The Story of Evolution/Chapter XVIII

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 distinguishing 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 classification 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 illustrate the evolution of a type which culminates in the horse, so the spreading and branching members of the Primate group illustrate the evolution of a type of organism which culminates in man. The particular relationship of the various families, living and dead, will need careful study.

That there is a general blood-relationship, 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 analogy 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 assuredly no possibility of their being alike in all organs, unless by common inheritance. Yet the essential identity 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 relationship 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 shell 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 shell 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 shell 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 space 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 canal, 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 breasts 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 sex. In any case the male breasts are vestigial organs. Another peculiarity of the mammary system is that sometimes three, four, or five pairs of breasts 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 breasts.

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 buttons 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 relationship 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 relationship—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 relationship.

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.

In the Miocene period we find a great expansion of the monkeys. These in turn enter the scene quite suddenly, and the authorities are reduced to uncertain and contradictory conjectures as to their origin. Some think that they develop not from the femurs, but along an independent line from the Insectivores, or other ancestors of the Primates. We will not linger over these early monkeys, nor engage upon the hopeless task of tracing their gradual ramification into the numerous families of the present age. It is clear only that they soon divided into two main streams, one of which spread into the monkeys of America and the other into the monkeys of the Old World. There are important anatomical differences between the two. The monkeys remained in Central and Southern Europe until near the end of the Tertiary. Gradually we perceive that the advancing cold is driving them further south, and the monkeys of Gibraltar to-day are the diminished remnant of the great family that had previously wandered as far as Britain and France.

A third wave, also spreading in the Miocene, equally obscure in its connection with the preceding, introduces the man-like apes to the geologist. Primitive gibbons (Pliopithecus and Pliobylobates), primitive chimpanzees (Palaeopithecus), and other early anthropoid apes (Oreopithecus, Dryopithecus, etc.), lived in the trees of Southern Europe in the second part of the Tertiary Era. They are clearly disconnected individuals of a large and flourishing family, but from the half-dozen specimens we have yet discovered no conclusion can be drawn, except that the family is already branching into the types of anthropoid apes which are familiar to us.

Of man himself we have no certain and indisputable trace in the Tertiary Era. Some remains found in Java of an ape-man (Pithecanthropus), which we will study later, are now generally believed, after a special investigation on the spot, to belong to the Pleistocene period. Yet no authority on the subject doubts that the human species was evolved in the Tertiary Era, and very many, if not most, of the authorities believe that we have definite proof of his presence. The early story of mankind is gathered, not so much from the few fragments of human remains we have, but from the stone implements which were shaped by his primitive intelligence and remain, almost imperishable, in the soil over which he wandered. The more primitive man was, the more ambiguous would be the traces of his shaping of these stone implements, and the earliest specimens are bound to be a matter of controversy. It is claimed by many distinguished authorities that flints slightly touched by the hand of man, or at least used as implements by man, are found in abundance in England, France, and Germany, and belong to the Pliocene period. Continental authorities even refer some of them to the Miocene and the last part of the Oligocene.

The question whether an implement-using animal, which nearly all would agree to regard as in some degree human, wandered over what is now the South of England (Kent, Essex, Dorsetshire, etc.) as many hundred thousand years ago as this claim would imply, is certainly one of great interest. But there would be little use in discussing here the question of the "Eoliths," as these disputed implements are called. A very keen controversy is still being conducted in regard to them, and some of the highest authorities in England, France, and Germany deny that they show any trace of human workmanship or usage. Although they have the support of such high authorities as Sir J. Prestwich, Sir E. Ray Lankester, Lord Avebury, Dr. Keane, Dr. Blackmore, Professor Schwartz, etc., they are one of those controverted testimonies on which it would be ill-advised to rely in such a work as this.

We must say, then, that we have no undisputed traces of man in the Tertiary Era. The Tertiary implements which have been at various times claimed in France, Italy, and Portugal are equally disputed; the remains which were some years ago claimed as Tertiary in the United States are generally disallowed; and the recent claims from South America are under discussion. Yet it is the general feeling of anthropologists that man was evolved in the Tertiary Era. On the one hand, the anthropoid apes were highly developed by the Miocene period, and it would be almost incredible that the future human stock should linger hundreds of thousands of years behind them. On the other hand, when we find the first traces of man in the Pleistocene, this development has already proceeded so far that its earlier phase evidently goes back into the Tertiary. Let us pass beyond the Tertiary Era for a moment, and examine the earliest and most primitive remains we have of human or semi-human beings.

The first appearance of man in the chronicle of terrestrial life is a matter of great importance and interest. Even the least scientific of readers stands, so to say, on tiptoe to catch a first glimpse of the earliest known representative of our race, and half a century of discussion of evolution has engendered a very wide interest in the early history of man.

