INTRODUCTORY. I 7 



will never transmit abnormality to any of their descendants in any 

 environment, the daughters will transmit (if bred to normal males) in 

 a suitable environment their peculiarity to half of their daughters and 

 to half of their sons. The experiment shows convincingly that the 

 abnormal abdomen appears in a special environment only in those flies 

 that have a given genotypic constitution. 



As the cultures dry out the abnormal males are the first to change 

 over to normal, then the heterozygous females, and lastly the homo- 

 zygous females. It is doubtful if any far-reaching conclusion can be 

 drawn from this series, because the first and second classes differ from 

 each other not only in the presence of one or of two factors for abnor- 

 mal, but also by the absence in the first case (male) of an entire X 

 chromosome with its contained factors. The second and third classes 

 differ from each other only by the abnormal factor. 



Similar results were found in the mutant type called reduplicated 

 legs, which is a sex-linked recessive character that appears best when 

 the cultures are kept at about io° C. As Miss M. A. Hoge has shown, 

 this character then becomes realized in nearly all of the flies that have 

 the proper constitution, but not in flies of normal constitution placed 

 in the same environment. Here the effect is produced by cold. 



SEXUAL POLYMORPHISM. 



Outside the primary and secondary sexual differences between the 

 male and the female, there is a considerable number of species of 

 animals with more than one kind of female or male. Darwin and his 

 followers have tried to explain such cases on the grounds that more 

 than one kind of female (or male) might arise through natural selection, 

 in consequence of some individuals mimicking a protected species. It 

 is needless to point out here how involved and intricate such a process 

 would be, because the mutation theory has cut the Gordian knot 

 and given a simpler solution of the origin of such diandromorphic and 

 digynomorphic conditions. 



In Drosophila a mutant, eosin eye-color, appeared in which the 

 female has darker eyes than the male. If such stock is crossed with 

 cherry (another sex-linked recessive mutant, allelomorphic to eosin) 

 the females in the F2 generation are alike (for the pure eosin and the 

 eosin-cherry compound are not separable), but the cherry males and 

 the eosin males are quite different in appearance. Here we have a simu- 

 lation, at least, of a diandromorphic species. Such a group perpetuates 

 itself, giving one type of female (inasmuch as eosin and cherry females 

 are very closely similar) and two types of males, only one of which is 

 like the females. A population of this kind is very directly comparable 

 to certain polymorphic types that occur in nature. In Colias philodice 

 there is one type of male, yellow, and two types of females, yellow and 



