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.
CHAPTER VIII.
CROSSING BETWEEN VEGETABLE AND ANIMAL RACES AND SPECIES; MONGRELS AND HYBRIDS; REALITY OF SPECIES.
I. From the very first, in the union of two individuals belonging to different stocks, the race and the species display very distinct and characteristic phenomena. We shall now see this opposition as strongly marked in the product of these unions in mongrels and hybrids.
Several questions are raised by the mixed nature of these beings. I shall confine myself to those which refer to filiation, and which have therefore a special interest for us. They may be stated generally as follows:—are mongrel races, that is, those derived from two distinct races, and hybrid races, that is those which are derived from the crossing of two species, formed naturally, or can they be obtained artificially? In other words, do mongrels and hybrids retain, during an indefinite number of generations, the faculty of reproducing and transmitting to their descendants the mixed character they inherited from the first parents which effected the cross?
II. In regard to mongrels there is not a shadow of doubt. Facts which frequently occur, often without our intervention, and sometimes in spite of our precautions, prove again and again that the mongrels of the first generation are as fertile as the parents, and transmit equal fertility to their offspring. Our gardeners and breeders always take advantage of this property of mongrels in order to vary, modify or ameliorate from their point of view the plants and animals in which they are interested; the careful experiments of Buffon, of Geoffroy St.-Hilaire, father and son, and the testimony of Darwin, on this point very significant, prove beyond a doubt that unions between different races remain fertile, whatever morphological differences there may be between them. I shall confine myself to quoting one example from Darwin. The niata will unite indifferently in both senses with the ordinary ox, and the offspring is fertile.