in every tooth—the crown and the root; the latter, as its name denotes, is imbedded in the gum, while the crown is the freely-projecting summit of the tooth. The varying proportions of these two regions of the tooth enables us to divide teeth into two series—the brachyodont and the hypselodont; in the latter the crown is developed at the expense of the root, which is small; the hypselodont tooth is one that grows from a persistent pulp or, at any rate, one that is long open. Brachyodont teeth on the contrary have narrow canals running into the dentine. The primitive form of the tooth seems undoubtedly to be a conical single-rooted tooth, such as is now preserved in the Toothed Whales and in the canine teeth of nearly all animals. The development of the teeth, that is, the simple bell-shaped form of the enamel organ, seems to go some way towards proving this; but it is quite another question whether we can fairly regard the Whales as having retained this early form of tooth. In their case the simplification, as is so often the case where organs are simplified, seems to be rather degeneration than retention of primitive characters. But this is a matter which must be deferred for the present.

The incisor teeth are generally of simple structure and nearly always single rooted. In the Rodents, in the extinct Tillodontia and in Diprotodont Marsupials, they have grown large, and, as has been already stated, they increase in size continuously from the growing pulp. These teeth have a layer of enamel only on the anterior face, which keeps a sharp chisel-like edge upon them by reason of the fact that the harder enamel is worn away more slowly than the comparatively soft dentine. The "horn" of the Narwhal is another modification of an incisor, as are the tusks of Elephants. Among the Lemurs the incisors are denticulate, and serve to clean the fur in a comb-like fashion. This is markedly the case in Galeopithecus. The incisors are sometimes totally absent, as in the Sloths, sometimes partially absent, as in many Artiodactyles, where the lower incisors bite against a callous pad in the upper jaw, in which no trace of incisors has been found.

Canine teeth are present in the majority of mammals, but are absent without a single exception from the jaws of the Rodentia. The canine tooth of the upper jaw is that tooth which comes immediately after the suture dividing the premaxillary from the maxillary bone. The canines are as a rule simple conical teeth, with but a single root; indeed they resemble what we may presume to have been the first kind of tooth developed in mammals. In this they resemble also as a general rule the foregoing incisors. But instances are known where the canines are implanted by two roots. This is to be seen in Triconodon, in the pig Hyotherium, in the Mole and some other Insectivores, and in Galeopithecus, where the incisors also may be thus implanted in the jaw. Furthermore, the simple condition of the crown of the tooth may be departed from. This is the case with a Fruit Bat belonging to the genus Pteralopex. In the more primitive Mammalia it is common to find no great difference between the canines and incisors; such is the case with the early Ungulate types of Eocene times, such as Xiphodon. In modern mammals, however, especially among the Carnivora, the canines tend to become larger and stronger than the incisors, and in some of the Cats and in the Walrus these teeth are represented by enormous offensive tusks. It is not rare for the canines of male animals to be larger than those of their mates. There are also cases such as the Musk-deer and the Kanchil where the male alone possesses these teeth, but only in the upper jaw. The teeth which follow the canines are known as the grinders or cheek teeth, or more technically as premolars and molars. These two latter terms separate teeth which arise at different periods, and their use will be explained later. In the meantime it may be pointed out that the cheek teeth are the teeth which show the greatest amount of variation in their structure; this is shown by the number and variety of the cusps in which the biting surface ends. The grinding teeth vary from simple one-cusped teeth, precisely like canines, to teeth with an enormous number of separate tubercles. In the former case it is hard to distinguish between incisors, canines, and cheek teeth in the lower jaw, where no suture separates the bone. Moreover it is quite common for the first cheek tooth in the lower jaw to have the characters of a canine, while the true canine approximates in its form to the antecedent incisors. This is so, for instance, with the Lemurs, where the first premolar is caniniform, and the canine shares in the curious procumbent attitude which distinguishes the lower incisors of many of those animals.

A variable number of the anterior cheek teeth may be little more than simple conical teeth; but the rest of the set are commonly more complicated. No definite laws can be laid down as to the complication of the posterior as compared with the anterior set. Broadly speaking, it is purely herbivorous creatures in which the least difference can be detected at the two extremities, and which are at the same time the most elaborately decorated with tubercles and ridges. The converse is true that in purely carnivorous animals, including insect- and fish-eating forms, there is the greatest difference between the anterior set of grinding teeth and those which follow. In these two respects such animals as a Lemur and a Rhinoceros occupy the extremes. Furthermore, it may be said that omnivorous creatures lie, as their diet would suggest, in an intermediate position. Generally speaking, when there is a marked difference between the first premolar and molars at the end of the series, there is a gradual approximation in structure of a progressive kind. The tubercles become more numerous in successive teeth; but the corollary which is apparently deducible from this, i.e. that the last molar is the most elaborate of the series, is by no means always true. The last cheek tooth indeed is often degenerate. On the other hand, it is very markedly the largest of the series in such diverse types as the Elephant, the hog Phacochoerus, and the Rodent Hydrochoerus. It is a rule that the cheek teeth of the upper jaw are more complicated than the corresponding teeth of the lower jaw.

The structure of the cheek teeth is very diverse among the Mammalia. Broadly, two types are to be recognised. There are teeth in which the grinding surface is raised into a series of two, to many, tubercles sharper or blunter as the case may be;—sharper and fewer at the same time in carnivorous and especially in insectivorous types, more abundant in omnivorous animals. To this form of tooth the term "bunodont" is applied. There is no doubt that this is the earliest type of tooth; but whether the fewer or the more cusped condition is the primitive one is a question that is reserved for consideration at the end of the present chapter. The other type of grinding tooth is known as "lophodont." This is exemplified by such types as the Perissodactyla and Ungulates generally, and by the Rodents. The tooth is traversed by ridges which have generally a transverse direction to the long axis of the jaw in which the tooth lies. The ridges may be regarded as having been developed between tubercles which they connect and whose distinctness as tubercles is thereby destroyed. Lophodont teeth are only found in vegetable-feeding animals.

Fig. 36.—Molar teeth of Aceratherium platycephalum. × ½. m.1-m.3., Molars; mh, metaloph; p.1-p.4, premolars; ph, protoloph; ps.f, parastyle fossa; te, tetartocone. (After Osborn.)

The special characteristics of the teeth of various groups of animals will be considered further under the accounts of the several orders of recent and fossil Mammalia.

A very general feature of the teeth of the Mammalia is what is usually termed the diphyodont dentition. In the majority of cases there are two sets of teeth developed, of which the first lasts for a comparatively short time, and is termed on account of its usual time of appearance the "milk dentition"; this is replaced later by the permanent dentition. In lower vertebrates the teeth are replaced as worn away. There is not, however, so great an antithesis in this matter between the Mammalia and other vertebrates as was at one time assumed. But in order to explain this very important part of the subject it will be necessary to give some account of the development of the teeth. The type selected is the Hedgehog, which has been recently and carefully described by Dr. Leche of Stockholm,

Fig. 37.—Two stages in the development of the teeth of a Mammal (diagrammatic sections). alv, Bone of alveolus; dent, dentine; dent.s, dental sac; en, enamel; en.m, enamel membrane; en.m², enamel membrane of permanent tooth; en.plp, enamel pulp; gr, dental groove; lam, dental lamina; lam′, part of dental lamina which grows downwards below the tooth germ; n, neck connecting germs of milk and permanent tooth; pap, dental papilla; pap², dental papilla of permanent tooth. (After O. Hertwig.)

which type has furthermore the advantage of being a "central" type of mammal. The first step in the formation of the teeth is a continuous invagination of the epithelium covering the jaw to form a deepish wall of tissue running in the thickness of the jaw; this is perfectly continuous from end to end of the lower jaw. From this "common enamel germ" (Schmelzleiste of the Germans^[23]) "special enamel germs" (Schmelzorgane, enamel organs) are developed here and there as thickenings in the form of buds which arise on the outer side of the fold of epithelium and some way above its lower termination. These ultimately acquire a bell-like form, and are as it were moulded on to a thickened concentration of the dermis beneath; they then become separate from the downgrowth of the epithelium whence they have arisen. Finally, each of the eight germs becomes one of the milk teeth of the animal. The lower end of the sheet of invaginated epithelium, the common enamel germ, is the seat of the formation of the second set of teeth, of which, however, in the animal under consideration, there are only two in each jaw. But corresponding to each of the enamel germs of the milk dentition, with the exception of the first two molars, there is a slight thickening of the end of the common enamel germ, which at a certain stage is indistinguishable from the thickening which will become one of the permanent teeth. We have thus the diphyodont arrangement. But this does not exhaust the series of rudimentary teeth, though no more come to maturity than those whose development has already been touched upon. In the upper jaw a small outgrowth of the common enamel germ arises above and to the outer side of the enamel germ of the third milk incisor; this does not develop any further, but its resemblance to the commencing germ of a tooth seems to indicate that it is the remnant of a tooth series antecedent to the milk series. Furthermore, there are indications in the fourth premolar of a fourth series of teeth posterior in appearance to the permanent dentition. We arrive therefore at the important conclusion that although here as elsewhere there are only two sets of calcified teeth ever developed, there are feeble though unmistakable remains of two other series, one antecedent to and the other posterior to the diphyodont dentition. The gap therefore which separates the mammalian dentition from that of reptiles is less than has hitherto appeared. Dr. Leche also carefully studied the tooth development of Iguana; he found that in this lizard there are four series of teeth which come to maturity, and a rudimentary series antecedent to these which never produces fully formed teeth.

