No. 570] FACTORS GOVERNING DISTRIBUTION 355 



nevertheless a few flies in the strain in which bristles are 

 lacking. 



Bristles normally present may be lacking in individuals 

 having additional bristles. 



GrENEKAL SuMMAKY AND CONCLUSIONS 



Taking a general summation of all the bred material of 

 Lucilia sericata, we find that reduction affects the males 

 while addition affects the females. Of the 5,367 flies bred, 

 2,708 are males and 2,659 are females, giving practical 

 equality. 



Reduction in the males is 748.5 bristles, while in the 

 females it is only 455.5 bristles. As has been noted before 

 the degree of reduction in the females is increased by the 

 later generations of strain 1913 — A, by reason of the fact 

 that reduction rarely goes beyond the loss of two bristles 

 in a single fly. Thus when most of the flies of a popula- 

 tion become reduced to this extent it is evident that reduc- 

 tion in the males would be but slightly in advance of that 

 in the females. 



There are 210 bristles added in the males, while there 

 are 343 added in the females. Thus addition affects the 

 females more than the males. These figures for bristles 

 added represent number of bristles, and thus no distinc- 

 tion is made between bristles of large and bristles of 

 small size. 



I wish to express my appreciation for the advice and 

 criticism offered me in this work by Professor W. M. 

 Wheeler, Messrs. H. D. Fish, S. G. Wright, and C. C. 

 Little. 



