The rest is a question of pre-historic geography.

Within the tropics, the habitat of the man-like apes, and presumably of the earliest men, a black skin protects against sunlight.

A white skin, on the other hand--though this is more doubtful--perhaps economizes sun-heat in colder latitudes.

Brown, yellow and the so-called red are intermediate tints suitable to intermediate regions.

It is not hard to plot out in the pre-historic map of the world geographical provinces, or "areas of characterization," where races of different shades corresponding to differences in the climate might develop, in an isolation more or less complete, such as must tend to reinforce the process of differentiation.
Let it not be forgotten, however, that individual plasticity plays its part too in the determination of human colour.