Popular Science Monthly/Volume 5/September 1874/The Chain of Species III
| THE CHAIN OF SPECIES. |
By Hon. LAWRENCE JOHNSON.
Part III.—The Passage from Annulosa to Mollusca.
ANOTHER plan, however, is proposed, which seeks to connect Annulosa and Mollusca as successive stages in the progress of evolution from the simplest types and stages necessary to be taken in order to reach the highest development. This is the chain: Evolution of Protozoa directly into Annulosa; or first into the cœlenterate type and these into the annulose, either of which routes seems feasible and easy; then Annulosa into Mollusca; and then Mollusca into Vertebrata. Pursuing this road, the only difficulty of importance is the passage from the articulate or annulose form to the mollusk. Enough has already been said to furnish the key to all the other transitions, and the few brief minutes left us must be devoted to this really obscure problem. If the bridge erected here is practicable, it puts a different aspect upon the whole question, and reflects light backward and forward on every link of the chain.
As remarked, the only real difficulty is to connect annulosa and mollusca. For from cephalopods it is easy to develop the vertebrate type, by elongating the ventral aspect of the creature; and bringing down therewith the ventral portion of the cephalic appendages, which subsequently assume, or rather return to, the place and functions of lateral limbs to the main trunk; the other cephalic appendages, properly belonging to the neurohæmal axis, easily taking the form and offices of œsophagus, branchiæ, and even internal bronchial apparatus. Evolution of amphioxus in this way, from some lost octopus is easier to account for than from salpidæ, as proposed by recent authors; even if amphioxus is a vertebrate animal, which is exceedingly doubtful. But the difficulty is to get to cephalopods.
It is confessedly easy to pass from the cœlenterate type to the annulose—or, in the old style, from Radiata to Articulata.
Nothing is more manifest in Nature than that she never loses the effect of a habit—never gives up a plan—never resigns the use of means and tools once adopted. When we have arrived at the involute cell as the type of the endothentic series—the animal kingdom—nothing is more obvious than the compounding of the simple form to constitute the rest of the kingdom. Nor is it necessary to suppose that the compounding takes place by any other means than that already observed in the case of exothentic creatures. If vegetal cells multiply and compound by a gemmating and fissiparous process, so also does the endothentic and celœnterate type.
Mr. Spencer has shown, triumphantly, how this may take place even mechanically. A simple endothen becomes two by ordinary growth, until division is forced at some median point, exactly as in the case of a cell. When the separation is complete we say this is an increase by gemmation. When there is differentiation without separation, we call it compounding—or, as Mr. Spencer has it, an aggregation. The same two great laws of aggregation and segregation, which rule in all things else, present themselves here also in explanation of the phenomena of life. As presented in annulose and annuloid creatures, the compounding is a segmentation. For an annulose or articulate segment is nothing else but one of the simple elements of a compound structure, of which the distinction of the parts is less pronounced. In sponges, in corals, in compound ascidians, the segregation is far advanced—the compounding is very evident. In these it is not denied any more than in the analogous compounding displayed in mosses, ferns, and trees. For, according to the laws of vegetal life already reviewed, every leaf, every node, is a distinct creature, and the bud of the node its progeny. But it is not at first sight so obvious that the real law of all creatures constituted of rings and joints is the law of compound association. It is difficult, without some reflection, to admit that every segment in these is a modification of the original unicellular creature—the mono-segmentarian from which the aggregation sprang. Yet the most casual anatomist cannot fail to perceive that each section in annulosa is but a repetition of the same structure with all its organs and appendages. Even where there are modification and differentiation of function, the structure is always and evidently identical—perfectly homologous.
Fully comprehending the nature of segments, we may proceed to the further efforts of Nature to obtain higher combinations and greater concentrations of energy. For this seems to be the end and object aimed at, if we may be allowed in our own minds to clothe Nature with conscious impersonation, i. e., personality. From the organization of the first drop of protoplasm, and the vivification of the first cell, the tendency has been, as the necessary result of natural selection, to concentrate force. In the history of evolution there is no passing from lower to higher forms—no foldings, no involutions, without an increase of power; and by the same law this development must continue until the highest point of physical energy is attained.
