THE MAMMALS AND MAN

The subject of our discussion is now narrowed down to the group of the mammals. The mammals are characterised by two very obvious features: a body-covering of hair, and a set of special glands in the female which secrete milk for the nourishment of the young. These are constant characters, and neither is ever found in any other group. As to how the hair originated, nothing definite is known; but (while there are certain difficulties in regard to the theory) it is on the whole reasonable to suppose that the mammalian hair arose, as the bird's feather undoubtedly did, as a modification of the reptile's scale. The mammary gland appears to represent a modification of other skin glands, either of sweat glands or more probably of the oil glands which exist in connection with the hairs.

Another important character, already mentioned at the end of the last chapter, is the diaphragm, a muscular partition separating the body cavity into a thoracic and an abdominal portion. The diaphragm has important functions in connection with the mechanism of breathing. By means of it the thoracic cavity can be increased or diminished in size, and air thus drawn into or expelled from the lungs. It is interesting to observe that a similar partition, with the same function, occurs in the Crocodiles, but this has different relations to the abdominal organs, and has evidently evolved quite independently.

Very characteristic of the mammals are, further, their teeth, for whereas the teeth of the reptiles are indefinite in number, and generally very numerous, those of the mammal are relatively few, and each species has a definite normal number. Moreover, the teeth of the reptile are all of a kind, and they may be replaced almost any number of times during the animal's life, whereas those of the mammal show differentiation according to their respective functions, and are only once changed, that is, when the milk teeth are replaced by the permanent set. The teeth of the mammal are of four kinds: incisors, or chisel-like cutting teeth, canines, which are especially well developed in carnivorous species, and which are used in the tearing up of flesh, etc., and two groups of grinders—premolars, which are replaced during the animal's life, and molars, which occur only as permanent teeth.

The mammals in all probability arose from a particular group of reptiles which flourished in the Permian and Triassic periods, and which disappeared very shortly afterwards. These pass under the name of the Theromorpha, and a typical specimen, Pareiosaurus, is illustrated in Fig. 102. The skull of another, illustrated in Fig. 103, shows distinct indications of a mammal-like differentiation of the teeth. The Tritylodon, whose skull is shown in Fig. 104, is an intermediate type between this group and the mammals, some zoologists regarding it, as it were, as the last reptile, others as the first of the mammals.

As to how the special mammalian features arose, or what special conditions called them into existence, we are of course without definite knowledge, for neither hair nor mammary glands are recognisable in fossils. But it seems likely that the warm blood and the hairy covering evolved in correlation with one another, and as adaptions to meet a gradual cooling of the climate. It is certain at all events that the present-day mammals (and also the birds) are far better adapted for a cold environment than the reptiles, which very easily get frozen to death; and it is also known that ice periods occurred in South Africa, where many fossil Theromorpha and the Tritylodon are found, at the time when these creatures existed; both of which facts support the theory indicated.

At the present time the mammals are the highest and on the whole the most successful of the vertebrate groups. They include the largest and strongest, the swiftest-footed and the most [125]
[126]
[127]intelligent of the animal kind. They show refinements of the senses of sight, hearing, and smell such as are met with nowhere else. They range from the Equator to the coldest regions of the earth in which any food is to be found; they people alike the forest and the plain, and have their representatives both in the air and in the sea.

The most lowly of the mammals are the Monotremes, which include the well-known Australian duck-mole or duck-billed platypus, and two species of spiny ant-eaters, one of which is found in Australia, the other in New Guinea. The two types are shown in Figs. 105 to 107. The best-known and most striking fact concerning these is that, like the birds and reptiles, and unlike all other mammals, they lay eggs. Beyond this feature they show many affinities with the reptiles, in their skeleton for example, and particularly in their reproductive organs. Another interesting fact is that the blood temperature, of the ant-eater at least, is low, and varies considerably. It has been found to range between 80 and 93 degrees Fahrenheit, whereas all other mammals, so far as is known, have a blood temperature between 97 and 105, and moreover one that is remarkably constant for each species during normal health. The brain development and the intelligence are also much lower than in other mammals, though distinctly superior to those met with among the reptiles. The method of rearing the young in the case of the ant-eater (that of the duck-mole not being yet fully known) is that the single egg is placed, as soon as it is laid, in the pouch under the belly of the female. Here it hatches in a very short time, and here the young animal remains for the first two or three months of its life, being nourished by milk produced by the mammary glands, which open into the pouch. It is certainly owing to the absence of the competition of the higher mammals in the regions where they are found, that these two creatures, so interesting from the standpoint of Evolution, have been preserved to us.

