178 GENETICS 



white with black flecks in their plumage. These black 

 flecks, however, disappear at the time of the first 

 molt. The complete dominance of white is, there- 

 fore, simply delayed. 



4. "Reversed" Dominance 



In certain instances there seems to be a reversal 

 of dominance, as may be illustrated by Lang's results 

 w^ith snails (Helix) . He has proven in his experiments 

 that red snails are generally dominant over yellow 

 snails, although in certain cases there is apparently 

 an exception to the rule, for snails with yellow shells 

 dominate those with red shells. 



Davenport also has shown that although extra 

 toes are usually dominant over the normal number 

 in poultry, yet, in something like 20 per cent of the 

 cases, the normal number is dominant. 



To speak of these cases as instances of "reversed 

 dominance," is open to serious objection, since such 

 an explanation does not agree with the generally 

 accepted "presence and absence" idea of heritable 

 characters. It is difficult to see how the presence of 

 a certain determiner can dominate in a part of the 

 offspring of any cross and the absence of the same 

 determiner be able to dominate the remainder. 



It is perhaps nearer the truth to conceive that in 

 cases of apparent "reversal" of dominance there is 

 an insufficient amount of a particular determiner 

 available to bring the character concerned into 

 expression. In other words, although a dominant 

 character may be present in two cases, yet in one 



