No. 600] PIEBALD RATS AND MULTIPLE FACTORS 735 



the mutant and the pins race at the time the mutant ap- 

 peared, this factor affords a critical test for the interpre- 

 tation of the modifications that result from crosses. 



Objection to the Multiple Factor Interpretation 

 One new point since 1914 has been urged against the 

 application of the multiple factor hypothesis to the re- 

 sults. By the strength of this evidence the authors of 

 the rat publication are "forced to conclude that this unit 

 (hoodedness) itself changes under repeated selection in 

 the direction of selection "; (Castle, :15b, p. 722). The 

 point follows : 



The changes effected by selection show permanency under crosses with 

 wild rats. They change no more nor less than an unselected hooded 

 race does. A first cross of the selected race seems to show a partial 

 undoing of the changes produced by selection, but a second cross made 

 on a still larger scale, involving over 1,000 second-generation individu- 

 als, showed no further change of this sort, but instead a return to about 

 what the selected race would have been had no crossing at all occurred 

 (Castle, :16, p. 96). 



If the grade of hooding of the plus race is reduced in 

 crosses with wild by the replacement of factors selected 

 out of the plus race, repeated crossing of the modified rats 

 should produce further reduction. On the basis of the 

 above claim that crosses do not produce such modifications 

 in the hooded pattern all the evidence formerly admitted to 

 favor the multiple factor interpretation has been swept 

 aside. No one would claim that a single strongly sup- 

 ported experiment may not upset large amounts of con- 

 trary evidence, but in such cases it is of utmost impor- 

 tance to have the validity of the crucial experiment fully 

 supported. Is the claim that crosses do not change the 

 selected races fully supported? The following are all 

 the data we are given on this point : 



Extracted hoodeds from 



Extracted hoodeds from extracted 

 hooded X wild 263 rats, average 3.33 ; advance on grandparents .32 



