In tracing man’s ancestry from fish upward we ought properly to describe three or four fish, an amphibian, a reptile, and then take up the series of mammalian ancestors. But we have not sufficient time for so extended a study, and a simpler method may answer our purpose fairly well. Let us fix our attention on the few organs which still show the capacity of marked development, and follow each one of these rapidly in its upward course.

We must remember that there are changes in the vegetative organs. The digestive and excretory systems improve. But this improvement is not for the sake of these vegetative functions. Brain and muscle demand vastly more fuel, and produce vastly more waste which must be removed. At almost the close of the series the reproductive system undergoes a modification which is almost revolutionary in its results. But we shall find that this modification is necessitated by the smaller amount of material which can be spared for this function; not by its increasing importance, still less its dominance for its own worth. The vertebrate is like an old Roman; everything is subordinated to mental and physical power. He is the world conqueror.

The important changes from fish upward affect the following organs: 1. The skeleton. A light, solid framework must be developed for the bod. The appendages start as fins, and end as the legs and arms of ma. The circulatory and respiratory systems developed so as to carry with the utmost rapidity and certainty fuel and oxygen to the muscular and nervous high-pressure engines. Or, to change the figure, they are the roads along which supplies and munitions can be carried to the army suddenly mobilized at any point on the frontie. Above all, the brain, especially the cerebrum, the crown and goal of vertebrate structure. The improvement is now practically altogether in the animal organs of locomotion and thought. Still, among these animal organs, the lower systems will lead in point of time. The brain must to a certain extent wait for the skeleton.

1. The skeleton. The axial skeleton consists, in the lowest fish, of the notochord, a cylindrical unsegmented rod of cartilage running nearly the length of the body. This is surrounded by a sheath of connective tissue, at first merely membranous, later becoming cartilaginous or gristly. Pieces of cartilage extend upward over the spinal marrow, and downward around the great aortic artery, forming the neural and haemal arches. These unite with the masses of cartilage surrounding the notochord to form cartilaginous vertebrae, which may be stiffened by an infiltration of carbonate of lime. The vertebral column of sharks has reached this stage. Then the cartilaginous vertebrae ossify and form a true backbone. I have described the process as if it were very simple. But only the student of comparative osteology can have any conception of the number of experiments which were tried in different groups before the definite mode of forming a bony vertebra was attained. At the same time the skull was developing in a somewhat similar manner. But the skull is far more complex in origin and undergoes far more numerous and important changes than the simpler vertebral column. Into its history we have no time to enter.

And what shall we say of bone itself as a mere material or tissue, with its admirable lightness, compactness, and flawlessness. And every bone in our body is a triumph of engineering architecture. No engineer could better recognize the direction of strain and stress, and arrange his rods and columns, arches and buttresses, to suitably meet them, than these problems are solved in the long bone of our thigh. And they must be lengthened while the child is leaping upon them. An engineer is justly proud if he can rebuild or lengthen a bridge without delaying the passage of a single train. But what would he say if you asked him to rebuild a locomotive, while it was running even twenty miles an hour? And yet a similar problem had to be solved in our bodies.

But the vertebral column is not perfected by fish. The vertebrae with few exceptions are hollow in front and behind, biconcave; and between each two vertebrae there is a large cavity still occupied by the notochord. Thus these vertebrae join one another by their edges, like two shallow wine-glasses placed rim to rim. Only gradually is the notochord crowded out so that the vertebrae join by their whole adjacent surfaces. Even in highest forms, for the sake of mobility, they are united by washer-like disks of cartilage. Biconcave vertebrae persisted through the oldest amphibia, reptiles, and birds. But finally a firm backbone and skull were attained.

2. The appendages. Of these we can say but little. The fish has oar-like fins, attached to the body by a joint, but themselves unjointed. By the amphibia legs, with the same regions as our own and with five toes, have already appeared. The development of the leg out of the fin is one of the most difficult and least understood problems of vertebrate comparative anatomy. The legs are at first weak and scarcely capable of supporting the body. Only gradually do they strengthen into the fore- and hind-legs of mammals, or into the legs and wings of birds and old flying reptiles.

3. Changes in the circulatory and respiratory systems. The fish lives altogether in the water and breathes by gills, but the dipnoi among fishes breathes by lungs as well as gills. As long as respiration takes place by gills alone, the circulation is simple; the blood flows from the heart to the gills, and thence directly all over the body; the oxygenated blood from the gills does not return directly to the heart. But the blood from the lungs does return to the heart; and there at first mixes in the ventricle with the impure blood which has returned from the rest of the body. Gradually a partition arises in the ventricle, dividing it into a right and left half. Thus the two circulations of the venous blood to the lungs, and of the oxygenated blood over the body, are more and more separated until, in higher reptiles, they become entirely distinct.

As the animal came on land and breathed the air, more completely oxygenated blood was carried to the organs, and their activity was greatly heightened. As more and more heat was produced by the combustion in muscular and nervous tissues, and less was lost by conduction, the temperature of the body rose, and in birds and mammals becomes constant several degrees above the highest summer temperature of the surrounding air.

