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In what I just said I left an “almost.” There is a great deal of saving virtue in that little adverb. Doubtless even animals low in the scale possess some faint traces of educability; but they are so very slight that it takes geologic ages to produce an appreciable result. In all the innumerable wanderings, fights, upturnings and cataclysms of the earth’s stupendous career, each creature has been summoned under penalty of death to use what little wit he may have had, and the slightest trace of mental flexibility is of such priceless value in the struggle for existence that natural selection must always have seized upon it, and sedulously hoarded and transmitted it for coming generations to strengthen and increase. With the lapse of geologic time the upper grades of animal intelligence have doubtless been raised higher and higher through natural selection. The warm-blooded mammals and birds of to-day no doubt surpass the cold-blooded dinosaurs of the Jurassic age in mental qualities as they surpass them in physical structure. From the codfish and turtle of ancient family to the modern lion, dog, and monkey, it is a very long step upward. The mental life of a warm-blooded animal is a very different affair from that of reptiles and fishes. A squirrel or a bear does a good many things in the course of his life. He meets various vicissitudes in various ways; he has adventures. The actions he performs are so complex and so numerous that they are severally performed with less frequency than the few actions performed by the codfish. The requisite nervous connections are accordingly not fully established before birth. There is not time enough. The nervous connections needed for the visceral movements and for the few simple instinctive actions get organized, and then the creature is born before he has learned how to do all the things his parents could do. A good many of his nervous connections are not yet formed, they are only formable. Accordingly he is not quite able to take care of himself; he must for a time be watched and nursed. All mammals and most birds have thus a period of babyhood that is not very long, but is on the whole longest with the most intelligent creatures. It is especially long with the higher monkeys, and among the man-like apes it becomes so long as to be strikingly suggestive. An infant orang-outang, captured by Mr. Wallace, was still a helpless baby at the age of three months, unable to feed itself, to walk without aid, or to grasp objects with precision.

But this period of helplessness has to be viewed under another aspect. It is a period of plasticity. The creature’s career is no longer exclusively determined by heredity. There is a period after birth when its character can be slightly modified by what happens to it after birth, that is, by its experience as an individual. It becomes educable. It is no longer necessary for each generation to be exactly like that which has preceded. A door is opened through which the capacity for progress can enter. Horses and dogs, bears and elephants, parrots and monkeys, are all teachable to some extent, and we have even heard of a learned pig. Of learned asses there has been no lack in the world.

But this educability of the higher mammals and birds is after all quite limited. By the beginnings of infancy the door for progressiveness was set ajar, but it was not all at once thrown wide open. Conservatism stilt continued in fashion. One generation of cattle is much like another. It would be easy for foxes to learn to climb frees, and many a fox might have saved his life by doing so; yet quickwitted as he is, this obvious device never seems to have occurred to Reynard. Among slightly teachable mammals, however, there is one group more teachable than the rest. Monkeys, with their greater power of handling things, have also more inquisitiveness and more capacity for sustained attention than any other mammals; and the higher apes are fertile in varied resources. The orang-outang and gorilla are for this reason dreaded by other animals, and roam the undisputed lords of their native forests. They have probably approached the critical point where variations in intelligence, always important, have come to be supremely important, so as to be seized by natural selection in preference to variations in physical constitution. At some remote epoch of the past–we cannot say just when or how–our half-human forefathers reached and passed this critical point, and forthwith their varied struggles began age after age to result in the preservation of bigger and better brains, while the rest of their bodies changed but little. This particular work of natural selection must have gone on for an enormous length of time, and as its result we see that while man remains anatomically much like an ape, be has acquired a vastly greater brain with all that this implies. Zoologically the distance is small between man and the chimpanzee; psychologically it has become so great as to be immeasurable.