We see that man has found no difficulty in breeding mongrels, and that, when he has wished to do so for any purpose whatever, he has been able to regulate it by merely choosing the animal or plant. This kind of union has, indeed, been long in daily practice for the amelioration, modification, and diversification of the living beings upon which human industry is exercised. It is useless to insist upon facts which are known to all gardeners and breeders, and I shall confine myself to one remark, the importance of which will be understood later.
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.
Among wild animals living in liberty hybrids are still more rare. It is unknown, for example, among mammalia, according to Isidore Geoffroy, whose experience has here a double value. The order of birds alone presents some facts of this kind, nearly all of which are in the order of Gallinæ. According to Valenciennes, they are unknown among fishes. In domestication and captivity spontaneous crossing between different species is a little less rare.
The intelligent intervention of man has multiplied unions of this kind in a remarkable manner, especially among plants, but without being able to extend their limits. Linnæus thought crossing was possible between species of different families. But in 1761 Koelreuter showed that he was mistaken. From these investigations, which were carried on for twenty-seven years, and from those of M. Naudin, his worthy rival, it appears that artificial crossing between species of different families never succeeds, and very rarely between species of different genera; that it is always very difficult, and demands the most minute precautions to insure success; that it often fails between species of the same genus closely allied in appearance, and finally, that there are whole families among which hybrids are impossible. Amongst the latter figures the family of the cucurbitaceæ, so thoroughly studied by M. Naudin, where the most perfect mongrels were produced spontaneously. We could not imagine, evidently, a more complete contrast.
This contrast is carried into the minutest details. For example, any flower which has in the least possible degree undergone the action of pollen of its own species becomes absolutely insensible to the action of pollen of a different species. How different to the equality of action displayed by the several pollens of most distant races!