Man is distinguished from the animals above all things by his power of learning. Whereas the behaviour of animals, even of the higher ones, consists almost entirely of purely instinctive actions, innate modes of response to a limited number of situations; man has an indefinitely great capacity for acquiring new modes of response, and so of adapting himself in new and more complex ways to an almost indefinite variety of situations. And his new mental acquisitions are not made only by the slow process of adaptation in the light of his own individual experience of the consequences of behaviour of this and that kind; as are most of the few acquisitions of the animals. By far the greater part of the mental stock-in-trade by which his behaviour is guided is acquired from his fellow men; it represents the accumulated experience of all the foregoing generations of his race and nation. Man’s life in society, together with the great plasticity of his mind, its great capacity for new adaptations, secures him this enormous advantage; the two things are necessarily correlated. Without the plasticity of mind, his life in society would benefit him relatively little. Many animals that lead a social life in large herds or flocks are not superior, but rather inferior, in mental power to animals that lead a more solitary life; and indeed this seems to be generally true, as we see on comparing generally the herbivorous gregarious animals with the solitary carnivores that prey upon them. The social life of such animals, rendering individual intelligence less necessary for protection and escape from danger, tends actually against mental development. On the other hand, man’s great plastic brain would be of comparatively little use to him if he lived a solitary unsocial life. His great brain is there to enable him to assimilate and make use of the accumulated experience, the sum of knowledge and morality, which is traditional in the society into which he is born a member; that is to say, the development of social life, which depended so much upon language and for the forwarding of which language came into existence, must have gone hand in hand with the development of the great brain, which enables full advantage to be secured from social co-operation and which, especially, renders possible the accumulation of knowledge, belief, and traditional sentiment.