The changes in the brain affect mainly the large and small brain. The cerebellum increases with the greater locomotive powers of the animal. But its development is evidently limited. The large brain, or cerebrum, is in fish hardly as heavy as the mid-brain; in amphibia the reverse is true. In higher recent reptiles the cerebrum would somewhat outweigh all the other portions of the brain put together. In mammals it extends upward and backward, has already in lower forms overspread the mid-brain, and is beginning to cover the small brain. But this was not so in the earliest mammals. Here the cerebrum was small, more like that of reptiles. But during the tertiary period the large brain began to increase with marvellous rapidity. It was very late in arriving at the period of rapid development, but it kept on after all the other organs of the body had settled down into comparative rest, perhaps retrogression.

We have given thus a rapid sketch in outline of the changes in the most characteristic systems between fish and mammals. Some of the changes which took place in mammals were along the same lines, but one at least is so new and unexpected that this highest class demands more careful and detailed examination.

The mammal is a vertebrate. Hence all its organs are at their best. But mammals stand, all things considered, at the head of vertebrates. The skeleton is firm and compact. The muscles are beautifully moulded and fitted to the skeleton so as to produce the greatest effect with the least mass and weight of tissue. The sense-organs are keen, and the eye and ear especially delicate, and fitted for perception at long range. Yet in all these respects they are surpassed by birds. As a mere anatomical machine the bird always seems to me superior to the mammal. It is not easy to see why it failed, as it has, to reach the goal of possibility of indefinite development and dominance in the animal world. Why he stopped short of the higher brain development I cannot tell. The fact remains that the mammal is pre-eminent in brain power, and that this gave him the supremacy.

But mammals came very late to the throne, and the probability of their ever gaining it must for ages have appeared very doubtful. They seem to have been a fairly old group with a very slow early development. Reptiles especially, and even birds, were far more precocious than these slower and weaker forms which crept along the earth. But reptiles and birds, like many other precocious children, soon reached the limit of their development. They had muscle, the mammal brain and nerve; the mammal had the staying power and the future. Bitter and discouraging must have been the struggle of these feeble early mammals with their larger, swifter, and more powerful, reptilian relatives. And yet, perhaps, by this very struggle the mammal was trained to shrewdness and endurance.

The primitive mammals laid eggs like reptiles or birds. Only two genera, echidna and platypus, survive to bear witness of these old oviparous groups, and these only in New Zealand. These retain several old reptilian characteristics. Their lower position is shown also by the fact that the temperature of their bodies is, at least, ten degrees Fahrenheit below that of higher mammals. One of these carries the egg in a pouch on the ventral surface; the other, living largely in water, deposits its eggs in a nest in a burrow in the side of the bank of the stream.

After these came the marsupials. In these the eggs develop in a sort of uterus; but there is no placenta, in the sense of an organic connection between the embryo and the uterus of the mother. The young are at birth exceedingly small and feeble. The adult giant Kangaroo weighs over one hundred pounds; the young are at birth not as large as your thumb. They are placed by the mother in a marsupial pouch on her ventral surface, and here nourished till able to care for themselves.

Pardon a moment’s digression. The marsupials, except the opossum, are confined to Australia, and the oviparous mammals, or monotrèmes, to New Zealand. Formerly the marsupials, at least, ranged all over Europe and Asia, for we have indisputable evidence in their fossil remains. But they have survived only in this isolated area, and here apparently only because their isolation preserved them from the competition with higher forms. If the Australian continent had not been thus early cut off from all the rest of the world, the only trace of both these lower groups would have been the opossum in America and certain peculiarities in the development of the egg in higher mammals. This shows us how much weight should be assigned to the formerly popular argument of the “missing links.” The wonder is not that so many links are missing, but that any of these primitive forms have come down to us. For we see here another proof of the fearful extermination of lower forms during the progress of life on the globe. It seems as if the intermediate forms were less common among these most recent animals than among the older types. This may not be true, for it is not easy to compare the gap between two mammals with that between two worms or insects, and mistakes are very easily made. But it seems as if extermination had done its work more ruthlessly among these highest forms than among their humbler and lower ancestors. I would not lay much weight on such an opinion; but, if true, it has a meaning and is worthy of study.

In higher, true, placental mammals the period of pregnancy is much longer, and the young are born in a far higher stage of development, or rather, growth. The stage of growth at which the young are born differs markedly in different groups. A new-born kitten is a much feebler, less developed being than a new-born calf. An embryonic appendage, the allantois, used in reptiles and birds for respiration, has here been turned to another purpose. It lays itself against the walls of the uterus, uterine projections interlock with those which it puts forth, and the blood of the mother circulates through a host of capillaries separated from those of the blood system of the embryo only by the thinnest membrane. This is the placenta, developed, in part from the allantois of the embryo, in part from the uterus of the mother. It is not a new organ, but an old one turned to better and fuller use. In these closely associated systems of blood-vessels, nutriment and oxygen diffuse from the blood of the mother into that of the embryo, and thus rapid growth is assured. The importance and far-reaching effect of this new modification in the old reproductive system cannot be over-estimated. The internal intra-uterine development of the young, and the mammalian habit of suckling them, far more than any other factors, have made man what he is. Some explanation must be sought for such a fact.

