In his embryonic development man closely resembles the lower animals, the human creature being almost indistinguishable in certain stages from the dog, the bat, the seal, and especially the monkeys. At a very early age he possesses a slight projecting tail; at another, the great toe is shorter than its neighbours, and projects like the thumb at a slight angle; and at a third, the convolutions of the brain reach a point of development about equivalent to that of the adult baboon. In his first stages man himself stands far more closely related to the apes than the apes in turn stand to cats or hyænas.
Rudiments of muscles not normally found in man occur in many aberrant human individuals. Some people possess the power of moving their scalps and wagging their ears like dogs and monkeys; others can twitch the skin of their bodies, as horses do when worried by flies. Mr. Woolner, the sculptor, pointed out to Darwin a certain little projecting point or knob on the margin of the ear, observed by him in the course of modelling, which comparison shows to be the last folded remnant or rudiment of the once erect and pointed monkey-like ear-tip. The nictitating membrane, or third eyelid, once more, which in birds can be drawn so rapidly across the ball of the eye, and which gives the familiar glazed or murky appearance, is fairly well developed in the ornithorhyncus and the kangaroo, as well as in a few higher mammals, like the walrus; but in man, as in the monkey group, it survives only under the degenerate form of a practically useless rudiment, the semilunar fold. Man differs from the other Primates in his apparently hairless condition; but the hair, though short and downy, still remains on close inspection, and in some races, such as the Ainos of Japan, forms a shaggy coat like an orang's or a gibbon's. A few long rough hairs sometimes project from the short smooth down of the eyebrows; and these peculiar bristles, occasional only in the human species, are habitual in the chimpanzee and in many baboons. Internal organs show similar rudiments, of less enthralling interest, it must be candidly confessed, to the unscientific outside intelligence. Even the bony skeleton contributes its share of confirmatory evidence; for in the lower monkeys and in many other mammals a certain main trunk nerve passes through a special perforation in the shoulder-blade, and this perforation, though now almost obsolete, sometimes recurs in man, in which case the nerve in question invariably passes through it, as in the inferior monkeys. What is still more remarkable is the fact that the perforation occurs far more frequently (in proportion) among the skeletons of very ancient races than among those of our own time. One chief cause why in this and other cases ancient races often present structures resembling those of the lower animals seems to be that they stand nearer in the long line of descent to their remote animal-like progenitors.
The conclusion at which, after fully examining all the evidence, Darwin finally arrives is somewhat as follows:
The early ancestors of man must have been more or less monkey-like animals, belonging to the great anthropoid group, and related to the progenitors of the orang-outang, the chimpanzee, and the gorilla. They must have been once covered with hair, both sexes possessing beards. Their ears were probably pointed and capable of movement, and their bodies were provided with a movable tail. The foot had a great toe somewhat thumb-like in its action, with which they could grasp the branches of trees. They were probably arboreal in their habits, fruit-eaters by choice, and inhabitants of some warm forest-clad land. The males had great canine teeth, with which they fought one another for the possession of the females. At a much earlier period, the internal anatomical peculiarities approached those of the lowest mammals, and the eye was provided with a third eyelid. Peering still further back into the dim abyss of the ages, Darwin vaguely describes the ancestors of humanity as aquatic animals, allied to the mudfish; for our lungs are known to consist of modified swim-bladders, which must once have served our remote progenitors in the office of a float. The gill-clefts on the neck of the human embryo still point to the spot where the branchiæ once, no doubt, existed. Our primordial birthplace appears to have been a shore washed twice a day by the recurrent tides. The heart then took the shape merely of a simple pulsating vessel; and a long undivided spinal cord usurped the place of the vertebral column. These extremely primitive ancestors of man, thus dimly beheld across the gulf of ages, must have been at least as simply and humbly organised as that very lowest and earliest of existing vertebrates, the worm-like lancelet.
