Full passage- Beginning of Mein Kampf Volume 1 - Chapter 11 "People and Race": There are certain truths which are so completely obvious that, for the very reason that they are so common, the average person does not see them or at least does not recognize them. A man may often pass blindly by such obvious truths and be utterly astonished when someone suddenly discovers a fact that everybody should have already known. Columbus’ eggs are lying around by the hundreds of thousands, but a Columbus is rare indeed. (The egg of Columbus refers to an idea or discovery that appears obvious after it is made; the reference is based on a common story about Christopher Columbus.) All men, without exception, stroll about the garden of Nature, arrogantly assuming they know and are familiar with all of it. Yet, with few exceptions, they pass blindly over one of the most striking principles of Nature’s rule: the inner separation of the various species of all earth’s living creatures. Even the most casual observer can see a firm, basic law that rules all the countless creatures in which Nature expresses her will to live. All of the different forms of animals keep within specific boundaries, within their own species, when propagating or multiplying their kind. Every animal only mates with another of the same species. The titmouse pairs with the titmouse, the finch with the finch, the stork with the stork, the field-mouse with the fieldmouse, the house-mouse with the house-mouse, the wolf with the she wolf, etc. Only extraordinary circumstances can alter this boundary rule and then it only happens under the pressure of captivity or for some other reason that makes mating within the same species impossible. But, even then, Nature begins to resist with all her might. Her clearest protest is demonstrated in the infertility of the bastard creature or in restricting the fertility of later descendants. In most cases, she deprives them of the ability to resist disease or deprives these unfortunates the means to defend against attacks from enemies. This is natural. Every breeding between two creatures that are not from the same level produces a result mid-way between the levels of the two parents. The offspring will be on a higher level than the racially lower one of its parents, but not as high as the higher one. Consequently, in battle, it will eventually yield to the higher species because of its deficiencies. That sort of mating runs counter to Nature’s will to breed life upwards. Nature’s will is accomplished by complete victory of the higher species, not by uniting superiority and inferiority. The stronger must rule. It must not unite with the weaker, thus sacrificing its own higher nature. Only those who are born as the weaker being can think this cruel and that is why he is a weak and defective man. If this Law did not hold, the evolution of life would be unthinkable. The consequence of this instinct for race purity, which is found universally throughout Nature, is not only the sharp outward difference between the separate races, but their uniform nature within themselves of their character that is unique to each species. The fox is always a fox, the goose a goose, the tiger a tiger, etc. The only possible difference is in varying degrees of health, strength, understanding, cleverness, and endurance among individual specimens. We will never find a fox which naturally has moments of kindness toward geese, just as there is no cat with a friendly affection for mice. The battle is born out of hunger and love, not because of any native hostility. In both cases, Nature watches with calm satisfaction. The struggle for daily bread conquers the weak, sickly, and undecided, while the males who win the contest for the female win the right or at least the opportunity of reproduction for the healthiest individuals. Struggle is the means that improves the health, strength, and stamina of the species and the reason for its evolution to a higher quality of being.