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Walking about in the garden of Nature, most men have the self-conceit to think that they know everything; yet almost all are blind to one of the outstanding principles that Nature employs in her work. This principle may be called the inner isolation which characterizes each and every living species on this earth.
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.
Deviations from this law take place only in exceptional circumstances. This happens especially under the compulsion of captivity, or when some other obstacle makes procreative intercourse impossible between individuals of the same species. But then Nature abhors such intercourse with all her might; and her protest is most clearly demonstrated by the fact that the hybrid is either sterile or the fecundity of its descendants is limited. In most cases hybrids and their progeny are denied the ordinary powers of resistance to disease or the natural means of defence against outer attack.
Such a dispensation of Nature is quite logical. Every crossing between two breeds which are not quite equal results in a product which holds an intermediate place between the levels of the two parents. This means that the offspring will indeed be superior to the parent which stands in the biologically lower order of being, but not so high as the higher parent. For this reason it must eventually succumb in any struggle against the higher species. Such mating contradicts the will of Nature towards the selective improvements of life in general. The favourable preliminary to this improvement is not to mate individuals of higher and lower orders of being but rather to allow the complete triumph of the higher order. The stronger must dominate and not mate with the weaker, which would signify the sacrifice of its own higher nature. Only the born weakling can look upon this principle as cruel, and if he does so it is merely because he is of a feebler nature and narrower mind; for if such a law did not direct the process of evolution then the higher development of organic life would not be conceivable at all.
This urge for the 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 or species. The fox remains always a fox, the goose remains a goose, and the tiger will retain the character of a tiger. The only difference that can exist within the species must be in the various degrees of structural strength and active power, in the intelligence, efficiency, endurance, etc., with which the individual specimens are endowed. It would be impossible to find a fox which has a kindly and protective disposition towards geese, just as no cat exists which has a friendly disposition towards mice.
That is why the struggle between the 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 the 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 its kind. 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 capacities for survival and for the procreation of their kind; and the final consequence would be