We have already seen that any animal devotes to reproduction the balance between income and expenditure of nutriment. Now, the digestive system is here well developed, and the income is large. But we have already noticed that, as animals grow larger, the ratio between the digestive surface and the mass to be supported grows continually smaller. On account of size alone the mammal has but a small balance. But the amount of expenditure is proportional to the mass and activity of the muscular and nervous systems. And the mammal is, and from the beginning had to be, an exceedingly active, energetic, and nervous animal. The income has increased, but the expenses have far outrun the increase. The mammal can devote but little to reproduction.

Moreover, it requires a large amount of material to form a mammalian egg, such as that of the monotreme. It requires indefinitely more nutriment to build a mammal than a worm, for the former is not only larger and more perfect at birth; it is also vastly more complicated. The embryonic journey has, so to speak, lengthened out immensely. One monotreme egg represents more economy and saving than a thousand eggs of a worm. Moreover, where the individuals are longer lived and the generations follow one another at longer intervals, the number of favorable variations and the possibility of conformity to environment through these is greatly lessened. In such a group it is of the utmost importance that every egg should develop; the destruction of a single one is a real and important loss to the species. It is not enough to produce such an egg; it must be most scrupulously guarded. Even the egg of the platypus is deposited in a nest in a hole in the bank, and the female Echidna carries the egg in a marsupial pouch until it develops.

Notice further that among certain species of fish, amphibia, and reptiles, the females carry the eggs in the body until the embryos or young are fairly developed. Viviparous forms are unknown by birds, probably because this mode of development is incompatible with flight, their dominant characteristic. Putting these facts together, what more probable than that certain primitive egg-laying mammals should have carried the eggs as long as possible in the uterus. The embryo under these conditions would be better nourished by a secretion of the uterine glands than by a very large amount of yolk. The yolk would diminish and the egg decrease in size, and thus the marsupial mode of development would have resulted. And, given the marsupial mode of development and an embryo possessing an allantois, it is almost a physiological necessity that in some forms at least a placenta should develop. That the placenta has resulted from some such process of evolution is proven by its different stages of development in different orders of mammals. And even the feeblest attachment of the allantois of the embryo to the wall of the uterus would be of the greatest advantage to the species.

This is not the whole explanation; other factors still undiscovered were undoubtedly concerned. But even this shows us that the internal development of the young and the habit of suckling them was a logical result of mammalian structure and position. The grand results of this change we shall trace farther on.

The changes from the lower true mammals to the apes are of great interest, but we can notice only one or two of the more important. The prosimii, or “half apes,” including the lemurs, are nearly all arboreal forms. Perhaps they were driven to this life by their more powerful competitors. The arboreal life developed the fingers and toes, and most of these end, not with a claw, but with a nail. The little group has much diversity of structure, and at present finds its home mainly in Madagascar; though in earlier times apparently occurring all over the globe. The brain is more highly developed than in the average mammal, but far inferior to that of the apes. They have a fairly opposable thumb.

The highest mammals are the primates. Their characteristics are the following: Fingers and toes all armed with nails, the eyes comparatively near together and fully enclosed in a bony case. The cerebrum with well-developed furrows covers the other portions of the brain. There is but one pair of milk-glands, and these on the breast. The differences between hand and foot become most strongly marked by the “anthropoid” apes. These have become accustomed to an upright gait in their climbing; hence the feet are used for supporting the body and the hands for grasping. Both thumb and great toe are opposable; but the foot is a true foot, and the hand a true hand, in anatomical structure. The face, hands, and feet have mainly lost the covering of hair. They have no tail, or rather its rudiments are concealed beneath the skin. These include the gibbon, the orang, the gorilla, and the chimpanzee.

We can sum up the few attainments of mammals in a line. The lower forms attained the placental mode of embryonic development; the higher attained upright gait, hands and feet, and a great increase of brain. Anatomically considered these were but trifles, but the addition of these trifles revolutionized life on the globe. The principal anatomical differences between man and the anthropoid ape are the following: Man is a strictly erect animal. The foot of the ape is less fitted for walking on the ground, where he usually “goes on all fours.” The skull is almost balanced on the condyles by which it articulates with the neck, and has but slight tendency to tip forward. The facial portion, nose and jaws, is less developed and retracted beneath the larger cranium or brain-case. This has greatly changed the appearance of the head. Protruding jaws and chin, even when combined with large cranium and brain, always give man the appearance of brutality and low intelligence.

The pelvis is broad and comparatively shallow. The legs, especially the thighs, are long. The foot is long and strong, and rests its lower surface, not merely the outer margin as in apes, on the ground. The elastic arch of the instep must be excepted in the above description, and adds lightness and swiftness to his otherwise slow gait. The great toe is short and generally not opposable. The muscles of the leg are heavy and the knee-joint has a very broad articulating surface. But the great result of man’s erect posture is that the hand is set free from the work of locomotion, and has become a delicate tactile and tool-using organ. The importance of this change we cannot over-estimate. The hand was the servant of the brain for trying all experiments. Had not our arboreal ancestors developed the hand for us we could never have invented tools nor used them if invented. And its reflex influence in developing the brain has been enormous. The arm is shorter and the hand smaller. The brain is absolutely and relatively large, and its surface greatly convoluted. This gives place for a large amount of “gray matter,” whose functions are perception, thought, and will. For this gray matter forms a layer on the outside of the brain.

