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 Mendelian 
ratio of 3:1 was obtained in the F, 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 