The next group, the Marsupials, is the lowest in which we get the true mammalian characteristic of the bearing of living young. For while occasional members of other groups, of the reptiles especially, produce live young, the actual state of affairs is fundamentally different. In these latter the egg is merely retained in the genital duct until the young creature emerges. It is merely hatched inside the body of the mother instead of outside. But in the Marsupials the developing young receive nourishment from the mother, during prenatal life, other than what is contained in the yolk. This nourishment is obtained in the form of a secretion from the wall of the uterus, there being as yet (with a single partial exception) no real connection between mother and young.

The peculiar method of nourishing and protecting the young Marsupial after birth is of course well known. The young are born in a very immature and helpless condition, and are placed by the mother in her pouch. The mouth becomes permanently attached to the nipple of the dam, and the young creature remains thus for a considerable time, the milk being pumped into it by the mammary gland rather than sucked in by the efforts of the creature itself.

The Marsupials are further characterised by the possession of an extra pair of bones in the pelvis, which function as supports for the pouch; by the peculiar and primitive arrangement of the reproductive organs; by their still poorly developed brain; and by their generally large number of teeth, which reach a total of fifty or fifty-two in some species, whereas forty-four is the ordinary maximum number in the main mammal group. This last is to be regarded as a character derived from the reptiles.

It is very interesting to observe how, in Australia, where the Marsupials have been free from the competition of other mammals, they have evolved along many of the same general lines as the higher group. We have indeed no marsupial whales, bats, or seals, but there is a mole, very similar in its appearance and habits to our European species, and a carnivorous type which closely resembles a wolf or jackal; again we have bandicoots, occupying the same place in nature as our rabbits and other rodents; tree-dwelling, squirrel-like forms; and kangaroos, which compare in their mode of life, if not in their appearance, with the cattle, deer, and antelopes of other countries. The Marsupials are illustrated in Figs. 108 to 113. This group, so far as can be judged from fossils, is considerably older than that of [131]
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[133]the higher mammals, and everything points to some marsupial type as a connecting link between the egg-laying Monotremes and the placental mammals.

As has previously been indicated, the most important characteristic of the third great group is a modification of the membranes of the embryo to form a connection between it and the wall of the uterus. The allantois develops as a highly vascular membrane, the small blood vessels of which are brought into very close contact with those in the wall of the uterus. So that, while the blood of the mother does not actually mix with that of the child, the two fluids are separated only by thin membranes, through which nutritive substances easily pass. The broad advantage of this is, of course, that the young animal passes the earlier stages of its life inside the mother's body, where it is exposed to a minimum of risk, is efficiently nourished, and from which it is not sent forth into the world until it is tolerably well able to look after itself.

We shall now ask the reader to conceive a primitive placental mammal type, and to consider briefly the relationships of the various groups that have sprung from it. The group whose evolution has been perhaps least progressive is that of the Edentates, including the sloths, armadillos, and American ant-eaters (Fig. 114). Then, on the one hand, we have the herbivorous types, including the remarkable sea-cows (Fig. 115), the rodents (Fig. 116), and the hoofed animals, leading up from such comparatively primitive forms as the coneys (Fig. 117) to such highly specialised types as the giraffe (Fig. 118). The other series of groups are believed to be more nearly related to each other than to the aforementioned, and the nearest approach to their [135]
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[137]common ancestor, among modern mammals, is probably to be found in the Insectivora (Figs. 119 and 120). Differing sharply from the Insectivora in their possession of wings, but otherwise closely similar, are the bats (Fig. 121). From this same insectivor-like type have probably evolved the whales (Fig. 122), the seals, etc. (Fig. 123), the carnivora (Fig. 124), and finally the primates, leading up through the lemurs to the monkeys and man. Thus merely indicating the many and devious roads of mammalian Evolution, we turn again to the one particular line that we set out to follow.