In a few mammals there is a kind of dentition known as the monophyodont, in which only one series of teeth reaches maturity; where in fact there is no replacement of a milk series by a permanent dentition. Of the monophyodont dentition Whales form an example. The Marsupials are very nearly an instance of the same phenomenon; for Sir W. Flower showed, and Mr. Thomas confirmed his discovery, that only one tooth, according to Mr. Thomas the fourth premolar, is replaced in that group. But even the purely monophyodont dentition of the Toothed Whales is a more apparent than real contrast to the diphyodont dentition elsewhere prevalent. An investigation of the embryos of various Toothed Whales by Dr. Kükenthal and by Dr. Leche has brought to light the highly important fact that two dentitions are present, but that one only comes to maturity; from this fact obviously follows the interesting question:—To which of the two dentitions of more normal Mammalia does the monophyodont dentition of the Whales and Marsupials belong? To this question a clear answer is fortunately possible. As has been pointed out in the foregoing sketch of tooth development, and has been illustrated in the figures, the milk teeth develop as lateral outgrowths of the common enamel germ, while the permanent teeth arise from the end of the same band of tissue. This fact enables it to be stated apparently beyond a doubt that in the Whales and in the Marsupials it is the milk dentition which is the only one to arrive at maturity. Thus the earlier theoretical conclusion that the Marsupial dentition "is a secondary dentition with only one tooth of the primary set left," is proved on embryological grounds to be untrue. But there are other monophyodont animals than those already mentioned.^[24] Orycteropus, the Cape Anteater, is an example. Mr. Thomas has lately discovered that in this Edentate there is a set of minute though calcified milk teeth which probably never cut the gum; here we have a different sort of monophyodontism, in which the teeth belong to the second and not to the first set. Between the latter condition and the diphyodont state are intermediate stages. Thus in the Sea Lions the milk teeth are developed but disappear early, probably before the animal is born.

In the typical diphyodont dentition, such as is exhibited for example in Man and the vast majority of mammals, the milk teeth eventually completely disappear and are entirely replaced by the permanent set of teeth, with the exception, of course, of the molars, which though they are developed late belong to the milk series.

Their correspondence with the milk series is shown in an interesting way by the close resemblance which the last milk premolar often bears to the first molar. These two extremes of dentition, i.e. purely monophyodont and, excepting for the molars, purely diphyodont, are however connected by an intermediate state of affairs, which is represented by more than one stage. In Borhyaena (probably a Sparassodont) the incisors and the canines and two out of the four premolars belong to the permanent dentition, while the two remaining premolars and of course the three molars are of the milk series. Prothylacinus, a genus belonging to the same group, has a dentition which is a step or two further advanced in the direction of the recent Marsupials. We find, according to Ameghino,^[25] whose conclusions are accepted by Mr. Lydekker, that the incisors, canines, and two premolars belong to the milk series, while the permanent series is represented only by the two remaining premolars. We can tabulate this series as follows:—

(1) Purely monophyodont, with teeth only of the first set—Toothed Whales.

(2) Incompletely monophyodont, as in the Marsupials, where there is a milk dentition with only one tooth replaced.^[26]

(3) Incompletely diphyodont, with the dentition made up partly of milk, partly of permanent teeth, as in Borhyaena.

(4) Diphyodont, where all the teeth except the molars are of the second set; this characterises nearly all the mammals.

As we pass from older forms to their more recent representatives there is as a rule a progressive development of the form of the teeth. This is especially marked among the Ungulata. The extremely complicated type of tooth found in such a form as the existing Horse can be traced back through a series of stages to a tooth in which the crown is marked by a few separated tubercles or cusps. Arrived at this point, the differences between the teeth of ancestral Horses and ancestral Rhinoceroses and Tapirs are hard to distinguish with accuracy; and the same difficulty is experienced in attempting to give a definition of other large orders by the characters of the teeth, such as will apply to the Eocene or even earlier representatives of these families. Fig. 36 (p. 51) illustrating a series of mammalian teeth will illustrate the above remarks. That there is such a convergence in tooth structure shows that it is, theoretically at least, possible to determine the ancestral form of the mammalian tooth. Practically, however, the difficulties which beset such theorising are great; that there are such divergent and such strongly-held antithetical views is sufficient proof of this. Two main views hold the field: one, which has found most favour in America, and is due chiefly to the labours and persuasiveness of Professors Cope, Scott, Osborn, and others, is known as "trituberculy."^[27] The alternative view, as urged by Forsyth Major, Woodward, and Goodrich, attempts to show that the dentition of the original mammal included grinding teeth which were multicuspidate or "multitubercular." There is much to be said for both views, and something to be said against both.

Fig. 38.—Molar teeth of A, Phenacodus, and B, the Creodont Palaeonictis. End, endoconid; hld, hypoconulid; hyd, hypoconid; med, metaconid; prd, protoconid. (After Osborn and Wortman.)

This question is, however, wrapped up in a wider one. Its solution depends upon the ancestry of mammals. If the Mammalia are to be derived from reptiles with simple conical teeth, then the first stage in the development of trituberculy is proved. On the other hand, however, the evidence is gradually growing that the Theromorpha represent more nearly than any non-mammalian group with which we are acquainted the probable ancestral form of the mammals. These animals offer some support to both the leading views. Cynognathus had triconodont teeth which, as will be pointed out later, are a theoretically intermediate stage in the evolution of tritubercular teeth; on the other hand, the teeth of Diademodon and some others are multituberculate, and have been very properly compared to the multitubercular teeth of such primitive mammalia as the Ornithorhynchus. Professor Osborn is no doubt correct in italicising a remark of an anonymous writer in Science to the effect that in Diademodon the teeth, though multitubercular, show the prevalence of three cusps arranged in the tritubercular fashion. But this may be only a proof that the multitubercular antedates the tritubercular. It may be, indeed, that the mammalian tooth was already differentiated among the mammal-like Saurians and that from such a form as Cynognathus the Eutheria and other forms in which a tritubercular arrangement can be detected were evolved, and from such form as Tritylodon the Monotrematous branch of the mammals. This way of looking at the matter harmonises a much-disputed question, but involves a diphyletic origin of the mammals—an origin which for other reasons is not without its supporters.

We shall now attempt to give a general idea of the facts and arguments which support or tend to support "trituberculy." As a matter of fact the name is inaccurate; for the holders of this view do not derive the mammalian molar from a trituberculate condition, but in the first place from a simple cone such as that of a crocodile!

To this main and at first only cusp came as a reinforcement an additional cusp at each side, or rather at each end, having regard to their position with reference to the long axis of the jaw. This stage is the "triconodont" stage, and teeth exist among living as well as extinct mammals which show this early form of tooth. We have, indeed, the genus Triconodon, so named on that very account. Among living mammals the Seals and the Thylacine all show some triconodont teeth. A Toothed Whale, it may be remarked, is a living example of a mammal with monoconodont teeth. The three primary cusps, as the supporters of Cope's theory of trituberculism denominate them, are termed respectively the protocone, paracone, and metacone, or, if they are in the teeth of the lower jaw, protoconid, paraconid, and metaconid. At a slightly later stage, or coincidently, a rim partly surrounded the crown of the tooth; the rim is known as the cingulum, and from a prominent elevation of this rim a fourth cusp, the hypocone, was developed. The three main cones then moved, or rather two of them moved, so as to form a triangle; this is the tritubercular stage. Teeth of this pattern are common, and occur in such ancient forms as Insectivora and Lemurs, besides numerous extinct groups. An amendment has been suggested, and that is to term the teeth with the simple primitive triangle "trigonodont," and to reserve the term tritubercular for those teeth in which the hypocone has appeared. The platform bearing the hypocone widened into the “talon"; and this ledge became produced into two additional cusps, the hypoconule or hypoconulid, and the ectocone or ectoconid. Thus the typical sextuberculate tooth of the primitive Ungulate, and indeed of many primitive Eutherians, is arrived at. From this the still further complicated teeth of modern Ungulates can be derived by further additions or fusions, etc.^[28] On the other hand, the development of the Primate molar stops short at the stage of four cusps.

Fig. 39.—Epitome of the evolution of a cusped tooth. 1, Reptile; 2, Dromatherium; 3, Microconodon; 4, Spalacotherium; me, metaconid; pa, paraconid; pr, protoconid; 5, Amphitherium. (After Osborn.)

That such a series can be traced is an undoubted fact. Every stage exists, or has existed. But whether the stages can be connected or not is quite another question. It is by three main lines of argument that the view here sketched out in brief is supported. In the first place, the tracing of the pedigrees of many groups of mammals has met with very considerable success; and it is clear that as we pass from the living Horse and Rhinoceros, with their complicated molars, to their forerunners, we find that both can be referred to a primitive Ungulate molar with but six cusps. Going still further back to the lowest Eocene and ancestral type as it appears, Euprotogonia, we still find in the molar tooth the sextubercular plan of structure. We can hardly get further back in the evolution of the Perissodactyles with any probability of security. On the other hand, many facts point to a fundamental relationship between the primitive Ungulates and the early Creodonts. The latter frequently show plainly tritubercular molars. Such Ungulates as Euprotogonia and Protogonodon, though sex- or quinque-tubercular as to their molars, have a distinctly prevailing trituberculism, when the size and importance of three of the cusps is taken into account. But this lacks finality as a convincing proof of the tritubercular tooth as a primitive Ungulate tooth.

