206 COOK 



method of descent in which the members of the same species 

 of two definite kinds or castes, so formed as to require or to 

 facilitate cross-fertilization, though without the separation of 

 the sexes in different individuals. The differences found in 

 semisexual species correspond in all respects, to the secondary- 

 sexual characters of completely sexual species, except that they 

 do not accompany sex-differentiation. Examples of semi- 

 sexual dimorphism are numerous among plants, as in the spe- 

 cies of Primula^ Houstonia, etc. In some species of Oxalis and 

 L,yth?'um there is even a trimorphism involving three kinds of 

 stamens, pistils and pollen-grains. 



Subsexual Dtmo?'p/iisfn and Polymorphism. — Subsexual 

 dimorphism is a method of descent in which the members of the 

 same species are of two or more different kinds, castes or " ele- 

 mentary species ", but without regard to sexuality or to provi- 

 sions for cross-fertilization. The phenomenon may prove to be 

 completely analogous with Mendelism and other related forms 

 of polarity of character-expression in crosses between narrow- 

 bred varieties. It is, in short, polar inheritance under natural 

 conditions of free interbreeding, not complicated by the more 

 highly specialized polarity of sex-differentiation.^ 



A recently recorded addition to this series of phenomena is 

 that of a color-diversity in Verbena striata^ discovered in Kan- 

 sas by Professor Schaffner. 



"The peculiar form which I discovered was very different 

 from the type and its variants. The color of the corolla was a 

 pinkish-white [instead of deep purplish blue] and the individ- 

 uals, numbering several thousands, were found to be remarkable 

 for the uniformity of this character. There were no transition 

 forms whatever. The mutant had a much smaller color varia- 

 bility than the parent species, and no other peculiarit}' than the 

 color of the corolla was discovered. 



"The mutants covered about a square mile of territory. In 



some directions, however, they have advanced for nearly a mile 



beyond what seems to have been the center of distribution. . . . 



The mutant was found indifferently on nearly all kinds of soil 



^Examples of Semisexual, Subsexual, and Supersexual differentiations are 

 given in Cook, in Aspects of Kinetic Evolution. Proc. Wash. Acad. Sci., 8: 

 320-376. 



