1920] . Forbes: General Discussion 197 



ments and avoidances of competition, exhibited in their relations 

 to each other. We have much reason to expect, indeed, that 

 an insect association of long standing in a geographical area, 

 or in a set of situations of fairly uniform character, will have 

 come to make many internal readjustments — adaptations of 

 one species to another in habit and life history, of each species, 

 perhaps, to several others, of different stages of the same species 

 each to the other, such that the whole association may avail 

 itself to the best advantage of the resources for existence and 

 multiplication offered to it by any environment. The strains 

 and pressures of competition will thus be in some measure 

 relieved, and an internal equilibrium of the ecological group 

 will be reached which will smooth and steady the system 

 of interactions within the association, to the general advantage 

 of all its members. It seems to me quite possible that a single 

 species of wide range may have become a permanent member 

 of unlike associations in different parts of its area of distribution ; 

 may have had to adjust itself, consequently, to different 

 systems of interaction with its associates; may have acquired 

 local peculiarities of life history not to be understood until these 

 internal systems have been studied and made out. 



The subject is, indeed, delightfully complex — a challenge to 

 the curiosity and ingenuity of the accomplished naturalist 

 equipped with apparatus for exact experiments with variations 

 of temperature, moisture, light, rates of air movement and 

 evaporation, such that he can produce any desired combination 

 of these natural factors of the insect environment and determine 

 their separate and conjoined effects on the life cycle of any 

 species which he wishes to study in detail. 



An equipment of this description is invaluable in testing 

 the inferences of the field observer and in detecting reactions 

 to features of the insect environment which are obscured or 

 lost in the complex of the natural system out-of-doors. By 

 such a means we are beginning to account for some, at least, of 

 the almost explosive outbreaks of insect multiplication in 

 certain species, which we find peculiarly sensitive to meteoro- 

 logical conditions by which others are little affected. Doctor 

 Shelford tells me, for example, as a result of his studies now in 

 progress on the chinch-bug, that these insects, whose numbers 

 fluctuate enormously in successive years, are extremely depend- 

 ent on relatively high temperatures; that with optimum 



