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Show 262 HYBRIDISM. CHAP. VIII. with different degrees of facility on the quince ; so ~o different varieties of the apricot and peach on certain varieties of the plum. . . As Gartner found that there was sometimes an Innate difference in different individualB of the same two species in crossing ; so Sagaret believes this to be th~ ca?e with different individuals of the same two spemes In being grafted together. ~s i~ reciprocal crosses, the facility of effecting an u~u~n IS of~en very far from equal, so it sometimes IS Ill grafting ; the common gooseberry, for instance, cannot be grafted on the currant whereas the currant will take, though with diffi-culty' , on the gooseberry. We have seen that the sterility of hybrids, which have their reproductive organs in an imperfect condition, is a very different case from the difficulty of uniting two pure species, which have. t~eir reproductive organs perfect ; yet these two dist~nct cases run to a certain extent parallel. Something 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 another 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 extraordinary case of Hippe~ astrum, Lobelia, &c., which seeded much more freely when fertilised with the pollen of distinct species, than when self-fertilised with their own pollen. We thus se~, that although there is a clea: and fundamental difference between the mere adhesiOn of grafted stocks, and the union of the male and ferr:aie elements in the act of reproduction, yet that th.ere IS a rude degree of parallelism in the results of grafting and CHAP. VIII. CAUSES OF STERILITY. 263 of crossing distinct species. And as we must look at the curious and complex laws governing the facility with which trees can be grafted on each other as incidental on unknown differences in their vegetative systems, so I believe that the still more complex laws governing the facility of first crosses, are incidental on unknown differences, chiefly in their reproductive systems. These differences, in both cases, follow to a certain extent, as might have been expected, systematic affinity, by which every kind of resemblance and dissimilarity between organic beings is attempted to be expressed. The facts by no means seem to me to indicate that the greater or lesser difficulty of either grafting or crossing together various species has been a special endowment; although in the case of crossing, the difficulty is as important for the endurance and stability of specific forms, as in the case of grafting it is unimportant for their welfare. OauBeB of the Sterility of jirBt OroBBeB and of HybridB.W e may now look a little closer at the probable causes of the sterility of first crosses and of hybrids. These two cases are fundamentally different, for, as just remarked, in the union of two pure species the male and female sexual elements are perfect, whereas in hybrids they are imperfect. Even in first crosses, the greater or lesser difficulty in effecting a union apparently depends on several distinct causes. There must sometimes be a physical impossibility in the male element reaching the ovule, as would be the case with a plant having a pistil too long for the pollen-tubes to reach the ovarium. It has also been observed that when pollen of one species is placed on the stigma of a distantly allied species, though the pollen-tubes protrude, they do not penetrate the stigmatic surface. Again, the |