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 puzzHng phenomenon of 

 reversion. We have already seen that in certaiu 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 consequence of the meeting of the factors C and 

 G in the same zygote. As soon as they are brought to- 

 gether, 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 con- 

 cerned, and one of them is brought in from each of the 



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