1906] Brainerd, Hybridism in the Genus Viola, — II 9 



closely allied forms of Viola. DeVries holds that ordinary species 

 "differ from each other partly in specific, partly in varietal charac- 

 ters. As to the first, they give in their hybrids stable peculiarities, 

 while as to the latter, they split up according to Mendel's law."^ This, 

 as I hope to show, is exactly what takes place, when two such species 

 as V. fimbriatula and V. sagittata interbreed. Dr. Sabransky also 

 observes that in Austria "among the stemless violets, hybrids present 

 for the most part a more or less distinctly intermediate character; 

 while among the stemmed violets, they present an unbroken series 

 of transitional forms between the parents, whose characters thus 

 become confluent."^ 



Ake specific Distinctions endangered by Hybridism? 



Some systematists seem disturbed at the discovery that certain 

 groups of nearly related species freely interbreed. It seems to admit, 

 as one of our keenest observers has expressed it, the existence of "a 

 mutually destructive influence, breaking down the individual charac- 

 ters of species, and merging them finally into one." Surely, hybridism 

 is a movement in this direction. But we must remember that the 

 advance of living organisms proceeds through the conflict of two 

 opposing forces, the one leading to multiform variation, the other 

 seeking to conserve the type by requiring double parentage. Thus 

 the aberrant traits that may arise in an individual by mutation or 

 fluctuation, are usually eliminated in succeeding generations by the 

 repeated process of pairing with normal individuals. This conflict 

 ordinarily ceases with the pronounced establishment of separate spe-_ 

 cies; but within several groups of Viola, consisting of forms but 

 recently ranked as species, the conflict seems to be exceptionally 

 prolonged. 



But we may, I fancy, discover the cause of this exceptional behav- 

 ior. The power of sexual reproduction to maintain uniformity in a 

 group of plants is so great, that for the evolution of two or more species 

 out of one the incipient species need in some way to be isolated from 

 each other. Darwin describes the fauna and flora of certain oceanic 



' "Species and Varieties, their Origin by Mutation," p. 307. 



» Translated from "Allgemeine Botanische Zeitschritt," xi. 162, Oct., 1905. 



