98 ANNALS NEW YORK ACADEMY OF SCIENCES 



acter of the coat shifted in later generations in the direction of selection ; 

 and Castle has shown for rats similar shifting by selecting the darker 

 coat pattern. If my interpretation is correct selection here affects the 

 stage at which the "segregation" occurs in development. The cause of 

 this shifting must still obviously go back to some change in the germ cells, 

 but the change so brought about does not, from this point of view, cause 

 a change in the character of the coat carried by the germ cells, but a 

 change in the factor that determines when the segregation takes place in 

 the cleavage of the egg. 



The uniform coat appears to bear the same relation to the spotted coat 

 that the uniformly colored hairs (yellow, black, chocolate or white) bear 

 to the gray or ticked hairs. From this point of view the "ticking factor" 

 has the same theoretical value as the "spotted factor." It is, of course, a 

 fiction to assume that all the hairs of a gray mouse are alike and consist 

 of the three bands — black, yellow, chocolate — in the same relation. A 

 casual examination of the hair under the microscope will suffice to show 

 the wide range of variability that exists and the numerous departures 

 from this rule. The variability is almost as great as that shown by the 

 spotted coat. Nevertheless, in a general way, most hairs of the body may 

 be said to conform to this scheme. 



I have met with this same problem in certain mutations in Drosophila. 

 In one case, a heterozygous fly had one white eye and one red one. Here 

 the separation must have occurred after the egg had been fertilized. The 

 tendency was not inherited. The point will be discussed below. On two 

 occasions, I have had a heterozygous fly that had one normal wing and 

 one small "proportionate" wing. These flies when bred to males of the 

 race with proportionate wings (the recessive) have produced both long- 

 and short-winged individuals, showing the heterozygous nature of the 

 original asymmetrical fly. I then crossed again the long and short flies 

 to bring about the same heterozygous combination, in the hope that the 

 "asymmetrical factor" would come to light again, but of several hundred 

 offspring not one was asymmetrical. 



Influence of the Spotted Coat on the White Belly of the Sport 



The wild sport was crossed on several occasions with spotted mice. The 

 hybrids were gray mice with white or gray bellies. The former when 

 inbred produced some spotted mice with white bellies. An examination 

 of these mice showed that whenever a white spot extended into the region 

 of the white belly, the black at the base of the hair that is present, as 

 stated, in the ordinary white belly is absent. In other words, the spot 



