No. 570] FACTORS GOVERNING DISTRIBUTION 349 



Barely does a fly occur lacking more than two. In the few 

 cases in which three or more bristles are lacking, the 

 absence of the third acrostichals or of the dorso-centrals 

 is as frequent as the absence of first and second acros- 

 tichals. Why this should be is difficult to understand, as 

 it would be expected that both first and both second post- 

 acrostichals might frequently be lacking in the same fly, 

 especially as flies asymmetrical for the loss of these 

 bristles are common. 



A further point of interest lies in the fact that not only 

 is number of bristles a hereditary matter, but their dis- 

 tribution is also hereditary.. Thus from Table I (first- 

 generation flies) we see that in general the first post- 

 acrostichals tend to be reduced more than the second. 

 This may be expressed as a fraction : 



First post-acrostichals lacking _ 40.5 _ ^ ^ 

 Second post-acrostichals lacking ~~ 34 " " 



It is possible that this tendency to reduce the first post- 

 acrostichal more than the second is evidence of relation- 

 ship to L. ccesar Linn., in which the absence of the former 

 and the presence of the latter is the normal condition. 

 Strain 1913— A (Table III), however, gives 



First post-acmstirhaF lacking 

 Second post-acrostirhal> lacking 



Considering the reduction in the first post-acrostichals 

 separately, we may express the effect of selection as 

 follows : 



2 