Thus, even anatomically, man differs from the anthropoid apes. His whole structure is moulded to and by the higher mental powers, so that he is the “Anthropos” of the old Greek philosophers, the being who “turns his face upward.” Yet in all these anatomical respects some of the apes differ less from him than from the lower apes or “half apes.” And every one of these can easily be explained as the result of progressive development and modification. Whoever will deny the possibility or probability of man’s development from some lower form must argue on psychological, not on anatomical, grounds; and it grows clearer every day that even the former but poorly justify such a denial.

But it is interesting to note that no one ape most closely approaches man in all anatomical respects. Thus among the anthropoids the orang is perhaps most similar to man in cerebral structure, the chimpanzee in form of skull, the gorilla in feet and hands. No evolutionist would claim that any existing ape represents the ancestor of man. The anthropoids represent very probably the culmination of at least three distinct lines of development. But we must remember that in early tertiary times apes occurred all over Europe, and probably Asia, many degrees farther north than now. In those days, as later, the fauna and flora of northern climates were superior in vigor and height of development to that of Africa or Australia. It is thus, to say the least, not at all improbable that there existed in those times apes considerably, if not far, superior to any surviving forms. Whether the palaeontologist will find for us remains of such anthropoids is still to be seen.

But you will naturally ask, “Is there not, after all, a vast difference between the brain of man and that of the ape?” Let us examine this question as fully as our very brief time will allow. Considerable emphasis used to be laid on the facial angle between a line drawn parallel to the base of the skull and one obliquely vertical touching the teeth and most prominent portion of the forehead. Now this angle is in man very large from seventy-five to eighty-five degrees, or even more, and rarely falling below sixty-five degrees. But this angle depends largely on the protrusion of the jaws, and varies greatly in species of animals showing much the same grade of intelligence. In some not especially intelligent South American monkeys the facial angle amounts to about sixty-five degrees. In this respect the skull of a chimpanzee reminds us of a human skull of small cranial capacity and large jaws, in which the cranium has been pressed back and the jaws crowded forward and slightly upward.

The weight of the brain in proportion to that of the body has been considered as of great importance, and within certain limits this is undoubtedly correct. Thus, according to Leuret, the weight of the brain is to that of the whole body: In fish, 1:5,668; in reptiles, 1:1,320; in birds, 1:212; in mammals, 1:186. These figures give the averages of large numbers of observations and have a certain amount of value. But within the same class the ratio varies extraordinarily. Thus the weight of the brain is to that of the whole body: In the elephant, 1:500; in the largest dogs, 1:305; in the cat, 1:156; in the rat, 1:76; in the chimpanzee, 1:50; in man, 1:36; in the field-mouse, 1:31; in the goldfinch, 1:24.

From this series it is evident that the relative weight of the brain is no index of the intelligence of the animal. Indeed if the brain were purely an organ of mind, there is no reason that it should be any larger in an elephant than in a mouse, provided they had the same mental capacity. As animals grow larger the weight of the brain, relatively to that of the body, decreases, and considering the size of man it is remarkable that it should form so large a fraction of his weight. Still the fraction in the chimpanzee is not so much smaller. It is still possible that this fraction is above the normal for the chimpanzee, for some of the observations may have been taken on animals which had died of consumption or some other wasting disease. I have not been able to find whether this possibility of error has been scrupulously avoided.

A fair idea of the size of the brain may be obtained by measuring the cranial capacity. This varies in man from almost one-hundred cubic inches to less than seventy. In the gorilla its average is perhaps thirty, in the orang and chimpanzee rather less, about twenty-eight. This is certainly a vast difference, especially when we remember that the gorilla far exceeds man in weight.

Le Bon tells us that of a series of skulls forty-five per cent, of the Australian had a cranial capacity of 1,200 to 1,300 c.c., while 46.7 per cent. of modern Parisian skulls showed a capacity of between 1,500 and 1,600 c.c. The skull of the gorilla contains about five hundred and seventy cubic centimetres. Broca found that the cranial capacity of 115 Parisian skulls, of probably the higher classes from the twelfth century, averaged about 1,426 cubic centimetres, while ninety of those of the poorer classes of the nineteenth century averaged about 1,484. His observations seemed to prove that there has been a steady increase in Parisian cranial capacity from the twelfth to the nineteenth century.

Turning to the actual weight of the brain, that of Cuvier weighed 64.5 ounces, and a few cases of weights exceeding 65 ounces have been recorded. The lowest limit of weight in a normal human brain has not yet been accurately determined. From 34 to 31 ounces have been assigned by different writers. The brain of a Bush woman was computed by Marshall at 31.5 ounces, and weights of even 31 ounces have been recorded without any note to show that the possessors were especially lacking in intelligence. As Professor Huxley says in his “Man’s Place in Nature,” a little book which I cannot too highly recommend to you all, “It may be doubted whether a healthy human adult brain ever weighed less than 31 or 32 ounces, or that the heaviest gorilla brain has ever exceeded 20 ounces. The difference in weight of brain between the highest and the lowest men is far greater, both relatively and absolutely, than that between the lowest man and the highest ape. The latter, as has been seen, is represented by 12 ounces of cerebral substance absolutely, or by 32:20 relatively. But as the largest recorded human brain weighed between 65 and 66 ounces, the former difference is represented by 33 ounces absolutely, or by 65:32 relatively.”

