The Story of Evolution Part 13

The rest of the Ungulates continued to develop through the Tertiary, and fortunately we are enabled to follow the development of two of the most interesting of them, the horse and the elephant, in considerable detail.

As I said above, the primitive Ungulate soon branches into three types which dimly foreshadow the tapir, the horse, and the rhinoceros, the three forms of the Perissodactyl. The second of these types is the Hyracotherium. It has no distinct equine features, and is known only from the skull, but the authorities regard it as the progenitor (or representative of the progenitors) of the horse-types. In size it must have been something like the rabbit or the hyrax. Still early in the Eocene, however, we find the remains of a small animal (Eohippus), about the size of a fox, which is described as "undoubtedly horse-like." It had only three toes on its hind feet, and four on its front feet; though it had also a splint-bone, representing the shrunken and discarded fifth toe, on its fore feet. Another form of the same period (Protorohippus) shows the central of the three toes on the hind foot much enlarged, and the lateral toes shrinking. The teeth, and the bones and joints of the limbs, are also developing in the direction of the horse.

In the succeeding geological period, the Oligocene, we find several horse-types in which the adaptation of the limbs to running on the firm gra.s.sy plains and of the teeth to eating the gra.s.s continues. Mesohippus has lost the fourth toe of the fore foot, which is now reduced to a splintbone, and the lateral toes of its hind foot are shrinking. In the Miocene period there is a great development of the horse-like mammals.

We have the remains of more than forty species, some continuing the main line of development on the firm and growing prairies of the Miocene, some branching into the softer meadows or the forests, and giving rise to types which will not outlive the Tertiary. They have three toes on each foot, and have generally lost even the rudimentary trace of the fourth toe. In most of them, moreover, the lateral toes--except in the marsh-dwelling species, with spreading feet--scarcely touch the ground, while the central toe is developing a strong hoof. The leg-bones are longer, and have a new type of joint; the muscles are concentrated near the body. The front teeth are now chopping incisors, and the grinding teeth approach those of the modern horse in the distribution of the enamel, dentine, and cement. They are now about the size of a donkey, and must have had a distinctly horsy appearance, with their long necks and heads and tapering limbs. One of them, Merychippus, was probably in the direct line of the evolution of the horse. From Hipparion some of the authorities believe that the zebras may have been developed.

Miohippus, Protohippus, and Hypohippus, varying in size from that of a sheep to that of a donkey, are other branches of this spreading family.

In the Pliocene period the evolution of the main stem culminates in the appearance of the horse, and the collateral branches are destroyed.

Pliohippus is a further intermediate form. It has only one toe on each foot, with two large splint bones, but its hoof is less round than that of the horse, and it differs in the shape of the skull and the length of the teeth. The true horse (Equus) at length appears, in Europe and America, before the close of the Tertiary period. As is well known, it still has the rudimentary traces of its second and fourth toes in the shape of splint bones, and these bones are not only more definitely toe-shaped in the foal before birth, but are occasionally developed and give us a three-toed horse.

From these successive remains we can confidently picture the evolution, during two or three million years, of one of our most familiar mammals.

It must not, of course, be supposed that these fossil remains all represent "ancestors of the horse." In some cases they may very well do so; in others, as we saw, they represent sidebranches of the family which have become extinct. But even such successive forms as the Eohippus, Mesohippus, Miohippus, and Pliohippus must not be arranged in a direct line as the pedigree of the horse. The family became most extensive in the Miocene, and we must regard the casual fossil specimens we have discovered as ill.u.s.trations of the various phases in the development of the horse from the primitive Ungulate. When we recollect what we saw in an earlier chapter about the evolution of gra.s.sy plains and the successive rises of the land during the Tertiary period, and when we reflect on the simultaneous advance of the carnivores, we can without difficulty realise this evolution of our familiar companion from a hyrax-like little animal of two million years ago.