From such a rude and indefinite beginning natural selection, aided by the various concomitant principles, has slowly built up the pedigree of man. Starting from these remote half-invertebrate forms, whose vague shape is still perhaps in part preserved for us by the soft and jelly-like larva of the modern ascidian, we rise by long stages to a group of early fishes, like the lancelet itself. From these the ganoids and then the lung-bearing mudfish must have been gradually developed. From such fish a very small advance would carry us on to the newts and other amphibians. The duck-billed platypus helps us slightly to bridge over the gap between the reptiles and the lower mammals, such as the kangaroo and the wombat, though the connection with the amphibians is still, as when Darwin wrote, highly problematical. From marsupials, such as the kangaroo, we ascend gradually to the insectivorous type represented by the shrews and hedgehogs, and thence once more by very well-marked intermediate stages to the lemurs of Madagascar, a group linked on the one hand to the insectivores, and on the other to the true monkeys. The monkeys, again, 'branched off into two great stems—the New World and Old World monkeys; and from the latter, at a remote period, man, the wonder and glory of the universe, proceeded.'
The word was spoken; the secret was out. The world might well have been excused for treating it scornfully. But as a matter of fact, the storm which followed the 'Descent of Man' was as nothing compared with the torrent of abuse that had pursued the author of the 'Origin of Species.' In twelve years society had grown slowly accustomed to the once startling idea, and it listened now with comparatively languid interest to the final utterance of the great biologist on the question of its own origin and destinies. In 1859 it cried in horror, 'How very shocking!' in 1871, it murmured complacently, 'Is that all? Why, everybody knew that much already!'
Nevertheless, on the moral and social side, the ultimate importance of the 'Descent of Man' upon the world's history can hardly be overrated by a philosophic investigator. Vast as was the revolution effected in biology by the 'Origin of Species,' it was as nothing compared with the still wider, deeper, and more subtly-working revolution inaugurated by the announcement of man's purely animal origin. The main discovery, strange to say, affected a single branch of thought alone; the minor corollary drawn from it to a single species has already affected, and is destined in the future still more profoundedly to affect, every possible sphere of human energy. Not only has it completely reversed our entire conception of history generally, by teaching us that man has slowly risen from a very low and humble beginning, but it has also revolutionised our whole ideas of our own position and our own destiny, it has permeated the sciences of language and of medicine, it has introduced new conceptions of ethics and of religion, and it threatens in the future to produce immense effects upon the theory and practice of education, of politics, and of economic and social science. These wide-reaching and deep-seated results began to be felt from the first moment when the Darwinian principle was definitely promulgated in the 'Origin of Species,' but their final development and general acceptance was immensely accelerated by Darwin's own authoritative statement in the 'Descent of Man.'
To some among us still, as to Lyell before us, this new belief in the animal origin of man seems far less beautiful, noble, and inspiriting than the older faith in his special and separate divine creation. Such thinkers find it somehow more pleasant and comfortable to suppose that man has fallen than that man has risen; the doctrine of the universal degradation of humanity paradoxically appears to them more full of promise and aspiration for the times to come than the doctrine of its universal elevation. To Darwin himself, however, it seemed otherwise. 'Man,' he says, 'may be excused for feeling some pride at having risen, though not through his own exertions, to the very summit of the organic scale; and the fact of his having thus risen, instead of having been aboriginally placed there, may give him hope for a still higher destiny in the distant future.' Surely this is the truer and manlier way of looking at the reversed and improved attitude of man. Surely it is better to climb to the top than to have been placed there—and fallen—at the very outset. Surely it is a nobler view of life that we may yet by our own strenuous exertions raise our race some places higher in the endless and limitless hierarchy of nature than that we are the miserable and hopelessly degenerate descendants of a ruined and degraded angelic progenitor. Surely it is well, while we boast with Glaucus that we indeed are far braver and better than our ancestors, to pray at the same time, in the words of Hector, that our sons may be yet braver and better than ourselves.