DIFFERENTIATION, HEREDITY, SEX 313 



well known as the constant relation in some Insects (Sinety, 

 McClung), and among these forms various degrees of the inti- 

 macy of the association occur (Fig. 146). 



Several stages can be found in the gradual increase in the 

 relative size of the Y-element, until in such forms as Nezara 

 hilaris, one of the Hemiptera-Heteroptera described by Wilson, 

 X and Y are nearly equal (Fig. 145), and finally in some of the 

 Lepidoptera and other forms, X and Y are quite equal and 

 indistinguishable from one another, although the XY pair may 

 be distinguished from the other chromosomes by staining prop- 

 erties and behavior. 



We thus reach through gradual transitions a condition where 

 the spermatozoa are no longer dimorphic with respect to chro- 

 mosome content. The conditions of such a series suggest, how- 

 ever, the possibility that spermatozoa that visibly appear 

 morphologically alike, may after all be physiologically dimor- 

 phic as regards chromosome characters; such an assumption 

 must of course be made with respect to traits other than sex, 

 which are inherited in an alternative fashion. 



Another series of modifications of the Anasa-type is illus- 

 trated by various genera of Hemiptera, where the X-chromo- 

 some is represented by more than a single element. Such a 

 series has been described by Payne, and is readily derived 

 from the condition in such a form as Euschistus (Fig. 145), 

 with unequal X- and Y-elements. Thus in Fitchia and several 

 others, Y is a fairly large chromosome while X is represented 

 by two somewhat smaller chromosomes; in Prionidus, Sinea, etc. 

 there are three X-elements to one Y; in Gelastocoris there are 

 four X and one Y, and in Acholla five X and one Y. In still 

 another series (Fig. 145) Y is entirely absent and X is repre- 

 sented by several chromosomes two in Phylloxera (Morgan), 

 Syromastes (Wilson), Agakna (Wallace), and man (Guyer), 

 five in Ascaris lumbricoides according to Edwards. 



In all of these cases where X is a multiple element, 

 different species show greatly varying relations among the 

 members of the X-group; they may be approximately equal 

 in size or very unequal, but it is important that the size 



