294 Retrograde Varieties 



ity. Read in the light of modern ideas on unit- 

 characters it is now one of the most important 

 works on heredity and has already widespread 

 and abiding influence on the philosophy of hy- 

 bridism in general. 



But from its very nature and from the choice 

 of the material made by Mendel, it is restricted 

 to bisexual or varietal crosses. It assumes 

 pairs of characters and calls the active unit of 

 the pair dominant, and the latent recessive, 

 without further investigations of the question 

 of latency. It was worked out by Mendel for a 

 large group of varieties of peas, but it holds 

 good, with only apparent exceptions, for a wide 

 range of cases of crosses of varietal characters. 

 Recently many instances have been tested, and 

 even in many cases third and later generations 

 have been counted, and whenever the evidence 

 was complete enough to be trusted, Mendel's 

 prophecy has been found to be right. 



According to this law of Mendel's the pairs 

 of antagonistic characters in the hybrid split up 

 in their progeny, some individuals reverting to 

 the pure parental types, some crossing with 

 each other anew, and so giving rise to a new 

 generation of hybrids. Mendel has given a 

 very suggestive and simple explanation of his 

 formula. Putting this in the terminology of 

 to-day, and limiting it to the occurrence of only 