We have not in many cases so rich a collection of intermediate forms as in the case of the horse, but our fossil mammals are numerous enough to suggest a similar development of all the mammals of to-day. The primitive family which gave birth to the horse also gave us, as we saw, the tapir and the rhinoceros. We find ancestral tapirs in Europe and America during the Tertiary period, but the later cold has driven them to the warm swamps of Brazil and Malaysia. The rhinoceros has had a long and interesting history. From the primitive Hyrochinus of the Eocene, in which it is dimly foreshadowed, we pa.s.s to a large and varied family in the later periods. In the Oligocene it spreads into three great branches, adapted, respectively, to life on the elevated lands, the lowlands, and the water. The upland type (Hyracodon) was a light-limbed running animal, well ill.u.s.trating the close relation to the horse. The aquatic representative (Metamynodon) was a stumpy and bulky animal.

The intermediate lowland type was probably the ancestor of the modern animal. All three forms were yet hornless. In the Miocene the lowland type (Leptaceratherium, Aceratherium, etc.) develops vigorously, while the other branches die. The European types now have two horns, and in one of the American species (Diceratherium) we see a commencement of the h.o.r.n.y growths from the skull. We shall see later that the rhinoceros continued in Europe even during the severe conditions of the glacial period, in a branch that developed a woolly coat.

There were also in the early Tertiary several sidebranches of the horse-tapir-rhinoceros family. The Palaeotheres were more or less between the horse and the tapir in structure; the Anoplotheres between the tapir and the ruminant. A third doomed branch, the t.i.tanotheres, flourished vigorously for a time, and begot some strange and monstrous forms (Brontops, t.i.tanops, etc.). In the larger specimens the body was about fourteen feet long, and stood ten feet from the ground. The long, low skull had a pair of horns over the snout. They perished like the equally powerful but equally sluggish and stupid Deinocerata. The Tertiary was an age of brain rather than of brawn. As compared with their early Tertiary representatives' some of our modern mammals have increased seven or eight-fold in brain-capacity.

While the horses and tapirs and rhinoceroses were being gradually evolved from the primitive types, the Artiodactyl branch of the Ungulates--the pigs, deer, oxen, etc.--were also developing. We must dismiss them briefly. We saw that the primitive herbivores divided early in the Eocene into the "odd-toed" and "even-toed" varieties; the name refers, it will be remembered, not to the number of toes, but to the axis of stress. The Artiodactyl group must have quickly branched in turn, as we find very primitive hogs and camels before the end of the Eocene. The first hog-like creature (Homacodon) was much smaller than the hog of to-day, and had strong canine teeth, but in the Oligocene the family gave rise to a large and numerous race, the Elotheres. These "giant-pigs," as they have been called, with two toes on each foot, flourished vigorously for a time in Europe and America, but were extinguished in the Miocene, when the true pigs made their appearance.

Another doomed race of the time is represented by the Hyopotamus, an animal between the pig and the hippopotamus; and the Oreodontids, between the hog and the deer, were another unsuccessful branch of the early race. The hippopotamus itself was widespread in Europe, and a familiar form in the rivers of Britain, in the latter part of the Tertiary.

The camel seems to be traceable to a group of primitive North American Ungulates (Paebrotherium, etc.) in the later Eocene period. The Paebrotherium, a small animal about two feet long, is followed by Pliauchenia, which points toward the llamas and vicunas, and Procamelus, which clearly foreshadows the true camel. In the Pliocene the one branch went southward, to develop into the llamas and vicunas, and the other branch crossed to Asia, to develop into the camels. Since that time they have had no descendants in North America.

The primitive giraffe appears suddenly in the later Tertiary deposits of Europe and Asia. The evidence points to an invasion from Africa, and, as the region of development is unknown and unexplored, the evolution of the giraffe remains a matter of speculation. Chevrotains flourished in Europe and North America in the Oligocene, and are still very primitive in structure, combining features of the hog and the ruminants. Primitive deer and oxen begin in the Miocene, and seem to have an earlier representative in certain American animals (Protoceras), of which the male has a pair of blunt outgrowths between the ears. The first true deer are hornless (like the primitive muskdeer of Asia to-day), but by the middle of the Miocene the males have small two-p.r.o.nged antlers, and as the period proceeds three or four more p.r.o.ngs are added. It is some confirmation of the evolutionary embryonic law that we find the antlers developing in this way in the individual stag to-day. A very curious race of ruminants in the later Tertiary was a large antelope (Sivatherium) with four horns. It had not only the dimensions, but apparently some of the characters, of an elephant.

