132 THE AMERICAN NATURALIST [VOL.L 



ciently extensive scale. Quite recently the view that 

 heterozygosis is responsible for the mutability of species 

 has again been advanced by Lotsy in an interesting article 

 published in the Archives Neerlandaises. This author 

 very definitely takes the position that variability in gen- 

 eral is due to hybridization, and that true species (not 

 necessarily those of Linnaeus and other systematists) are 

 invariable. With this view I am personally in agree- 

 ment, with the limitation that the statement goes much 

 too far. 



It is one of the commonplaces of breeding that the off- 

 spring resulting from hybridization is extremely variable 

 and may be characterized by a greater or less degree of 

 sterility. Taking the particular case of the Angiosperms, 

 it is found that when species of lilies, irises, honeysuckles, 

 etc., are crossed, the result is a highly mutable progeny 

 with a greater or less degree of sexual sterility, the latter 

 condition most easily recognized in the microspores or 

 pollen. The main purpose of the present statement is to 

 make it clear to my fellow naturalists that in nature a 

 high degree of variability often exists in the case of the 

 Angiosperms, expressed either in terms of difficulty of 

 systematic determination in view of intergrading forms, 

 or often in the less obtrusive form of multiplication of 

 species in a given genus. This extreme degree of vari- 

 ability is very largely accompanied by the highly signifi- 

 cant phenomenon of pollen sterility. 



A family of Angiosperms much in the foreground in 

 recent years is the Onagraceae or Evening Primrose 

 family. In the case of the genus Oenothera remarkable 

 conditions have been discovered by De Vries. The plants 

 of 0. lamarckiana, when grown in large numbers, show a 

 number of individuals, sometimes as high as one twentieth 

 of the total number, markedly different in character from 

 typical 0. lamarckiana. This phenomenon was at first 

 thought to be peculiar to this species of Oenothera and a 

 great deal of importance was consequently attached to 

 clearing up its somewhat dubious systematic position. 