Our ordinary discoveries teach us that force can be multiplied by a multiplication of the elements or organs evolving force. The simplest child knows how to obtain two pounds instead of one by putting another weight in the scale; and the electrician obtains more power by adding another jar, or another plate, or another cell to his battery. In like manner Nature has in various directions seized every means to increase and concentrate what for convenience is termed vital force. For there is the same tendency of the forces to aggregation that we see in matter.
Watching the progress as we have ascended the scale from the first evolution of life, the greatest concentration of force yet reached in organic life is in articulate creatures—or, as we may call them, by a name of more general application, segmentarians. A segmentary such as a centipede, a bee, a lobster, or even the humblest worm, is as truly a compound zoary as any other collection of zooids, whether cœlenterate or molluscoid; and no argument is needed to show a physicist that the closely-united segmentarian zoary evolves more force than the looser aggregation of a branch of ascidians, however highly organized the individuals of the latter may be. There is more force evolved from the gigantic oak consisting of such a closely-united system as presented by the nodes, and which is capable of appropriating such volumes of inorganic matter, than in the loose sheets of ulva or protococcus creeping upon damp walls and slimy pools. Again, in physics, we may multiply force, not only by the number of our elements, but by their size and arrangement. In galvanism two large elements or cells may be made to exhibit greater energy than many small ones in the aggregate of equal superficial extent, yet not precisely the same energy. It is modified as well as increased. Organic Nature presents us exactly as good illustrations of this law as the experiments of Mr. Grove and Dr. Faraday.
The first experiment, so to speak, of Nature to multiply force from the cœlenterate or monosegmental creature, the last stage arrived at in our progress, is by multiplying the segments upon one or more axes. When upon more than one, as in annuloida, Nature seems to break down early, on account of the complication of the machinery, and soon seeks greater simplicity. This is attained, first, by selecting one longitudinal axis, and multiplying the units or elements of organization and force indefinitely. In some worms the number of segments is incredible. Instances of iulidæ, according to Mr. Newport, have one hundred and fifty rings, at least during embryonic life; and, by the same rule, some geophilidæ, mentioned by Dr. Carpenter, must reach to three hundred and twenty—each segment, remember, having a quasi-separate organization, separate circulation, separate nerves, and separate appendages for aëration and locomotion. Their close and intimate connection does not prevent easy analysis into distinct systems for each segment.
Again, however, Nature soon discovers her mistake. The highest concentration of energy cannot be reached in this way, and she makes trial of a new plan. She seeks to modify and expand the elements, and to bring them into closer contiguity. Gathering up her forces into a few central segments, the rest are lopped off. This is actually the history seen in some embryones. In insects, finally, the highest of the segmentarians, there are only about twenty well-pronounced segments. Some of these, as in the cephalic and thoracic regions, are so intimately united that the divisions cannot be traced, except in the embryo. In adults the thoracic segments are further modified, for great extension and concentration of force, by expansions, primarily, of the aërating apparatus into legs, wings, or other appendages. In short, the thoracic region is evidently the concentration of the life of these creatures.
Now to advance beyond this type. Nature never changes her tools—her means; and even her plans present but slight modifications. Her collection and concentration of energy in the thoracic region of polysegmental annulosa is a gathering of her strength at that point whence the next march will begin.
Exactly as we may see an individual segmentarian zoary doubling itself up for its own comfort, increase of heat, or of force—exactly as we have seen in vegetal life, after a longitudinal or a superficial evolution of the elements of the compound, energy concentrated by a folding down of the compound upon itself, and an adhesion—so works Nature here. The concentration of life or force at any one point diminishes its intensity in remoter segments; a more intimate union of a few segments causes atrophy in the more distant frontier provinces; and this process continues, in fact, until the segments are reduced to two. The very fact of a closer folding and adhesion of two would have this result. This is a position we might, at this stage, comprehend a priori. Now let us see if it has any foundation in fact.
