On Some Interactions of Organisms. 15 



deavor briefly to show the bearing of some of these ideas 

 upon practical conduct. 



To man, as to nature at large, the question of adjust- 

 ment is of vast importance, since the eminently destruc- 

 tive species are the widely oscillating ones. Those insects 

 Avhich are well adjusted to their environments, organic and 

 inorganic, are either harmless or inflict but moderate inju- 

 ry (our ordinary crickets and grasshoppers are examples) ; 

 while those that are imperfectly adjusted, whose numbers 

 are, therefore, subject to wide fluctuations, like the Col- 

 orado grasshopper, the chinch-bug and the army-worm, 

 are the enemies which we have reason to dread. Man 

 should then especially address his efforts, first, to prevent 

 any unnecessary disturbance of the settled order of the 

 life of his region which will convert relatively stationary 

 species into widely oscillating ones; second, to destroy or 

 render stationary all the oscillating species injurious to 

 him ; or, failing in this, to restrict their oscillations within 

 the narrowest limits possible. 



For example, remembering that every species oscillates 

 to some extent, and is held to relatively constant numbers 

 by the joint action of several restraining forces, we see 

 that the removal or weakening of any check or barrier is 

 sufficient to widen and intensify this dangerous oscillation ; 

 may even convert a perfectly harmless species into a 

 frightful pest. Witness the maple bark-louse, which is so 

 rare in natural forests as scarcely ever to l)e seen, limited 

 there as it is by its feeble locomotive power and the scat- 

 tered situation of the trees it infests. With the multiplica- 

 tion and concentration of its food in towns, it has increased 

 enormously, and if it has not done the gravest injury 

 it is because the trees attacked by it are of comparatively 

 slight economic value, and because it has finally reached 

 new limits which hem it in once more. 



We are therefore sure that the destruction of any species 

 of insectivorous bird or predaceous insect is a thing to be 

 done, if at all, only after the fullest acquaintance with the 

 facts. The natural presumptions are nearly all in their favor. 

 It is also certain that the species best worth preserving are 

 the mixed feeders and not those of narrowly restricted diet- 



