giraffes flourished during the Pliocene in Asia (where they may have
originated), they survive only in Africa. An African origin has also
been suggested for the hollow-horned ruminants (Bovidae); and if
this were substantiated it would explain the abundance of that family
in Africa and the absence from the heart of that continent of the
deer-tribe. Some confirmation of this theory is afforded by the fact
that whereas we can recognize ancestral deer in the Tertiaries of
Europe we cannot point with certainty to the forerunners of
the Bovidae. Whether its birthplace was in Africa or to the north,
it is, however, clear that the hollow-horned ruminants are essentially
an Old World group, which only effected an entrance into North
America at a comparatively recent date, and never succeeded in
reaching South America. So far as it goes, this fact is also in favour
of the African ancestry of the group.
The Antilocapridae (prongbuck), whose relationships appear to be rather with the Cervidae than with the Bovidae, are on the other hand apparently a North American group. The chevrotains (Tragulidae), now surviving only in West and Central Africa and tropical Asia, are conversely a purely Old World group.
The camels (Tylopoda) certainly originated in the northern hemisphere, but although their birthplace has been confidently claimed for North America, an equal, if not stronger, claim may be made on the part of Central Asia. From the latter area, where wild camels still exist, the group may be assumed to have made its way at an early period into North America; whence, at a much later date, it finally penetrated into South America. In the Old World it seems to have reached the fringe of the African continent, where its wanderings in a wild state were stayed.
The pigs (Suidae) and the hippopotamuses (Hippopotamidae) are essentially Old World groups, the former of which has alone succeeded in reaching America, where it is represented by the collateral branch of the peccaries (Dicotylinae). An African origin would well explain the present distribution of both groups, but further evidence on this point is required before anything decisive can be affirmed, although it is noteworthy that the earliest known pig (Geniohyus) is African. The Suinae are at present spread all over the Old World, although the African forms (other than the one from the north) are markedly distinct from those inhabiting Europe and Asia. Hippopotamuses, on the contrary, are now exclusively African, although they were represented in tropical Asia during the Pliocene and over the greater part of Europe at a later epoch.
A brief notice with regard to the distribution of the Primates must suffice, as their past history is too imperfectly known to admit of generalizations being drawn. The main facts at the present day are, firstly, the restriction of the Prosimiae, or lemurs, to the warmer parts of the Old World, and their special abundance in Madagascar (where other Primates are wanting); and, secondly, the wide structural distinction between the monkeys of tropical America (Platyrrhina), and the Old World monkeys and apes, or Catarrhina. It is, however, noteworthy that extinct lemurs occur in the Tertiary deposits of both halves of the northern hemisphere—a fact which has induced Dr J. L. Wortman to suggest a polar origin for the entire group—a view we are not yet prepared to endorse. For the distribution of the various families and genera the reader may be referred to the article Primates; and it will suffice to mention here that while chimpanzees and baboons are now restricted to Africa and (in the case of the latter group) Arabia, they formerly occurred in India.
As regards aquatic mammals, the greater number of the Cetacea, or whales and dolphins, have, as might be expected, a very wide distribution in the ocean. A few, on the other hand, have a very restricted range, the Greenland right whale (Balaena mysticetus) being, for instance, limited to the zone of the northern circumpolar ice, while no corresponding species occurs in the southern hemisphere. In this case, not only temperature, but also the peculiar mode of feeding, may be the cause. The narwhal and the beluga have a very similar distribution, though the latter occasionally ranges farther south. The bottle-noses (Hyperöodon) are restricted to the North Atlantic, never entering, so far as known, the tropical seas. Other species are exclusively tropical or austral in their range. The pigmy whale (Neobalaena marginata), for instance, has only been met with in the seas round Australia, New Zealand and South America, while a beaked whale (Berardius arnouxi) appears to be confined to the New Zealand seas.
The Cetacea, however, are by no means limited to the ocean, or even to salt water, some entering large rivers for considerable distances, and others being exclusively fluviatile. The susu (Platanista) is, for instance, extensively distributed throughout nearly the whole of the river systems of the Ganges, Brahmaputra and Indus, ascending as high as there is water enough to swim in, but apparently never passing out to sea. The individuals inhabiting the Indus and the Ganges must therefore have been for long ages isolated without developing any distinctive anatomical characters, those by which P. indi was separated from P. gangetica having been shown to be of no constant value. Orcella fluminalis, again, appears to be limited to the Irrawaddy; and at least two distinct species of dolphin, belonging to different genera, are found in the Amazon. It is remarkable that none of the great lakes or inland seas of the world is inhabited by cetaceans.
