as seen in the mule. This hybrid-vigor is called, also, heterosis.
The second result of hybridization is increased variability, especially in the second generation descended from the hybrids. New combinations of traits arise that were carried genotypically in the parental forms. Among these new combinations may well be some, little suited to pass the censor of natural selection; others may be superior to the combinations previously existing, and thus give the hybrids an advantage in the struggle for existence.
Of the genotypical mutations that occur many are not compatible with development. They are, as it were, still-born. Those that bring the phenotype to full development, must still pass the censor of environment. For example, I once reared 300 hybrid chickens, about 40% white, 40% black, and 20% of mixed colors. All were healthy and vigorous. One morning a flock of crows descended on my chickens as they were scattered over a lot of feeding. Twenty four were killed, all but one self white or self black birds- the other one coarsely gray and buff. Consequently I bred relatively few self-colored descendants from that flock. White, and uniform black because of their conspicuousness, could not pass the censor of environment. Chiefly the pencil-marked chickens could survive in an environment of predaceous birds.
Now, of human adaptations many are adaptive. Thus the black skin of the tropical negro protects him from sunburn of the skin and even of the viscera, without uses of clothing. The fat deposits of the Eskimo form a useful blanket of protection against severe cold. The long legs of the Australian aborigines serve them well in tracking the kangaroo. The broad pelves[sic] of the European women, who give birth to large and big-headed babies, are an obvious physiological adjustment.