But there is another characteristic of the brain which seems to bear a close relation to the degree of intelligence. The surface of the human brain is not smooth but covered with convolutions, with alternating grooves or sulci, which vastly increase its surface and thus make room for more gray matter. Says Gratiolett: “On comparing a series of human and simian brains we are immediately struck with the analogy exhibited in the cerebral forms in all these creatures. There is a cerebral form peculiar to man and the apes; and so in the cerebral convolutions, wherever they appear, there is a general unity of arrangement, a plan, the type of which is common to all these creatures.” Professor Huxley says: “It is most remarkable that, as soon as all the principal sulci appear, the pattern according to which they are arranged is identical with the corresponding sulci in man. The surface of the brain of the monkey exhibits a sort of skeleton map of man’s, and in the man-like apes the details become more and more filled in, until it is only in minor characters that the chimpanzee’s or orang’s brain can be structurally distinguished from man’s.”

The facts of anatomy, at least, are all against us. Struggle as we may, be as snobbish as we will, we cannot shake off these poor relations of ours. Our adult anatomy at once betrays our ancestry, if we attempt to deny it. Read the first chapter of that remarkable book by Professor Drummond on the “Ascent of Man,” the chapter on the ascent of the body, and the second chapter on the scaffolding left in the body. The tips of our ears and our rudimentary ear muscles, the hair on hand and arm, and the little plica semilunaris, or rudimentary third eyelid in the inner angle of our eyes, the vermiform appendage of the intestine, the coracoid process on our shoulder-blades, the atlas vertebra of our necks to say nothing of the coccyx at the other end of the backbone many malformations, and a host of minor characteristics all refute our denial.

If we appeal from adult anatomy to embryology the case becomes all the worse for us. Our ear is lodged in the gill-slit of a fish, our jaws are branchial arches, our hyoid bone the rudiment of this system of bones supporting the gills. Our circulation begins as a veritable fish circulation; our earliest skeleton is a notochord; Meckel’s cartilage, from which our lower jaw and the bones of our middle ear develop, is a whole genealogical tree of disagreeable ancestors. Our glándula thyreoidea has, according to good authorities, an origin so slimy that it should never be mentioned in polite society. The origin of our kidneys appears decidedly vermian. Time fails me to read merely the name of the witnesses which could be summoned from our own bodies to witness against us.

Even if the testimony of some of these witnesses is not as strong as many think, and we have misunderstood several of them, they are too numerous and their stories hang too well together not to impress an intelligent and impartial jury. But what if it is all true? What if, as some think, our millionth cousin, the tiger or cat, is anatomically a better mammal than I? His teeth and claws and magnificent muscles are of small value compared with man’s mental power.

What a comedy that man should work so hard to prove that his chief glory is his opposable thumb, or a few ounces of brain matter! Man’s glory is his mind and will, his reason and moral powers, his vision of, and communion with, God. And supposing it be true, as I believe it is true, that the animal has the germ of these also, does that cloud my mind or obscure my vision or weaken my action? It bids me only strive the harder to be worthy of the noble ancestors who have raised me to my higher level and on whose buried shoulders I stand. Whatever may have been our origin, whoever our ancestors, we are men. Then let us play the man. If we will but play our part as well as our old ancestors played theirs, if we will but walk and act according to our light one-half as heroically and well as they groped in the darkness, we need not worry about the future. That will be assured.

Says Professor Huxley: “Man now stands as on a mountain-top far above the level of his humble fellows, and transfigured from his grosser nature by reflecting here and there a ray from the infinite source of truth. And thoughtful man, once escaped from the blinding influences of traditional prejudice, will find in the lowly stock whence man has sprung the best evidence of the splendor of his capacities, and will discern in his long progress through the past a reasonable ground of faith in his attainment of a nobler future.”

We have sketched hastily and in rude outline the anatomical structure of the successive stages of man’s ancestry; let us now, in a very brief recapitulation, condense this chronicle into a historical record of progress.

We began with the amoeba. This could not have been the beginning. In all its structure it tells us of something earlier and far simpler, but what this earlier ancestor was we do not know. Rather more highly organized relatives of the amoeba, the flagellata, have produced a membrane, and swim by means of vibratile, whiplash-like flagella. We must emphasize that these little animals correspond in all essential respects to the cells of our bodies; they are unicellular animals. And the cell once developed remains essentially the same structure, modified only in details, throughout higher animals. And these unicellular animals have the rudiments of all our functions. Their protoplasm and functions seem to differ from those of higher animals only in degree, not in kind. And the more we consider both these facts the more remarkable and suggestive do they become.

