254 MULTIPLE FACTORS 



experiments had been begun to test the Darwinian 

 problem of whether selection of a fluctuating 

 character in a given direction would tend to further 

 variation in the same direction, and so enable the 

 establishment of a genetic type with a new mode, 

 and a new range of variation. When Castle found 

 that the selection was in fact successful in these 

 respects, he interpreted the results to mean that 

 through selection, or after selection, a ''unit charac- 

 ter" can be changed. He has used at times a word 

 familiar to readers of Darwin, namely, "potency." 

 The potency of a factor as well as of a character is 

 supposed to be a somewhat variable element, and 

 it was apparently presumed that this property of 

 the factor was responsible for the observed fluctu- 

 ation, rather than any recombination of modifying 

 factors. 



In support of the view that the particular charac- 

 ter of the hooded rat differs from the wild rat by a 

 single (fluctuating) factor, rather than by multiple 

 factors, it was pointed out that the Mendehan 

 ratio of 3 : 1 was obtained in the E. generation 

 when these types were crossed. It should be ob- 

 served, however, that this ratio only shows that a 

 recessive factor for hoodedness must be present in 

 order that the rats may be hooded at all. One- 

 fourth of the rats will receive this factor and only 

 these will appear as some grade of hooded. Other 

 pairs of factors that modify the coat will be dis- 

 tributed independently of the former factor through- 

 out the F. individuals, but they may produce 



