But let us set aside the words _species_ and _genera_, which, in nature, have not the same value as we assign to them in our museums and in our books, and let us, instead, take from the world of the living a handful of animals, haphazard, so that we may gather together brothers, cousins, nephews, individuals of the same or affinitive classes, genera, orders, and let us place them in line, in the order of their degrees of similarity. Should we try to couple them, or study their spontaneous loves, we would find cases of sterility in beings too similar and in beings too dissimilar; therefore, generation moves between these two opposite poles, too great similarity and too great dissimilarity. That is the reason why we may see a woman with a mustache, atrophied breasts and deep voice remain sterile with a robust man: they do not generate because they have too close a resemblance. That is the reason why a dog and a cat are sterile: they do not generate, because they are too dissimilar. Nature said to living beings: "If you wish to love, be neither too similar nor too dissimilar." Let us try and discover the reason of this law. Germs that are too similar cannot concur in fecundation, or fecundate unsatisfactorily, perhaps through the same laws of elementary physics and chemistry which cause bodies to repel other bodies equally electrified or with which they have too close a resemblance in their physico-chemical characteristics. Try the combination of sulphur with phosphorus, of iodine with bromine, and, on the other hand, observe the ardent loves of chlorine and hydrogen, of potassium and oxygen. The fecundity of two different organisms is, besides, an energy bearing in one direction; it is the sum of resistances all of them equal, while two quantities, different but susceptible of being summed, give a greater number of diverse resistances and have, therefore, a greater possibility of living and resisting external enemies. An individual is the sum of many victories over exterior elements, the result of many and infinite adaptations to the ambient which surrounds it. Two individuals dissimilar, but not enough to impede generation, will bring together those adaptations and those victories through which the new creature enjoys the possibility of resistance and will meet with fewer dangers.