Cells with membranes can unite in colonies capable of division of labor and differentiation. And magosphaera is just such a little spheroidal colony. But the cells are still all alike, each one performs all functions equally well. But in volvox division of labor and differentiation of structure have taken place. Certain cells have become purely reproductive, while the rest gather nutriment for these, but are at the same time sensitive and locomotive, excretory and respiratory. The first function to have cells specially devoted to it is the reproductive; this is a function absolutely necessary for the maintenance of the species. For the nutritive cells die when they have brought the reproductive cells to their full development. These few nutritive cells represent the body of all higher animals in contrast with the reproductive elements. And with the development of a body, death, as a normal process, enters the world. The dominant function is here evidently the reproductive, and the whole body is subservient to this.

In hydra the union and differentiation of cells is carried further. But the cells are still much alike and only slowly lose their own individuality in that of the whole animal. This is shown in the fact that each entodermal cell digests its own particles of food, although the nutriment once digested diffuses to all parts of the body. Also almost any part of the animal containing both ectoderm and entoderm can be cut off and will develop into a new animal.

But beside the reproductive cells and tissues hydra has developed a very simple digestive system, in which the newly caught food at least macerates and begins to be dissolved. This is the second essential function. The animal can, and the plant as a rule does, exist with only the lowest rudiments of anything like nervous or muscular power; but no species can exist without good powers of digestion and reproduction. These essential organs must first develop and the higher must wait. And the inner, digestive, layer of cells persists in our bodies as the lining of the mid-intestine. We compared hydra therefore to a little patch of the lining of our intestine covered with a flake of epidermis; only these layers in hydra possess powers lost to the corresponding cells of our bodies in the process of differentiation. Notice, please, that when cell or organ has once been developed it persists, as a rule, modified, but not lost. Nature’s experiments are not in vain; her progress is very slow but sure. But hydra has also the promise of better things, traces of muscular and nervous tissue. There are still no compact muscles, like our own, much less ganglion or brain or nerve-centre of individuality. The tissues are diffuse, but they are the materials out of which the organs of higher animals will crystallize, so to speak. Notice also that these higher muscles and nerves are here entirely subservient to, and exist for, digestion and reproduction.

In the turbellaria the reproductive system has reached a very high grade of development. It is a complex and beautifully constructed organ. The digestive system has also vastly improved; it has its own muscular layers, and often some means of grasping food. But it is slower in reaching its full development than the reproductive system. But all the muscles are no longer attached to the stomach; they are beginning to assert their independence, and, in a rude way, to build a body-wall. But they are in many layers, and run in almost all directions. Some of these layers will disappear, but the most important ones, consisting of longitudinal and transverse fibres, will persist in higher forms. Locomotion by means of these muscles is slowly coming into prominence. They are no longer merely slaves of digestion.

But a muscular fibril contracts only under the stimulus of a nervous impulse. More nerve-cells are necessary to control these more numerous muscular fibrils. The animal now moves with one end foremost, and that end first comes in contact with food, hindrances, or injurious surroundings. Here the sensory cells of feeling and their nerve fibrils multiply. Remember that these neuro-epithelial sensory cells are suited to respond not merely to pressure, but to a variety of the stimuli, chemical, molecular, and of vibration, which excite our organs of smell, taste, and hearing. Such organs and the directive eyes appear mainly at this anterior end. But a ganglion cell sends an impulse to a muscle because it has received one along a sensory nerve from one or more of these sensory cells. Hence the ganglion cells will increase in number. The old cobweb-like plexus condenses into a little knot, the supra-oesophageal ganglion. This ganglion cannot do much, if any, thinking; it is rather a steering organ to control the muscles and guide the animal. It is the servant of the locomotive system. Yet it is the beginning of the brain of higher animals, and probably still persists as an infinitesimal portion of our human brain. And all this is the prophecy of a head soon to be developed. An excretory system has appeared to carry off the waste of the muscles and nerves.

In the schematic worm and annelid the reproductive system is simpler, though perhaps equally effective. It takes the excess of nutriment of the body. The muscular system has taken the form of a sack composed of longitudinal and transverse fibres. The perivisceral cavity, formed perhaps by cutting off and enlarging the lateral pouches of the turbellarian digestive system, serves as a very simple but serviceable circulatory system. But in the annelid and all higher forms a special system of tubes has developed to carry the nutriment, and usually oxygen also, needed to keep up the combustion required to furnish the energy in these active organs. The digestive system has attained its definite form with the appearance of an anal opening and the accompanying division of labor and differentiation into fore-, mid-, and hind-intestine.

The digestive and reproductive systems have thus nearly attained their final form. From the higher worms upward the digestive system will improve greatly. Its lining will fold and flex and vastly increase the digestive and absorptive surfaces. The layer of cells which now secrete the digestive fluids will in part be replaced by massive glands. Far better means of grasping food than the horny teeth of annelids will yet appear. But all these changes are inconsiderable compared with the vast advance made by the muscular and nervous systems. Reproduction and digestion are losing their supremacy in the animal body. Their advance and improvement will require but little further attention.