The Insectivora are by many of their features recognisable as low types of placental mammals. They have a primitive type of skull, and frequently show a rather marked similarity between the several classes of teeth. The brain is relatively ill-developed, reminding one rather strongly of the Marsupials, and the hemispheres show little if any tendency to develop those wrinkles and fissures which always accompany the higher type of intelligence. Probably the most primitive member of the whole group is the Centetes from Madagascar, shown in Fig. 119. The hedgehog and the shrews are its best-known examples. An interesting point regarding the hedgehog is that, like few other mammals, it has persisted almost unchanged from early tertiary times. It is thus to be regarded as a very antique type, not only in its main features, but in its details.

The Lemurs are an interesting group, standing, as they do, midway between the primitive placental forms and the monkeys. Their special home, as already mentioned, is Madagascar, to which some thirty-six of the fifty known species are confined, but they occur also in Africa and in South-Eastern Asia. They are arboreal and mostly nocturnal in habits, and their food consists partly of fruit, etc., partly of insects. They were formerly much more widely distributed, and many fossils have been unearthed, for example, in North America. They show certain characters of a distinctly primitive kind, such, for instance, as their habit of hibernation. Their typical number of teeth is thirty-six, the same as in the lower monkeys, but fossil forms are known which possessed the full number of forty-four.

In their general build they show marked adaptation to their arboreal life, and approach, some more and some less, the appearance of the monkeys. The fore-limbs are considerably modified from the condition in which they occur in ordinary mammals, in which they are placed vertically under the body. They are placed in a more lateral position, so that they can be moved through greater angles, and extended over the head. In common language, they are ceasing to be legs, and are becoming arms. As in the monkeys, the thumb and the great toe are opposed to the other digits so as to render the hand and foot more efficient as grasping organs. Hence the Lemurs may be included with the apes as 'Quadrumana,' or four-handed animals. The fingers and toes either bear claws, as in the lower animals, or flattened nails like those of the higher apes and man, many species possessing [139]
[140]
[141]the two types of structures on different digits. The face is fox-like, and lacks the human expression that is seen in the monkeys. The brain shows considerable variation, being in some species far more primitive, in others rather more highly developed, than in the lower monkeys.

The history of the group has been very completely made out from fossils, and it is possible to work back to forms which, apart from their known subsequent evolution, could not be definitely separated from the ancestors of other mammal groups. The Lemurs are illustrated in Figs. 125 to 127.

The monkeys, to which we now turn, are divided into two groups, the American or Western and the Eastern or Old-World types, each of which is definitely confined to the regions indicated by the names. The Western apes (Figs. 128 and 129) are by much the lower group of the two, and in fact they lead back to fossil forms which cannot be definitely distinguished from the Lemurs. They are a side branch of the monkey stem, but are in all probability the nearest living representatives of the first of our ape ancestors. They are characterised by a wide septum between the nostrils, which makes the latter open in an outward direction, a feature which enables them to be distinguished at a glance from the other group. They have further a prehensile tail, which is of much use in climbing, and generally thirty-six teeth, or four more than the Old-World group. The lowest of them are the well-known Marmosets, which have claws on all the digits except the great toe, which last has a nail. These are further characterised by the presence of four nipples, all other monkeys having only two. Correlated with this feature, the Marmosets normally bear two or three young at a birth, whereas in all other apes one is the usual number.

The Eastern monkeys have thirty-two teeth, the same number as in man; the septum between the nostrils is narrow, so that these open downwards and forwards; the tail is never prehensile and is frequently absent. They include such well-known forms as the baboons, the Gibraltar ape, the sacred Hanuman of India, the Diana monkey (Fig. 130), and the comical-looking Nasalis [143]
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[145](Fig. 131). These represent a further step towards the final climax of the primate group.

