Four fundamental classes of effects can be distinguished.

1. The first class includes direct physical effects of environment,
similar to those exerted on plants and animals by their habitat. Certain
geographic conditions, more conspicuously those of climate, apply
certain stimuli to which man, like the lower animals, responds by an
adaption of his organism to his environment. Many physiological
peculiarities of man are due to physical effects of environment, which
doubtless operated very strongly in the earliest stages of human
development, and in those shadowy ages contributed to the
differentiation of races. 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