compulsion. In the other, the men who practised the old usage disappear and are replaced by others who do not practise it; these may be strangers, or they may be the descendants of the first set of men, but educated in a different manner. This renewing of the generations seems, in our day, to be the most active cause of evolution. It is natural to suppose that the same holds good of the past; evolution has been slower, the more exclusively each generation has been formed by the imitation of its forerunners.
There is still one more question to ask. Are men all alike, differing merely in the conditions under which they live (education, resources, government), and is evolution produced solely by changes in these conditions? Or are there groups of men with hereditary differences, born with tendencies to different activities and with aptitudes leading to different evolutions, so that evolution may be the product, in part at least, of the increase, the diminution, and the displacement of these groups? Taking the extreme cases, the white, black, and yellow races of mankind, the differences in aptitude are obvious; no black people has ever developed a civilisation. It is thus probable that smaller hereditary differences may have had their share in the determination of events. If so, historical evolution would be partly produced by physiological and anthropological causes. But history provides us with no sure means of determining the action of these hereditary differences between men; it goes no further than the conditions of their existence. The last question of history remains insoluble by historical methods.
CHAPTER V
EXPOSITION
We have still to study a question whose practical interest is obvious: What are the forms in which