The origin of this instinct, which helps to keep even closely allied species in a state of nature distinct, seems to be sufficiently clear. The number of species which have proved fertile together are very limited, and the fertility of the hybrid offspring is almost constantly diminished, often even to a very great extent. Of course, no one now talks of the sterility of hybrids as a moral necessity—hybrids being _animalia adulterina_,—or as the result of a special divine decree, that new species should not be multiplied indefinitely.[1636] M. Isidore Geoffroy has shown not only that hybrids may be fertile, but that “infertile” hybrids are, properly speaking, merely the hybrids which are most rarely fertile, their sterility never being absolute.[1637] Moreover, as has been pointed out by Mr. Wallace, in almost all the experiments that have hitherto been made in crossing distinct species, no care has been taken to avoid close interbreeding; hence these experiments cannot be held to prove that hybrids are in all cases infertile _inter se_.[1638] But looking to all the ascertained facts on the intercrossing of plants and animals, we may with Mr. Darwin conclude that some degree of sterility in hybrids is an extremely general result.[1639] This being the case with the hybrids of our domesticated animals, it must be so all the more with animals in a state of nature, which generally live under conditions less favourable to mutual fertility. It is easy to understand, then, that instincts leading to intercrossing of different species, even if appearing occasionally, never could be long-lived, as only those animals which preferred pairing with individuals of their own species, gave birth to an offspring endowed with a normal power of reproduction, and thus became the founders of numerous generations that inherited their instincts.