These facts with respect to plants show that in some few cases certain varieties have had their sexual powers so far modified, that they cross together less readily and yield less seed than other varieties of the same species. We shall presently see that the sexual functions of most animals and plants are eminently liable to be affected by the conditions of life to which they are exposed; and hereafter we shall briefly discuss the conjoint bearing of this and other facts on the difference in fertility between crossed varieties and crossed species. _Domestication eliminates the tendency to Sterility which is general with Species when crossed._ This hypothesis was first propounded by Pallas,[239] and has been adopted by several authors. I can find hardly any direct facts in its support; but unfortunately no one has compared, in the case of either animals or plants, the fertility of anciently domesticated varieties, when crossed with a distinct species, with that of the wild parent-species when similarly crossed. No one has compared, for instance, the fertility of _Gallus bankiva_ and of the domesticated fowl, when crossed with a distinct species of Gallus or Phasianus; and the {110} experiment would in all cases be surrounded by many difficulties. Dureau de la Malle, who has so closely studied classical literature, states[240] that in the time of the Romans the common mule was produced with more difficulty than at the present day; but whether this statement may be trusted I know not. A much more important, though somewhat different, case is given by M. Groenland,[241] namely, that plants, known from their intermediate character and sterility to be hybrids between Aegilops and wheat, have perpetuated themselves under culture since 1857, _with a rapid but varying increase of fertility in each generation_. In the fourth generation the plants, still retaining their intermediate character, had become as fertile as common cultivated wheat.