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. Whether these have been acquired by centuries of extreme lung
expansion, or represent the survival of a chance variation of undoubted
advantage, they are a product of the environment. They are a serious
handicap when the Aymara Indian descends to the plains, where he either
dies off or leaves descendants with diminishing chests.[36] [See map page
101.]

[Sidenote: Stature and environment]

Darwin holds that many slight changes in animals and plants, such as
size, color, thickness of skin and hair, have been produced through food
supply and climate from the external conditions under which the forms
lived.[37] Paul Ehrenreich, while regarding the chief race distinctions
as permanent forms, not to be explained by external conditions,
nevertheless concedes the slight and slow variation of the sub-race
under changing conditions of food and climate as beyond doubt.[38]