is sufficient for our present purpose to point out that in the structure of the foot alone we discern the evidence for evolution as clearly as in the entire organisation of the animal. An increase of speed, and obvious advantage over its enemies, would be gained by the horse, as its toes grew "small by degrees and beautifully less;" and the single-toed race has thus practically come to the front in the world of to-day, as the plain and favourable result of the work of degradation amongst its digits. It may likewise be mentioned, that the conclusions of evolution and geology are strengthened by the evidence of teratology, or the science of abnormalities. Occasionally horses are born with several toes; this fact being explicable only on the idea of "reversion" to a multiple-toed ancestry.

Two bony shreds or rudiments thus lay the foundation of a grave conclusion regarding the horse and its manner of develop. ment, and exemplify the adage that great and unlooked-for results sometimes spring from beginnings of apparently the most trifling kind. The "splint-bones" form, in fact, a clue which, when rightly pursued, leads not merely to a knowledge of the evolution of the horse, but to an understanding of the entire scheme of nature. For if evolution is the law of the horse's history, it must logically follow that it represents the scheme of nature throughout: since the uniformity of nature, in which we are bound to believe, and to which we are bound to appeal, would utterly negative the idea that evolution should hold good for the horse, and be inapplicable to any other living thing. Because the missing links are not so completely supplied to us in other cases as in the horse, we are not on that account entitled to assume that the theory of development is invalid. We may not see an oak tree grow inch by inch, but we are as positive as our mental nature will admit, that the oak was once an acorn, and that there has been a progressive growth and increase which might not be apparent to us were we to watch the tree for weeks together. Applying this reasoning to the case before us, it would be as illogical to deny that the order of nature was that of development, as to insist that the oak was created as it stands. The extent of human knowledge, and the duration of human existence, are together inadequate to enable us to discern the progress of this world's order after the fashion whereby, from a lofty elevation, we may trace every winding of a stream. But the probabilities of the case are as overwhelmingly for progressive development, as the direct evidence at hand-exemplified by the horse's pedigree-tells against special and independent creation having been the way of developmental law in the making of the world and its living things.

97

VI.

THE EVIDENCE FROM THE

TAILS, LIMBS, AND LUNGS OF ANIMALS.

THE extreme respect occasionally paid by the scientific investigator to the merest rudiments of parts and structures in animals and plants, or to apparently insignificant phenomena in the physical universe around us, naturally presents a source of wonder and curiosity to the uninitiated mind. Circumstances which to the latter appear "trifles light as air," may in truth afford "proofs of confirmation" of the strongest character to the man of science. He has learned from the successes of the past, the wisdom of seeing a possible clue to some of the deepest of nature's problems in the veriest byways and in the most unlikely paths into which his researches may lead. The connection of one fact with another may not at first sight be apparent; and the isolated truth may remain, for years, a detached fragment of knowledge, possessing no evident relationship with the arranged facts constituting the main body of the science. But the patience of science must be equal to its hope; and the experience of the past has taught us many a lesson regarding the real value of facts which seemingly were of little import as year by year they remained disconnected and solitary offshoots of the tree of knowledge. Thus one of the first precepts of scientific inquiry is that which inculcates the wisdom of gathering up the fragments which deep research often leaves behind after its "golden reaping" is past and over. For a second harvest of veritable treasures may not unfrequently reward the patient searcher in science-pastures, after the larger toil has apparently left no corner of the field of inquiry unexplored. The application of the foregoing commonplaceisms is nowhere better exemplified than in many facts supporting evolution which have been elicited from quarters of the most unlikely nature, and from natural-history details. which, in former years, might have been regarded as antagonistic to the first principles of the development theory. One of the most convincing circumstances of the general truth of evolution, indeed, consists in the amount of spontaneous support which has flowed towards this theory from all directions in biology; whilst, in turn, the theory of development has strengthened its own case by affording the only rational explanation of hitherto unexplained facts,

H

and in no less degree by supplying the theoretical connection required to connect detached facts with the main body of scientific knowledge.

