MENDELISM AND SELECTION 



traits. Thus the extra toe of polydactylous 

 guinea-pigs made its appearance as a poorly 

 developed fourth toe on the left foot only. 

 Only 6 % of the offspring of this animal by 

 normal unrelated mothers were polydactylous, 

 but among his offspring were some with better 

 developed fourth toes than the father pos- 

 sessed. Such individuals were selected through- 

 out five successive generations, at the end of 

 wliich time a good four-toed race had been 

 established. It was found in general that those 

 animals which had best-developed fourth toes 

 transmitted the character most strongly in 

 crosses with unrelated normal animals. The 

 percentage of polydactylous individuals pro- 

 duced in such crosses varied all the way from 

 to 100 %. By selection this percentage was 

 increased, as was also the degree of develop- 

 ment of the fourth toe in crosses. 



Another character which made its appear- 

 ance among our guinea-pigs,, at first feebly 

 expressed, was a silvering of the colored fur, 

 due to interspersing of white hairs with the 

 colored ones (see Fig. 37). The first indi- 

 viduals observed to have this character bore 



121 



