334 ANIMAL AGGREGATIONS 



responsible for these 'crowding' and 'age of culture' effects, indicate 

 that they are due largely, perhaps entirely, to competition for food. 

 This factor seems to affect especially the younger larvae when in 

 competition with older ones." 



When virgin Fi females from a cross of a wild type by vestigial 

 winged Drosophila were backcrossed to vestigial males, an equal 

 number of eggs destined to produce hybrid and homozygous vestig- 

 ial flies was laid. Under conditions of overcrowding, Harnly (1929) 

 found that the proportion of vestigial to the wild-type hybrid flies 

 was reduced, thus demonstrating a pronounced selective effect of 

 overcrowding. Similar results were obtained whether crowding was 

 produced by using a reduced surface area of food or by increasing 

 the length of the egg-laying period with the food area (and depth) 

 remaining constant. Clausen (1924) reported a similarly low survi- 

 val value for vestigial flies reared under crowded conditions. 



Evidence from these students of genetics concerning the effective- 

 ness of an environmental factor is the more trustworthy since they 

 do not have the reputation of being easily convinced of the effective- 

 ness of environmental factors upon morphological characters. 



