84 Darwin, and after Darwin. 



I concluded that these two new forms had, by joining their 

 forces, expelled the H. mllosum from its primitive abode, but 

 had not succeeded in displacing one another. As to their origin, 

 they had evidently developed in two different directions from 

 a common point of departure, namely H. mllosum. They had 

 succeeded, not only in separating themselves from the original 

 form, but also in preventing any intermediary form from inter- 

 posing. I thought myself therefore justified in considering this 

 as a case of varieties which have come into existence subsequently 

 to the Glacial epoch. The morphological characteristics of the 

 three forms are sufficiently distinct for them to be designated as 

 species by a good many writers. They are better defined than 

 some of MM. Frolich and Fries' weaker species, and as well 

 defined as some of MM. Koch and Grisebach's (p. 222). 



Now it is clear, without comment, that all this is 

 exactly as it ought to be, if allied species have been 

 differentiated on common areas by selective fertility. 

 For if, as Nageli elsewhere says, "one meets forms 

 in nature associated with one another, and severally 

 distinguished by every possible degree of differen- 

 tiation," not only as Nageli adds, does this general 

 fact lead to the inference that species are (usually) 

 developed when plants grow intimately associated 

 together; but as certainly it leads to the further 

 inference that such development must be due to 

 a prior development of cross-infertility between the 

 diverging varietal forms, cross-infertility which is 

 therefore afterwards so characteristic of the allied 

 species, when these are found, in their fully dif- 

 ferentiated condition, still occupying the same area 

 in large and intimately mingled populations, 



To my mind there could not be any inference more 

 strongly grounded than this, because, with the one 

 exception of the physiological form, no other form 

 of homogamy can be conceived which shall account 



