456 



services of a nurse. The corn field contains an artificial "association" 

 persistently maintained by human agency in the midst of a hostile en- 

 vironment to which it would promi)tly succumb if left to itself, and as 

 such it would seem to offer to the ecologist all the advantages of a vast 

 and long-continued experiment, by a study of whose results he may learn 

 something of the manner in which ecological relations may be affected 

 when a plant takes advantage of a single favoring condition to push its 

 way into a territory foreign to its former habits. 



This corn plant, at least, which has certainly lived in our territory 

 under the care of man for several centuries, and perhaps for some millen- 

 niums, has even yet no specialized friends active in its service, and no 

 structurally adapted enemies enlisted against it, such specializations of 

 injurious relationship as one detects being clearly due to other than struc- 

 tural differentiations. During all this long period, it has been widely 

 and steadily forced into a strange ecological system which has neverthe- 

 less scarcely yielded to it at any point. It has produced, it is true, by its 

 enormous multiplication and extension, a profound eft'ect on the num- 

 bers and distribution of some insect species, reducing the area of multi- 

 l)lication for several, which, like the cutworms and the army-worm, for- 

 merly 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 longicornis and Aphis maidiradicis. were almost unknown 

 fifty years ago within our territory, have now, through their increase in 

 corn fields, arisen to the rank of dominant species. 



But the few discernible insect adaptations to the offerings 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 insect ; both parties 

 of the association may have been substantially 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 un'.il 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 pri- 

 marily, psychological — that they are often, in the first instance, specializa- 

 tions of preference or choice, or, as we may perhaps more safely say, 

 of tropic reaction. Species which would otherwise compete with each 

 other, with disadvantageous consequences to each, escape these disad- 

 vantages 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. Physio- 