Again I repeat, this is not an imaginary process I am going to amuse you with—it is not speculation—but an irrefragable chain of facts and demonstrations. The entomostracous group of crustacea affords us illustrations of every step of the progress. First, an excessive development of two segments of the thorax; then, atrophy or dwarfing of the cephalic and abdominal regions, in various degrees. In cypris, for instance, two segments are so enormously developed as to usurp the mass of its substance, if not yet all the functions of its life. The dorsal scales of these two segments are actually so enlarged as to present a bivalve box, or case, to inclose the creature as snugly as you see effected for a clam or an oyster. Loss of head and tail would leave the little cypris as perfectly a testacean mollusk as any of these.
But you say, if—if it should lose head and tail! The exaggerated fable of the fairy tale is actually the method adopted by the magician Nature to disenchant the fair princess Life from the thraldom of larval and infusorial forms. The cypris does lose head and tail, and so becomes molluscan, before proceeding to higher evolution. But not as a cypris—oh, no! she changes her name, as she changes her type; and we must advance to another group of the entomostracous class—one which has been divided from it, more on account of general superior size than for any other reason—the cirrhipeds.
In this group we find the most striking metamorphoses in the life of the same individual. Passed out of the mere embryo, the larval form resembles more an ordinary macrourous decapod than the entomostracous its nearest relations. Take one generally known, the genus lepas—the common goose-barnacle—in extreme youth, an exceedingly active little shrimp-like tenant of the deep. During the latter part of this stage, the hypertrophied segments of the thorax continuing to grow as the cephalic and abdominal parts dwindle, finally, by his jaws, he first seizes hold of some solid support, when the whole head and neck become entirely changed, and remain a mere stipe, for the support of the creature. Every part becomes metamorphosed. The head having disappeared, a new mouth is opened in the breast; and the abdominal portions, although not entirely lost in this genus, are differentiated to other functions. The most important change to notice, in this creature of change, is the change of axis. It is now exactly at right angles to the original axis of vitality in the young crustacean. Yet, mark well: this is only a return to the true axis of nerve-force in the segments, which, in all annulosa, is at right angles to the longitudinal development of the sections. The interminable gemmations and addition of segments to the zoary being arrested, and life confined to one or two, most naturally the current of the dominant force remaining controls the direction or axis of all the rest. So completely have the method of vitality and the appearances of the little animal been changed, that earlier naturalists classed the young lepas as a crustacean, and the adult barnacle as a multivalve mollusk. And so, indeed, it is a mollusk, with a few crustacean characteristics not yet lost.
The steps from this to the perfect mollusk are too plain to be dwelt on here. Indeed, the difference is so small that, had not the larval cirrhiped been discovered, the position of the group would never have been assailed. They should, like human aspirants to rank, have concealed their plebeian origin. If the observations of recent embryologists are to be credited, many mollusks and molluscoidea exhibit a similar evolution. At any rate, even in undoubted mollusca, the elements of their segmentarian origin are abundantly visible.
If we take the genus arca, for instance, as a typical mollusk, it is easy to follow the segments of which it is composed. Each half of this bivalve has its own organization, as complete as in any segment of any annulosa. Each has its own heart, its own nerve-ganglia and branches, its own aërating apparatus and systemic circulation, such as it is. You may even trace, still, the crossing of muscles and nerve-fibres, at the back or hinge, exactly as between the two articulate segments from which they sprang.
Here it may be remarked, again, how tenacious Nature is of her plans. This mysterious crossing and anastomosing of nerve-fibres, so unaccountable in the brain of the higher animals, and of such important consequences even in man himself, had its origin in the primal union of two annulose segments.
Having overcome the principal difficulty by noticing the change of axis, nothing more remains but to pursue this bisegmental arrangement to its full development in the grand class of mollusks. Possibly not one of existing species had any part in the chain of development. But this is of small interest at present, since we are looking for the method, the steps, the finger-boards of the road traveled, and care net now to count the milestones.