Professor Osborn has ingeniously utilised certain deviations from the normal type of tooth structure (for the group) in favour of his strongly-urged opinions. If the stages of development have been as he suggests, a retrogression would naturally be in the inverse order; thus the "apparently 'triconodont' lower molar of Thylacinus" may be interpreted as a retrogression from a tritubercular tooth. In the same way may be explained the triconodont teeth of Seals and of the Cetacean Zeuglodon. Finally, the modern Toothed Whales have retrograded into "haplodonty."

Embryological evidence has also been called in, and with some success, to contribute towards the proof of the tritubercular theory of teeth. Taeker has shown that in the Horse and the Pig, and some other Ungulates, there is first of all a single hillock or cusp, and that later the additional cones arise separately. An apparent stumbling-block raised by these investigations is that it is not always the protocone or its equivalent in the upper jaw which arises first, as it obviously ought to do phylogenetically. This, however, is not a final argument in either direction. We know from plenty of examples that ontogenetic processes sometimes do not correspond in their order with phylogenetic changes. Thus in the mammalian heart the ventricle divides before the auricle; and of coarse, phylogenetically, the reverse ought to occur, since a divided auricle precedes a divided ventricle. This method of development has, moreover, been interpreted otherwise. It has been held to signify that the complex teeth of mammals are indeed derived from simple cones but by the fusion of a number of those cones.

On the other hand there are the claims of the multitubercular theory of the origin of mammalian teeth to be considered. The palaeontological evidence has been already, to some extent, utilised. The occurrence of such teeth among the possible forerunners of mammals, and in some of the most primitive types of Mammalia, has been referred to. Señor Ameghino dwells upon the sextubercular condition of many primitive mammals even belonging to the Eutheria. In a recent communication^[29] he attempts to identify six tubercles in the molars of types belonging to a variety of Orders. The same condition, as has been noted, characterises that ancient Ungulate form Euprotogonia. Even where the teeth seem at first sight to be tritubercular a detailed study shows traces of otherwise vanished cusps.

It must be remembered in basing arguments upon the early Jurassic and Cretaceous mammals, that our knowledge of them mainly depends upon lower jaws, the teeth of which are usually simpler in pattern than those of the upper jaws. Moreover, another fact, not always insisted upon, must not be lost sight of. In many of those creatures the jaws were of small size, and yet accommodated a large series of molar teeth. Amphitherium, for example, had six molar teeth, and five is a number frequently met with. As the teeth are so numerous and the jaws so small it seems reasonable to connect the simplicity of the structure of the teeth with the need for crowding a number together. The same argument may partly account for the superabundant teeth of many Toothed Whales. It is true that the Manatee has very numerous grinders which are yet complex; but then in this animal there is a succession, and the jaw does not hold at a given time the entire series, with which it is provided in relays. On the other hand, where there are few molars they are often of the multitubercular type, or at least approach it; of this the Multituberculate Polymastodon is a good example; so, too, the molars of Hydrochoerus, and of many other Rodents.

It is well known that the fourth deciduous molar of the upper jaw, which is replaced by a permanent premolar in the fully adult animal, is of a more complex structure than its successor. This may indeed be extended to premolars earlier in the series. In the Dog "the second and first milk molars closely resemble the third and second premolars"; now the milk premolars belong evidently to the same dentition as the permanent molars, and they are earlier teeth than the later-developed replacing teeth. It is therefore significant that these earlier teeth should be more cuspidate than the later teeth. It tells distinctly in favour of the simplification as opposed to the complication of teeth in time, in the groups concerned.

These facts may possibly be applied in explanation of the simple teeth of some of the Jurassic and Cretaceous mammals. It has been mentioned that absolute trituberculy is exceedingly rare among those ancient creatures; more generally there are to be found at least traces of more cusps. Now in some of them we may be dealing with instances of a complete tooth change; the suppression, save for one tooth, which is found in Marsupials, was probably not developed in at least some of these early mammals. The simplicity may therefore have been preceded by complexity, and may have been merely an adaptation to an insectivorous diet.

Alimentary Canal.—The mouth of the Mammalia is remarkable for the fact that with a few exceptions, such as the Whales, there are thick and fleshy lips. The office of these is to seize the food. The roof of the mouth is formed by the "hard palate" in front, which covers over the maxillary and palatine regions. This region is often covered with raised ridges, which have a symmetrical disposition, and are particularly strong in Ruminant animals. They are much reduced in the Rodents, where the anterior part of the palate is ill-defined owing to the way in which its sides fade into the lateral surface of the face. It has been shown that these ridges, in the Cat at least, develop as separate papilliform outgrowths, and it has been suggested that these papillae, which later become united to form the ridges, are the last remnant of palatine teeth such as occur in lower vertebrates.

Fig. 40.—Palatal folds of the Raccoon (Procyon lotor). p.p, Papilla palatina; r.p, palatal folds. (From Wiedersheim's Structure of Man.)

The tongue is a well-developed organ, usually playing a double part. It acts as an organ of prehension, especially in such animals as the Giraffe and the Anteater, where it is long and protrusible beyond the mouth for a considerable distance. It also carries gustatory organs, which serve for the discrimination of the nature of the food. Beneath the tongue there may be a hardish plate, known as the sublingua. This is especially prominent in the Lemurs, where it projects as a horny structure below the tongue, and has an independent and free tip. It is supported in some of these animals by a cartilaginous structure. It is held by Gegenbaur that this organ is the equivalent of the reptilian tongue, and that in the skeletal vestiges which it contains are to be found the equivalents of the hyoid skeletal cartilages which support the tongue in lizards. In this case the tongue of mammals is a subsequently added structure.

The oesophagus leads from the mouth cavity to the stomach. The latter organ has commonly a distinctive shape in mammals. This is well shown in Man. The orifices of the oesophagus and intestine are somewhat approximated; and this causes a bulging of the lower border of the organ, usually spoken of as the greater curvature. A stomach of this typical form is found in many orders of mammals, and is unlike the stomach in any of the groups of lower vertebrates in shape. Sometimes the shape of the organ is greatly altered: it may be drawn out, sacculated, or divided, as in the Ruminants and Whales, into a series of differentiated chambers, each of which plays some special part in the phenomena of digestion.

The intestine of mammals is always long and much coiled, though the length and consequent degree of coiling naturally varies. On the whole it is perhaps safe to say that it is shorter in carnivorous than in vegetable-feeding beasts. Thus the Paca has an intestine of 39 inches total length, while the Cat, an animal of about the same size, has an intestine which is only 36 inches long. A fish diet, however, to judge from the Seals, is associated with a long intestinal tract. The intestine is divisible in the vast majority of mammals into a small and a large intestine. The two are separated by a valvular constriction save in certain Carnivores; and in the majority of cases the distinction is also emphasised by the presence at the junction of a blindly-ending diverticulum, the caecum. This latter organ varies greatly in length, being very short in the Cat-tribe and exceedingly long in Rodents. Its size is, to some extent, dependent upon the flesh-eating or grass-eating propensities of the animal in which it occurs. One of the longest caeca is possessed by the Vulpine Phalanger, in which the organ is one-fifth of the length of the small intestine; while the opposite extremity is reached by Felis macroscelis, which has a small intestine one hundred times the length of the caecum. An interesting point in connexion with the gut of mammals

Fig. 41.—Different forms of the stomach in Mammals. A, Dog; B, Mus decumanus; C, Mus musculus; D, Weasel; E, scheme of the Ruminant stomach, the arrow with the dotted line showing the course taken by the food; F, Human stomach. a, Minor curvature; b, major curvature; c, cardiac end. G, Camel; H, Echidna aculeata. Cma, Major curvature; Cmi, minor curvature. I, Bradypus tridactylus. Du, Duodenum; MB, coecal diverticulum; **, outgrowths of duodenum; †, reticulum; ††, rumen. A (in E and G), Abomasum; Ca, cardiac division; O, psalterium; Oe, oesophagus; P, pylorus; R (to the right in E and to the left in G), rumen; R (to the left in E and to the right in G), reticulum; Sc, cardiac division; Sp, pyloric division; WZ, water-cells. (From Wiedersheim's Comparative Anatomy.)

is the varying proportion of the small to the large intestine. As a general rule the former is very considerably longer than the latter; in Paradoxurus, for instance, the small intestine may be fifteen times the length of the large. The excess of length of one section over the other is not generally so marked as this. In Phalanger maculatus the two sections of the gut are as nearly as possible equal in length, while in Phaseolarctos the large intestine is considerably longer than the small, the lengths being respectively 160 inches and 111 inches. It is common among the Marsupials and also among the Rodents for these proportions to exist, i.e. for the large intestine to be as long as, or longer than, the small. But there are so many exceptions that no general statements can be extracted from the facts.

Some few details will be found in the systematic part of this book. Mr. Chalmers Mitchell has brought forward some reasons for associating a great length of large intestine with an archaic systematic position, in the birds at any rate. The facts here briefly touched upon are not at variance with the extension of such a view to the mammals.

Fig. 42.—Diagrammatic plan of the liver of a Mammal (posterior surface). c, Caudate lobe; cf, cystic fissure; dv, ductus venosus; g, gall-bladder; lc, left central lobe; ll, left lateral lobe; llf, left lateral fissure; p, portal vein entering transverse fissure; rc, right central lobe; rl, right lateral lobe; rlf, right lateral fissure; s, Spigelian lobe; u, umbilical vein; vc, post-caval vein. (After Flower and Lydekker.)

