A powerful instinct keeps animals from pairing with individuals belonging to another species than their own. “L’animal,” says M. Duvernoy, “a l’instinct de se rapprocher de son espèce et de s’éloigner des autres, comme il a celui de choisir ses aliments et d’éviter les poisons.”[1629] Among Birds, there are found a small number of wild hybrids, nearly all of which are in the order of Gallinae, and most of which belong to the genus Tetrao.[1630] But among Insects, Fishes, and Mammals, living in a state of nature, hybridism is unknown or almost so.[1631] And, even among domesticated mammals, some tricks are often required to deceive the male, and so to conquer its aversion to a female of a different species. The stallion, for instance, who is to cover a she-ass, is frequently first excited by the presence of a mare, for which, at the proper moment, the she-ass is substituted.[1632] We may be sure that, were it not for this instinctive feeling, many more animal hybrids would be naturally produced than is the case. In the vegetable kingdom, where the play of instincts is altogether out of the question, bastards occur much more frequently;[1633] and in captivity a considerable number of animal hybrid forms are produced that are never met with in a state of nature.[1634] Yet, according to Mr. Darwin, there are good grounds for the doctrine of Pallas, that the conditions to which domesticated animals and cultivated plants have been subjected, generally eliminate the tendency towards mutual sterility, so that the domesticated descendants of species which in their natural state would have been in some degree sterile when crossed, become perfectly fertile.[1635]