462 E. ELEANOR CAROTHERS 



B is only about one-half as heterozygous as form A and con- 

 sequently may be expected to be less variable taxonomically. 

 That it really is less variable is indicated by the- fact that tax- 

 onomists have separated the individuals that largely compose 

 form A into four species, while they have treated the group 

 that chiefly composes form B as a single species. 



Since chromosome number 1 is the only tetrad subject to heter- 

 omorphism which could be recognized without possibility of 

 confusion, its composition was studied in the two groups. Its 

 behavior in the whole collection has been described on page 458. 

 It occurs as follows: 



In both cases it approximates closely a 1-2-1 ratio as it did 

 for the entire collection. While it is evident then, that a rather 

 general change has taken place in the two forms in respect to the 

 number of atelomitic and telomitic chromosomes, it has not af- 

 fected chromosome number 1. Likewise, a glance through 

 the plates will show that the accessory (no. 4) and chromosome 

 number 2 have remained constant. The remaining nine tetrads 

 intergrade in size so closely that it is impossible to identify any 

 given one with certainty. It was, therefore, necessary to study 

 them collectively. The number of atelomitic dyads per indi- 

 vidual was determined and the results plotted. The curves thus 

 derived are shown in text figure 1. Numbers 10 ; 11 and 12 have 

 remained constantly atelomitic, if I have been correct in my iden- 

 tification of them. But on the chance that I may not have been 

 able to identify these correctly I shall include them in the curves; 

 if their form is constant, they will not affect the shape of the 

 graphs, while, on the other hand, there is a possibility that they 

 are concerned in the changes. In text figure 1 the ordinates rep- 

 resent the number of animals; the abscissae the number of atelo- 

 mitic dyads per cell. That is, each Stenobothrus ring is made up 

 of a pair of atelomitic dyads, while the J-shaped tetrads have only 



