No. 570] FACTORS GOVERNING DISTRIBUTION 351 



In the 137 offspring of a single mating of this strain, cF x c, 

 there are 23 first post-acrostichals lacking, showing that 

 it is due to this mating especially that the strain is so 

 lacking in first post-acrostichals. 



It can not as yet be said that the factors governing the 

 first post-acrostichals are altogether independent of those 

 governing the second. That a certain degree of inde- 

 pendence obtains is evident from a comparison of the 

 ratio of reduction in first to reduction in second post- 

 acrostichals in flies in general (Table I), with the same 

 ratio for strain 1913 — A. In the former case we have 

 40.5/34 or 1.19. In the latter we have 329/750, or 0.43. 

 In order to establish the independence of the factors un- 

 derlying these two tendencies it will be necessary to 

 obtain, either by selection from a strain showing both 

 tendencies or by breeding from wild stock, two strains, 

 one tending to lack the first while retaining the second, 

 and the other tending to lack the second while retaining 

 the first. 



A point of interest in strain 1913— A is the presence of 

 twelve small second post-acrostichals in the progeny of 

 AF 2 b in which the female had one of these reduced to half 

 size. The progeny of AF 2 a in which there was total ab- 

 sence of these bristles showed either presence or absence 

 of the same but no reduced bristles. In F 2 , however, we 

 have eight reduced bristles. The occurence of these 

 small bristles in the progeny of certain matings is taken 

 as an indication of recombinations of multiple factors, 

 but the numbers are too small to establish this with cer- 

 tainty. 



A glance at the tables shows that third post-acrosti- 

 chals are rarely lacking. These are normally present in 

 all related species, while in a ie^,—Cynomyia mor- 

 tuorum, Musca domestica, Pseitdopi/rrllin cornicina, and 

 others, there is normally but one post-acrostichal, and this 

 is always the last. 



Posterior dorso-centrals are very rarely absent. Thus 

 in the 2,273 flies recorded in Table I only one had a single 

 post-dorso-central missing. Reduction in post-acrosti- 