The transition from the highest mollusca to vertebrata, as already remarked, is plain enough. Mollusca already have the internal structure of vertebrata—the same digestive system—similar nerves—and identical circulatory machinery. The highest mollusks have as much brain as the lowest fishes, and decidedly more than the famous amphioxus—a creature which just now is an obstruction instead of a help in the establishment of a sound biological theory of development; and this for the reason that it is leading us away from the true relations of these orders, and helping to keep up the old misconception of the nature, origin, and importance of a jointed spinal column. Unfortunately, it is this want of vertebræ, of backbone in mollusks, that prevents us from seeing the near alliance of cephalopods to vertebrates. Had the latter been supplied with the more appropriate and distinctive title Cerebrata, these highest mollusks might better have claimed admission to the class than many species now found in its ranks. For, besides the homologues already mentioned, do but notice the optic and auditory apparatus of sepia, for instance. These are perfectly identical with the eyes and ears of vertebrates. Notice, again, the organization of the mouth. Only vertebrates have such. In annulosa the jaws have lateral motion, and are modifications of the feet. One other consideration must be mentioned—the size of these animals. Only vertebrata and mollusca seem to have unlimited powers of increase. They only have been distinguished for the magnitude of individuals. Among the latter especially are to be noted the cephalopods for furnishing giants. Did space permit, a thousand homologues might be pointed out.
From all of which results the general conclusion, worked out more fully in my paper on the "Homologues of Organic Creatures," that here is the true route of organic development; the vertebrates from the mollusks, and these from the articulates proceed; and that the basis of the history we are seeking must be searched for in the history of segmentation.
All are familiar with the construction of an annulose ring or segment. To the rest of the compound creature each segment is a microcosm, precisly repeating similar parts, or their homologues. To each segment belongs a neural ganglion, or rather a pair of ganglia; a dorsal vessel swelling in the middle to a pulsating organ, or heart, or rudiments thereof; and each is furnished with stigmata, spiracles, or appendages, which are homologous of each other, or, as before, with rudiments thereof. And all these by sufficient analysis are to be traced as actually present in every true segment, however often the function of the homologous structure may be changed, or however it may be reduced to a rudiment, or atrophied, or absorbed and apparently lost. These homologues constitute the basis of biology, as they do of the life-functions of all creatures made of segments, which includes, as we have seen, mollusks and vertebrates, as well as annulosa—possibly, also, molluscoidea; unless the latter should be considered a modified cœlenterate, and then it would be the beginning and unit of the series. If, as sometimes contended (and these are unsettled questions), the molluscoid is an evolution of the annulose type, and its embryonic history tends to prove this, then all the segments have been lost but one. For the molluscoids are monosegmental. Nor is this proposition strange or improbable. Undoubtedly such a reduction to one segment takes place in high orders of genuine mollusks, as in gasteropods (of which you have instances in common snails), in whom one segment has become atrophied, leaving generally a rudiment behind.
In vertebrata, which more immediately concern us, two segments, and only two, are always present, and always bear with them their distinctive elements or appendages, however rudimentary some of the latter may sometimes appear. These segments really constitute the well-known bilateral arrangement of parts and organs so general in animals of this class. Here, again, it is necessary to go back a little in order that we may make the greater speed forward. It is necessary distinctly to understand what we are talking about, and what we mean by segments. In the discussion of the bisegmental organization of vertebrates, the question comes up whether any true homologues exist in the two great classes, vertebrata and annulosa. In the present advanced condition of biology, this question receives a decided affirmative. Until recently the contrary was generally supposed.
When it is seen that two sides of a vertebrate, if it be split asunder down the backbone, present exact counterparts of each other, and also of two segments of any annulose animal—a crustacean or a centipede, for instance; and when we have seen in the progress from unicellular and from monosegmental structures, to the multi-segmental; and from these again to the bisegmental, by lopping off surplus machinery, and by modifying and enlarging what is left in order to increase and intensify the forces evolved; when we actually see these changes, with the attendant change of axis, taking place and becoming the normal conditions of some creatures—all mystery vanishes.
