Great Men, Great Thoughts, and the Environment

What are the causes that make communities change from generation to generation? ... The difference is due to the accumulated influences of individuals, of their examples, their initiatives, and their decisions... The causes of production of great men lie in a sphere wholly inaccessible to the social philosopher. He must simply accept geniuses as data, just as Darwin accepts his spontaneous variations.
For him, as for Darwin, the only problem is, these date being given, How does the environment affect them, and how do they affect the environment? Now, I affirm that the relation of the visible environment to the great man is in the main exactly what it is to the "variation" in the Darwinian philosophy. It chiefly adopts or rejects, preserves or destroys, in short selects him. And whenever it adopts and preserves the great man, it becomes modified by his influence in an entirely original and peculiar way.