A special class has to be made for the four genera of Old-World apes illustrated in Figs. 132 to 136. These are the Gorilla and Chimpanzee, which are African in distribution, and the Gibbon and Orang, which are Asiatic. They are termed the Anthropoid or man-like apes, and there can be no question that they are the nearest living relatives of the human species. This is seen in their general build, which is man-like in a high degree; by many similarities, even in the minutest details, in their skeleton and muscular system; and by the fact that their brain, while still greatly inferior to that of man, is by as much superior to that of any other animal. The voice is frequently well modulated, and the expression of the emotions, whether by it or by the countenance, is very man-like. The position of the group was summed up by Huxley, after a most thorough investigation of their anatomy, in these words: "Thus, whatever system of organs be studied, the comparison of their modification in the ape series leads to one and the same result—that the structural differences which separate man from the Gorilla and Chimpanzee are not so great as those which separate the Gorilla from the lower apes." An interesting physiological proof of the close relationship between the anthropoids and man has more lately been discovered. It is found that the blood serum of any animal destroys the blood corpuscles of any other when these are mixed with it, except those of closely related species. Now the human blood serum is destructive of the corpuscles of all the lower animals, so far as is known, except those of the anthropoids.

As regards the inter-relationships of the four species, it is certain that the Gibbon is the lowest, and the nearest to the common ancestor of the other three and of man. It has indeed the man-like characteristic of walking in the erect position; but it has thirteen or fourteen pairs of ribs, as against the normal twelve in man; it has long arms, like the lower Old-World apes, and large canine teeth. It is further the smallest of the group, and in habits the most like the lower monkeys.

The Gorilla and Chimpanzee are closely related. The Gorilla is not only the largest ape of the four, but in shape and build the most man-like, which is accounted for by the fact that it is less strictly arboreal than the others, and confines itself largely to the ground. Its skull is superficially much less human in appearance than that of the Chimpanzee, due to the strongly developed crests, which serve as attachments for the powerful muscles of the lower jaw. The teeth of the Chimpanzee are more uniform in size, and the skull smoother. There is a marked difference in temperament between the two species, the Gorilla being fierce and gloomy and quite untamable, while the Chimpanzee is of a pleasant and lively disposition, and can, as is well known, be trained to wear clothes, eat with a fork and knife, etc.

The Orang-Utan is found inhabiting forest ground in Borneo and Sumatra, and living largely in trees, in which it builds nests as temporary sleeping-places. It is a clumsy-looking animal, supporting itself on the knuckles of its hands when travelling along the ground, and moving but slowly. It has twelve pairs of ribs, the same number as in man, and one fewer than in the Gorilla and Chimpanzee.

Practically all the important anatomical distinctions between man and the anthropoids are reducible to two causes: the change from the arboreal existence to a life on the ground and in the open country, and the great development of the intelligence. When the change in the mode of life occurred, it is obvious that an erect carriage would possess an advantage over the stooping gait of the ape, in which it is neither definitely a biped nor a quadruped. The erect position necessitated a better-developed heel, stronger calf and hip muscles, and a more parallel position and stronger development of the great toe. It also brought about a shortening of the fore-arms and a widening of the pelvis. Much has been made of the difference in the foot, between the condition of a grasping organ, with an opposable great toe, and that seen in man. But the change is just what we should devise in order to make the foot stronger for propelling the body on the ground. Moreover, the difference is more obvious than fundamental, and it is well known that in armless persons, who develop the possibilities of their feet, the latter members can still be turned into wonderfully efficient 'hands.'

In brain development the anthropoids are, as already mentioned, greatly inferior to man; in the Gorilla, the largest-brained type, the cranial capacity never, so far as is known, exceeds 600 cubic centimetres, whereas the average in man is about 1500, and the smallest known about 930. Apart from the difference in size, however, there is a surprisingly close similarity between the anthropoid and the human brains, which may be followed even in the particular arrangement of the fissures. That the gap between the two types in respect of the character in question is by no means extraordinarily wide may be seen from Fig. 137, in which a series of brains are depicted. The size of the hemispheres and the wrinkling of their surface are the characters from which we judge the brain development, and it is obvious that in these respects the step from the lower monkeys to the Chimpanzee is greater than that between the Chimpanzee and the lowest human type. In the teeth also, as is shown on Fig. 138, there is a gradual change from the lemur to the human type.

