However the intermixture of colliding groups kept the gene pool a world-wide reservoie. Nevertheless different environments have left their mark on his physique. Tropical peoples tend to be ectomorphic-slenderly constructed, the better to radiate body heat. There are differences in skin color, a function of exposure to solar radiation. Northern peoples have thin nostrils to warm and moisten cold air before it reaches fragile lung tissues.
Tropical man has broad, open nostrils for the opposite reason.
Peoples of northeast Asis, cut off by Himalayan mountains during the last glaciation acquired eyes deeply buried in their sockets and protected by fatty folds, noses flattened into their faces and a diminution of facial hair, as defenses against frost bite. Baltic fog and reduced exposure to sunlight produced the Nordics. The examples are many. In his arrogance he has believed that he was independent of natural controls, that he could create his own environment. He should have been warned by his own written history, by the fate of the great irrigation civilizations which were brought low by salting of the soil, by the desertification of lands once flowing with milk and honey because of pasturing of goats and sheep, by the soil erosion and sedimentation and malarial swamp created by deforestation especially of hilly areas.