The influences of climate, of food, of temperature, of domesticity upon
the variation of species is well known. These mediate and external causes
act with more vigor when the immediate and internal causes favor the
effect. "All the mechanism of the formation of varieties," says Flourens,
"turns upon these two internal causes--the tendency of the species to
vary, and the transmission of the acquired variations." Cultivated plants
and domesticated animals, when deprived of the modifying influence of man,
return to the state of nature, and undergo new modifications, alterations,
degenerations, even so far as to disguise and conceal the primitive type.
A few generations suffice to restore a variety to the primitive stock
without retaining any of the organic elements which would debase it.
The more the influence of civilized man makes itself felt, the more the
superior species overpower, absorb, or modify the inferior species.
The more rude the people and the less polished their societies, the more
powerful and rapid will be the influences of climate. Civilized men are
continually exercising their talents to conform their conditions to the
necessities of the time and place, and by their ingenuity remedy the
defects, and by the resisting powers of a cultivated and occupied mind
resist many of the morbid influences of climate. But plants and animals
succumb at once if not protected by man.