Seite - 306 - in The Origin of Species

306 ORIGIN OF SPECIES gard to hybrid animals, I have been surprised to find how generally the same rules apply to both kingdoms. It has been already remarked, that the degree of fertility, both of first crosses and of hybrids, graduates from zero to- perfect fertility. It is surprising in how many curious ways this gradation can be shown; but only the barest outline of the facts can here be given. When pollen from a plant of one family is placed on the stigma of a plant of a distinct family, it exerts no more influence than so much inorganic dust. From this absolute zero of fertility, the pollen of dif- ferent species applied to the stigma of some one species of the same genus, yields a perfect gradation in the number of seeds produced, up to nearly complete or even quite complete fertility; and, as we have seen, in certain abnormal cases, even to an excess of fertility, beyond that which the plant's own pollen produces. So in hybrids themselves, there are some which never have produced, and probably never would produce, even with the pollen of the pure parents, a single fertile seed: but in some of these cases a first trace of fer- tility may be detected, by the pollen of one of the pure parent- species causing the flower of the hybrid to wither earlier than it otherwise would have done; and the early withering of the flower is well known to be a signof incipient fertilisa- tion. From this extreme degree of sterility we have self- fertilised hybrids producing a greater and greater number of seeds up to perfect fertility. The hybrids raised from two species which are very diifi- cult to cross, and which rarely produce any offspring, are generally very sterile; but the parallelism between the diffi- culty of making a first cross, and the sterility of the hybrids thus produced—two classes of facts which are generally con- founded together—is by no means strict. There are many cases, in which two pure species, as in the genus Verbascum, can be united with unusual facility, and produce numerous hybrid-offspring, yet these hybrids are remarkably sterile. On the other hand, there are species which can be crossed very rarely, or with extreme difficulty, but the hybrids, when at last produced, are very fertile. Even within the limits of the same genus, for instance in Dianthus, these two opposite cases occur.