The unity of the human species is as clearly established as the diversity of races and peoples, whose divergences must be interpreted chiefly as modifications in response to various habitats in long periods of time.
[Sidenote: Variation and natural conditions.] Such modifications have probably been numerous in the persistent and unending movements, shiftings, and migrations which have made up the long prehistoric history of man.

If the origin of species is found in variability and inheritance, variation is undoubtedly influenced by a change of natural conditions.

To quote Darwin, "In one sense the conditions of life may be said, not only to cause variability, either directly or indirectly, but likewise to include natural selection, for the conditions determine whether this or that variety shall survive."[34] The variability of man does not mean that every external influence leaves its mark upon him, but that man as an organism, by the preservation of beneficent variations and the elimination of deleterious ones, is gradually adapted to his environment, so that he can utilize most completely that which it contributes to his needs.

This self-maintenance under outward influences is an essential part of the conception of life which Herbert Spencer defines as the correspondence between internal conditions and external circumstances, or August Comte as the harmony between the living being and the surrounding medium or _milieu_.
According to Virchow, the distinction of races rests upon hereditary variations, but heredity itself cannot become active till the characteristic or _Zustand_ is produced which is to be handed down.[35] But environment determines what variation shall become stable enough to be passed on by heredity.

For instance, we can hardly err in attributing the great lung capacity, massive chests, and abnormally large torsos of the Quichua and Aymara Indians inhabiting the high Andean plateaus to the rarified air found at an altitude of 10,000 or 15,000 feet above sea level.