172 INHERITANCE IN RATS. 



equilibrium toward which regression occurs serves to show that geno- 

 typic as well as phenotypic fluctuations occur in the material on 

 which selection is brought to bear. DeVries and Johannsen have 

 damned the word fluctuation by ascribing to it purely phenotypic sig- 

 nificance. Is it not worth while to rescue the term from its present 

 odious position, since it is clear that variation having a genetic basis 

 may in every way resemble somatic fluctuation, except in its behavior 

 under selection? Fluctuation may conceivably be either somatic or 

 genetic or both. No one, in advance of actual experiment, can tell 

 what its nature is in a particular case. In the case under discussion 

 the fluctuation is obviously partly somatic and partly genetic. The 

 somatic fluctuation occasions regression, the genetic fluctuation per- 

 mits a change (under selection) of the point toward which regression 

 occurs that is, in the general average of the race. 



Tables 156 and 157 show (generation by generation) the progress 

 made by selection in modifying the racial character. It will be 

 observed that as the mean advances in the direction of the selection 

 both the upper and the lower limits of variation move in the same 

 direction. The amount of the variation as measured by the standard 

 deviation is less in the last half of the experiment than in the first half. 

 It is also steadier, owing in part doubtless to the fact that the numbers 

 are larger, and in part to a more stable genetic character of the selected 

 races. But the genetic variability is plainly still large enough to per- 

 mit further racial modification and there is no indication that it will 

 cease until the hooded character has been completely selected out of 

 existence, producing at one extreme of the series all-black rats, and at 

 the other end of the series black-eyed white rats. 



