Heredity 183 



green and yellow pigment in yellow and in green peas 

 has also been demonstrated. 



Green-seeded varieties therefore contain two pigments 

 in their cotyledons, a yellow and a green ; neither of them 

 fades during the process of ripening, and inasmuch as 

 the green masks the yellow, the ripe seed is green. Yellow- 

 seeded varieties also contain the same two pigments, but 

 the green fades in the process of ripening, so that the 

 ripe seed is yellow. This fading of the green pigment in 

 the yellow pea is supposed to be brought about by the 

 presence of some substance which is absent from the green 

 pea. Similarly, when the apparent absence of a character 

 is dominant, as in the case of dominance of hornlessness in 

 cattle and of white color in swine, there is believed to be 

 present an inhibiting factor or " inhibitor" which pre- 

 vents the formation of the black pigment. In other 

 words, it is the presence of the inhibitor (causing white) 

 over its absence (black) which explains the phenomena of 

 the dominance of white over black. 



It is not the dominance of an absent factor, but the 

 presence of an unseen inhibitor, which reacts upon the 

 otherwise visible character, causing it to disappear. 



Let us now consider another type of cause which may 

 be explained on the basis of the presence-and-absence 

 hypothesis. The heredity of the combs of fowls has been 

 carefully studied by Bateson, Davenport, Punnett, and 

 others. The latter gives an excellent description l of this 

 on the presence-and-absence hypothesis. 



Four types of combs are recognized ; namely, rose, pea, 

 walnut, and single. (See Fig. 46.) 



1 Punnett, " Mendelism," pp. 35, 36. 