First, let us notice that the so-called vertebrate segments or sections—the vertebræ of the spinal column, are not segments in the sense used here—are not segmenta, the biological bracelets, in any true sense; although their development takes place after the change of axis by a growth analogous to the original evolution of annulose segments. The vertebræ are not homologues of the segments of the great segmentarian series.
Having thus indicated what the segments are not, let us call to mind again what they are. This will be the best way to establish the chain we are seeking to comprehend, as it will also be the readiest plan of exhibiting the homologues of these classes, upon which depends all our knowledge of transcendental biology. It is best done, in the few moments left us, by tracing a few of these homologues. For this purpose let us select prominent and obvious organs, for instance, the heart, the brain, and the extremities or appendages.
The first to be noted, being most obvious to popular inquiry, are the extremities or limbs—the true articulæ. Fins, wings, legs, and arms—can these truly be homologues of the arthritic appendages of annulosa? They are indeed. Not half so plain, when first announced, was that first wonderful revelation of comparative anatomy, which displayed the fin of a fish, the paddle of a whale, the wing of a bird, the leg of a horse, and the arm of a man, to be composed of the same organic elements, as is this which now proclaims that these so varied and beauteous limbs of vertebrate animals made the first essays of their evolution as the lateral appendages of aëration or locomotion of the segments of the lowest orders of annulosa.
The proofs of this are now obvious, but to appreciate them we have need to travel back again through all the grades of life, and weigh the homologues of every organic element.
Yet some of the evidences are plain, and almost superficial, in the vertebrate class itself. Every anatomist, in order to comprehend the physiology and anatomy of this class, begins his analysis of the vertebrate skeleton by observing two distinct systems of organization—the dermal and the neurohæmal, heretofore supposed to be peculiar to vertebrates. Now, to which system, the dermal or the neurohæmal, do the extremities belong? No amount of ingenuity can satisfy the thoughtful student that the limbs are evolutions of the appendages of the neurohæmal axis. They are dermal. Even in mammalia they have no proper union, no true articulation with the spinal column. Even in those cases where the bones of the pelvis and of the sternum are most securely fastened to the neural axis, it is by a mere anchylosis completed only in the adult age.
The dermal system is the counterpart and homologue of that wondrous external skeleton of articulata, as you must see if you have followed me through the metamorphoses of cœlenterata, entomostraca, and cirrhipeda to mollusca, and from mollusca to vertebrata. Comparative anatomy will not hesitate now to point out every correspondence of every part, every organ, every articulation. Not a bone, nor a scale, nor a hair, is ever lost. In each limb there are so many joints and no more; and with a certain definite relation to each other, clear and true; to be seen in the contorted limbs of articulata, as well as in the arm of man.
Our very limited amount of time compels me to make this address a mere introduction to the subject; compels us to be content with mere suggestions for studies in biology, to be pursued by after-investigations.
One of these suggestions is made to us by the manner of the folding of vertebrate limbs. The difficulties of the question, as put by Dr. Wyman, for example, are well worth consideration, and by this transcendental history of evolution find solution. How is it that in all vertebrates the forward limbs fold one way, forward, and the hinder limbs the reverse? But take you any two segments of an articulate creature, a crab or a grasshopper, and observe how the appendages extend away from the body, and the mystery is explained. For the effect of the magnetic and other forces causing polarity or arrangement is to be searched for and found first in the segments, always remembering that in articulates the axis of the neural system—the life of each segment—is transverse to the general axis of the compound organism. Now, separate two such segments of your articulate—fold them together in the manner indicated by the history of cypris and of lepas—revolve the involuted creature ninety degrees, stretch the limbs forward and backward as their organization fits them to move; and you cannot fail to perceive a perfect counterpart of the motions of vertebrate extremities.