Appended to the alimentary tract are three glands or sets of glands. Opening into the mouth cavity are the salivary glands, which are of enormous size in Anteaters, and small or absent in Whales. In their number and position these glands are characteristic of mammals. Into the intestine open the ducts of the pancreas and liver, two glands which the mammals share with lower vertebrates. The form of the liver is, however, generally characteristic of mammals. It is divided as a rule into a right and a left half, the line of division being marked by the insertion of the umbilical ligament, a vestige of the primitive ventral mesentery. Each half is again commonly subdivided into central and lateral lobes. In addition to these, two other divisions are often to be seen—the Spigelian and the caudate lobe. The liver is less divided in Cetacea and some others, very much subdivided in Rodents and other groups. The degree of subdivision and the proportions of the several lobes frequently offer valuable systematic characters. The gall-bladder may be present or absent; it is always a diverticulum of the hepatic duct. The two are never separate, as in birds, for instance.

Organs of Circulation.—The heart of all mammals is a completely four-chambered organ. In the adult heart there is no communication between the right and left halves. The auricles are comparatively thin-walled, the ventricles thick-walled, in relation to the amount of work that they have severally to perform. The right ventricle, moreover, which has only to drive the blood into the lungs, is much thinner-walled than the left ventricle, which is concerned with the entire systemic circulation. The exits of the arteries and the auriculo-ventricular orifices are guarded by valves, which are so arranged as only to permit the blood to flow in the proper direction. But these valves have a morphological as well as a physiological interest. At the origin of each artery, the aorta and the pulmonary, there is a row of three watch-pocket valves, as they have been generally termed on account of their form. These three valves meet accurately in the middle of the lumen of the arterial tube when liquid is poured into them from above, and thus completely occlude the orifice. The auriculo-ventricular valves differ in structure in the two ventricles. That of the left ventricle has only two flaps, and is therefore often spoken of as the bicuspid or mitral valve. Both these flaps are membranous, and together they completely surround the exit from the auricle into the ventricle. The edges of the valve are bound down to the parietes of the heart by numerous branching tendinous threads, the chordae tendineae, which often take their origin from pillar-like muscles arising from the walls of the heart, the so-called musculi papillares. The valve of the right ventricle is composed of three flaps, and is therefore often spoken of as the tricuspid valve; it is in the same way membranous, and has chordae tendineae and musculi papillares connected with it. The disposition of the musculi papillares and their number differ in different mammals, but no exhaustive study has as yet been made of the arrangements in different groups; the amount of individual variation even is not known, though it is certainly considerable in some cases, for instance in the heart of the Rabbit. The heart of the Monotremata presents differences of some importance from those of other Mammalia; the modern knowledge of the Monotrematous heart is mainly due to Gegenbaur^[30] and Lankester,^[31] in whose memoirs references to the older literature will be found. The principal features of interest in which the heart of the Monotremata differs from that of the higher Mammalia are these. When the two ventricles are cut across transversely, the cavity of the right is seen to be wrapped round that of the left in a fashion precisely like that of the bird's heart; on the other hand in the higher mammal the two cavities lie side by side. The main difference between Monotremes and other Mammals concerns the right auriculo-ventricular valve. The differences which it presents from the corresponding structure of the rest of the Mammalia are two: in the first place, the valve itself does not completely surround the ostium; it is only developed on one side; the septal half (i.e. that turned towards the interventricular septum) is either entirely absent or more generally represented by a small bit of membrane; nevertheless I found^[32] recently in an Ornithorhynchus heart a complete septal half to the right auriculo-ventricular valve. The second point of interest in connexion with this valve is, that the musculi papillares instead of ending in chordae tendineae attached to the free edge of the valve are directly attached to the valve, and in some cases pass through its membranous flap, to be attached to its origin at the boundary of the auricle and of the ventricle. The invading of the valve-flap by muscle in this way is highly interesting, as it recalls the heart of the bird and of the crocodile. The imperfect condition of the valve (from which, as has already been stated, the septal half is as a rule nearly absent) is a point of resemblance to the heart of the bird; the corresponding valve of the crocodile's heart being complete.

There are also features in the system of arteries and veins which are eminently distinctive of mammals. In the first place, the aorta leaving the heart and conveying blood to the body is only a half arch, and bends to the left side as seen in Fig. 43. The right and left halves are present in reptiles, and meet behind the heart. In the bird the right half alone has remained. This fact, therefore, shows that the mammal cannot have been derived from a bird-like ancestor, but that

Fig. 43.—Lepus cuniculus. Ventral view of the vascular system. The heart is somewhat displaced towards the left of the subject; the arteries of the right and the veins of the left side are in great measure removed. a.epg, internal mammary artery; a.f, anterior facial vein; a.m, anterior mesenteric artery; a.ph, anterior phrenic vein; az.v, azygos vein; br, brachial artery; c.il.a, common iliac artery; c.il.v, common iliac vein; cœ, coeliac artery; d.ao, dorsal aorta: e.c, external carotid artery; e.il.a, external iliac artery; e.il.v, external iliac vein; e.ju, external jugular vein; fm.a, femoral artery; fm.v, femoral vein; h.v, hepatic veins; i.c, internal carotid artery; i.cs, intercostal vessels; i.il.a, internal iliac artery; i.il.v, internal iliac vein; i.ju, internal jugular vein; i.l, iliolumbar artery and vein; in, innominate artery; l.au, left auricle; l.c.c; left common carotid artery; l.pr.c, left pre-caval vein; l.v, left ventricle; m.sc, median sacral artery; p.a, pulmonary artery; p.epg, epigastric artery and vein; p.f, posterior facial vein; p.m, posterior mesenteric artery; p.ph, posterior phrenic veins; pt.c, post-caval vein; p.v, pulmonary vein; r, renal artery and vein; r.au, right auricle; r.c.c, right common carotid artery; r.prc, right pre-caval vein; r.v, right ventricle; s.cl.a, right subclavian artery; s.cl.v, subclavian vein; spm, spermatic artery; s.vs, vesical artery; ut, uterine artery and vein; vr, vertebral artery. (From Parker's Zootomy.)

both must have independently come from an ancestor with both halves of the aortic arch present, of which one half has disappeared in one group, and the other half in the other. It is an interesting fact, too, to notice that the four cavities of the mammal's heart, which fourfold division it shares with birds alone, do not exactly correspond compartment for compartment with those of the bird's heart, at least in so far as concerns the ventricles. For the reptilian heart is provided with only one ventricle, and therefore the division of that cavity must have been independently accomplished in mammals and in birds.

There are two features in the venous system which distinguish all the Mammalia (with the exception of Echidna in one of these points) from vertebrates standing lower in the series. The hepatic portal system is limited to a vein which conveys to the liver blood derived from the alimentary tract; in no mammal except in Echidna is there any representative of the anterior abdominal vein of lower vertebrates. In that animal there is such a vein, which apparently arises from a capillary network upon the bladder and passes up, supported by a membrane, along the ventral wall of the abdomen to the liver, thus emptying blood into that organ exactly as does the anterior abdominal vein of the frog. In no mammal is there any trace of a renal portal system. The kidneys derive their blood from the renal arteries only.

Many mammals have two superior venae cavae; this is the case, for instance, in the Elephant and the Rodents and other types lying comparatively far down in the series. In most if not in all mammals there are considerable remains of one of the posterior cardinals, in the form of the azygos vein, which opens into the vena cava superior or pre-caval vein, i.e. the superior cardinal just before the latter debouches into the heart. This one posterior cardinal is usually on the right side; but it may be on the left side, for instance in Trichosurus vulpecula. In Halmaturus bennettii there are two azygos veins, one left and one right, of which the left is rather the larger.^[33]

Urinary Organs.—The kidneys in the Mammalia have a compact form, which contrasts with the somewhat diffuse and vaguely-outlined kidneys of the Sauropsida. In mammals the organ is as a rule of that peculiar shape which is called "kidney-shaped"; a depression termed the hilum, which receives the ducts of the glands, indenting the border of an otherwise oval-shaped gland. In some few mammals the kidney is broken up into lobules; this is the case with the Whales, the Bears, the Oxen, and a few other forms. A curious fact about the kidneys of the Mammalia is their very general asymmetry of position. One of them usually lies in a more advanced position than the other. The ureters lead from the kidneys to the urinary bladder, which in its form and relations is quite distinctive of the Mammalia. The bladder is formed out of the remains of the allantois, and is therefore not the exact homologue of the bladder of the frog, which is the equivalent of the entire sac which grows out of the cloaca in the mammal, and is the foetal allantois. The ureters open into the bladder in the higher Mammalia, but lower down in the urino-genital passage in the more primitive mammals.

The Body Cavity.—The Mammalia differ from all other living vertebrates by the arrangement of the body cavity in which lie the viscera. That cavity is divided into two by a partly muscular and partly tendinous partition, the diaphragm. No other vertebrate has this precise disposition of the coelom. The diaphragm lies usually transversely to the longitudinal axis of the body, but gets a much more oblique arrangement in the Cetacea and the Sirenia, whose needs demand a more expanded chamber for the lungs. For in front of the diaphragm lie the lungs and heart; behind it the stomach, liver, intestines, and the organs of reproduction and excretion. The diaphragm is used in respiration; when its muscles contract, the surface directed toward the pleural cavity becomes less convex, and the cavity of the lungs is thus increased, allowing them to expand under the pressure of the entering air.

