564 THE AMERICAN JOURNAL OF SOCIOLOGY
side for a long time without knowing each other, and thanks to the continued isolation, varieties of the same race are thus formed in spite of their common origin.
Emigration, with its correlative aspect, invasion, followed by a new isolation, favors therefore the formation of new species or races. This is a law of static balancing, and it is the same among all organic beings wherever there exists differentiation of sexes. On the contrary, these variations are always counter- acted in their tendency through the casual or even habitual sexual mixture of posterity with the primitive type. These physiological laws have been recognized for a long time, at least empirically, by positive legislations and by the customs of the most opposite societies. Crossings within the same groups rarely cause variations from the original type, and, furthermore, if the existing varieties emigrate and become sufficiently separated from the mother-country, either by distance or other natural barriers, seas, mountains, etc., time aiding, which is itself also a great natural barrier, the separated groups continuing to differentiate, and these differences becoming more and more fixed, they cannot, by crossing, return to the former type. The crossing in this case produces only mixed types, that is to say, new varieties capable of new extensions.
The history of societies shows us that the individuals and groups that emigrate are precisely those whose interests, charac- teristics, and opinions, or even vague tendencies, are no longer in harmony with those of the majority. They are therefore those who are the better prepared to organize new societies. Such were the Phoenician and Greek colonies, the English colonies in the United States formed by the Puritans and dissenters from the mother-country, the Australian formed of the convicts, etc., etc.
Crossing, favored by the breaking down of the natural or social barriers, is itself the general and principal factor in the unification of races, as heredity is the chief agent in the forma- tion and conservation of variations. As to the external environ- ment, comprehending all of the external conditions of life, it is the most powerful factor in the differentiations. Its instrument is selection. Heredity is the quality in living beings of repeating
