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.