A popular excursion into the domain of comparative anatomy will present us with several apt illustrations of these remarks, and will serve to prove the truth of the assertion regarding the import to science at large of the veriest "odds and ends" in natural-history trifles. Of such "ends," in one sense, the tails of fishes may be said to present us with examples of the most literal kind. The class of fishes unquestionably presents an interesting field of inquiry to zoology of the most popular nature. There might possibly exist, however, a shade of hesitation on the part of even enthusiastic students of fishlore, in affirming the truth of the assertion that in the tails of fishes we may perchance find a study of more than usual interest. These structures are unquestionably elegant enough in their way, and, whether as constituting the propelling agents or the steering-gear of their possessors, claim a just share of zoological attention. But that on the caudal appendages of fishes we may presume to "hang a tale"

of the probable origin and evolution of the race at large, is an expectation by no means warranted on a brief review and consideration of the apparently trivial nature of the subject. In the history of scientific speculation, however, "tails" have played more than one prominent part. On more than one occasion a theory of tails. has been gravely discussed and hotly debated; and it is indeed difficult to assign a reason why the apparent insignificance of the subject should disguise and conceal its real importance. Possibly owing to the deterioration of the caudal region in the human subject, the importance of the "tail" in lower life acquires thus a tendency to become thoroughly overlooked. Were a spider monkey (Fig. 35), however, capable of forming and expressing an adequate opinion on the value of his tail, consisting, as it does, of some

CERVICAL
OR ATLAS
2ND
CERVICAL
OR AXIS

thirty-three joints, our estimate of tails in general might undergo a complete revolution. Such an appendage constitutes a veritable fifth hand to that agile denizen of the South American forests. Grasping the bough of a tree with its prehensile tip, he is enabled to swing himself hither and thither, with his hands and feet free and ready for action in any desired direction. He might be inclined to regard his higher neighbours, in which the tail is reduced to a mere rudiment, as degenerate and reduced creatures when compared with himself and his terminal organisation-so much in thoughts and thinking, as we all know, depends upon one's special point of view.

It is, of course, a patent fact to any one who will take the trouble to compare the backbone of man with that of a spider monkey, or indeed with the spine of well-nigh any other quadruped that the four small bones forming the end of the human spine, and collectively named the coccyx (Fig. 36), represent a rudimentary tail. These bones are seen to be degraded and deteriorated in structure when compared with the other joints of the spine (or vertebræ) with the typical structure of which they undoubtedly correspond. As any tail is merely the hinder extension of the vertebrate spine, so the coccyx, representing in its feeble way the terminal part of man's spine, is certainly a veritable appendage of the kind in question. Man is, however, not the only animal in which degradation of the tail exists, and is propagated by descent as a natural condition of animal existence. The Manx cat has a truly rudimentary tail in this latter aspect; certain higher monkeys (e. g., orang, chimpanzee, and gorilla) possess the merest traces of this appendage; and tailless varieties of sheep are known, the latter being well exemplified by a Chinese breed in which, as Mr. Darwin, quoting from Pallas, tells us, the tail is reduced "to a little SIDE VIEW OF HUMAN SPINE. button, suffocated in a manner by fat." It

FIG. 36.

should also be remembered that, in lower life, tails of considerable length may dwindle and disappear, leaving their possessors as abso

lutely tailless as man. One has but to compare the young crab with the adult, or the fish-like tadpole with the frog (Fig. 48), to witness a most typical case of the disappearance of a tail. And it is worth remembering that the frogs have the advantage of humanity in point of antiquity; since the advancement of the tailed tadpole race to become the tailless frogs of to-day must have taken place, according to geological evidence, long ages anterior to the advent of the "imperial race" of man.

But if so much may be proved and said regarding the rudimentary nature of "tails," it must also be borne in mind that the opposite case of a special development of the tail in man is by no means unknown. Occasionally in the human subject a short but free tail is found to be developed, this fact constituting at once a surgical abnormality and a physiological "reversion" to an ancient order of things. Let us consider for a moment what development teaches us concerning the exact place assumed by the end of the spine in higher animals. Primarily, we are struck by the close resemblance to each other presented by the embryos or young of vertebrate animals (Fig. 37) in their earlier stages of development. Even Von

D

A
PIG.

B

CALF: [FIG. 37.] RABBIT.

MAN.

Baer himself, an authority in matters relating to embryology, said of this likeness that "the embryos of mammalia, of birds, lizards, and snakes, and probably also of Chelonia (tortoises and turtles), are, in their earliest states, exceedingly like one another, both as a whole and in the mode of development of these parts; so much so, in fact, that we can often distinguish the embryos only by their size. In my possession," he continues, "are two little embryos in spirit, whose names I have omitted to attach, and at present I am quite unable to say to what class they belong. They may be lizards, or small birds, or very young Mammalia, so complete is the similarity in the mode of formation of the head and trunk in these animals. The extremities, however, are still absent in these embryos. But even if they had existed in the earliest stage of their development, we should learn nothing, for the feet of lizards and mammals, the wings and feet of birds, no less than the hands. and feet of man, all arise from the same fundamental form." The close likeness between vertebrates in their early stages of growth, so plainly lescribed in Von Baer's words, extends to the caudal or tail