The Lungs.—The lungs of the Mammalia differ from those of animals lying lower in the series by the fact, just referred to, that they occupy a pleural cavity completely shut off from the abdomen by the diaphragm. As a rule the lungs of the Mammalia are to be distinguished by their more or less extensive lobation. In the Whales, however, and in the Sirenia, they are not much divided, but present the appearance of the simple sac-like lungs of the reptiles. In some mammals there is a median and posterior unpaired lobe of the lung, which lies in the post-pericardial cavity behind the pericardium. This is not universally present. The lungs are very frequently not symmetrical in their lobation, the number of separate lobes on the right side and on the left being different. The lungs of mammals agree with those of the lower reptiles in being freely suspended within their coelomic cavity, and in not being, as in birds, crocodiles, and the Varanidae among lizards, tied down to the dorsal surface of that cavity by a sheet of peritoneum covering them.

Fig. 44.—Part of a sagittal section of an ovary of a child just born. bl.v, Blood-vessels; foll, strings and groups of cells derived from the germinal epithelium becoming developed into follicles; g.ep, germinal epithelium; in, ingrowing cord of cells from the germinal epithelium; pr.ov, primitive ova. (From Hertwig, after Waldeyer.)

The Gonads (Ovaries and Testes).—The ovary in the Mammalia is always paired; there is never a partial or complete abortion of one gonad as in birds—except of course in pathological cases. The ovaries are small, and lie in the abdominal cavity behind the kidneys. In the immense majority of the Mammalia the ova which are produced within the ovaries are of minute size; those of even the colossal Rorqual are, so far as we know, not markedly larger than the ova of a Mouse. The smallness of size of these reproductive elements implies necessarily an absence of much nutritive yolk; and as a consequence the developing embryo, since it is not hatched in an early stage as a free living larva, has to be nourished by the mother, to whose tissues it is attached through the intermediary of the placenta, a structure partly composed of foetal structures derived from the embryo, and partly of portions of the lining membranes of the uterus of the mother. The ova of the Eutherian mammals, including the Marsupials, are very small as compared with those of any other vertebrates, excepting only Amphioxus, where the young are hatched early as free swimming larvae. They also differ in a highly characteristic way in the mode of their development within the ovary. These processes are to some extent illustrated in Fig. 44. The main framework of the ovary is formed of the so-called "stroma," which is a mass of tissue formed of more or less connective-tissue-like cells. Within this are numerous cavities, the Graafian follicles. The very young follicles consist of but a single layer of follicular cells surrounding the ovum, which lies centrally. The follicular cells gradually increase in number until the ovum lies in the midst of several layers of cells. At this period a vacuity is formed between some of these cells, and grows into a large cell-free cavity; the ovum does not lie loosely in this space, but is connected at one side with the follicular cells, which still line the interior of the Graafian follicle by the so-called discus or cumulus proligerus. The egg or ovum has, moreover, a layer of cells immediately surrounding itself. All these facts can be gathered by an inspection of Fig. 45. It has been shown that, as in lower vertebrates, the cells immediately surrounding the ovum are connected with it directly by delicate processes which penetrate the actual membrane of the egg.

Fig. 45.—Two stages in the development of the Graafian follicle. A, With the follicular fluid beginning to appear; B, after the space has largely increased. caps, Capsule; disc, cumulus proligerus; memb, membrana granulosa; ov, ovum; sp, space containing fluid. (After Hertwig.)

Fig. 46.—Ovarian egg of Echidna. b, Basilar membrane; fe, follicular epithelium; o, oil globules; vm, vitelline membrane; y¹, y², yolk. (Partly after Caldwell.)

The only ova which depart at all in structure from that above described are those of the Monotremata. The credit of this discovery rests with Owen and with Professor Poulton, who pointed out in 1884,^[34] that the ovum of Ornithorhynchus is very large as compared with those of other Mammalia (6 mm. as against .2 mm.), that it is filled with yolk, and that it completely fills the follicle, being surrounded by two layers of follicular cells only. This latter fact was proved by Caldwell. Subsequently Gyldberg^[35] and I^[36] described the ovarian ovum of Echidna, showing it to be identical with that of Ornithorhynchus. Later still a more elaborate and beautifully illustrated paper was published by Caldwell^[37] upon the early stages of development in the Monotremata and Marsupials, in which the ovum of the former was accurately described (see Fig. 46). In the particulars mentioned above, the ovum of the Monotremata is practically identical with that of the large-yolked ova of the Sauropsida.

Fig. 47.—Lepus cuniculus. The anterior end of the vagina, with the right uterus, Fallopian tube, and ovary. (Nat. size.) Part of the ventral wall of the vagina is removed, and the proximal end of the left uterus is shown in longitudinal section, fl.t, Fallopian tube; fl.t′, its peritoneal aperture; l.ut, left uterus; l.ut′, left os uteri; ov, ovary; r.ut, right uterus; r.ut′, right os uteri; s, vaginal septum; va, vagina. (From Parker's Zootomy.)

It is the general rule among vertebrate animals that the ovaries are completely independent of the ducts which convey their products to the exterior. In certain fishes, however, there is an absolute continuity between the two structures, which is believed to be due to a simple concrescence between the originally distinct ovary and oviduct. The latter has grown round the former, an obvious advantage in preventing the eggs from wandering into the abdominal cavity and becoming lost. In the Mammalia we find discontinuity as a general rule. But in quite a number of forms folds of the lining membrane of the abdominal cavity are developed, which practically ensure the passage of the ova into the oviduct when they are extruded from the ovaries. The oviduct, moreover, has a large and fimbriated mouth, called in human anatomy—which is provided with a number of fanciful names—the morsus diaboli. This almost wraps round the ovary, and thus prevents the ova from straying in the wrong direction. Moreover, the ovary itself is often so arranged that it can easily be withdrawn into a pocket of the peritoneum, from which the obvious exit is by the gaping mouth of the oviduct. This disposition of the generative parts is still further modified in a few animals, such as the Rat^[38] and the Kinkajou.^[39] In these animals the mouth of the oviduct actually opens into the interior of a closed chamber which contains the ovary. In this case there is but one route for the extruded ova to follow. This series of steps in the perfecting of the mode of safe extrusion of the ova is highly interesting,

Fig. 48.—Female urino-genital apparatus of various Marsupials. A, Didelphys dorsigera (young); B, Trichosurus; C, Phascolomys wombat. B, Urinary bladder; Cl, "cloaca"; Fim, fimbriae; g, clitoris; N, kidney; Od, Fallopian tube; Ot, aperture of Fallopian tube; Ov, ovary; r, rectum; Sp, septum dividing vagina; Sug, urino-genital sinus; Ur, ureter; Ut, uterus; Ut′, opening of the uterus into the median vagina (VgB); Vg, lateral vagina; Vg′, its opening into the urino-genital sinus; † (in B), point of approximation of uteri; † (in C) and *, rectal glands. (From Wiedersheim's Comparative Anatomy.)

and is a piece of evidence in favour of the high position of the mammals.

The oviducal apparatus of the mammal is more specialised than that of lower vertebrates. It is most simple, as might be imagined, in the egg-laying Monotremes, where, indeed, it is on the same level as that of reptiles. But in the Eutheria the fimbriated mouth of the oviduct passes into a narrow and winding tube, the Fallopian tube; this widens into a uterus, and the two uteri combine into a single tube in the higher forms. They are called the Monodelphia on this account. In the Marsupials the uteri are distinct though they often join above, and from this junction depends a median "uterus." After the uterus or the uteri follows in every case a single vagina.

The testes of the Mammalia, like those of other vertebrates, occupy primitively a position within the body cavity precisely corresponding to that of the ovaries. And in the lowly-organised Monotremata, and some other forms, such as the Whales, they retain that primitive position within the body. It is, however, distinctive of the Mammalia as opposed to lower vertebrates that the testes descend later into a scrotum, which is simply a protrusion of the skin of the body surrounded by muscles, and, of course, containing a section of the body cavity in which lie the testes. The penis of the Mammalia, represented by the clitoris and associated structures in the female, is of a structure entirely peculiar to this group.

The Brain.—Inasmuch as Professor Wiedersheim has said with perfect truth that "the brain of the extinct Ungulate Dinoceras shows so striking a likeness to that of a lizard that one would be compelled to explain it as that of a lizard without a knowledge of the skeleton," it is clear that to define the mammalian brain is a difficult matter. The existing Mammalia, however, all possess brains which can be readily distinguished from those of vertebrates lying lower in the scale. They are of relatively large size, brought about mainly by the dimensions of the cerebral hemispheres, which have an importance in this class of vertebrates that they have not elsewhere. Coupled with this large size of the hemispheres is a more elaborate system of transverse commissures uniting the two; and this culminates in the higher Mammalia, where the corpus callosum attains a large size and great physiological importance. A very marked feature, moreover, of the mammal's brain is the development of regular fissures upon its surface, which fissures are only absent from Ornithorhynchus, various small Rodents,

Fig. 49.—Brain of Dog. A, ventral; B, dorsal; C, lateral aspect. B.ol, Olfactory lobe; Cr.ce, crura cerebri; Fi.p, great longitudinal fissure; HH, HH¹, lateral lobes of cerebellum; Hyp, hypophysis; Med, spinal cord; NH, medulla oblongata; Po, pons Varolii; VH, cerebral hemispheres; Wu, middle lobe (vermis) of cerebellum; I-XII, cerebral nerves. (From Wiedersheim's Comparative Anatomy.)