In the annelid especially, and to some extent in the schematic worm, the supra-oesophageal ganglion is relieved in part of the direct control of the muscular fibrils and has become an organ of perception and the seat of government of lower nervous centres. In all higher forms it innervates directly only the principal sense-organs of the head. And at this stage the light-perceiving directive eye has developed into a form-perceiving, eidoscopic organ. The eye was short of range and its images were perhaps rude and imperfect, but it was a visual eye and had vast possibilities. The animal is taking cognizance of ever more subtle elements in its environment. Perhaps it is not too much to say that the eidoscopic eye first awakened the slumbering animal mind, for its reflex effect upon the supra-oesophageal ganglion cannot be over-estimated. The animal will very soon begin to think.

Between the turbellarian and the annelid many aberrant lines diverged. Some of these attained a comparatively high level and then seemed to meet insuperable obstacles, while others came to an end or turned downward very early. Three of these demanded attention, those leading to mollusks, insects, and vertebrates. And it is interesting to notice that the fundamental difference between these three lines was the skeleton, or perhaps we ought to say it was the habit of life which led to the development of such a skeleton.

The mollusk took to a sluggish, creeping mode of life, under an external purely protective skeleton; the insect to a creeping mode of life, with an external but almost purely locomotive skeleton; the vertebrate kept on swimming and developed an internal locomotive skeleton. And it must already have become clear to you that the destiny of these different lines was fixed not so much directly by the skeleton itself as by its reflex effect in moulding the muscular, and ultimately the nervous, system.

The insects formed their skeleton by thickening the horny cuticle of the annelid. They transformed the annelid parapodia into legs and developed wings. They attained life in the air. They devoted the muscles of the body largely to the extremities and gained swift locomotion. They have a fair circulatory and an excellent respiratory system. Best of all, they developed a head and a brain by fusing the three anterior ganglia of the body. The insect could and does think. Such a structure ought to lead to great and high results. But actually their possibilities were very limited. They have not progressed markedly during the last geological period. Their external skeleton was easily attained and brought speedy advantages, which for a time placed them far above all competitors. But it limited their size and length of life and opportunities, and finally their intelligence. They remained largely the slaves of instinct. They followed an attractive and exceedingly promising path, but it led to the bottom of a cliff, not to the summit.

The mollusks, clams, and snails took an easier, down-hill road. They formed a shell, and it developed large enough to cover them. It hampered and almost destroyed locomotion and reduced nerve to a minimum. But nerves are nothing but a nuisance anyhow. And why should they move? Food was plenty down in the mud, and if danger threatened, they withdrew into the shell. They stayed down in the mud and let the world go its way. If grievously afflicted by a parasite they produced a pearl to save themselves from further discomfort. They developed just enough muscle and nervous system to close the shell or drag it a little way; that was all. Digestion and reproduction retained the supremacy. They were fruitful and multiplied, and produced hosts of other clams and snails. The present was enough for them and they had that.

For if the winner in the struggle for existence is the one who gains the most food, the most entire protection against discomfort, danger from enemies or unfavorable surroundings, and the most fruitful and rapid reproduction and these are all good then the clam is the highest product of evolution. It never has been surpassed I venture to say it never can be except possibly by the tape-worms. I can never help thinking with what contempt these primitive oysters, if they had had brains enough, would have looked down upon the toiling, struggling, discontented, fighting, aspiring primitive vertebrates. How they would have wondered why God allowed such disagreeable, disturbing, unconventional creatures to exist, and thanked him that he had made the world for them, and heaven too, if there be such a place for mollusks. Their road led to the Slough of Contentment.

But even in molluscan history there was a tragic chapter. The squids and cuttle-fishes regained the swimming life, and in their latest forms gave up the protective shell. But its former presence had so modified their structure that any great advance was impossible. It was too late. The sins of the fathers were visited upon the children in the thousandth generation.

The vertebrate developed an internal skeleton. This was necessarily a slow growth, and the type came late to supremacy. The longitudinal muscles are arranged in heavy bands on each side of the back, and the animal swims rapidly. The sense-organs are keen. The brain contains the ganglia of several or many segments and is highly differentiated. It has a special centre of perception, thought, and will; it is an organ of mind. The vertebrate has the physical and mental advantages of large size.

First the definite form and mode of developing a vertebra is attained. Then the vertebral column is perfected. The fins are modified into legs. The lungs increase in size and the heart becomes double. The animal emerges on land; and, with a better supply of oxygen and less loss of heat, all the functions are performed with the highest possible efficiency. First, apparently, amphibia, then reptiles, and finally mammals of enormous size and strength appeared. It looked as if the earth were to be an arena where gigantic beasts fought a never-ending battle of brute force. But these great brutes reproduced slowly, had therefore little power of adaptation, were fitted to special conditions, and when the conditions changed they disappeared. The bird tried once more the experiment of developing the locomotive powers to the highest possible extent. It became a flying machine, and every organ was moulded to suit this life. Every ounce of spare weight was thrown aside, the muscles were wonderfully arranged and of the highest possible efficiency. The body temperature is higher than that of mammals. The whole organization is a physiological high-pressure engine. The sense-organs are perhaps the finest and keenest in the whole animal kingdom. The brain is inferior only to that of mammals. The experiment could not have been tried under more favorable conditions; it was not a failure, it certainly was not a success when compared with that of mammals.