The elephant itself, the last type of the Ungulates, has a clearer line of developments. A chance discovery of fossils in the Fayum district in Egypt led Dr. C. W. Andrews to make a special exploration, and on the remains which he found he has constructed a remarkable story of the evolution of the elephant. [*] It is clear that the elephant was developed in Africa, and a sufficiently complete series of remains has been found to give a good idea of the origin of its most distinctive features.

In the Eocene period there lived in the Egyptian region an animal, something like the tapir in size and appearance, which had its second incisors developed into small tusks and--to judge from the nasal opening in the skull--a somewhat prolonged snout. This animal (Moeritherium) only differed from the ordinary primitive Ungulate in these incipient elephantine features. In the later Eocene a larger and more advanced animal, the Palaeomastodon, makes its appearance. Its tusks are larger (five or six inches long), its molars more elephantine, the air-cells at the back of the head more developed. It would look like a small elephant, except that it had a long snout, instead of a flexible trunk, and a projecting lower jaw on which the snout rested.

*See this short account, "Guide to the Elephants in the British Museum," 1908.

Up to the beginning of the Miocene, Africa was, as we saw, cut off from Europe and Asia by the sea which stretched from Spain to India. Then the land rose, and the elephant pa.s.sed by the new tracts into the north. Its next representative, Tetrabelodon, is found in Asia and Europe, as well as North Africa. The frame is as large as that of a medium-sized elephant, and the increase of the air-cells at the back of the skull shows that an increased weight has to be sustained by the muscles of the neck. The nostrils are s.h.i.+fted further back. The tusks are from twenty to thirty inches long, and round, and only differ from those of the elephant in curving slightly downward, The chin projects as far as the tusks. The neck is shorter and thicker, and, as the animal increases in height, we can understand that the long snout--possibly prehensile at its lower end--is necessary for the animal to reach the ground. But the snout still lies on the projecting lower jaw, and is not a trunk.

Pa.s.sing over the many collateral branches, which diverge in various directions, we next kind that the chin is shortening (in Tetrabelodon longirostris), and, through a long series of discovered intermediate forms, we trace the evolution of the elephant from the mastodon. The long supporting skin disappears, and the enormous snout becomes a flexible trunk. Southern Asia seems to have been the province of this final transformation, and we have remains of some of these primitive elephants with tusks nine and a half feet long. A later species, which wandered over Central and Southern Europe before the close of the Tertiary, stood fifteen feet high at the shoulder, while the mammoth, which superseded it in the days of early man, had at times tusks more than ten feet in length.

It is interesting to reflect that this light on the evolution of one of our most specialised mammals is due to the chance opening of the soil in an obscure African region. It suggests to us that as geological exploration is extended, many similar discoveries may be made. The slenderness of the geological record is a defect that the future may considerably modify.

From this summary review of the evolution of the Ungulates we must now pa.s.s to an even briefer account of the evolution of the Carnivores. The evidence is less abundant, but the characters of the Carnivores consist so obviously of adaptations to their habits and diet that we have little difficulty in imagining their evolution. Their early Eocene ancestors, the Creodonts, gave rise in the Eocene to forms which we may regard as the forerunners of the cat-family and dog-family, to which most of our familiar Carnivores belong. Patriofelis, the "patriarchal cat," about five or six feet in length (without the tail), curiously combines the features of the cat and the seal-family. Cyonodon has a wolf-like appearance, and Amphicyon rather suggests the fox. Primitive weasels, civets, and hyaenas appear also in the Eocene. The various branches of the Carnivore family are already roughly represented, but it is an age of close relations.h.i.+ps and generalised characters.

In the Miocene we find the various groups diverging still further from each other and from the extinct stocks. Definite wolves and foxes abound in America, and the bear, civet, and hyaena are represented in Europe, together with vague otter-like forms. The dog-family seems to have developed chiefly in North America. As in the case of the Ungulates, we find many strange side-branches which flourished for a time, but are unknown to-day. Mach.o.e.rodus, usually known as "the sabre-toothed tiger,"

though not a tiger, was one of the most formidable of these transitory races. Its upper canine teeth (the "sabres") were several inches in length, and it had enormously distensible jaws to make them effective.

