Now this traditional stock of knowledge and morality has been very slowly accumulated, bit by bit; and every bit, every least new addition to it, has been a difficult acquisition, due in the first instance to some spontaneous variation of some individual’s mental structure from the ancestral type of mental structure. That is to say, throughout the evolution of civilisation, progress of every kind, increase of knowledge or improvement of morality, has been due to the birth of more or less exceptional individuals, individuals varying ever so slightly from the ancestral type and capable, owing to this variation, of making some new and original adaptation of action, or of perceiving some previously undiscovered relation between things. These new acquisitions, first made by individuals, are, if true or useful, sooner or later imitated or accepted by the society of which the original-minded individual is a member, and then, becoming incorporated in the traditional stock of knowledge and morality, are thereby placed at the service of all members of that society. Thus favourable spontaneous variations do not, as with the animals, render possible mental evolution merely by conducing to the survival of, and the perpetuation of the qualities of, those individuals in whom the variations occur. They may do this, or they may not; but, in addition and more importantly, they contribute to the stock of traditional knowledge and morality, and so raise the social group as a whole in the scale of civilisation; they render it more capable of successfully contending against other groups and against the adverse influence of the physical environment; and they promote the solidarity of the group by adding to its stock of common tradition; thus the acquisitions of each member benefit the group as a whole and all its members, quite apart from any philanthropic purpose or intention of producing such a result. The achievement of this unconscious undesigned solidarity of human societies is one of two great steps in the evolution of the human race by which the process is rendered very different from, and is raised to a higher plane than, the mental evolution of the animal world. The second and still more important step is one which is only just beginning to be achieved in the present age; I shall have to touch on it in a later chapter. The original or primary divergence of mental type between any two peoples must, then, have been due to these fundamental causes—namely, differences of physical environment and spontaneous variations of mental structure, the latter adding to the traditional stock of knowledge and belief, of moral precepts and sentiments.