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cal powers of easy digestion and profitable assimilation ; and within its 

 habitual range and location ; and which are consistent with its usual pref- 

 erences and habits of action, and are well adapted to furnish continuously 

 amounts of food answering to its varying necessities during the different 

 stages of its life. 



Advantages of Biographical Adaptation 



It is obviously to the advantage of any insect species that it shall 

 have its largest possible food supply coincident with its own largest de- 

 mand for food — that is, at the climax of its period of growth. In a 

 species restricted to one annual food ])lant the most favorabl? relation 

 will usually be that in which the life history of the plant and that of the 

 insect coincide, the egg-laying period of the one corresponding to the 

 seeding period of the other, the hatching of the insect being virtually 

 simultaneous with the germinating period of the plant, and the period of 

 most rapid growth being coincident in both. This kind of adaptation is 

 well illustrated by the life histories of Diabrofica 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 yovmg 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 mem- 

 bers 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 injury can be borne, and concentrates them, on the other hand, 

 where the largest injury can be supported with the least serious conse- 

 quences. 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 necessary to the maintenance 

 of the insect. 



The same reasoning applies and the same rule holds good for spe- 

 cies with a more heterogeneous food, exce])t 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 differ- 

 ence 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 



