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.
Adolf Hitler (2016). “Mein Kampf: My Struggle: (Vol. I & Vol. II) - (Complete & Illustrated Edition)”, p.282, eKitap Projesi