Bats, and Insectivores, among living mammals. It is sometimes, but erroneously, said that the more complicated the fissures of the brain are, the higher in intelligence and "zoological position" is the possessor of that brain. Instances can undoubtedly be quoted to support such a view; but they are merely selected cases, which do not indicate a wide applicability of such a generalisation. Thus it is true that the brain of a Man is more elaborate in its furrows and convolutions than is that of a Cat. The real fact of the matter is, that the complexity of the brain from this point of view increases with the size of the animal within the group.

Fig. 50.—Lepus cuniculus. Longitudinal vertical section of the brain. (Nat. size.) a.co, Anterior commissure; b.fo, body of the fornix; cb, cerebellum, showing arbor vitae; c.c, crus cerebri; c.h¹, parencephalon or cerebral hemisphere; c.h², temporal lobe; c.ma, corpus mammillare; cp.cl, corpus callosum; f.m, foramen of Monro; inf, infundibulum; l.t, lamina terminalis; ly, lyra; m.co, middle commissure; m.o, medulla oblongata; o.ch, optic chiasma; o.l¹, o.l², corpora quadrigemina or optic lobes; olf, olfactory lobe; p.co, posterior commissure; pd.pn, peduncle of the pineal "gland," pn; p.fo, anterior pillar of the fornix; pty, pituitary body; pv.a, pons Varolii; sp.lu, septum lucidum; v⁴, fourth ventricle; vl.ip, velum interpositum; v.vn, valve of Vicussens; II, optic nerve. (From Parker's Zootomy.)

The Gorilla and the Chimpanzee have a more furrowed brain than has the little Marmoset; the Bear a more complicated brain than the Weasel, etc. The most highly-convoluted brains of all mammals are those of the Elephants, and there does not seem in the Ungulates to be so marked a relation between size and abundance of fissures as there is among other mammals. A regular plan of the fissures can be detected with certainty for each group considered by itself; but it is not so easy to homologise the details of arrangement from group to group. This is so far in accord with the view that the existing groups of mammals have diverged from each other ab initio.

Another marked characteristic of the mammalian as opposed to other brains is the relatively small importance in size and yet the fourfold nature of the optic lobes. What was the case with the optic lobes of the early Ungulates is difficult to understand, on account of the fact that the casts are necessarily imperfect. Altogether the enormous progress in the complexity of the brain from the early Tertiary mammals down to the present, is one of the most remarkable revelations of palaeontology. It goes perhaps some way in explaining the remarkable diversity in mode of life exhibited by the mammals as compared, for example, with the birds, whose brains have not diverged so much or in so many directions from the primitive form.

The present Distribution of the Mammalia.—In the following pages some of the principal facts in the geographical range of the orders, families, and many of the genera of Mammalia will be given. It has been justly observed by Mr. Sclater that the habitat of an animal is as much a part of its definition as is its structure or external form. No systematic account of the Mammalia would therefore be complete without such geographical facts. But that branch of zoology which is concerned with the past and present distribution of animals is wider in scope than this. Zoogeography deals not only with the actual facts in the range of animals, but with the inferences as to past changes in the relations of land and sea which the facts seem to indicate, and with speculations as to the place of origin of the different groups, of which more than hints are sometimes given by their past and present distribution. In addition to this, the earth can be mapped out into provinces and regions which are definable by their animal inhabitants. In the present volume, dealing only with the Mammalia, it will be obviously impossible to enter fully into the entire subject of zoogeography. All that will be attempted is a brief general survey of the science so far as it can be illustrated by the Mammalia. For fuller knowledge the reader is referred to the treatises mentioned below.^[40]

There are certain facts in the distribution of animals which are commonplaces of knowledge, but which may be set forth with definiteness. Everybody knows that an animal has a given range: Elephants, for example, are found in India and certain adjacent parts of Asia, and again in Africa; the Rhinoceroses have roughly the same range; the Tiger is limited to Asia; the Jaguar to America, and so forth. The entire expanse of country which is inhabited by an animal is called its area of distribution. Such areas are larger or smaller. The Lion ranges over the whole of Africa, a small part of India, and some neighbouring countries; on the other hand, the Insectivore Solenodon is limited to Cuba and Hayti, a separate species to each. Among other groups of animals are instances of an even more restricted range. There are humming-birds confined to the slopes of a single mountain, and fishes limited in their range to a single small lake.

A species may be found everywhere within the area of its distribution, or it may be confined to a number of limited tracts within that area. In this case it is usual to speak of "stations." In such cases the species in question is generally suited to some particular kind of environment. Thus the Otter and other aquatic mammals will only be found where there is water; and intervening tracts of waterless country will contain no Otters. Goats and Chamois live only upon mountains; the intervening plains are destitute of them. This discontinuity of distribution within the area is very general. But a discontinuity of area is also seen—not so commonly however; and, indeed, when it does occur, it is a matter of a genus and not of a species. Thus the Tapir is found in the East Indies on the one hand and in South and Central America on the other, being absent in the intermediate tracts.

It is clear that tracts of country eminently suitable for the housing of a particular mammal do not always possess that kind, or even an allied form. Africa, for example, possesses no arboreal Anteaters; there are no Anteaters at all (of the order Edentata) in Australia, though there are plenty of ants for them to feed upon, and tropical conditions of climate prevail. But as in these cases the inference may be denied on the grounds that no experiments exist to prove or to disprove the assertion, the matter may be better emphasised by such cases as the introduction of the Rabbit into Australia, and various mammals, such as Goats, into oceanic islands. The plague caused by the former is a matter of notoriety. But although climate and conditions and animal inhabitants do not march accurately together, there is certainly some connexion between temperature and the range of animals. Mr. Lydekker writes on this point as follows: "The llama-like animals, respectively known as vicunas and guanacos, are met with in company on the highlands of the Cordillera in Peru and Ecuador, but as we go farther south the latter are found on the plains of southern Argentina and Patagonia, as well as on the island of Tierra del Fuego at the sea level. Here then is a clear proof of the intimate connexion existing between temperature and station; the guanaco being an animal which can only live in cold or temperate climates, finds suitable conditions for its existence in tropical latitudes solely at a height of so many thousands of feet, although farther south it is able to thrive at the sea level." This, however, cannot be pushed too far—the world cannot be mapped out into areas bounded by parallels of temperature as was once attempted—since there are plenty of cases like that of the Tiger, which is as much at home in a tropical jungle as on the icy plains of Northern Asia.

Seeing that there are in many cases no climatic barriers to the spreading of a given race of animals over a larger area of distribution than it actually occupies, it becomes important to inquire why there are so many cases of restriction in range.

It is possible to see, at any rate, three causes which are responsible for a large number of such cases. In the first place, a given species of animal must have originated at a certain spot; its multiplication in individuals must always be a slow matter, since enemies, and untoward events generally, would conspire to check the natural multiplication by geometrical progression. A long time might therefore elapse before the species greatly extended its range. A restricted distribution may therefore, in some cases, mean a modern race. In the second place, there are definite physical barriers which check the migration of species. The terrestrial Mammalia cannot cross wide arms of the sea; that they can and do swim for considerable distances has been proved in several instances; but, as has been pointed out, it is unlikely that a purely terrestrial mammal would voluntarily swim out into an unknown sea. And then if it did, and successfully reached the opposite side, nothing would happen unless it were a pregnant female; or, if not pregnant, till a male swam very soon afterwards in exactly the same direction. Many travellers have told of floating islands, formed of torn-up trees and brushwood, which have been seen at the mouths of large rivers, with animal passengers upon them. These are, however, so much at the mercy of currents and storms, that but little reliance can be placed on them as a means of transit; besides, here again, two individuals, or a pregnant female, would be required to effect a settlement on a foreign shore. The existence of oceanic islands is often urged as a proof of this inability to cross tracts of sea; even those which are comparatively near an extensive continent, such as, for example, Fernando Noronha in the Atlantic, are destitute of mammals (except, indeed, the ubiquitous Mouse, which is believed to have been carried there, often in company with the equally widely-spread Rat, in ships). This argument, however, is not so conclusive as might appear; it doubtless is in the case of far-distant islands. But the size of the islands has to be taken into account. For there are islands, such as the Galapagos, or, to take a less contested instance, some of the islands of the Malagasy Archipelago, undoubtedly continental, which have an exceedingly reduced number of mammals. An area of a certain size seems to be a necessity.

The converse of this is in many cases easy to show, that is, the wide range of animals when there are no marine barriers to stop their spreading. John Hunter, the celebrated anatomist and surgeon (not often quoted, however, as an authority upon geographical distribution), observes: "It is a curious circumstance in the natural history of animals to find most of the northern animals the same both on the continent of America and what is called the Old World, while those of the warmer parts of both continents are not so. Thus we find the bear, fox, wolf, elk, reindeer, ptarmigan, etc., in the northern parts of both.... The reason why the same animals are to be found in the northern parts is the nearness of the two continents. They are so near as to be within the power of accident to bring the animals, especially the large ones, from one continent to the other either on the ice or even by water. But the continents diverging from each other southward, so as to be at a very considerable distance from each other even beyond the flight of birds, is the reason why the quadrupeds are not the same."

There is no doubt, in fact, that the ocean is the most insuperable of all barriers to the dispersal of mammals. In a less degree mountain ranges and deserts are also barriers. The Desert of Sahara is a striking instance to the point; it separates two exceedingly different faunas.

