210 Life and Death, Heredity and Evolution 



heritable variation may occur so small as to be barely de- 

 tectible. Although the variations do not usually occur in 

 this way, the case present the conditions which would allow 

 of a gradual transition from one extreme to the'other, by 

 means of numerous intermediate conditions. 



2. But, as we have seen, the gradual changes in heredi- 

 tary characters seen in selective breeding usually do not oc- 

 cur in this way, but rather by the slow accumulation of 

 many factors each having a slight effect, the multiple modi- 

 fying factors. But what sort of things are these factors and 

 what is their relation to actual changes in the heritable con- 

 stitution of the organism? 



Our direct experimental knowledge of these "modifying 

 factors" is scanty; it comes mainly from the studies of 

 Drosophila. We find data as to certain known modifying 

 factors by Bridges (1916) in his 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 par- 

 ticular member of the series, the middle member, called eosin. 

 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 is to modify eosin in the direction of 

 a darker color ; this factor he calls "dark." None of these 

 factors has any effect save on eosin-eyed flies. 



As you see, these things add tremendously to our grada- 



