240 LAWS GOVERNING STERILITY. [Chap. IX 



of the same species, can usually, but not invariably, be grafted with, 

 ease. But this capacity, as in hybridisation, is by no means abso- 

 lutely governed 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 difference 

 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 different 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. Something analogous occurs 

 in grafting ; for Thoum found that three species of Eobini.o. 

 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 extraordinary cases of Hippeastrum. Passifiora, &c, which seed 

 much more freely when fertilised with the pollen of a distinct 

 species, than when fertilised with pollen from the same plant. 



We thus see, that, although there is a clear and great difference 

 between the mere adhesion of grafted stocks, and the union of 

 the male and female elements in the act of reproduction, yet that 

 there is a rude degree of parallelism in the results of grafting and 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 in their reproductive systems. These differences in both 

 cases, follow to a certain extent, as might havs been 3xpected, 



