Even a superficial glance is sufficient to show that all the innumerable forms in which the life-urge of Nature manifests itself are subject to a fundamental law--one may call it an iron law of Nature--which compels the various species to keep within the definite limits of their own life-forms when propagating and multiplying their kind. Each animal mates only with one of its own species. The titmouse cohabits only with the titmouse, the finch with the finch, the stork with the stork, the field-mouse with the field-mouse, the house-mouse with the house-mouse, the wolf with the she-wolf, etc.