THE AMERICAN NATURALIST [Vol. XLIX 



later, but it may be stated beforehand that no certain 

 evidence could be found in favor of this view. The num- 

 ber of larvae in a culture brings about a rapid alteration 

 in the condition of the food, so that it changes more 

 quicky from an acid to a neutral or alkaline condition. 

 If the black flies used gave vigorous F 1 offspring the 

 effect in question could be explained as due to numbers, 

 and not as connected with the black factor. 



Gray, Bed, Abnormal by Black, White, Normal 

 The results of this cross and of its reciprocal are given 

 in Tables VI and VI 1. The RN class (cross-over) is 

 relatively too large, but the increase is due to the transi- 

 tion from abnormal to normal. 



F,: GRAb?— GBAb c 



>ll7Ql...Uol2[ 41 1 SO 



The Link ao e of Abnormal, Yellow, and White 

 In the following crosses three pairs of sex-linked fac- 

 tors characters are involved, viz., yellow, white, abnor- 

 mal and their normal allelomorphs whose location at one 

 end of the X chromosome is shown in Diagram II. 



Gray, Red, Abnormal by Yellow, White, Normal 

 When a Y WN $ is crossed to a GRAb <? the daughters are 

 GKAb and the sons YWN. The F 1 male is a triple re- 



