No. 570] 



FACTORS GOVERNING DISTRIBUTION 



347 



5 4 = 3,2,2,3. 

 from AF.d = <? and $ = 3, ac, ac 

 3; male died August 18 and anot 



The record of 1913-J, recorded in tabular form is given in Table EH. 



We are now in a position to consider the nature of re- 

 duction of bristles in Lucilia sericata. 



It is evident from Table I (record of first generation 

 flies), that reduction and addition of bristles are both 

 hereditary. It is further evident from Table III, (inbred 

 strain), that reduction yields readily to selection. This 

 effect may be expressed by making the number of bristles' 

 lost the numerator of a fraction of which the denominator 

 is the number of bristles normal. We then have a ratio 

 for each generation of 1913— A as follows : 



*i * 55S5 " 0-006 * .010, F 3 -^x = 0.093 * .003, 



• F ~ ' 1788 = °- 055 ± - 004, Fi * 5101) = 0,104 * ■° 03 ' 



It may be readily seen by glancing at these figures that 

 selection has a very rapid effect. It also appears that as 

 we pass from F t to F 4 the effect of selection gradually 

 diminishes. This may be expressed by dividing the above 

 decimals for each generation by that of the preceding 

 generation. 



F t 0.055 Ft 0.093 



^" 0.006 " 9 ' Ft 0.055 • ' 



Ft 0.093 1 



The reason for this decrease in the effect of selection in 

 the later generations is that as the selection advances the 

 majority of the flies become reduced in two bristles only. 



