The Descent of Man Volume 1 Chapter VII - On the Races of Man Part 3

In which we observe that many aboriginal races experience a marked decline in fertility and health upon modification of their native habits and environments, much as certain wild animals succumb to sterility when removed from natural conditions. It is further discussed how variations in skin colour among human races resist simple climatic explanations, suggesting more complex hereditary and constitutional influences akin to those observed in other species.