74 HEREDITY [ch. 



produced ' reversion on crossing ' and tlie young had 

 reverted to the ancestral wild form. It is not of 

 course necessary that the albino used to produce 

 such a 'reversion' should itself be the offspring of 

 a grey ; such grey-bearing albinos may be bred 

 together for an indefinite number of generations, 

 and still carry the factor (? ; or if they were ori- 

 ginally derived from a black stock they would bear 

 the factor B. When such stocks are crossed together 

 heterozygous GB albinos are produced, and G and B 

 segregate from one another in the albino just as in 

 the coloured rats in which the colour-factor P is 

 present. The fact that colour in animals and plants 

 depends on the concurrent action of distinct factors 

 thus explains the phenomena of 'reversion on crossing' 

 which have so long been a puzzle to biologists. 



Among the varieties of the brown (grey) rat 

 only two colour types occur, grey (wild-colour) and 

 black, but in the rabbit, mouse and other animals 

 more are found. In the mouse there are four funda- 

 mental colour-types, yellow, grey, black and chocolate. 

 The behaviour of yellow is complicated and not yet 

 thoroughly understood, but of the others, grey crossed 

 with either black or chocolate gives grey ; black with 

 chocolate gives black, and chocolate can only appear 

 in the absence of all the others. This was formerly 

 described by saying that grey was dominant over 

 black and chocolate, and black over chocolate, but 



