The adaptation of habits of thought is the growth of institutions.
But along with the growth of institutions has gone a change of a more substantial character.

Not only have the habits of men changed with the changing exigencies of the situation, but these changing exigencies have also brought about a correlative change in human nature.

The human material of society itself varies with the changing conditions of life.
This variation of human nature is held by the later ethnologists to be a process of selection between several relatively stable and persistent ethnic types or ethnic elements.

Men tend to revert or to breed true, more or less closely, to one or another of certain types of human nature that have in their main features been fixed in approximate conformity to a situation in the past which differed from the situation of today.
There are several of these relatively stable ethnic types of mankind comprised in the populations of the Western culture.

These ethnic types survive in the race inheritance today, not as rigid and invariable moulds, each of a single precise and specific pattern, but in the form of a greater or smaller number of variants.