Thus there can be no reasonable doubt that man has evolved from an ancestor which, if it existed to-day, we should without hesitation class as an anthropoid ape. Could any doubt have remained, it would have been set aside by the discovery, in 1891, of a being occupying a position about midway between the highest apes and the most primitive known man. This is the Pithecanthropus, whose remains were discovered in Java in a volcanic deposit of somewhat doubtful age, but probably belonging to an era when a primitive type of man was already in existence. The remains were indeed somewhat scanty, consisting of the roof of the skull, a thigh bone, and a fragment of the lower jaw, the former two of which are illustrated in Figs. 139 and 140. From these fragments of the skeleton numerous deductions have been drawn, of greater or less probability. It may be said with practical certainty, however, that this ape-man was of the size of a smallish man, and that he was accustomed to walk and stand in the characteristically human erect attitude. The cranial capacity has been calculated at from 850 to 1000 cubic centimetres, or considerably greater than the highest existing apes, and about equal to that of the lowest known human specimens. His statue, as his discoverer conceived him, is illustrated in Fig. 141.

A restoration of the skull is shown in Fig. 142, and it is apparent that in respect of the shape of the roof, at least, the Pithecanthropus stands just about midway between the Chimpanzee and the most primitive living man. The gradual approach to the human type, as we move upwards in the primate scale, is very striking. The Pithecanthropus is by some regarded as the result of an abortive attempt at 'man making,' by others as a true transition form. We cannot in any case be very far from the truth if we hang up his picture among the portraits of our ancestors, for the transitional form would necessarily be closely similar to him in its main features.

The next ancestor of whom we have any knowledge is definitely a human being. This is the primitive man who inhabited Europe in earlier Diluvial times, particularly in the interval between the first and the second great ice ages. To him, from the place of his first discovery, the name of the Neandertal man has been applied, and he is classed by scientists as belonging to a different species from modern man, the latter being named Homo sapiens, while he is given the less flattering name of Homo primigenius. He was characterised, as may readily be seen from the skull illustrated in Fig. 143, by a very low and receding forehead, with heavy ridges of bone over the eyes. In shape of head he stands midway between the Javan ape-man and the Australian native, the lowest type existing at the present day. Further ape-like peculiarities are the prominence of the lower part of the face, the very large and massive lower jaw, and the receding chin, shown in Fig. 144. From the skulls which have been found it is possible to form a good idea of the man's appearance, which idea has been expressed by a German sculptor in the bust illustrated in Fig. 145.

One may be permitted to hazard a guess at the cause of the process "running to brain," which is the main feature of the last phases of man's Evolution. The most probable theory seems to be that man came into existence owing to the disappearance of forest over an area inhabited by some high anthropoid ape. Ill-adapted as this ape would certainly be for a life on the plains, it was saved from extinction only by its high intelligence. And as cunning and reason would now, in the new environment, be the most important assets, the process of natural selection made for progress chiefly in respect of these characters.

The faculty of articulate speech, which we must regard as an accidental result of the great brain development, has given the human species that great advantage which it possesses over all other animals of being able to accumulate knowledge and experience from generation to generation. It is this mass of experience, which is not inherent in man's nature, but has to be impressed afresh on each successive generation, which accounts for man's unique position in the animal world. But it is no part of the scheme of this book to deal with the evolution of language or invention or culture, and we must conclude.

Mankind are divisible into many types and races, some of which, like the Australian aborigines and the Veddas of Ceylon, are relatively primitive, others like the Germanic races being undoubtedly high in the series. None of the differences are sufficient, however, to make it necessary to regard mankind except as members of a single species. The lower races have from time to time disappeared before the higher, and the process continues [153]
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[157]at the present time. However much we may regret it, this process has undoubtedly been a great factor in the progress of the species as a whole.

Within the cultivated races, however, man has practically ceased to evolve, at least in so far as concerns the main lines on which his Evolution has thus far proceeded. For in creating the artificial conditions of civilisation, he refuses any longer to be governed by the stern law of nature, which decides that the fit shall live and multiply, and the unfit surely perish. There is in fact evidence that conditions of civilisation are making for retrogression rather than for progress, a state of affairs that is worthy of the most serious consideration. The only rational and scientific remedy that has been offered for this state of affairs is the institution of some moderate system of artificially guiding man's further Evolution.

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