"In a few cases, the union between two different species may be fruitful, but the offspring cannot reproduce. For example, the union between a horse and an ass. The product of this union is the mule. All the mules in the world are the descendants of the ass and the mare. These animals are so numerous in Spain and South America that they are preferred to horses, on account of their great strength and powers of endurance. The genet, which is less desirable because it is not so robust, is the fruit of the inverse crossing of the horse and the female ass. The genet, no more than the mule, can reproduce. If one or the other is desired, of necessity recourse is had to the two _species_. In extremely rare cases, fecundity remains among some of their descendants, but it diminishes gradually from the second generation down to the third, fourth, and fifth. The same result is shown in the union of the canary bird. I could here accumulate a crowd of analogous details. Above all, two great general facts appear that comprehend all, and are the expression of the law; they are that, notwithstanding the accumulated observations of years, made from experiments on certain species, not a single example is known of an intermediate species being obtained by the _crossing_ of animals belonging to _two different species_. "This general fact explains how order is maintained in the actual living creation. Were it otherwise, the animal and vegetable world would have been filled with intermediate groups, passing from one to the other insensibly, and in the confusion, it would be impossible for naturalists to recognize them. The general conclusion to draw from these precedents is, that infecundity is _the law of union between animals of different species_. "Unions are always more fruitful when between two animals of the same race. Their descendants are as fruitful as the parents and the grandparents, where pains are taken to preserve the race pure, and to prevent strange blood from debasing it. "When, on the contrary, a union is effected between two different races belonging to the same species, producing a _mongrel race_, the contrary takes place. "There is no difficulty in obtaining a mongrel race--the result of a crossing of races; but the difficulty is when there is a pure race, and it is desirable to have it maintained, that great care is needed to prevent strange blood from changing it. "Races crossed by mongrels--that is to say, by animals of the same species, but belonging to different races, multiply around us. There are the dogs in the streets, the cats of the alleys, the coach-horses; all beasts among whom the race is undecided in consequence of crossing indiscriminately, their characteristics becoming confounded.