BRISTLE INHERITANCE IN DROSOPHILA 115 



and evident responses to environment not present, such as are 

 found in generations 26 to 30, the curves might be treated math- 

 ematically, and a theoretical curve fitted, but since such clear 

 evidence of the irregular influence of environment is present, 

 this mathematical treatment seems out of place. The critical 

 period is between generations' 11 and 23. In this period it 

 seems possible that the ideal condition is represented by the 

 series of high points at grade 4f for the females, that the long 

 gradual decline is due to less and less favorable conditions, 

 which later are improved till the high point is again reached. 

 It may be somewhat problematical whether the high or the 

 low points (at 3^) in this period should be considered ideal, 

 yet it does seem probable that an intermediate position would 

 not be so considered. Moreover there can be found no tend- 

 ency for the later generations in this period to be higher than 

 the earlier ones. The irregularities in generations 24, 25 and 

 26 may be discounted, since, as explained, these include very 

 small families. Although the high peak in generations 29 to 

 31 may not be directly due to the conditions of the constant 

 temperature room, yet its occurrence immediately upon the 

 introduction of the flies into that room, and the absence of any 

 comparable high region in all the twenty months of the experi- 

 ment, strongly suggest a causal connection between the two. 

 However it is evident that whatever stimulus the constant 

 humidity and temperature may have been at first, the effective- 

 ness of this stimulus became weakened long before the flies 

 were removed from this room. The portions of the curves of 

 the sons and daughters following this high region show general 

 fluctuations, but one can- not detect a tendency for the earlier 

 generations to have lower means than the later ones. Between 

 generations 36 and 38 there is a continuous rise, but this rise 

 does not exceed the limits of fluctuation shown by the later 

 generations. 



To conclude, it may be said that there is a rise at the beginning 

 of selection ; that this is followed by a period in which no rise is 

 discernible; that the irregularities in generations 24, 25 and 26, 

 and the sudden rise in generations 29 to 31, are not due to the 



