HEEEDITY 



of pigment formation without alternation in the 

 character of the pigments produced. 



In rabbits as well as in guinea-pigs rever- 

 sion to the original wild type, in this case 

 gray, may be obtained by crossing a black ani- 

 mal with a yellow one. In guinea-pigs the 

 yellow (or red) animal which will yield this 

 result cannot be distinguished in appearance 

 from one which will not; but in rabbits the 

 yellow animal which will give reversion has 

 a white belly and tail, while the one which 

 will not give reversion is not so distinguished. 



We now know what is implied in the fixa- 

 tion of a heterozygous character obtained by 

 crossing. When A and B are crossed we ob- 

 tain a third condition, C. C is due either to 

 the simple coexistence of A with B, or to the 

 coexistence with them of a third factor intro- 

 duced with one or the other. In either case 

 fixation will consist in getting into the gamete 

 all the factors necessary to the production of 

 C. In the first supposed case the zygote is 

 A-B and the resultant is equivalent to C. Fix- 

 ation will consist in getting a zygote of the 

 formula AB • AB. In the second supposed 



70 



