They sometimes pass by such truisms as though blind and are most astonished when someone suddenly discovers what everyone really ought to know. Columbus's eggs lie around by the hundreds of thousands, but Columbuses are met with less frequently. Even the most superficial observation shows that Nature's restricted form of propagation and increase is an almost rigid basic law of all the innumerable forms of expression of her vital urge.
Every animal mates only with a member of the same species.
The titmouse seeks the titmouse, the finch the finch, the stork the stork, the field mouse the field mouse, thedormouse the dormouse, the wolf the she-wolf, etc. Only unusual circumstances can change this, primarily the compulsion of captivity or any other cause that makes it impossible to mate within the same species. This is only too natural. Any crossing of two beings not at exactly the same level produces a medium between the level of the two parents. Consequently, it will later succumb in the struggle against the higher level.