SEGREGATION AND DOMINANCE 129 



other hand, they are handicapped by being unable 

 usually to obtain more than one generation in a year, 

 while zoologists may secure from many animals like 

 rabbits and mice several generations in a year, al- 

 though ordinarily the number of progeny is much 



smaller and the ratios obtained have a larger chance 

 of error than is the case with the more numerous 

 plant offspring. 



Semi-microscopic animals, as, for example, the 

 pomace fly, Drosophila, which produces a large progeny 

 every two weeks or so, may combine the general 

 advantages mentioned for the two groups of organisms 



