Careful analysis supersedes it by a whole group of geographic factors working directly and indirectly.

The first of these was the dividing ocean which, prior to the introduction of cheap ocean transportation and bustling steerage agents, made a basis of artificial selection.

Then it was the man of abundant energy who, cramped by the narrow environment of a Norwegian farm or Irish bog, came over to America to take up a quarter-section of prairie land or rise to the eminence of Boston police sergeant.

The Scotch immigrants in America who fought in the Civil War were nearly two inches taller than the average in the home country.[24] But the ocean barrier culled superior qualities of mind and character also--independence of political and religious conviction, and the courage of those convictions, whether found in royalist or Puritan, Huguenot or English Catholic.
[Sidenote: Indirect effect through isolation.] Such colonists in a remote country were necessarily few and could not be readily reinforced from home.

Their new and isolated geographical environment favored variation.