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.