X 



Chap. IX.] OP FIRST CROSSES AND OF HYBRIDS. 19 



eies in being grafted together. As in reciprocal crosses, 

 the facility of eflecting an union is often very far from 

 equal, so it sometimes is in grafting; the common goose- 

 berry, for instance, cannot be grafted on the currant, 

 whereas the currant will take, though with difiaculty, on 

 the gooseberry. 



We have seen that the sterility of hybrids, which 

 have their reproductive organs in an imperfect con- 

 dition, is a different case from the difliculty 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 Thouin found that three species of 

 Eobinia^ 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 fertil- 

 ised 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 dis- 

 tinct 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 differ- 

 ences in their vegetative systems, so I believe that the 

 still more complex laws governing the facility of first 

 37 



