"This urge for maintenance of the unmixed breed, which is a phenomenon that prevails throughout the whole of the natural world, results not only in the sharply defined outward distinction between one species and another but also in the internal similarity of characteristic qualities which are peculiar to each breed and species. [...] That is why the struggle between various species does not arise from a feeling of mutual antipathy but rather from hunger and love. In both cases, nature looks on calmly and is even pleased with what happens. The struggle for daily livelihood leaves behind in the ruck everything that is weak or diseased or wavering; while the fight of the male to possess the female gives to the strongest, the right, or at least the possibility, to propagate. And this struggle is a means
of furthering the health and powers of resistance in the species. Thus it is one of the causes underlying the process of development towards a higher quality of being. If the case were different, the progressive process would cease, and even retrogression might set in. Since the inferior always outnumber the superior, the former would always increase more rapidly if they possessed the same capacity for survival and for procreation of their kind; and the final consequence would be that the best in quality would be forced to recede in the background. Therefore a corrective measure in favour of better quality must intervene. Nature supplies this by establishing rigorous conditions of life to which the weaker will have to submit and will thereby be numerically restricted...
— Adolf Hitler"