The possibilities of every system except one had been practically exhausted. Only brain development remained as the last hope of success. Here was an untried line, and the mammals followed it. During the short tertiary period the brain in many of their genera seems to have increased tenfold. By the arboreal life of the highest forms the hand is developed as the instrument of the thinking brain. The battle is beginning to become one of wits, and the crown will soon pass from the strongest to the shrewdest. Mind, not muscle, much less digestion or reproduction, is the goal of the animal kingdom. And we shall see later that the mammalian mode of reproduction and of care of the young led to an almost purely mental and moral advance. For these could have but one logical outcome, family life. And the family is the foundation of society. And family and social life have been the school in which man has been compelled to learn the moral lessons, the application of which has made him what he is.

You must all, I think, have noticed that the different systems of organs succeed one another in a certain definite order; and that each stage from the lowest to the highest is characterized by the predominance of a certain function or group of functions. This sequence of functions is not a deduction but a fact. Place side by side all possible genealogical trees of the animal kingdom, whether founded on comparative anatomy, embryology, palaeontology, or all combined. They will all disclose this sequence of functions arranged in the same order. Let me call your attention to the fact that this order is not due to chance, but rests upon a physiological basis. We might almost claim that if the evolution of man from the single cell be granted, no other order of their occurrence is possible.

The protozoa are mostly, though not purely, nutritive and reproductive. These functions are essential to the existence of the species. Naturally in the early protozoan colonies, and in forms like hydra, these functions predominated. But mere digestive tissue is not enough for digestion. Muscles are needed to draw the food to the mouth, to keep the digestive sack in contact with it, and for other purposes. A little higher they are used to enable the animal to go in search of its food. They are still, however, more or less entirely subservient to digestion. But in the highest worms we are beginning to see signs that muscles are predominating in the body; and we feel that, while mutually helpful, the digestive system exists for the muscles, and these latter are becoming the aim of development. From worms upward there is a marked advance in physical activity and strength. The muscles thicken and are arranged in heavier bands. Skeleton and locomotive appendages and jaws follow in insects and vertebrates. The direct battle of animal against animal, and of strength opposed to strength or activity, becomes ever sharper. The strongest and most active are selected and survive.

And yet this is not the whole truth. Some power of perception is possessed by every animal. But until muscles had developed the nervous system could be of but little practical value. Knowledge of even a great emergency is of little use, if I can do nothing about it. But when the muscles appeared, nerves and ganglion cells were necessary to stimulate and control them. And this highest system holds for a long time a position subordinate to that of the lower muscular organ. Its development seems at first sight extraordinarily slow. Only in insects and vertebrates has it become a centre of instinct and thought. Through the sense-organs it is gaining an ever clearer, deeper, and wider knowledge of its environment. First it is affected only by the lower stimuli of touch, taste, and smell. Then with the development of ear and eye it takes cognizance of ever subtler forces and movements. Memory comes into activity very early. The animal begins to learn by experience. The brain is becoming not merely a steering but a thinking organ. More and more nervous material is crowded into it and detailed for its work. Wits and shrewdness are beginning to count for something in the battle. Not only the animal with the strongest muscles, but the one with the best brain survives. And thus at last the brain began to develop with a rapidity as remarkable as its long delay. Thus each higher function is called into activity by the next lower, serves this at first, and only later attains its supremacy.

And yet the advance of the different functions is not altogether successive. Muscle and nerve do not wait for digestion and reproduction to show signs of halting before they begin to advance. They all advance at once. But the progress of reproduction and digestion is most rapid at first, and it appears as if they would outrun the others. But in the ascending series the others follow after, and soon overtake and pass by them. And these lower functions, when out-marched, do not lag behind, but keep in touch with the others, forming the rear-guard and supply-train of the army. And notice that each organ holds the predominance about as long as it shows the power of rapid improvement. The length of its reign is pretty closely proportional to its capacity of development. The digestive system reaches that limit early, the muscular system is capable of indefinitely higher complexity, as we see in our hand. But the muscular system has nearly or quite reached its limit. The body had seen its day of dominance before man arrived on the globe.

But where is the limit to man’s mental or moral powers? Every upward step in knowledge, wisdom, and righteousness only opens our eyes to greater heights, before unperceived and still to be attained. These capacities, even to our dim vision, are evidently capable of an indefinite, perhaps infinite, development. What, as yet only partially developed, faculty remains to supersede them? As being capable of an endless development and without a rival, may we not, must we not, consider them as ends in themselves? They are evidently what we are here for. Everything points to a spiritual end in animal evolution. The line of development is from the predominantly material to the predominance of the non-material. Not that the material is to be crowded out. It is to reach its highest development in the service of the mind. The body must be sustained and perfected, but it is not the end. The goal is mind, the body is of subordinate importance.

But if this is true, we must study carefully the development of mind in the animal. The question presses upon us; if there is a sequence of physical functions in animal development, is there not perhaps also a sequence in the development of the mental faculties? What is the crowning faculty of the human mind and how is its fuller development to be attained? Let us pass therefore to the question of mind in the animal kingdom.