HEREDITY 



to which a character develops in the zygote, 

 i. e. the completeness or incompleteness of its 

 dominance in a particular case. For example, 

 in salamanders, which apparently, like mam- 

 mals, form skin-pigments of different sorts, 

 such as yellow, brown, and black, Tornier has 

 found that by feeding one may control the 

 proportions in which chromatophores of the 

 several sorts are formed in the skin. Abundant 

 feeding causes preponderance of pigment of 

 one sort, scanty feeding causes preponderance 

 of pigment of another sort. Here external 

 conditions determine the degree of development 

 of characters. In other cases internal condi- 

 tions may exercise a controlling influence. Thus 

 in cattle the capacity to develop horns is a 

 semi-potent unit-character, behaving as a re- 

 cessive in crosses, heterozygotes developing 

 only " scurs," that is, mere thickenings of the 

 skin, or else no trace of horns at all. In sheep, 

 moreover, horns are more strongly developed 

 in males than in females, the presence of the 

 male sex-gland in the body, or rather probably 

 some substance given off into the blood from 

 the sex-gland, favoring growth of the horns. 



102 



