860 THE AMERICAN NATURALIST. 
the walls this is unnecessary. The disappearance of distinct 
tracheal channels has restored open competition between the 
tracheze, which accounts for the more rapid disappearance of 
all save those most favorably situated in relation to the source 
of the air supply — usually only two in the most specialized 
insects, which still stand as representatives of the two groups 
of tracheze with which we begin our series. 
Fortunately for the study of homologies in insect wings, the 
veins had attained an arrangement so useful that it could be 
held by natural selection after the tracheae had ceased to 
determine their position. ; 
