But in the non-human animal world all these phenomena are but slightly developed.  They are but the dim adumbrations of what was by and by to bloom forth in the human race.  They can scarcely be said to have served as a prophecy of the revolution that was to come.  One generation of dumb beasts is after all very like another, and from studying the careers of the mastodon, the hipparion, the sabre-toothed lion, or even the dryopithecus, an observer in the Miocene age could never have foreseen the possibility of a creature endowed with such a boundless capacity of progress as the modern Man.  Nevertheless, however dimly suggestive was this group of phenomena, it contained the germ of all that is preeminent in humanity.  In the direct line of our ancestry it only needed that the period of infancy should be sufficiently prolonged, in order that a creature should at length appear, endowed with the teachableness, the individuality, and the capacity for progress which are the peculiar prerogatives of fully-developed Man.[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