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THE AMEBIC AX NATUBALIS T [Vol. XLIII 



Diabrotira longicornis and the corn plant. This beetle 

 lays its eggs in fall when the ear is maturing, and the 

 larvae hatch in spring when the corn plant is young and 

 growing slowly, and they feed on the roots during the 

 entire growing season of the plant. It is evident that 

 such a well-adjusted insect will have an advantage, other 

 things being equal, over a poorly adjusted competitor for 

 food from the same plant, since it will be able, as a rule, 

 to leave a more vigorous and abundant progeny; and 

 similarly, any part of a species which, by aberration of 

 life history, may come to be poorly adjusted to its food 

 plant, will suffer as a consequence in comparison with 

 the normal members of the species, with the result that 

 these biographical characters of the insect will tend to 

 become permanent and characteristic in the same sense 

 in which its structural characters are. 



It should be noticed, also, that such an adjustment is 

 an advantage to the host plant as well as to the dependent 

 insect, since it distributes the depredations of the latter in 

 a way to make them relatively slight when but little in- 

 jury can be borne, and concentrates them, on the other 

 hand, where the largest injury can be supported with the 

 least serious consequences. Such a well-adjusted insect 

 will get the maximum amount of food with the minimum 

 injury to the plant, and such a plant-insect pair will have 

 a competitive advantage over a poorly adjusted pair in 

 which a greater injury is done to the plant than is neces- 

 sary to the maintenance of the insect. 



The same reasoning applies, and the same rule holds 

 good, for species with a more heterogeneous food, except 

 that in respect to them we must substitute for the single 

 plant the entire group of plants to which the insect resorts 

 for food. At this point, however, the facts become too 

 complicated for successful analysis, especially in view of 

 the difference of abundance from year to year of the 

 plants of a considerable list, and the effects, on the food 

 supply, of variable competitions among the various species 

 resorting to it. It may be said, in general terms, how- 