The great development of such animals, with large numbers of hyaenas, civets, wolves, bears, and other Carnivores, in the middle and later Tertiary was probably the most effective agency in the evolution of the horse and deer and the extinction of the more sluggish races. The aquatic branch of the Carnivores (seals, walruses, etc.) is little represented in the Tertiary record. We saw, however, that the most primitive representatives of the elephant-stock had also some characters of the seal, and it is thought that the two had a common origin.

The Moeritherium was a marsh-animal, and may very well have been cousin to the branch of the family which pushed on to the seas, and developed its fore limbs into paddles.

The Rodents are represented in primitive form early in the Eocene period. The teeth are just beginning to show the characteristic modification for gnawing. A large branch of the family, the Tillodonts, attained some importance a little later. They are described as combining the head and claws of a bear with the teeth of a rodent and the general characters of an ungulate. In the Oligocene we find primitive squirrels, beavers, rabbits, and mice. The Insectivores also developed some of the present types at an early date, and have since proved so unprogressive that some regard them as the stock from which all the placental mammals have arisen.

The Cetacea (whales, porpoises, etc.) are already represented in the Eocene by a primitive whale-like animal (Zeuglodon) of unknown origin.

Some specimens of it are seventy feet in length. It has large teeth, sometimes six inches long, and is clearly a terrestrial mammal that has returned to the waters. Some forms even of the modern whale develop rudimentary teeth, and in all forms the bony structure of the fore limbs and degenerate relic of a pelvis and back limbs plainly tell of the terrestrial origin. Dolphins appear in the Miocene.

Finally, the Edentates (sloths, anteaters, and armadilloes) are represented in a very primitive form in the early Eocene. They are then barely distinguishable from the Condylarthra and Creodonta, and seem only recently to have issued from a common ancestor with those groups.

In the course of the Tertiary we find them--especially in South America, which was cut off from the North and its invading Carnivores during the Eocene and Miocene--developed into large sloths, armadilloes, and anteaters. The reconnection with North America in the Pliocene allowed the northern animals to descend, but gigantic sloths (Megatherium) and armadilloes (Glyptodon) flourished long afterwards in South America.

The Megatherium attained a length of eighteen feet in one specimen discovered, and the Glyptodon often had a dorsal s.h.i.+eld (like that of the armadillo) from six to eight feet long, and, in addition, a stoutly armoured tail several feet long.

The richness and rapidity of the mammalian development in the Tertiary, of which this condensed survey will convey some impression, make it impossible to do more here than glance over the vast field and indicate the better-known connections. It will be seen that evolution not only introduces a lucid order and arrangement into our thousands of species of living and fossil mammals, but throws an admirable light on the higher animal world of our time. The various orders into which the zoologist puts our mammals are seen to be the branches of a living tree, approaching more and more closely to each other in early Tertiary times, in spite of the imperfectness of the geological record. We at last trace these diverging lines to a few very primitive, generalised, patriarchal groups, which in turn approach each other very closely in structure, and plainly suggest a common Cretaceous ancestor. Whether that common ancestor was an Edentate, an Insectivore, or Creodont, or something more primitive than them all, is disputed. But the divergence of nearly all the lines of our mammal world from those patriarchal types is admirably clear. In the mutual struggle of carnivore and herbivore, in adaptation to a hundred different environments (the water, the land, and the air, the tree, the open plain, the underground, the marsh, etc.) and forms of diet, we find the descendants of these patriarchal animals gradually developing their distinctive characters. Then we find the destructive agencies of living and inorganic nature blotting out type after type, and the living things that spread over the land in the later Tertiary are found to be broadly identical with the living things of to-day. The last great selection, the northern Ice-Age, will give the last touches of modernisation.