Fortunately, although these patriarchal bones are very scanty—two teeth, a thigh-bone, and the skull-cap—we are now in a position to form some idea of the nature of their living owner. They have been subjected to so searching a scrutiny and discussion since they were found in Java in 1891 and 1892 that there is now a general agreement as to their nature. At first some of the experts thought that they were the remains of an abnormally low man, and others that they belonged to an abnormally high ape. The majority held from the start that they belonged to a member of a race almost midway between the highest family of apes and the lowest known tribe of men, and therefore fully merited the name of "Ape-Man" (Pithecanthropus). This is now the general view of anthropologists.

The Ape-Man of Java was in every respect entitled to that name. The teeth suggest a lower part of the face in which the teeth and lips projected more than in the most ape-like types of Central Africa. The skull-cap has very heavy ridges over the eyes and a low receding forehead, far less human than in any previously known prehistoric skull. The thigh-bone is very much heavier than any known human femur of the same length, and so appreciably curved that the owner was evidently in a condition of transition from the semi-quadrupedal crouch of the ape to the erect attitude of man. The Ape-Man, in other words, was a heavy, squat, powerful, bestial-looking animal; of small stature, but above the pygmy standard; erect in posture, but with clear traces of the proneness of his ancestor; far removed from the highest ape in brainpower, but almost equally far removed from the lowest savage that is known to us. We shall see later that there is some recent criticism, by weighty authorities, of the earlier statements in regard to the brain of primitive man. This does not apply to the Ape-Man of Java. The average cranial capacity (the amount of brain-matter the skull may contain) of the chimpanzees, the highest apes, is about 600 cubic centimetres. The average cranial capacity of the lowest races of men, of moderate stature, is about 1200. And the cranial capacity of Ape-Man was about 900

It is immaterial whether or no these bones belong to the same individual. If they do not, we have remains of two or three individuals of the same intermediate species. Nor does it matter whether or no this early race is a direct ancestor of the later races of men, or an extinct offshoot from the advancing human stock. It is, in either case, an illustration of the intermediate phase between the ape and man The more important tasks are to trace the relationship of this early human stock to the apes, and to discover the causes of its superior evolution.

The first question has a predominantly technical interest, and the authorities are not agreed in replying to it. We saw that, on the blood-test, man showed a very close relationship to the anthropoid apes, a less close affinity to the Old World monkeys, a more remote affinity to the American monkeys, and a very faint and distant affinity to the femurs. A comparison of their structures suggests the same conclusion. It is, therefore, generally believed that the anthropoid apes and man had a common ancestor in the early Miocene or Oligocene, that this group was closely related to the ancestral group of the Old World monkeys, and that all originally sprang from a primitive and generalised femur-group. In other words, a branch of the earliest femur-like forms diverges, before the specific femur-characters are fixed, in the direction of the monkey; in this still vague and patriarchal group a branch diverges, before the monkey-features are fixed, in the direction of the anthropoids; and this group in turn spreads into a number of types, some of which are the extinct apes of the Miocene, four become the gorilla, chimpanzee, orang, and gibbon of to-day, and one is the group that will become man. To put it still more precisely, if we found a whole series of remains of man's ancestors during the Tertiary, we should probably class them, broadly, as femur-remains in the Eocene, monkey-remains in the Oligocene, and ape-remains in the Miocene. In that sense only man "descends from a monkey."

The far more important question is: How did this one particular group of anthropoid animals of the Miocene come to surpass all its cousins, and all the rest of the mammals, in brain-development? Let us first rid the question of its supposed elements of mystery and make of it a simple problem. Some imagine that a sudden and mysterious rise in intelligence lifted the progenitor of man above its fellows. The facts very quickly dispel this illusion. We may at least assume that the ancestor of man was on a level with the anthropoid ape in the Miocene period, and we know from their skulls that the apes were as advanced then as they are now. But from the early Miocene to the Pleistocene is a stretch of about a million years on the very lowest estimate. In other words, man occupied about a million years in travelling from the level of the chimpanzee to a level below that of the crudest savage ever discovered. If we set aside the Java man, as a possible survivor of an earlier phase, we should still have to say that, much more than a million years after his departure from the chimpanzee level, man had merely advanced far enough to chip stone implements; because we find no other trace whatever of intelligence than this until near the close of the Palaeolithic period. If there is any mystery, it is in the slowness of man's development.

Let us further recollect that it is a common occurrence in the calendar of life for a particular organ to be especially developed in one member of a particular group more than in the others. The trunk of the elephant, the neck of the giraffe, the limbs of the horse or deer, the canines of the satire-toothed tiger, the wings of the bat, the colouring of the tiger, the horns of the deer, are so many examples in the mammal world alone. The brain is a useful organ like any other, and it is easy to conceive that the circumstances of one group may select it just as the environment of another group may lead to the selection of speed, weapons, or colouring. In fact, as we saw, there was so great and general an evolution of brain in the Tertiary Era that our modern mammals quite commonly have many times the brain of their Tertiary ancestors. Can we suggest any reasons why brain should be especially developed in the apes, and more particularly still in the ancestors of man?

