HYBRIDISM 25 



We have seen that the sterility of hybrids, which 

 have their reproductive organs in an imperfect condi- 

 tion, 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 Thouin found that three species of Ro- 

 binia, 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, Passiflora, etc., 

 which seed much more freely when fertilized with the 

 pollen of a distinct species than when fertilized 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 dif- 

 ferences 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 re- 

 productive systems. These differences in both cases fol- 

 low to a certain extent, as might have been expected, 

 systematic affinity, by which term every kind of resem- 

 blance and dissimilarity between organic beings is at- 



