{ 248} <Note in original.> Animals seem more often made sterile by being taken out of their native condition than plants, and so are more sterile when crossed.
We have one broad fact that sterility in hybrids is not closely related to external difference, and these are what man alone gets by selection.
{ 249} See _Var. under Dom._, Ed. ii. vol. II. p. 132; for the case of the cheetah see _loc cit._ p. 133.
{ 250} _Var. under Dom._, Ed. ii. vol. II. p. 148.
{ 251} Quoted in the _Origin_, Ed. i. p. 9.
{ 252} See _Var. under Dom._, Ed. ii. vol. II. p. 147.
{ 253} _Var. under Dom._, Ed. ii. vol. II. p. 89.
{ 254} See _Var. under Dom._, Ed. ii. vol. II. p. 147.
{ 255} _Origin_, Ed. i. p. 267, vi. p. 392. This is the principle experimentally investigated in the author's _Cross-and Self-Fertilisation_.
But it may be objected{ 256} (however little the sterility of certain hybrids is connected with the distinct creations of species), how comes it, if species are only races produced by natural selection, that when crossed they so frequently produce sterile offspring, whereas in the offspring of those races confessedly produced by the arts of man there is no one instance of sterility. There is not much difficulty in this, for the races produced by the natural means above explained will be slowly but steadily selected; will be adapted to various and diverse conditions, and to these conditions they will be rigidly confined for immense periods of time; hence we may suppose that they would acquire different constitutional peculiarities adapted to the stations they occupy; and on the constitutional differences between species their sterility, according to the best authorities, depends. On the other hand man selects by external appearance{ 257}; from his ignorance, and from not having any test at least comparable in delicacy to the natural struggle for food, continued at intervals through the life of each individual, he cannot eliminate fine shades of constitution, dependent on invisible differences in the fluids or solids of the body; again, from the value which he attaches to each individual, he asserts his utmost power in contravening the natural tendency of the most vigorous to survive. Man, moreover, especially in the earlier ages, cannot have kept his conditions of life constant, and in later ages his stock pure. Until man selects two varieties from the same stock, adapted to two climates or to other different external conditions, and confines each rigidly for one or several thousand years to such conditions, always selecting the individuals best adapted to them, he cannot be said to have even commenced the experiment. Moreover, the organic beings which man has longest had under domestication have been those which were of the greatest use to him, and one chief element of their usefulness, especially in the earlier ages, must have been their capacity to undergo sudden transportals into various climates, and at the same time to retain their fertility, which in itself implies that in such respects their constitutional peculiarities were not closely limited. If the opinion already mentioned be correct, that most of the domestic animals in their present state have descended from the fertile commixture of wild races or species, we have indeed little reason now to expect infertility between any cross of stock thus descended.