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THE AMERICAN NATURALIST [VoL.XLVlII 



With these are accompanying structural and physiolog- 

 ical characters, which, however, play a subordinate part. 



4. Biographical Characters. — These may consist in 

 timing the life-history of the animal with that of the 

 food-species (plant or animal) in such a way that the 

 period of greatest activity of the former coincides with 

 the period of greatest growth or abundance of the latter. 

 This feature may be incidental to seasonal change of 

 physical environment. Whatever its cause, it is very 

 general in an established association, so general that it 

 is seldom recognized. It is of advantage to both animal 

 and food species. 



5. Numerical Characters. — The rate of reproduction 

 must be so adjusted to its food-supply (plant or animal) 

 ' ' that only the unessential surplus of this food shall be 

 appropriated, leaving the essential maximum product 

 undiminished" (Forbes, 1909: 293). Species of re- 

 stricted food-habits must remain less numerous in indi- 

 viduals than general feeders, as the available food-supply 

 is very much less. 



(II) Characters Which Remove the Animal from the 



1. Structural Characters.— Structures which permit 

 animals to live in varied habitats, to take varied foods, 

 or to time their activities differently, remove each group 

 of animals from competition of all the others, resulting 

 in advantage to all. To that extent the fossorial forelegs 

 of the mole, the long proboscis of the butterfly, and modi- 

 fications of the eyes of nocturnal animals, are characters 

 which do away with competition. The structural char- 

 acters are, however, accompaniments of modifications of 

 behavior, and are secondary to the latter. 



2. Physiological Characters. — Ability to digest food- 

 materials unavailable to other animals is an advantage- 

 ous physiological character. Thus the leaf-beetle Chry- 

 sochus auratus, which lives on dogbane (Apocynutn), 

 and the " skin-beetle" Trox, which eats animal tissues 

 in an advanced stage of decomposition, have few com- 