But this is not half the demonstration. Indeed, it is now so well settled, that Science can, dare make one of those predictions always regarded as affording by their verification the highest order of certainty. After noticing the remarkable fact that all vertebrates having limbs have four, or the rudiments of them, and no more. Science predicts with the repose of conscious systematic truth, derived from this doctrine of segmental genesis, that no vertebrate animal, nor the remains of any, ever will be found, on the earth, or in the waters under the earth, or in its caverns or quarries, having more than these four homologous appendages—and these always dermal.
But, it may be objected, this is only of external matters, the limbs, the appendages; and these may be accidental correspondences.
Very well! Select any other set of organs. Are there any more internal and peculiar to vertebrates than the brain and nerves? Let us take the neurohæmal system itself, that which confessedly has no perfect counterpart in the articulate class. It is not suggested that any creature lower than cephalopod mollusks presents any thing exactly corresponding to the cerebral hemispheres, and the optic and auditory lobes of the vertebrate brain. But let us see if we cannot trace some homologues? Without this historical derivation from annulose segments there will remain many things in cerebral anatomy entirely inexplicable. To pursue this theme no further at present, the approximation of the vertebrate and the molluscan systems of nerves cannot be doubted. In higher cephalopods, as sepia, the principal ganglia are brought into near proximity—at least resembling a brain. They are even covered by a bony framework, rudimentary of a neural skeleton—in all of which the great cephalopod is decidedly more cerebrate than some fishes, to say nothing of the doubtful amphioxus. But if all else were wanting, the auditory lobes and the optical apparatus would establish this correspondence with the highest orders. The cuttle-fish has an eye with a retina, lens, iris, and cornea; and the optical ganglia are as truly lobes of the brain as they are in mammalia. Nothing like this is seen in acephalous mollusks, nor in articulata. Where articulata have a machinery for vision, it is not organized upon this plan. But we do see centres of nerve-force, or ganglia, appertaining to every segment; and we do see also that the nervous system of sepia is only an advance upon that of the inferior mollusca. Even in the oyster the ganglia are brought nearly to a common centre; and this arrangement does not differ essentially from that in the perfectly equilateral mollusks, as in area, for instance, except in the nearer contiguity of the ganglia. Finally, comparing area with lepas and cypris, it is manifest that we have essentially the same plan as in the nervous system of any two segments of articulata. For you will find in each segment two ganglia, one on each side of the median line; and these being brought together, as they generally are in annulosa, appear as one. Now, this one twofold ganglion is, if our explanation of the metamorphoses of the segments be correct, the homologue of the ganglia of muscular motion, of the two principal valves of cypridæ, of cirrhipeds, and of bivalve mollusca; and finds its final evolution in the quadruple structure of a mammalian brain. In fact, the homology is complete, with the additions, or rather modifications, already explained, which endow vertebrates with olfactory, auditory, and optic lobes and systems apparently peculiar.
There is another branch of the internal structure of vertebrates inexplicable upon any other hypothesis than this chain of specific descent. I mean the hæmal system—the system appropriated to the circulation of the blood. Comparative physiology, guided by the light of this metamorphosis of the segments, can have no difficulty in tracing the history of the heart back from mammals, and reptiles, and typical mollusks, to crustacea; and finding it at last reduced to its elementary form in the twofold pulsatile vesicle of the dorsal artery of each double segment of, for instance, iulidæ—lowest of the annulose series. The folding together and stricter union of these segments, as we have seen, bringing these pulsatile vesicles into juxtaposition, perfectly account for the two double hearts of bivalve mollusca, and of fishes; while the approximation and union on the median line of this double machinery of the mollusk explain the fourfold heart and the circulation of the highest types of life.
