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THE AMERICAN NATURALIST [Vol. XLVIII 



leaf -beetles are thus moderately specialized plant-eaters. 

 There are also non-selective predaceous animals, as tiger- 

 beetles and lycosid spiders, which eat any kind of small 

 animal. These are also moderately specialized. The 

 moderately specialized animals carry on the gross metab- 

 olism of the association; they constitute the dominant 

 group, and include the dominant species. 



Selective feeders belong with the highly specialized 

 animals. In the bunch-grass association Languria bi- 

 color, an erotylid beetle, bores in the stems of the com- 

 posite Cacalia (Indian plantain), while Lygceus bicrucis 

 (hemipterous) feeds on the same plant; Perillus circum- 

 cinctus eats Blephar'ula rhois. Others of the associa- 

 tion eat selectively. The majority of parasites are 

 greatly restricted in their selection of hosts. Such ani- 

 mals are particularly dependent upon special kinds of 

 food, which in many cases are not available to general 

 feeders. Highly specialized forms are thus enabled to 

 avail themselves of opportunities denied to animals of 

 generalized type; but while they avoid competition by 

 the adoption of special kinds of food, or by special habit 

 of some other kind, they lack the versatility of the less 

 specialized animals, being unable to adjust themselves 

 to changed conditions. They may, therefore, become 

 abundant at times ; but as they depend wholly upon one 

 variable condition (perhaps the presence of a particular 

 plant species, which may be quite infrequent) they never 

 can become dominant species. Absolute numbers of the 

 insects which live upon Cacalia, for example, are insig- 

 nificant in comparison with such animals as the grass- 

 hoppers. 



On the other hand, animals of relatively non-specialized 

 habits would also be ineffective in the association, for 

 whatever field of activity they were to enter, they usu- 

 ally would find already occupied by some animal better 

 constituted for that activity. Such non-specialized forms 

 would assume particular importance only when some 

 animal on which they might feed should become unusually 



