The Method of Evolution 57 



offspring of this animal by normal unrelated mothers were 

 polydactylous, but among his offspring were some with 

 better developed fourth toes than the father possessed. 

 Such individuals were selected throughout five successive 

 generations, at the end of which 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 

 produced in such crosses varied all the way from o to 100 



Fig. 30. — A, hind-feet of an ordinary guinea-pig; B, of a four-toed guinea- 

 pig; C, of an imperfectly four-toed guinea-pig. 



per cent. By selection this percentage was increased, as 

 was also the degree of development of the fourth toe in 

 crosses. 



Another character which made its appearance 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 (Fig. 28). The first individuals observed 

 to have this character bore white hairs on the under surface 

 of the body only. By inbreeding, a homozygous strain of 

 the silvered animals was soon obtained, one in which all the 

 offspring were silvered to a greater or less extent. Selection 



