CHAPTER VI 



REVERSION 



• As soon as the idea was grasped that characters in 

 plants and animals might be due to the interaction 

 of complementary factors, it became evident that this 

 threw clear light upon the hitherto puzzling pheno- 

 menon of reversion. We have already seen that in 

 certain cases the cross between a black mouse or 

 rabbit and an albino, each belonging to true breeding 

 strains, might produce nothing but agoutis. In other 

 words, the cross between the black and the white in 

 certain instances results in a complete reversion to the 

 wild grey form. Expressed in Mendelian terms, the 

 production of the agouti was the necessary conse- 

 quence of the meeting of the factors C and G in the 

 same zygote. As soon as they ai-e brought together, * 

 no matter in what way, the reversion is bound to 

 occur. Reversion, therefore, in such cases we may 

 regard as the bringing together of complementary 

 factors which had somehow in the course of evolution 

 become separated from one another. In the simplest 

 cases, such as that of the black and the white rabbit, 

 only two factors are concerned, and one of them is 

 brought in from each of the two parents. But in 



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