CHARACTERS IN HEREDITY 149 



these bred inter se produce both black offspring 

 and brown ones, in the proportion three black 

 to one brown. We thus learn that black is a 

 unit character. It was contributed by one par- 

 ent to the cross, but not by the other, and trans- 

 mitted by the cross-bred individual to half its 

 offspring, but not to the other half. This is 

 Mendel's explanation of the 3:1 ratio, now fa- 

 miliar to every biologist. 



But if we cross the same black parent in the 

 foregoing case, not with a brown individual, but 

 with a white one or with a yellow one, we may 

 obtain not black offspring, but wild-colored 

 " agouti " ones, which bred inter se will produce 

 agouti, black, white (or else yellow) young, with 

 perhaps those of other new classes in addition. 

 Such a result as this puzzled Darwin, and would 

 naturally puzzle any one, but in the hght of Men- 

 del's law becomes capable of ready explanation. 

 The production of black pigment is a process in 

 which more than one unit character is concerned; 

 the production of a gray coat involves more units 

 still ; how many, can in part be determined by a 

 study of the number of classes of individuals 

 occurring in the second generation from the 

 cross, and the numerical proportions in which the 

 individuals occur in these classes. The point 

 may be made clearer by following through 

 a particular case, to which Darwin makes 

 reference. 



