The change from a fruit-eating to a hunting life subserved the great utility of opening fresh supplies of food; and, possibly, a failure of the normal supply of the old customary food was the direct cause of the new habit. If our ape lived near the northern limits of the tropical forest, and a fall of temperature there took place, such as to reduce (especially in winter) the yield of fruit and other nutritious vegetation on which he had subsisted, famine may have driven him to attack other animals;[1] whilst more southerly anthropoids, not suffering from the change of climate, continued in their ancient manner of life. A large anthropoid (_Dryopithecus_) inhabited Central Europe in the Miocene, for his bones have been found; there may have been others; and during that period the climate altered from sub-tropical to temperate, with corresponding changes in fauna and flora. Hence it formerly occurred to me that perhaps the decisive change in the life of our Family happened there and then. It seems, however, that good judges put the probable date of the great differentiation much earlier, in the Oligocene;[2] and since I cannot find that any extensive alteration of climate is known to have happened during that period, it seems necessary to fall back upon “spontaneous” variation (as one must in many other cases); that is to say, from causes which are at present beyond our vision, the fateful ape did, in fact, prefer animal food so decidedly as to begin a-hunting for it. That being granted, the rest of the history was inevitable. The new pursuit was of a nature to engross the animal’s whole attention and co-ordinate all his faculties; and to maintain and reinforce it, his structure in body and mind may reasonably be supposed to have undergone rapid modification by natural selection; because those individuals that were in any organ or faculty best adapted to the new life had an advantage, which was inherited and gradually intensified.[3]
Let me run rapidly through the chief differences between Man and his nearest congeners: some of them are obvious and can be stated very briefly; others I shall return to in the next chapter. We shall see that they all follow naturally from the above hypothesis.