We have already seen that in the endeavour to _perfect_ a vegetable or animal race, the physiological equilibrium has sometimes been destroyed at the expense of the reproductive power. In such cases, crossing with another race which is less modified, generally revives the extinguished fertility. For example, the English pigs imported into the middle of France by M. de Ginestous became sterile after several generations. Upon crossing them with a leaner and less perfect local race, their fertility returned. All these facts, and their inevitable consequences, have been admitted by every naturalist who has studied the question. Even Darwin has recognised the truth of them in his valuable work upon the _Variation of Animals and Plants_. At that time he confined himself to the conclusion that the crosses between some races of plants are less fertile than between others, a proposition which no one would think of denying. He has gone further in the latest editions of his work upon the Origin of Species. Without bringing forward clear facts, the meaning of which would go further than the wise conclusions he had previously admitted, he invokes our relative ignorance of what takes place among wild _varieties_, and concludes that we must admit that the crosses between varieties must always be perfectly fertile. This is one of those appeals to the unknown, one of those arguments where even our ignorance is invoked as a proof, which we too often meet with in Darwin, who is often carried away by his convictions. I shall have to return to this point, but I here make the statement as an established fact, on the authority even of Darwin, that all _known_ facts attest the _perfect fertility_ of mongrels. Finally, the formation of _crosses between races_, or the production of _mongrels_, is spontaneous, and may be promoted by man without the least difficulty; the results are as certain as those with the union of individuals of the same race; in certain cases, indeed, fertility is increased or revived under the influence of this crossing. Crosses _between species, or hybrids,_ will exhibit facts of an entirely contrary nature. III. The formation of hybrids, as of mongrels, may be either natural or artificial. The former is so rare that eminent naturalists have doubted its reality. There are, however, according to M. Decaisne, a score of well proved examples among plants. What is this number compared with the thousands of mongrels produced every day under our eyes. And yet the material conditions of fertility are identically the same with races as with species, and our botanical gardens, which group numbers of species side by side, facilitate crossing still more.