A third cause of more or less limited range is the barrier due to competition. If the ground is already taken up, there is no room for new immigrants. There is obviously a limit to the number of Antelopes or Deer that can graze upon a given tract of grassy plain. These two groups of Ungulates illustrate the matter well: the Antelopes are African and Indian, especially the former, while Africa has no Deer at all; America, on the other hand, has plenty of Deer but no Antelopes, save the Prong-horn. The more nearly akin the two species or groups of species are, the fiercer will be the competition; for a near kinship will at least often imply similar habits, the need for similar food, and other likenesses which will prevent both from successfully occupying the same tract of country. The remarkable fauna of Australia is believed to afford an example of this. In that country the prevalent inhabitants are the Marsupials. The Monotremes are found there also, and nowhere else save in New Guinea and Tasmania. The remaining mammals are inconspicuous; they embrace a few Rodents and Bats, and the doubtfully indigenous Dingo-dog. Now the Marsupials are fitted to every variety of life. We have the grazing Kangaroos and Wallabies, the burrowing Wombats, the arboreal Phalangers, and the carnivorous Dasyures. In the second place, it is an unquestioned fact that the Marsupials are an older race than are the existing Eutherian mammals; they were the dominant mammals during the Secondary epoch. At that time they were more widely distributed than at present. In most parts of the world they are now absent, since they have been successfully ousted by the more highly organised groups of Eutheria. But at that period, when the higher Eutheria were in the ascendant, Australia and the islands to the north became cut off from Asia, and thus became freed from inroads of Eutheria, which were partly prevented by the physical barrier of the sea from effecting a settlement, and partly perhaps prevented owing to the ground being already taken up by the Marsupials. Likeness of habit gave the older inhabitants victory in the struggle for existence.

The general statements that have been here made are in accord with current opinion upon the factors of geographical distribution. But the past range of animals appears to be less consonant with the received views. In the Tertiary period, groups of animals had often a far wider range than at present. To-day the Rhinoceroses are limited to Asia and Africa, and to quite limited parts of the former continent. In the past, these animals were abundant in Europe and North America. Wild Horses now have a range which is not widely different from that of the Rhinoceroses, save that they extend into the more northern regions of Asia. Their remains are abundant both in North and South America. The Hippopotamus, now confined to Africa, once ranged over Europe, Madagascar, and India. There were plenty of American and European Lemurs. Elephants were nearly world-wide in their range; and, in short, restricted distribution seems to be on the whole a characteristic of animals of the present day.

These statements, however, though perfectly true, must not lead to erroneous inferences. It is rather impressed upon the reader, in books which contain sections dealing with geographical distribution, that animals on the whole occupy more restricted areas at present than in the past. There are, however, plenty of examples of groups of extinct creatures which had, so far as we know, quite a restricted range. Thus the Toxodonts were purely South American, as were the Glyptodonts and some other forms. And, on the other hand, the Cervidae of to-day are as widely, if not more widely, distributed than at any other time. The Hares and Rabbits are now nearly universal in range; the Cats almost so. We meet with Bovidae, even excluding the Sheep and Goats, in all the four quarters of the globe, excluding only South America and, of course, Australia. The Camelidae are still common to both the Old and the New Worlds.

During certain periods of the Tertiary epoch it is true that there was more similarity between Europe and North America than there is at present. It would have been quite necessary to unite both into a Holarctic area, such as is now insisted upon by many; but the reasons for this union would then have been stronger. The fact is, however, that the closer resemblances were due to the larger number of families of animals which existed then than now; these have decayed away from both continents, and allowed the unlikenesses between the mammalian fauna of both to become evident. But the likenesses which still survive have led many to associate the two regions closely together.

So far as the history of a genus or family or larger division can be traced, it results as a conclusion that from a given area of origin the group in question migrated in all directions where possible to a varying degree; it then died out in intervening tracts, or was left only in a certain part of its former and more extensive area of range.

Zoological Regions.—Seeing that each species of animal has its own definite range, it is clear that the earth's surface can be apportioned into divisions which are characterised by their animal inhabitants. We shall divide the earth into realms, which are the largest divisions; then into regions; and finally into subregions. It must be borne in mind that the various groups of the animal kingdom are of different ages, geologically speaking, and have therefore had less or more time, as the case may be, to settle down into their present distribution, and that different animals differ greatly in their rate of multiplication, their power of migration, and their susceptibility to the effectiveness of various natural and other barriers to distribution. It is not, therefore, possible to divide the world into realms and regions which shall express the facts of distribution of the entire animal kingdom. Such divisions, which are common in text-books of zoology having but a small section devoted to zoogeography, are at best mere approximations and averages; no good is gained by taking such a comprehensive view of the matter, as the essential object of subdividing the earth's surface is thereby lost sight of. The zoogeographical division of the earth which will be adopted here is that originally recommended by Dr. Blanford, and now accepted by a number of authorities. There are three "realms," to which a fourth may perhaps be added—though on negative grounds, and merely for the purpose of emphasising the parts of the world to which mammals have not gained access. The realms are again divisible into regions, at least in the case of one of them, and the regions may be again separated into more or less distinct subregions or provinces. The three primary divisions or realms which contain mammals are the Notogaean, including Australia and certain islands to the north of it; the Neogaean, or the South American continent and Central America; the Arctogaean, including the continents of North America, Europe, Asia, and Africa, together with the adjacent islands, such as the West Indies, East Indies (exclusive of those which fall within the realm of Notogaea), and Madagascar; and finally, the realm of Antarctogaea or Atheriogaea, which embraces New Zealand, the Antarctic continent, and a series of islands such as South Georgia and Kerguelen, and possibly even the extreme south of Patagonia. This latter quarter of the globe will need no further reference, as it has no truly indigenous terrestrial mammalian inhabitants. We cannot include the Bats in this statement, as their distribution is due to different powers of extending their range, and to different barriers from those which govern the range of other groups of mammals.

(1) Notogaea.^[41] This realm is characterised by the exclusive possession of the Monotremes:—that is to say, one of the two primary divisions of the Mammalia is absolutely restricted to this area. It contains, moreover, the vast majority of the Marsupials. Further, the realm of Notogaea is to be distinguished by the entire absence of the higher mammals, with the exception of a few small Rodents. (The Bats are ignored for the reasons stated, and the Dingo is believed to have been an importation.) It cannot be disputed that this is a very distinctly-marked area of the earth's surface.

(2) Neogaea. The continent of South America has no Monotremes and only a few Marsupials, all of which, with the exception of Caenolestes, belong to the Polyprotodont division of that order, and to a peculiar family, Didelphyidae. The recent discovery of other fossil Marsupials, however, to some extent favours Huxley's view that Neogaea and Notogaea form one realm as opposed to the rest of the world. Besides this, Neogaea possesses the Edentata, which are found nowhere else;—that is, the division of the Edentata to which the name is now restricted by some authorities. It is also characterised by the nearly entire absence of the important order of Insectivora; and, as minor marks of distinction, by the absence of Antelopes, Oxen and Sheep, of the Ichneumon tribe, of Horses, and of Lemurs. It has the exclusive possession of the Hapalidae and Cebidae, and of several families of Rodents.

(3) Arctogaea. This vast realm is clearly capable of subdivision into four regions, which will be considered in detail later. In the meantime the points of likeness between these subdivisions is more marked than are either the resemblances or the differences of any one of them to either of the two realms which have just been defined. The two realms that have been discussed retain their distinctness from each other and from Arctogaea for a considerable way back into the Tertiary period. It is not until we reach very early Tertiary times that Edentates are met with in North America; and then it cannot be regarded as absolutely settled that the Ganodonta are really the forerunners of the Armadillos, Sloths, etc. Nor do we find Marsupials in Europe until far back in time, and at a corresponding period in North America. Indeed the fauna of South America in late Tertiary times was even more distinct than it is now; for then we had confined to that region the Toxodonts, Glyptodonts, Macrauchenia, and other forms, while in Australia there were still Marsupials. In late Tertiary times Europe and India were by no means so distinct from Africa as they are to-day. North America does not resemble the Old World quite so much as the subdivisions of the Old World resemble each other; but, as will be pointed out later, there are and were very substantial agreements. The Elephants, Rhinoceroses, Giraffe, Hippopotamus, Orycteropus, are now distinctively African or Indian animals; but all these genera, or at least families (in the case of the Giraffe), have occurred in Europe during quite recent times. Lycaon indeed, now confined to Africa, is thought to have had a European origin from its occurrence in caves there. The Hyaena and the Lion, certain members of the Horse tribe, Apes, and other animals, were also but are not now European.

India again, and the Oriental region generally, once possessed the Hippopotamus, the Chimpanzee, Giraffidae, the Antelopes, Cobus, Hippotragus, Strepsiceros, and Orias, which are now purely African animals. It shares at present with the Ethiopian region the Catarhines, including the Anthropoid Apes, the Lemurs, Tragulina (the genus Dorcatherium is also known from fossils in India), Manis, Hyaena, the Cheetah, Elephant, Rhinoceros, and the Ratel. There is, in fact, no order of mammals which is now absent from one of these three regions though present in the others, save the Lemurs, and they occurred in past times in Europe. The Tapir of India is known fossil in Europe, and the latter continent had its Monkeys and even Anthropoids. On the other hand, North America is more distinct. It has no Lemurs, Apes, Elephants, Rhinoceroses, Tapirs, Old World Edentates (Effodientia), Viverridae, Horses, or Antelopes, excepting Antilocapra, a type of a separate division of Bovidae. But since several of these groups have been represented in recent times, no primary line of division can be profitably drawn.

