156 THE BEHAVIOR OF UNIT 



in a reproductive cell transmitting the color char- 

 acters of a gray rabbit : — 



U 



I 

 A— C— B— E 



It is possible that in protoplasm we have or- 

 ganic molecules built up in some such way, and 

 that regressive variations arise by dropping off 

 the constituent parts of the molecule one by one. 

 Certainly it is loss or extreme modification of 

 factors that produces the ordinary color varia- 

 tions. If A drops out, we have a black rabbit 

 instead of a gray; if E is replaced by R, a yellow 

 one is produced; if both these changes occur, a 

 sooty one; if U is replaced by S, we have a 

 spotted gray rabbit; if both U is replaced by S, 

 and A is lost, a spotted black rabbit results; if 

 C is lost, we have an albino, whose breeding ca- 

 pacity varies with the number of other invisible 

 factors which remain. 



The list of known color factors is not ex- 

 hausted by those which I have enumerated. One 

 other, a factor for brown pigmentation, Br, has 

 been revealed in .the case of the guinea-pig, the 

 mouse, and the dog, by loss of factor B. Brown 

 pigmentation then everywhere replaces black. 

 This factor bears the same relation as does B to 

 both C and E. Some time doubtless we shall see 



