174 On the Reproductive Functional Relations of Verlasca. [No. 3, 



several undoubtedly distinct species. There is also a parallelism 

 between the results of reciprocal hybridisation of varieties of distinct 

 species, on the one hand with those of the reciprocal inter-crossing of 

 varieties of a single species on the other. The sole difference in the 

 two lines at least is merely as to the degree of extension ; species re- 

 lative to species occupying a higher point in the divergingly extended 

 line, than do the varieties of a species relatively to each other, and 

 accordingly yielding in general more intensified results, harmoniously 

 testifying to the truth of Mr. Darwin's remark that sterility is simply 

 a superinduced quality due to incidental differences in the reproduc- 

 tive system.... As in the varieties of a species, however, we find that the 

 relative amount of physiological divergence, — as judged by the fertility 

 of their reciprocal conjunctions, — is by no means regularly or definitely 

 co-ordinated with their morphological ; so in the hybridisation of the 

 different species of a genus, the most distinct morphologically are 

 often found to be most nearly allied in their physiological charac- 

 teristics, and thus there being no necessary co-ordination of these 

 characteristics we can readily understand how the sterility of the first 

 crosses of varieties of a species may, and occasionally does, exceed that 

 of well -marked and undoubtedly distinct species. 



