256 HEREDITY AND ENVIRONMENT 



may be determined by the original factor which 

 determines whether the germ cells shall be ova 

 or spermatozoa. 



On the other hand two or more factors may 

 be concerned in the production of a single 

 character. In many cases among both plants 

 and animals the development of color appears 

 to depend upon the presence in the germ cells 

 and the cooperation in development of at least 

 two factors, viz. (1) a pigment factor for 

 black B, for brown Br, for yellow Y ' , for red 

 R, etc., and (2) a color developer C. When 

 both of these factors are present color develops, 

 when either one is absent no color appears. 



Such cases have been described for mice, 

 guinea-pigs, and rabbits as well as for several 

 species of plants. Bateson and Punnett found 

 two varieties of white sweet peas which were 

 apparently alike in every respect except the 

 shapes of their pollen grains, one of them hav- 

 ing long and the other round pollen. But 

 when these were crossed a remarkable thing 

 occurred for the progeny "instead of being 

 white were purple like the wild Sicilian plant 

 from which our cultivated sweet peas are de- 



