ANALOGY. 105 



mutual sterility is a mark of specific distinct- 

 ness. But Huxley, in his essay on the "Com- 

 ing of Age of the ' Origin of Species,' " said: 

 " In my earliest criticisms of the ' Origin' I 

 ventured to point out that its logical founda- 

 tion was insecure so long as experiments in 

 selective breeding had not produced varieties 

 which were more or less infertile; and that 

 insecurity remains up to the present time." 1 

 Such was the serious nature of the facts of 

 hybridism which needed explanation. And it 

 was the close parallelism between hybridism 

 and heterostylism that led Darwin to seek in 

 the latter an explanation of the difficulties pre- 

 sented by the former. From the study of the 

 illegitimate offspring of heterostyled plants he 

 drew the conclusion that the sterility is due, 

 not to structural differences, but to functional 

 differences between the sexual elements; and 

 that it is not due directly to natural selection, 

 but is an incidental result accompanying the 

 adaptation of the sexual elements of the dif- 

 ferent forms of plants of the same species to 

 fertilize each other. By inverting the analogy 

 he transferred the conclusion to the facts of 

 hybridism. He said that it was this consider- 

 ation, that the sterility of species when first 



1 Life and Letters, Vol. I. p. 551. 



