NEW VIEW OF REVERSION 133 



6. Mendelian Interpretation of Reversion 



As we have already indicated, the number of alleged rever- 

 sions has been greatly reduced by the results of the study of 

 Mendelian inheritance. An interesting re-interpretation of " re- 

 versions " has been supplied. 



Some red guinea-pigs, as Castle has shown, produce in crosses with 

 a black race the " agouti " type of coat found in all wild guinea-pigs, 

 and various experiments prove that this is due to the coming to- 

 gether of three colour-factors simple red, simple black, and a third 

 which is carried by the red but can become visible only in the presence 

 of both black and red. 



In certain instances, which are quite well denned by the Mendelian 

 experimenters, a cross between a black and an albino mouse, or be- 

 tween a black and an albino rabbit, results in a complete reversion 

 to the wild grey form. 



Crosses between the tall, upright, bush-like " Bush " sweet-pea 

 and the dwarf prostrate " Cupid " variety resulted in a procumbent 

 plant with long internodes, like the wild type that is found growing 

 in Sicily. 



In these and in similar cases it has been possible by various 

 experimental tests to give convincing proof that the reversion is 

 a re-synthesis of characters that had been analysed apart. As 

 Prof. R. C. Punnett concludes : " 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 " (1911, p. 54). 



7. Reversion in Crosses 



False Reversion or Yicinism. In his criticism of cases which 

 have been labelled " reversions," De Vries draws a sharp dis- 

 tinction between "true re version/ 'due to unknown internal causes 

 which induce long-lost latent ancestral characters to assert 

 themselves, and " false atavism or vicinism," which is due to 

 crossing. His investigation of a large number of cases led him 



