230 LAWS GOVERNING THE STERILITY [CHAP. IX- 



no means absolutely 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 re- 

 productive organs perfect ; yet these two distinct classes of cases run 

 to a large 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 a fourth species, when thus grafted were rendered 

 barren. On the other hand, certain species of Sorbus, when graf ted 

 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, Passiflora, fec., 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 vegeta- 

 tive systems, so I believe that the still more complex laws govern- 

 ing 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 have been expected, systematic 

 affinity, by which term every kind of resemblance and dissimilarity 

 between organic beings is attempted to be expressed. The facts 

 by no means seem to indicate that the greater or lesser difficulty 

 of either grafting or crossing various species has been a special 

 endowment ; although in the case of crossing, the difficulty is as 