CHAPTER XVIII. THE EVOLUTION OF MAN

We have reserved for a closer inquiry that order of the placental mammals to which we ourselves belong, and on which zoologists have bestowed the very proper and distinguis.h.i.+ng name of the Primates. Since the days of Darwin there has been some tendency to resent the term "lower animals," which man applies to his poorer relations. But, though there is no such thing as an absolute standard by which we may judge the "higher" or "lower" status of animals or plants, the extraordinary power which man has by his brain development attained over both animate and inanimate nature fully justifies the phrase. The Primate order is, therefore, of supreme interest as the family that gave birth to man, and it is important to discover the agencies which impelled some primitive member of it to enter upon the path which led to this summit of organic nature.

The order includes the femurs, a large and primitive family with ape-like features--the Germans call them "half-apes"--the monkeys, the man-like apes, and man. This cla.s.sification according to structure corresponds with the successive appearance of the various families in the geological record. The femurs appear in the Eocene; the monkeys, and afterwards the apes, in the Miocene, the first semi-human forms in the Pleistocene, though they must have been developed before this. It is hardly necessary to say that science does not regard man as a descendant of the known anthropoid apes, or these as descended from the monkeys.

They are successive types or phases of development, diverging early from each other. Just as the succeeding horse-types of the record are not necessarily related to each other in a direct line, yet ill.u.s.trate the evolution of a type which culminates in the horse, so the spreading and branching members of the Primate group ill.u.s.trate the evolution of a type of organism which culminates in man. The particular relations.h.i.+p of the various families, living and dead, will need careful study.

That there is a general blood-relations.h.i.+p, and that man is much more closely related to the anthropoid apes than to any of the lower Primates, is no longer a matter of controversy. In Rudolph Virchow there died, a few years ago, the last authoritative man of science to express any doubt about it. There are, however, non-scientific writers who, by repeating the ambiguous phrase that it is "only a theory," convey the impression to inexpert readers that it is still more or less an open question. We will therefore indicate a few of the lines of evidence which have overcome the last hesitations of scientific men, and closed the discussion as to the fact.

The very close a.n.a.logy of structure between man and the ape at once suggests that they had a common ancestor. There are cases in which two widely removed animals may develop a similar organ independently, but there is a.s.suredly no possibility of their being alike in all organs, unless by common inheritance. Yet the essential ident.i.ty of structure in man and the ape is only confirmed by every advance of science, and would of itself prove the common parentage. Such minor differences as there are between man and the higher ape--in the development of the cerebrum, the number of the teeth or ribs, the distribution of the hair, and so on--are quite explicable when we reflect that the two groups must have diverged from each other more than a million years ago.

Examining the structure of man more closely, we find this strong suggestion of relations.h.i.+p greatly confirmed. It is now well known that the human body contains a number of vestigial "organs"--organs of no actual use, and only intelligible as vestiges of organs that were once useful. Whatever view we take of the origin of man, each organ in his frame must have a meaning; and, as these organs are vestigial and useless even in the lowest tribes of men, who represent primitive man, they must be vestiges of organs that were of use in a remote pre-human ancestor. The one fact that the ape has the same vestigial organs as man would, on a scientific standard of evidence, prove the common descent of the two. But these interesting organs themselves point back far earlier than a mixed ape-human ancestor in many cases.

The sh.e.l.l of cartilage which covers the entrance to the ear--the gristly appendage which is popularly called the ear--is one of the clearest and most easily recognised of these organs. The "ear" of a horse or a cat is an upright mobile sh.e.l.l for catching the waves of sound. The human ear has the appearance of being the shrunken relic of such an organ, and, when we remove the skin, and find seven generally useless muscles attached to it, obviously intended to pull the sh.e.l.l in all directions (as in the horse), there can be no doubt that the external ear is a discarded organ, a useless legacy from an earlier ancestor. In cases where it has been cut off it was found that the sense of hearing was scarcely, if at all, affected. Now we know that it is similarly useless in all tribes of men, and must therefore come from a pre-human ancestor.

It is also vestigial in the higher apes, and it is only when we descend to the lower monkeys and femurs that we see it approaching its primitive useful form. One may almost say that it is a reminiscence of the far-off period when, probably in the early Tertiary, the ancestors of the Primates took to the trees. The animals living on the plain needed acute senses to detect the approach of their prey or their enemies; the tree-dweller found less demand on his sense of hearing, the "speaking-trumpet" was discarded, and the development of the internal ear proceeded on the higher line of the perception of musical sounds.

