Seite - 311 - in The Origin of Species

LAWS GOVERNING THE STERILITY 311 capacity, as in hybridisation, is by no means absolutely gov- erned by systematic affinity. Although many distinct genera within the same family have been grafted together, in other cases species of the same genus will not take on each other. The pear can be grafted far more readily on the quince, which is ranked as a distinct genus, than on the apple, which is a member of the same genus. Even different varieties of the pear take with different degrees of facility on the quince ; so do different varieties of the apricot and peach on certain varieties of the plum. As Gartner found that there was sometimes an innate dif- ference in different individuals of the same two species in crossing: so Sageret believes this to be the case with different individuals of the same two species in being grafted together. As in reciprocal crosses, the facility of effecting an union is often very far from equal, so it sometimes is in grafting; the common gooseberry, for instance, cannot be grafted on the currant, whereas the currant will take, though with difficulty, on the gooseberry. We have seen that the sterility of hybrids, which have their reproductive organs in an imperfect condition, is a dif- ferent case from the difficulty of uniting two pure species, which have their reproductive organs perfect; yet these two distinct classes of cases run to a large extent parallel. Some- thing analogous occurs in grafting; for Thouin found that three species of Robinia, which seeded freely on their own roots, and which could be grafted with no great difficulty on a fourth species, when thus grafted were rendered barren. On the other hand, certain species of Sorbus, when grafted on other species yielded twice as much fruit as when on their own roots. We are reminded by this latter fact of the extra- ordinary cases of Hippeastrum, Passiflora, &c., which seed much more freely when fertilised with the pollen of a dis- tinct species, than when fertilised with pollen from the same plant. We thus see, that, although there is a clear and great dif- ference between the mere adhesion of grafted stocks, and the union of the male and female elements in the act of repro- duction, yet that there is a rude degree of parallelism in the results of grafting and of crossing distinct species. And as