Even if present in equal numbers one of the two contrasted types will have some small advantage or capacity which the other lacks toward a perfect adjustment to surroundings. Those possessing these favorable variations will flourish at the expense of their rivals and their offspring will not only be more numerous, but will also tend to inherit such variations. In this way one type gradually breeds the other out. In this sense, and in this sense only, do races change.
Man continuously undergoes selection through the operation of the forces of social environment.
Among native Americans of the Colonial period a large family was an asset and social pressure and economic advantage counselled both early marriage and numerous children. Two hundred years of continuous political expansion and material prosperity changed these conditions and children, instead of being an asset to till the fields and guard the cattle, became an expensive liability. They now require support, education and endowment from their parents and a large family is regarded by some as a serious handicap in the social struggle. These conditions do not obtain at first among immigrants and large families among the newly arrived population are still the rule, precisely as they were in Colonial America and are today in French Canada where backwoods conditions still prevail. The result is that one class or type in a population expands more rapidly than another and ultimately replaces it.