CHAPTER VII.
CROSSING OF RACES AND SPECIES IN THE ANIMAL AND
VEGETABLE KINGDOMS.—MONGRELS AND HYBRIDS.
I. Sexual unions in plants, as in animals, can take place between individuals of the same species and the same race; further, between different races of the same species, and, finally, between different species. In the two latter cases we have what is called a cross. This crossing itself is differently named according to whether it takes place between different races or different species. In the first case it produces a mongrel, in the second a hybrid. When the cross-unions are fertile the product of the union of mongrels is called a mongrel, the product of the union of hybrids a hybrid.
If the difference of the relations existing between the race and the species has been properly understood, we ought to be inclined to admit that mongrels and hybrids would not present the same phenomena; experience and observation confirm this presentiment.
We have, therefore, in this crossing a means of judging whether the human groups are only races of a single species, or rather distinct species. For this purpose it will be sufficient to study the phenomena which, in other organised and living beings, accompany the production of mongrels and hybrids, and then to compare with both the phenomena which characterise the crosses effected between human groups. If, in the latter case, the phenomena are those which characterise hybridism, we must conclude that the groups are specifically distinct, and admit the multiplicity of human species. If, however, crosses between human groups, morphologically different, are accompanied by phenomena peculiar to the production of mongrels, we shall only be justified in considering these groups as races of one species; we must take our stand upon the doctrine of the Specific Unity of all mankind.
The question before us becomes then entirely a physiological one, and depends simply upon observation and experiment. For its solution we must again turn our attention to plants as well as to animals. It is in the phenomena of reproduction that the two kingdoms show the greatest resemblance. This is not a case of mere analogy, but almost of identity, and it is not the superior which lowers itself but the inferior which is raised. We might say that, ennobled by the importance of the function, the plant, as far as its reproductive system is concerned, becomes, for the time, animal.
II. In these kingdoms the unions between races of the same species, that is to say, the production of mongrels, may be accomplished without any intervention on the part of man, or it may take place under his direction. It is consequently either natural or artificial.
Mongrels among plants could only be recognised after the discovery of the distinction of the sexes in 1744. The honour of this great discovery belongs to Linnæus. He at once comprehended the importance of the subject, and even exaggerated it, as we shall presently see. Linnæus admitted that cross-unions, which had been observed for centuries between animals, might be repeated between plants. And he thus explained the appearance of variegated tulips in the midst of borders originally formed of uniformly coloured flowers. Observation and experiment have confirmed the views of the founder of the natural sciences again and again. Moreover, it has been observed that the crossing may become apparent in all parts of the plant by a mixture of characters similar to that exhibited by the colouring of the tulips. M. Naudin, among others, who, during one year, watched the development of more than 1200 gourds, saw the seeds of a single fruit reproduce all the races contained in the garden in which his observations were made. Superfetation had taken place. It is a fact of great importance, as it demonstrates the equality of action enjoyed by the pollen of all these races, which, morphologically, differ so widely from each other. No better example could be given of the faculty of crossing between races.
The natural and spontaneous production of mongrels among animals presents the same characters. Facilitated by locomotion it is accomplished every day in our houses, our poultry-yards, and our farms. The difficulty does not consist in the accomplishment of the cross but in its prevention, and in the preservation of the purity of the race. The careful observations made by Isidore Geoffroy at the Paris Museum, have shown that with sheep, dogs, pigs, and fowls, mongrels between the most different races were invariably fertile. Here again the phenomena of superfetation was often proved. Bitches produced, by males of several races successively, young which showed three or four distinct sources. Here the case was the same as with the gourds of M. Naudin.
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!
All experimenters agree further in declaring that even in the unions between species which have been most successful, the fertility is constantly diminished, and often in immense proportions. The head of the Papaver somnifera generally contains 2000 seeds or more. In a hybrid of this species Gœrtner only found six which had been matured; all the rest were more or less abortive. Here again, what a contrast between the crossing productive of such fertility in M. De Ginestous’ English pigs.
Hybridism in animals presents exactly the same phenomena as in plants. Man has been able, by diverting and deceiving animal instincts, to multiply crosses between species. But he has not been able to extend the very narrow limits at which these phenomena cease. Not one fertile union has taken place between different families; they are very rare between genera, and even between species they are far from numerous, a fact the more remarkable as animal hybridation is an ancient institution. The mule was known to the Hebrews before the time of David, and to the Greeks in the age of Homer. Titires and musmons, products of crossings between the he-goat and the sheep and the ram with the she-goat, received their distinctive names from the Romans.
The uncertainty of the result is another point of resemblance between animal and vegetable hybrids. The same experiments executed with the same care and by equally clever experimenters have sometimes succeeded and sometimes failed without any apparent cause. Buffon and Daubenson often tried to reproduce titires and musmons. They succeeded twice, while Isidore Geoffroy has invariably failed. The formation of crosses between the hare and the rabbit, which has frequently been attempted in various parts of the globe, appears only to have been successful four or five times at the most. The pretended cross between the camel and the dromedary, admitted by Buffon and quoted by Nott, is certainly a fable, after the details which M. De Khanikoff kindly gave me, and which I have published elsewhere. We may, therefore, draw this conclusion from known facts, that there are only two species of mammals, the ass and the horse, the crossing of which is almost universally and invariably fertile.
Finally, crossing between species, or hybridation, is extremely exceptional among plants and animals when left to themselves; man can only produce them with great difficulty in the two kingdoms, and then only between a very limited number of species; when he has succeeded, the fertility is almost constantly diminished, and often to a very considerable extent.