CLASSES OF GEOGRAPHIC INFLUENCES Into almost every anthropo-geographical problem the element of environment enters in different phases, with different modes of operation and varying degrees of importance.

Since the causal conception of geography demands a detailed analysis of all the relations between environment and human development, it is advisable to distinguish the various classes of geographic influences.
[Sidenote: Physical effects.] 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.