HYBRIDS

—One of the chief causes of the separation of species of animals and plants has been the fact that members of a species are fertile, amongst themselves, but are not fertile with members of another species. This rule is not universal, however, for occasionally species are found which interbreed with Other species, the progeny of such a union being known as hybrids, and the phenomencn as hydridism. But it must © remembered that where hybridism occurs the species concerned are closely related, for while a Hare and a Rabbit may interbreed occasionally, it is absurd to think of either breeding with a Porcupine, although distantly related in the same natural order of Mammals. Another general fact is that hybrids are seldom fertile amongst themselves, although there is no apparent reason why this should be so. Mules—the offspring of horses and a es—are infertile with each other, but sometimes produce young when mated again with the pure parental forms. In the animal world hybrids seem to occur in nature not infrequently amongst deer, game-birds, ducks, and fishes, eg. Roach mating with Bream or Rudd, and Carp with Tench or Bream. And artificial hybrids among the and many others, such as the species of such lowly creatures as Sea Urchins, are not uncommon. In the ree table world the artificial cro ing or hybridising of species of plants has led to many new forms. It is unfortunate, for clearne ’ sake, that the word ‘hybrid’ is used for the progeny not only of two species, but also of two varieties of one species, when the word ‘mongrel’ would have indicated the latter cla more distinctly. Varietal mongrels or hybrids are sO much under man’s control and have given rise to so many useful breeds, that they are of the utmost importance.