104 THE METHOD OF DARWIN. 



In a more complex case, analogy led to a con- 

 clusion which, although it could not be verified, 

 possesses great importance in relation to one of 

 the principal difficulties in the way of the gen- 

 eral theory of natural selection. In the course 

 of the investigation on " Different Forms of 

 Flowers on Plants of the same Species," he 

 noticed the striking parallelism between the 

 phenomena of hybridism and those of the 

 heterostyled plants which he was studying. 1 

 When once the parallelism was established, 

 the remarkable and puzzling facts of hybridism 

 doubtless furnished a solid analogical basis 

 from which to foresee and scrutinize the results 

 of crossing the different forms of heterostyled 

 plants. 



Difficulty in uniting two forms and sterility 

 of their offspring had been almost universal-ly 

 regarded as a test of specific distinctness. 

 Darwin showed clearly that this belief, al- 

 though very generally true, is by no means 

 universally so; and his work on heterostyled 

 plants showed that all the phenomena of hy- 

 bridism were displayed among forms that 

 certainly belonged to the same species. He 

 triumphantly overthrew the doctrine that 



1 Different Forms of Flowers on Plants of the same 

 Species, pp. 242, 243. 