We might take a very large number of parts of the actual human body, and discover that they are similar historical or archaeological monuments surviving in a modern system, but we have s.p.a.ce only for a few of the more conspicuous.

The hair on the body is a vestigial organ, of actual use to no race of men, an evident relic of the thick warm coat of an earlier ancestor. It in turn recalls the dwellers in the primeval forest. In most cases--not all, because the wearing of clothes for ages has modified this feature--it will be found that the hairs on the arm tend upward from the wrist to the elbow, and downward from the shoulder to the elbow. This very peculiar feature becomes intelligible when we find that some of the apes also have it, and that it has a certain use in their case. They put their hands over their heads as they sit in the trees during ram, and in that position the sloping hair acts somewhat like the thatched roof of a cottage.

Again, it will be found that in the natural position of standing we are not perfectly flat-footed, but tend to press much more on the outer than on the inner edge of the foot. This tendency, surviving after ages of living on the level ground, is a lingering effect of the far-off arboreal days.

A more curious reminiscence is seen in the fact that the very young infant, flabby and powerless as it is in most of its muscles, is so strong in the muscles of the hand and arm that it can hang on to a stick by its hands, and sustain the whole weight of its body, for several minutes. Finally, our vestigial tail--for we have a tail comparable to that of the higher apes--must be mentioned. In embryonic development the tail is much longer than the legs, and some children are born with a real tail, which they move as the puppy does, according to their emotional condition. Other features of the body point back to an even earlier stage. The vermiform appendage--in which some recent medical writers have vainly endeavoured to find a utility--is the shrunken remainder of a large and normal intestine of a remote ancestor. This interpretation of it would stand even if it were found to have a certain use in the human body. Vestigial organs are sometimes pressed into a secondary use when their original function has been lost. The danger of this appendage in the human body to-day is due to the fact that it is a blind alley leading off the alimentary ca.n.a.l, and has a very narrow opening. In the ape the opening is larger, and, significantly enough, it is still larger in the human foetus. When we examine some of the lower mammals we discover the meaning of it. It is in them an additional storage chamber in the alimentary system. It is believed that a change to a more digestible diet has made this additional chamber superfluous in the Primates, and the system is slowly suppressing it.

Other reminiscences of this earlier phase are found in the many vestigial muscles which are found in the body to-day. The head of the quadruped hangs forward, and is held by powerful muscles and ligaments in the neck. We still have the shrunken remainder of this arrangement.

Other vestigial muscles are found in the forehead, the scalp, the nose--many people can twitch the nostrils and the scalp--and under the skin in many parts of the body. These are enfeebled remnants of the muscular coat by which the quadruped twitches its skin, and drives insects away. A less obvious feature is found by the anatomist in certain blood-vessels of the trunk. As the blood flows vertically in a biped and horizontally in a quadruped, the arrangement of the valves in the blood-vessels should be different in the two cases; but it is the same in us as in the quadruped. Another trace of the quadruped ancestor is found in the baby. It walks "on all fours" so long, not merely from weakness of the limbs, but because it has the spine of a quadruped.

A much more interesting fact, but one less easy to interpret, is that the human male has, like the male ape, organs for suckling the young.

That there are real milk-glands, usually vestigial, underneath the teats in the breast of the boy or the man is proved by the many known cases in which men have suckled the young. Several friends of the present writer have seen this done in India and Ceylon by male "wet-nurses." As there is no tribe of men or species of ape in which the male suckles the young normally, we seem to be thrown back once more upon an earlier ancestor.

The difficulty is that we know of no mammal of which both parents suckle the young, and some authorities think that the b.r.e.a.s.t.s have been transferred to the male by a kind of embryonic muddle. That is difficult to believe, as no other feature has ever been similarly transferred to the opposite s.e.x. In any case the male b.r.e.a.s.t.s are vestigial organs.

Another peculiarity of the mammary system is that sometimes three, four, or five pairs of b.r.e.a.s.t.s appear in a woman (and several have been known even in a man). This is, apparently, an occasional reminiscence of an early mammal ancestor which had large litters of young and several pairs of b.r.e.a.s.t.s.

