Mein Kampf

 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 characteristics qualities which are peculiar to each breed and species . That is why the struggle between various species does not arise from 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 disased or wavering, while the flight of the male to process 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.