JENNINGS: CHANGES IN HEREDITARY CHARACTERS 291 



his rats. The difference in the two cases is, that in Drosophila 

 variations which are large steps occur as well as do the small 

 ones; and that, according to Castle's conception of the matter, 

 such minute heritable variations occur more frequently in the 

 rat than in Drosophila. But on the showing of the students of 

 Drosophila, there is scarcely any other difference in principle 

 between what happens in Drosophila and what Castle believes 

 to happen in the rat. 



2. But as we have seen, the mutationists reject the view that 

 the changes in the coat color of the rat are due to alterations in 

 a single unit factor; they explain this and other cases of the 

 effectiveness of selection on a single character by multiple modify- 

 ing factors. Accepting again their contention, the question is 

 shifted to the nature of such factors. What sort of things are 

 these modifying factors? What is their relation to actual changes 

 in the heritable constitution of organisms? 



Our direct experimental knowledge of these "modifying fac- 

 tors" is scanty. What we have comes again mainly from the 

 studies of Drosophila, so that we need not suspect it of being 

 colored in such a way as to favor the selectionist point of view. 

 We find data as to certain known modifying factors by one of the 

 workers on Drosophila, Bridges (1916), in his recent important 

 paper on non-disjunction of the chromosomes. And here we 

 are taken back again to the series of eye colors, and indeed to 

 one particular member of the series, the middle member, called 

 eosin. s Bridges tells us that he found a factor whose only 

 effect was to lighten the eosin color in a fly with eosin eyes; this 

 factor indeed nearly or quite turns the eosin eye white. This 

 factor Bridges calls "whiting." Another factor has the effect 

 of lightening the eosin color a little less, giving a sort of cream 

 color; this is called "cream b." A third factor dilutes the eosin 

 color not so much; it is called "cream a." In addition to these, 

 Bridges tells us that he has discovered three other diluters of the 

 eosin color; we will call them the fourth, fifth, and sixth diluters. 

 And finally Bridges tells us of another factor whose only effect 



s Bridges, 1916, p. 148 (See Bibliography.) 