The great difference in the manner of life of the sea-cows, or Sirenia, as compared with that of the Cetacea, causes a corresponding difference in their geographical distribution. Slow in their movements, and feeding on vegetable substances, they are confined to the neighbourhood of rivers, estuaries or coasts, although there is a possibility of accidental transport by currents across considerable distances. Of the three genera existing within historic times, one (Manatus) is exclusively confined to the shores of the tropical Atlantic and the rivers entering into it, individuals scarcely specifically distinguishable being found both on the American and the African. The dugong (Halicore) is distributed in different colonies, at present isolated, throughout the Indian Ocean from Arabia to North Australia; while the Rhytina or northern sea-cow was, for some time before its extinction, limited to a single island in the extreme north of the Pacific Ocean.
The seals (Pinnipedia) although capable of traversing long reaches of ocean, are less truly aquatic than the last two groups, always resorting to the land or to ice-floes for breeding. The geographical range of each species is generally more or less restricted, usually according to climate, as they are mostly inhabitants either of the Arctic or Antarctic seas and adjacent temperate regions, few being found within the tropics. For this reason the northern and the southern species are for the most part quite distinct. In fact, the only known exception is the case of a colony of elephant-seals (Macrorhinus leoninus), whose general range is in the southern hemisphere, inhabiting the coast of California. In this case a different specific name has been given to the northern form, but the characters by which it is distinguished are of little importance, and probably, except for the abnormal geographical distribution, would never have been discovered. The most remarkable circumstance connected with the distribution of seals is the presence of members of the order in the three isolated great lakes or inland seas of Central Asia—the Caspian, Aral and Baikal—which, notwithstanding their long isolation, have varied but slightly from species now inhabiting the Polar Ocean.
Authorities.—The above article is partly based on that of Sir W. H. Flower in the 9th edition of this work. The literature connected with mammals is so extensive that all that can be attempted here is to refer the reader to a few textbooks, with the aid of which, combined with that of the annual volumes of the Zoological Record, he may obtain such information on the subject as he may require: F. E. Beddard, “Mammals,” The Cambridge Natural History, vol. x. (1902); W. H. Flower and R. Lydekker, The Study of Mammals (London, 1891); Max Weber, Die Säugethiere (Jena, 1904); W. T. Blanford, The Fauna of British India—Mammalia (1888–1891); D. G. Elliot, Synopsis of the Mammals of North America (Chicago, 1901) and The Mammals of Middle America and the West Indies (Chicago, 1904); W. L. Sclater, The Fauna of South Africa—Mammals (Cape Town, 1901–1902); W. K. Parker, Mammalian Descent (London, 1885); E. Trouessart, Catalogus mammalium, tam viventium quam fossilium (Paris, 1898–1899); and supplement, 1904–1905; T. S. Palmer, Index generum mammalium (Washington, 1904); W. L. and P. L. Sclater, The Geography of Mammals (London, 1899); R. Lydekker, A Geographical History of Mammals (Cambridge, 1896). (W. H. F.; R. L.*)
MAMMARY GLAND (Lat. mamma), or female breast, the organ by means of which the young are suckled, and the possession of which, in some region of the trunk, entitles the animal bearing it to a place in the order of Mammalia.
Anatomy.—In the human female the gland extends vertically from the second to the sixth rib, and transversely from the edge of the sternum to the mid axillary line; it is embedded in the fat superficial to the pectoralis major muscle, and a process which extends toward the arm-pit is sometimes called the axillary tail. A little below the centre of the glandular swelling is the nipple, surrounding which is a pigmented circular patch called the areola; this is studded with slight nodules, which are the openings of areolar glands secreting an oily fluid to protect the skin during suckling. During the second or third month of pregnancy the areola becomes more or less deeply pigmented, but this to a large extent passes off after lactation ceases. In structure the gland consists of some fifteen to twenty lobules, each of which has a lactiferous duct opening at the summit of the nipple, and branching in the substance of the gland to form secondary lobules, the walls of which are lined by cubical epithelium in which the milk is secreted. These secondary lobules project into the surrounding fat, so that it is difficult to dissect out the gland cleanly. Before opening at the nipple each lactiferous duct has a fusiform dilatation called the ampulla.
After the child-bearing period of life the breasts atrophy and tend to become pendulous, while in some African races they are pendulous throughout life. Variations in the mammary glands are common; often the left breast is larger than the right, and in those rare cases in which one breast is suppressed it is usually the right, though suppression of the breast does not necessarily include absence of the nipple.
