STRUCTURAL EFFECTS 143 



dominant factor in controlling wing development 

 in this species of aphid. The survival value of this 

 arrangement is immediately apparent when one 

 remembers that the winged individuals are capable 

 of migrating through the air to new host plants 

 and hence can keep the colony alive by such trans- 

 fers after conditions become crowded and the 

 original host begins to fail. 



Other bodily changes are produced in different 

 animals as a result of crowding. The marine 

 ascidian, Salpa, exists in two distinct forms, the 

 asexual, solitary, cask-like form and the crowded 

 sexual salpa chains whose individuals, apparently 

 through crowding, have lost their barrel-like 

 form. Crowded snails are not only smaller than 

 non-crowded ones but if the crowding is extreme, 

 other physical changes are produced as well. 

 The male organs may be suppressed, the liver 

 becomes smaller in proportion to the other organs 

 and the shell proportions are so changed that ex- 

 perienced experts in the identification of snails 

 recognized the dwarfed forms as sufficiently dis- 

 tinct to be placed in a distinct species from that 

 of their uncrowded relatives. 



The recent mass of careful work in the culture 

 of the common fruit-fly, Drosophila, in the course 

 of studies on its heredity, have shown that over- 

 crowding is one of the three main causes of dis- 



