777 K . 1 ME IUC. 1 A' A\ t TURALIST [Vol. XLIII 



extension, a profound effect on the numbers and distribu- 

 tion of some insect species, reducing the area of multipli- 

 cation for several, which, like the cutworms and the 

 army-worm, formerly bred in the turf of our native 

 prairies, but can not breed in fields of corn ; and immensely 

 extending the range and increasing the number of others 

 which have found in this plant a better and far more 

 abundant food supply than that originally available to 

 them. Insect species which, like Diabrotica longkomis 

 and Aphis )»<ii<Hra(ii< is, were almost unknown fifty years 

 ago within our territory, have now, through their increase 

 in cornfields, arisen to the rank of dominant species. 



But the few discernible insect adaptations to the offer- 

 ings of the corn plant are physiological, psychological, 

 synethic and biographical, and apparently not structural 

 at all. Slight and seemingly incipient as they are, we 

 have no sufficient reason to conclude that they are recent 

 results of the association of the corn plant with the in- 

 sect ; both parties of the association may have been sub- 

 stantially what they now are when they first found each 

 other, and such mutual fitness as they exhibit may be 

 merely like that of angular stones shaken together in a 

 box until like surfaces seem to cohere, simply because 

 in this position the fragments can not readily be shaken 

 apart. 



We may also derive, from this discussion, support for 

 the idea that adaptations of insects to their environment 

 are largely, and often primarily, psychological— that they 

 are often, in the first instance, specializations of prefer- 

 ence or choice, or, as we may perhaps more safely say, 

 of tropic reaction. Species which would otherwise com- 

 pete with each other, with disadvantageous consequences 

 to each, escape these disadvantages by acquiring, one or 

 both, different habits of reaction, under the influence of 

 which they separate, one going for its principal food to 

 the corn plant, for example, and the other continuing 

 on the strawberry, although structurally each remains 

 equally fit to feed on either. Physiological, or even strue- 