But there are features of the human body which recall an ancestor even earlier than the quadruped. The most conspicuous of these is the little fleshy pad at the inner corner of each eye. It is a common feature in mammals, and is always useless. When, however, we look lower down in the animal scale we find that fishes and reptiles (and birds) have a third eyelid, which is drawn across the eye from this corner. There is little room to doubt that the little fleshy vestige in the mammal's eye is the shrunken remainder of the lateral eyelid of a remote fish-ancestor.

A similar reminiscence is found in the pineal body, a small and useless object, about the size and shape of a hazel-nut, in the centre of the brain. When we examine the reptile we find a third eye in the top of the head. The skin has closed over it, but the skull is still, in many cases, perforated as it is for the eyes in front. I have seen it standing out like a ball on the head of a dead crocodile, and in the living tuatara--the very primitive New Zealand lizard--it still has a retina and optic nerve. As the only animal in nature to-day with an eye in this position (the Pyrosome, a little marine animal of the sea-squirt family) is not in the line of reptile and mammal ancestry, it is difficult to locate the third eye definitely. But when we find the skin closing over it in the amphibian and reptile, then the bone, and then see it gradually atrophying and being buried under the growing brain, we must refer it to some early fish-ancestor. This ancestor, we may recall, is also reflected for a time in the gill-slits and arches, with their corresponding fish-like heart and blood-vessels, during man's embryonic development, as we saw in a former chapter.

These are only a few of the more conspicuous instances of vestigial structures in man. Metchnikoff describes about a hundred of them. Even if there were no remains of primitive man pointing in the direction of a common ancestry with the ape, no lower types of men in existence with the same tendency, no apes found in nature to-day with a structure so strikingly similar to that of man, and no fossil records telling of the divergence of forms from primitive groups in past time, we should be forced to postulate the evolution of man in order to explain his actual features. The vestigial structures must be interpreted as we interpret the b.u.t.tons on the back of a man's coat. They are useless reminiscences of an age in which they were useful. When their witness to the past is supported by so many converging lines of evidence it becomes irresistible. I will add only one further testimony which has been brought into court in recent years.

The blood consists of cells, or minute disk-shaped corpuscles, floating in a watery fluid, or serum. It was found a few years ago, in the course of certain experiments in mixing the blood of animals, that the serum of one animal's blood sometimes destroyed the cells of the other animal's blood, and at other times did not. When the experiments were multiplied, it was found that the amount of destructive action exercised by one specimen of blood upon another depended on the nearness or remoteness of relations.h.i.+p between the animals. If the two are closely related, there is no disturbance when their blood is mixed; when they are not closely related, the serum of one destroys the cells of the other, and the intensity of the action is in proportion to their remoteness from each other. Another and more elaborate form of the experiment was devised, and the law was confirmed. On both tests it was found by experiment that the blood of man and of the anthropoid ape behaved in such a way as to prove that they were closely related. The blood of the monkey showed a less close relations.h.i.+p--a little more remote in the New World than in the Old World monkeys; and the blood of the femur showed a faint and distant relations.h.i.+p.

The FACT of the evolution of man and the apes from a common ancestor is, therefore, outside the range of controversy in science; we are concerned only to retrace the stages of that evolution, and the agencies which controlled it. Here, unfortunately, the geological record gives us little aid. Tree-dwelling animals are amongst the least likely to be buried in deposits which may preserve their bones for ages. The distribution of femur and ape remains shows that the order of the Primates has been widespread and numerous since the middle of the Tertiary Era, yet singularly few remains of the various families have been preserved.

Hence the origin of the Primates is obscure. They are first foreshadowed in certain femur-like forms of the Eocene period, which are said in some cases (Adapis) to combine the characters of pachyderms and femurs, and in others (Anaptomorphus) to unite the features of Insectivores and femurs. Perhaps the more common opinion is that they were evolved from a branch of the Insectivores, but the evidence is too slender to justify an opinion. It was an age when the primitive placental mammals were just beginning to diverge from each other, and had still many features in common. For the present all we can say is that in the earliest spread of the patriarchal mammal race one branch adopted arboreal life, and evolved in the direction of the femurs and the apes. The generally arboreal character of the Primates justifies this conclusion.