288 THE AMERICAN NATURALIST [Vol. LVI 



sex chromosome, mass mating in every other generation, and 

 complications resulting from only being able to test the female, 

 it is to be expected that selection will progress slowly at first 

 and come suddenly to the climax of reduced crossing over. 



Maeie S. Go wen, 

 John "W. Gowen 



Orono, Maine 



