We must recognise not only the selection, through survival in the struggle for existence, of new mental qualities arising as spontaneous variations of individual mental structure. This, which is the only, or almost the only, process at work in the mental evolution of animals, is immensely complicated and overshadowed in importance by two processes. The first is the accumulation of knowledge and morality in traditional forms. The traditional accumulation, which so far outweighs the mental equipment possible to any individual isolated from an old society, not only constitutes in itself a most important evolutionary product, but it modifies profoundly the conditions of evolution of the individual innate qualities of mind; for example, the greater and more valuable the stock of traditional knowledge and morality becomes, the more does fitness to survive consist in the capacity to assimilate this knowledge and to conform to these higher moral precepts, the less does it consist in the purely individualistic qualities, such as quickness of eye and ear, fleetness of foot, or strength and skill of hand. Secondly, the processes of natural selection are complicated by the social evolution, which tends progressively to abolish the struggle for existence between individuals, and to replace it by a struggle between groups; in which struggle success is determined not only by the qualities of individuals, but also very largely by the social organisation and by the traditional knowledge and morality of the groups.
Each variety of the human species, each race considered as a succession of individuals having certain innate mental qualities, has been evolved, then, not merely under the influence of the physical environment, like the animal species, but also and to an ever increasing extent under the influence of the social environment. The social environment we regard as consisting of two parts; namely, the social organisation and the body of social tradition; for these, though interdependent and constantly interacting, may yet with advantage be kept apart in thought. We must, then, bear constantly in mind the fact that man creates for himself an environment which becomes ever more complex and influential, overshadowing more and more in importance the physical environment.