Arctogaea as a whole may be characterised by both negative and positive characters. As negative features may be mentioned;—the entire absence of Edentates (Necrodasypus of Filhol is rather doubtful, see p. 164, n.), though a few crept up into the Nearctic region from Neogaea during past times; and of Hapalidae, Cebidae, and Marsupials, except an Opossum in North America. This realm has, on the other hand, all the Lemurs, all the Insectivores with the exception of the West Indian Solenodon, all the Proboscidea, Rhinoceroses, Horses, Deer, Antelopes, the last group including the Oxen and a variety of other important families. It is in fact the headquarters of all the Eutheria with the exception of the Edentata and Marsupials.

The subdivisions of this realm have been variously effected. The classical subdivisions are of course those of Mr. Sclater, who would recognise (1) the Nearctic, North America; (2) the Palaearctic, including Europe, Northern Asia, and Japan; (3) the Oriental, including Asia south of the Himalayas and the islands of the Malay Archipelago as far east as the Australian region; and (4) the Ethiopian, i.e. tropical Africa and Madagascar. Some would alter this by uniting America and the north of the Old World into a Holarctic region, separating off the southern parts of the North American continent into a Sonoran region. To some, the claims of Madagascar to form a separate region are convincing. To distinguish the boundaries of the several regions is a difficult task; they dovetail into each other on the frontiers with the complex curves of a puzzle-map. The difficulty has been grappled with by the suggestion of intermediate transitional areas; but this proceeding really doubles the difficulty, for there are then two frontiers to delimit in each case instead of only one. The animal inhabitants must be expected to mingle somewhat at the lines of junction of one region with another.

The Sonoran region does not appear to us to have great claims to recognition. It shows a mingling of southern with northern forms exactly as might be expected. An Armadillo and Didelphys have, as it is believed, invaded it from the Neogaeic realm; it possesses also the South American genera, Dicotyles, Nasua Conepatus, Sigmodon. On the other hand, the Sonoran genera Antilocapra, Cynomys, Procyon, and the Insectivora Blarina and Scapanus, extend further north. Peculiar to this region are only six genera of Rodents, which seems an insufficient reason for raising the Sonoran province to the dignity of a region. Considered from the point of view of numbers of peculiar forms, the Thibetan subregion has more claims to distinction as a region; for confined to that area we have the genera Nectogale, Aeluropus, Eupetaurus, Pantholops, Budorcas; while by slightly extending its limits, a number of other peculiar forms might be added. Madagascar has distinctly more claims to regional division. Absolutely confined to it are eleven of the seventeen existing genera of Lemurs, the family Centetidae among the Insectivora, which contains seven genera, and another recently discovered and peculiar genus, Geogale; it has six peculiar genera of Viverridae; it has five peculiar genera of Rodents. In addition to this it is negatively characterised by the absence of the following typical African animals, Felidae, Proboscidea, Rhinocerotidae, Equidae, Monkeys, etc. It seems to be impossible to avoid allowing the rank of a region to this part of the world.

In separating the Nearctic from the Palaearctic region, stress must be laid rather upon the absence of Asiatic and European forms from North America than upon the existence in the northern half of the New World of many peculiar forms. Peculiar to the Nearctic are the Goat genus Haploceros, the Rodents Erethizon, Zapus, and the family Haplodontidae. The Mole genus Condylura is also restricted to this part of the New World. Even so it has more peculiar forms than the Sonoran. If we add to this the absence of Horses, Antelopes except Antilocapra, Pigs, Hyaenas, etc., there are strong grounds for retaining this division. It must be agreed, however, that it comes rather nearer to the Eurasian district than the latter does to the Oriental.

The Oriental region has many characteristic animals. It has among the Anthropoid Apes the Orangs and Gibbons; of Old World Apes it has confined to its own area the genera Semnopithecus and Nasalis. Of Lemurs there are Loris and Nycticebus, and Tarsius, representing a family of that order, or even a sub-order. The Galeopithecidae are entirely Malayan. There are many Rodent, Carnivorous, and Insectivorous genera; the Rhinoceroses and the Elephant of this region differ from those of Africa. Tragulus concludes a sample from a very rich list of peculiar forms.

The Ethiopian region has also its Anthropoids, the Gorilla and the Chimpanzee, but they belong to genera or a genus different from those which include the Oriental forms. There are five peculiar genera of Cercopithecidae. The Lemurs restricted to this region are Galago, Perodicticus and Arctocebus. The peculiar Insectivorous families Macroscelidae and Chrysochloridae are only found here, besides many other peculiar genera. Africa is especially the home of Antelopes, and the Giraffe is not found now outside its borders. The Elephant and the Rhinoceroses are of different species from those of India. There are many peculiar Rodents and Ungulates.

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1. "Über die Haare der Säugethiere," Morph. Jahrb. xxi. 1894, p. 312.
2. "Bemerkungen über den Ursprung der Haare," Anat. Anz. 1893, p. 413.
3. See for this matter, p. 90. Dr. Bonavia has recently advanced (Studies in Evolution, London, 1895) the somewhat fantastic view that the pigment-patches of Carnivorous and other mammals are a reminiscence of an earlier scaly condition. There is no direct evidence that the primitive mammals were scaly, nor are the Monotremata or Marsupials furnished with any more traces of such a condition than are other mammals; and they are the most lowly organised of existing Mammalia.
4. Proc. Zool. Soc. 1887, p. 527.
5. "Über Marsupialrudimente bei Placentaliern," Morph. Jahrb. xx. 1893, p. 276.
6. See Haacke, "On the Marsupial Ovum, the Mammary Pouch, etc., of the Echidna," Proc. Roy. Soc. 1885, p. 72; and "Über die Entstehung der Säugetiere," Biol. Centralbl. viii. 1889, p. 8.
7. See Gegenbaur's Elements of Comp. Anat. Transl. by Bell, 1878, p. 421.
8. "Über die Beziehungen zwischen Mammartasche u. Marsupium," Morph. Jahrb. xvii. 1891, p. 483.
9. Catalogue of Marsupials in British Museum, 1886.
10. Its independence from the epistropheus is emphasised in Monotremes and some Marsupials by its late fusion with that vertebra.
11. Intercentra are but rarely met with anterior to the caudal series. Mr. Parsons has, however, recorded their occurrence in the lumbar vertebrae of Atherura.
12. Tufts College Studies, No. 6, 1900.
13. Cf. the Armadillo Peltephilus, p. 186.
14. Gegenbaur, Vergl. Anat. Wirbelth. Leipzig, 1898, p. 404.
15. Ehler's Zool. Miscellen, i. 1894.
16. Proc. Zool. Soc. 1865, p. 567.
17. Vergl. Anat. der Wirbelth. Leipzig, 1898, p. 497.
18. To this category are perhaps to be referred cartilaginous pieces occurring in the Rabbit, Mus and Sorex (see Fig. 29 above).
19. "On the Coracoid of the Terrestrial Vertebrates," P.Z.S. 1893, p. 585.
20. Horny matter is apt to be formed upon extremities; instances which are well known are the "claws" upon the tail of the Lion and Leopard and the Kangaroo Onychogale. For an account of the first see Proc. Zool. Soc. 1832, p. 146.
21. Cf. Tomes, A Manual of Dental Anatomy, 5th ed. London, 1898.
22. Materials for the Study of Variation, London, 1894.
23. Morph. Jahrb. xix. 1892, p. 502.
24. It would be of the greatest interest in relation to this and many other problems to ascertain the precise meaning of the monophyodont dentition of Ornithorhynchus.
25. Proc. Zool. Soc. 1899, p. 922.
26. Mr. M. Woodward, however (P.Z.S. 1893, p. 467), is disposed to think that in some Macropodidae at any rate the supposed tooth of the second set really belongs to the milk dentition, arising late between Pm_{3} and Pm_{4}.
27. See for a summary, Osborn, American Nat. Dec. 1897, p. 993.
28. e.g. the "protoloph," "metaloph," etc. (see Fig. 36, p. 51), of the modern Ungulate form of tooth.
29. "On the Primitive Type of the Plexodont Molars of Mammals," Proc. Zool. Soc. 1899, p. 555.
30. Jen. Zeitschr. ii. 1866, p. 365.
31. Proc. Zool. Soc. 1883, p. 8.
32. Proc. Zool. Soc. 1894, p. 715.
33. Beddard, Proc. Zool. Soc. 1895, p. 136.
34. Quart. Journ. Micr. Sci. xxiv. 1884, p. 9.
35. S.B. Jen. Gesells. 1885, p. 1.
36. Proc. Roy. Phys. Soc. Edin. viii. 1885, p. 354.
37. Phil. Trans. clxxviii. 1887, p. 463.
38. Robinson, Studies Biol. Lab. Owens Coll. ii. 1890, p. 35.
39. Beddard, Proc. Zool. Soc. 1900, p. 667.
40. Wallace, The Geographical Distribution of Animals, 1876. Heilprin, The Distribution of Animals, Internat. Scientific Series, 1887. Beddard, A Text-book of Zoogeography, Cambridge Natural Science Manuals, 1895. Lydekker, Geographical History of Mammals, Cambridge Geographical Series, 1896. W. L. and P. L. Sclater, The Geography of Mammals, Kegan Paul and Co. 1899.
41. This term is sometimes used in a wider sense; cf. vol. viii. p. 74.