On Some Interactions of Organisms. 15 
felt, and intelligent effort will mitigate some evils and 
avoid others. Without attempting to go into deatils — 
which would be quite unnecessary for my purpose — I will 
endeavor 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 
which are well adjusted to their environments, organic 
and inorganic, are either harmless or inflict but moderate 
injury (our ordinary crickets and grasshoppers are ex- 
amples) ; while those that are imperfectly adjusted, whose 
numbers are, therefore, subject to wide fluctuations, like 
the Colorado 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 sta- 
tionary 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 oscil- 
lations within the narrowest limits possible. 
For example, remembering that every species oscillates 
to some extent, and is held to relatively constant num- 
bers by the joint action of several restraining forces, we 
see that the removal or weakening of any check or barrief 
is sufficient to widen and intensify this dangerous oscilla- 
tion; 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 be seen, lim- 
ited there as it is by its feeble locomotive power and the 
scattered situation of the trees it infests. With the multi- 
plication and concentration of its food in towns, it has in- 
creased enormously, and if it has not done the gravest 
injury it is because the trees attacked by it are of com- 
paratively 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 spe- 
cies of insectivorous bird or predaceous insect is a thing 
to be done, if at ail, only after the fullest acquaintance 
