class="pginternal" tag="{http://www.w3.org/1999/xhtml}a">7 In this direct line the manlike apes of Africa and the Indian Archipelago have advanced far beyond the mammalian world in general. Along with a cerebral surface, and an accompanying intelligence, far greater than that of other mammals, these tailless apes begin life as helpless babies, and are unable to walk, to feed themselves, or to grasp objects with precision until they are two or three months old. These apes have thus advanced a little way upon the peculiar road which our half-human forefathers began to travel as soon as psychical variations came to be of more use to the species than variations in bodily structure. The gulf by which the lowest known man is separated from the highest known ape consists in the great increase of his cerebral surface, with the accompanying intelligence, and in the very long duration of his infancy. These two things have gone hand in hand. The increase of cerebral surface, due to the working of natural selection in this direction alone, has entailed a vast increase in the amount of cerebral organization that must be left to be completed after birth, and thus has prolonged the period of infancy. And conversely the prolonging of the plastic period of infancy, entailing a vast increase in teachableness and versatility, has contributed to the further enlargement of the cerebral surface. The mutual reaction of these two groups of facts must have gone on for an enormous length of time since man began thus diverging from his simian brethren. It is not likely that less than a million years have elapsed since the first page of this new chapter in the history of creation was opened: it is probable that the time has been much longer. In comparison with such a period, the whole recorded duration of human history shrinks into nothingness. The pyramids of Egypt seem like things of yesterday when we think of the Cave-Men of western Europe in the glacial period, who scratched pictures of mammoths on pieces of reindeer-antler with a bit of pointed flint. Yet during an entire geologic æon before these Cave-Men appeared on the scene, “a being erect upon two legs,” if we may quote from Serjeant Buzfuz, “and wearing the outward semblance of a man and not of a monster,” wandered hither and thither over the face of the earth, setting his mark upon it as no other creature yet had done, leaving behind him innumerable tell-tale remnants of his fierce and squalid existence, yet too scantily endowed with wit to make any written disclosure of his thoughts and deeds. If the physiological annals of that long and weary time could now be unrolled before us, the principal fact which we should discern, dominating all other facts in interest and significance, would be that mutual reaction between increase of cerebral surface and lengthening of babyhood which I have here described.

Thus through the simple continuance and interaction of processes that began far back in the world of warm-blooded animals, we get at last a creature essentially different from all others. Through the complication of effects the heaping up of minute differences in degree has ended in bringing forth a difference in kind. In the human organism physical variation has well-nigh stopped, or is confined to insignificant features, save in the grey surface of the cerebrum. The work of cerebral organization is chiefly completed after birth, as we see by contrasting the smooth ape-like brain-surface of the new-born child with the deeply-furrowed and myriad-seamed surface of the adult civilized brain. The plastic period of adolescence, lengthened in civilized man until it has come to cover more than one third of his lifetime, is thus the guaranty of his boundless progressiveness. Inherited tendencies and aptitudes still form the foundations of character; but individual experience has come to count as an enormous factor in modifying the career of mankind from generation to generation. It is not too much to say that the difference between man and all other living creatures, in respect of teachableness, progressiveness, and individuality of character, surpasses all other differences of kind that are known to exist in the universe.

VII.

Change in the Direction of the Working of Natural Selection.

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In the fresh light which these considerations throw upon the problem of man’s origin, we can now see more clearly than ever how great a revolution was inaugurated when natural selection began to confine its operations to the surface of the cerebrum. Among the older incidents in the evolution of organic life, the changes were very wonderful which out of the pectoral fin of a fish developed the jointed fore-limb of the mammal with its five-toed paw, and thence through much slighter variation brought forth the human arm with its delicate and crafty hand. More wondrous still were the phases of change through which the rudimentary pigment-spot of the worm, by the development and differentiation of successive layers, gave place to the variously-constructed eyes of insects, mollusks, and vertebrates. The day for creative work of this sort has probably gone by, as the day for the evolution of annulose segments and vertebrate skeletons has gone by,—on our planet, at least. In the line of our own development, all work of this kind stopped long ago, to be replaced by different methods. As an optical instrument, the eye had well-nigh reached extreme perfection in many a bird and mammal ages before man’s beginnings; and the essential features of the human hand existed already in the hands of Miocene apes. But different methods came in when human intelligence appeared upon the scene. Mr. Spencer has somewhere reminded us that the crowbar is but an extra lever added to the levers of which the arm is already composed, and the telescope but adds a new set of lenses to those which already exist in the eye. This beautiful illustration goes to the kernel of the change that was wrought when natural selection began to confine itself to the psychical modification of our ancestors. In a very deep sense all human science is but the increment of the power of the eye, and all human art is the increment of the power of the hand.8 Vision and manipulation,—these, in their countless indirect and transfigured forms, are the two coöperating factors in all intellectual progress. It is not merely that with the telescope we see extinct volcanoes on the moon, or resolve spots of nebulous cloud into clusters of blazing suns; it is that in every scientific theory we frame by indirect methods visual images of things not present to sense. With our mind’s eye we see atmospheric convulsions on the surfaces of distant worlds, watch the giant ichthyosaurs splashing in Jurassic oceans, follow the varied figures of the rhythmic dance of molecules as chemical elements unite and separate, or examine, with the aid of long-forgotten vocabularies now magically restored, the manners and morals, the laws and superstitions, of peoples that have ceased to be.9 And so