JOSEPH KRAFKA., JR. 



421 



differences are produced in the high mean stocks while the lesser are 

 produced in the low mean stocks. 



The average difference in facet number accompanying a difference 

 of 1°C. in the environment in which the flies were reared is 3.09 for 

 the Ultra-bar and 14.01 for the Low Selected bar stock. 



(b) Per Cent of Change in the Mean Facet Number Accompanying a 

 Unit Change in Temperature. — In the lower rows of Tables I to VI, the 

 change per °C. is expressed in per cents of the mean. These values 

 are found by dividing the difference in facet number per °C. for the 



Per cent 

 of mean 



15° 20' 25° 30" 



Tempemture 



Fig. 7. Per cent of change in mean facet number accompanying a change of 1° 

 in temperature. The solid line represents the Unselected bar stock, the broken line 

 the Low Selected, and the dot and dash line the Ultra-bar. The upper curve in 

 each pair represents the males. 



interval by the average of the mean values for the two temperatures. 



The per cent of change per °C. in Ultra-bar varies from 17.39 to 

 4.69; in Low Selected, from 32,45 to 3.49; in Unselected, from 13.71 to 

 5.84, Thus the proportional change per °C. is (1) not constant for 

 all temperatures or (2) constant for the three stocks. 



The average per cent change per °C. in the mean of the Ultra-bar 

 is 9.22 ; for Low Selected it is 14.01. 