If you have followed the chain of evolution here briefly sketched, although in mere suggestions, you can have no difficulty in perceiving the unbroken succession of allied links, all the way from the lowest cell-formations up to man. Many forms, doubtless, belong to subordinate systems, and take no part, directly, in the chain; yet the study of all will enable us more fully to comprehend the whole process. In fact, we have no right to expect to find, unless perhaps in fossil forms, the precise species—the very links of the chain—by which the transmission of form and life has been actually effected. It would be unreasonable to require this of the advocates of evolution, since the exact contrary would follow, as an a priori conclusion, from the very terms of the proposition. It would be a wonder, indeed, in view of all the transitions and transmutations of matter, of force, of time, of place, of forms, if we could find, now living, a single species actually concerned in the long process of evolution—a single "bark which brought us hither." It is just as unreasonable as to demand of us to produce, alive, every individual through whom our descent has been accomplished—just as unreasonable as to demand the resurrection of all the members of our race, for the last thousand years, to prove ourselves Anglo-Saxons. Time necessarily devoured these when the period of life was finished. So has it done with species; for, as the species is continued through individuals, and always with variation, the distance of removal from any special form is only a question of time. The cosmos is always
"Ein wechselnd Wehen,
Ein glühend Leben,"
and, therefore, it takes but a few generations—a few thousands, or a few millions of years—to leave behind any specific type, as completely as the forgotten bones of our progenitors that lie hid in Batavian bogs. Why, with all the lights of human history, we cannot trace the line of any human family more than a few hundred years! And what is the historic period, upon which all doubts and objections are based, compared to the ages multitudinous that have passed away without recognition in human calendars? Take the simplest tree, a few years old. We see a crowning bud at the apex of its principal axis of growth; and we see long lateral branches, likewise, similarly tipped with leaf, bud, and bloom; and we do not doubt their common origin, because there stands the common connecting trunk. Yet neither of these had any part in the production of the others; nor remains there a single one of the individual buds and tender leaves which, in the years gone by, really did take part in the development, the consummation whereof we now behold.
Nor, in regarding man, whom we fondly believe the crowning glory of creation, should we expect to see the precise yearly growths which have finally lifted him to this elevation. The steps of the evolution may still be traced, but not a single individual of the lineal ancestry remains. When, therefore, we speak of the chain of species—of the line of man's descent—upon a priori grounds, we do not expect to find, extant in life, the very links of the chain. The loss of them is another proof of evolution. It is thus, according to the plan proposed, we would expect to find it exemplified. It is a rational conception of creation we are attempting to reach, and no other rational hypothesis has ever been proposed.
We do not find, then, the very species through which the ascent to man has been accomplished, and do not seek to find them; but, if this plan of derivation is well founded, as it is clearly rational, we must conclude that the various races of man, now upon the earth, sprang from some common stock, of the order of primates; which, in turn, must have been derived from a lower simian form; and this, again, must have come of a trunk leading back to aplacental mammals; and these lead on to amphibious reptiles; these to fishes; these to cephalopod mollusks; these to bivalve mollusks; these to cirrhiped crustaceæ; which last, in fœtal life, possess all the characteristics of the general articulate or annulose type; annulosa being derived directly from primordial cœlenterata, whence probably issued, also, annuloids and molluscoids. If molluscoidea be the offspring of cœlenterata, then the part played in evolution by the molluscoid type was not to furnish a stage of transition, but to illustrate the power of a segment. In any case, we end with the cœlenterate type, whether fixed, as in actinia, or occasional only, as in rhizopods. This last, being the first animal form, causes us to remember that here branches off another great kingdom, of which, the life is always exothentic; and which, therefore, has no direct part in this chain, except that its first forms furnish the common stock whence has arisen all organic life.
This wonderfully intimate relationship of the innumerable forms of living creatures, properly considered, is calculated to elevate our conception of the creation, and of man himself; while, to the glory of the Creator, it is held out to us as another "bow of promise"—another assurance of the certainty of the universal reign of law. Nor must it be forgotten that it does not exclude, but, contrariwise, encourages, moral reflections. While it tells man of his dignity, it tells him also of his debasement—that he is sublime in being possessed of so much of his Maker's image as enables him to contemplate all this glorious mechanism; but that he is also "a brother to the insensible clod, which the rude swain turns with his share, and treads upon." It also enforces the reflection of the old poet:
" . . . . Except above himself he can
Erect himself, how mean a thing is man!"