The Primate group generally is a race of tree-climbers. The appearance of fruit on early Tertiary trees and the multiplication of carnivores explain this. The Primate is, except in a few robust cases, a particularly defenceless animal. When its earliest ancestors came in contact with fruit and nut-bearing trees, they developed climbing power and other means of defence and offense were sacrificed. Keenness of scent and range of hearing would now be of less moment, but sight would be stimulated, especially when soft-footed climbing carnivores came on the scene. There is, however, a much deeper significance in the adoption of climbing, and we must borrow a page from the modern physiology of the brain to understand it.

The stress laid in the modern education of young children on the use of the hands is not merely due to a feeling that they should handle objects as well as read about them. It is partly due to the belief of many distinguished physiologists that the training of the hands has a direct stimulating effect on the thought-centres in the brain. The centre in the cerebrum which controls the use of the hands is on the fringe of the region which seems to be concerned in mental operations. For reasons which will appear presently, we may add that the centres for controlling the muscles of the face and head are in the same region. Any finer training or the use of the hands will develop the centre for the fore limbs, and, on the principles, may react on the more important region of the cortex. Hence in turning the fore foot into a hand, for climbing and grasping purposes, the primitive Primate entered upon the path of brain-development. Even the earliest Primates show large brains in comparison with the small brains of their contemporaries.

It is a familiar fact in the animal world that when a certain group enters upon a particular path of evolution, some members of the group advance only a little way along it, some go farther, and some outstrip all the others. The development of social life among the bees will illustrate this. Hence we need not be puzzled by the fact that the lemurs have remained at one mental level, the monkeys at another, and the apes at a third. It is the common experience of life; and it is especially clear among the various races of men. A group becomes fitted to its environment, and, as long as its surroundings do not change, it does not advance. A related group, in a different environment, receives a particular stimulation, and advances. If, moreover, a group remains unstimulated for ages, it may become so rigid in its type that it loses the capacity to advance. It is generally believed that the lowest races of men, and even some of the higher races like the Australian aboriginals, are in this condition. We may expect this "unteachability" in a far more stubborn degree in the anthropoid apes, which have been adapted to an unchanging environment for a million years.

All that we need further suppose is—and it is one of the commonest episodes in terrestrial life—that one branch of the Miocene anthropoids, which were spread over a large part of the earth, received some stimulus to change which its cousins did not experience. It is sometimes suggested that social life was the great advantage which led to the superior development of mind in man. But such evidence as there is would lead us to suppose that primitive man was solitary, not social. The anthropoid apes are not social, but live in families, and are very unprogressive. On the other hand, the earliest remains of prehistoric man give no indication of social life. Fire-places, workshops, caves, etc., enter the story in a later phase. Some authorities on prehistoric man hold very strongly that during the greater part of the Old Stone Age (two-thirds, at least, of the human period) man wandered only in the company of his mate and children.

We seem to have the most plausible explanation of the divergence of man from his anthropoid cousins in the fact that he left the trees of his and their ancestors. This theory has the advantage of being a fact—for the Ape-Man race of Java has already left the trees—and providing a strong ground for brain-advance. A dozen reasons might be imagined for his quitting the trees—migration, for instance, to a region in which food was more abundant, and carnivores less formidable, on the ground-level—but we will be content with the fact that he did. Such a change would lead to a more consistent adoption of the upright attitude, which is partly found in the anthropoid apes, especially the gibbons. The fore limb would be no longer a support of the body; the hand would be used more for grasping; and the hand-centre in the brain would be proportionately stimulated. The adoption of the erect attitude would further lead to a special development of the muscles of the head and face, the centre for which is in the same important region in the cortex. There would also be a direct stimulation of the brain, as, having neither weapons nor speed, the animal would rely all the more on sight and mind. If we further suppose that this primitive being extended the range of his hunting, from insects and small or dead birds to small land-animals, the stimulation would be all the greater. In a word, the very fact of a change from the trees to the ground suggests a line of brain-development which may plausibly be conceived, in the course of a million years, to evolve an Ape-Man out of a man-like ape. And we are not introducing any imaginary factor in this view of human origins.

The problem of the evolution of man is often approached in a frame of mind not far removed from that of the educated, but inexpert, European who stands before the lowly figure of the chimpanzee, and wonders by what miracle the gulf between it and himself was bridged. That is to lay a superfluous strain on the imagination. The proper term of comparison is the lowest type of human being known to us, since the higher types of living men have confessedly evolved from the lower. But even the lowest type of existing or recent savage is not the lowest level of humanity. Whether or no the Tasmanian or the Yahgan is a primitive remnant of the Old Stone Age, we have a far lower depth in the Java race. What we have first to do is to explain the advance to that level, in the course of many hundreds of thousands of years: a period fully a hundred times as long as the whole history of civilisation. Time itself is no factor in evolution, but in this case it is a significant condition. It means that, on this view of the evolution of man, we are merely assuming that an advance in brain-development took place between the Miocene and the Pleistocene, not similar to, but immeasurably less than, the advance which we know to have been made in the last fifty thousand years. In point of fact, the most mysterious feature of the evolution of man was its slowness. We shall see that, to meet the facts, we must suppose man to have made little or no progress during most of this vast period, and then to have received some new stimulation to develop. What it was we have now to inquire.