The modification of human development by environment is a natural process; like all other natural processes, it involves the cumulative effects of causes operating imperceptibly but persistently through vast periods of time.
Slowly and deliberately does geography engrave the subtitles to a people's history.

Neglect of this time element in the consideration of geographic influences accounts equally for many an exaggerated assertion and denial of their power.

A critic undertakes to disprove modification through physical environment by showing that it has not produced tangible results in the last fifty or five hundred years.

This attitude recalls the early geologists, whose imaginations could not conceive the vast ages necessary in a scientific explanation of geologic phenomena.
The theory of evolution has taught us in science to think in larger terms of time, so that we no longer raise the question whether European colonists in Africa can turn into negroes, though we do find the recent amazing statement that the Yankee, in his tall, gaunt figure, "the colour of his skin, and the formation of his hair, has begun to differentiate himself from his European kinsman and approach the type of the aboriginal Indians."[28] Evolution tells the story of modification by a succession of infinitesimal changes, and emphasizes the permanence of a modification once produced long after the causes for it cease to act.

The mesas of Arizona, the earth sculpture of the Grand Canyon remain as monuments to the erosive forces which produced them.