INSECTS IN RELATION TO OTHER ANIMALS 295 
“We notice incidentally that it is thus made unlikely that an 
injurious species can be exterminated, can even be permanently 
lessened in numbers, by a parasite strictly dependent upon it,— 
a conclusion which remarkably diminishes the economical role 
of parasitism. The same line of argument will, of course, 
apply, with shght modifications, to any animal, or even to any 
plant dependent upon any other animal or any other plant for 
existence. 
“Tt is a general truth, that those animals and plants are 
least likely to oscillate widely which are preyed upon by the 
greatest number of species, of the most varied habit. Then 
the occasional diminution of a single enemy will not greatly 
affect them, as any consequent excess of their own numbers 
will be largely cut down by their other enemies, and especially 
as, in most cases, the backward oscillations of one set of ene- 
mies will be neutralized by the forward oscillations of another 
set. But by the operations of natural selection, most animals 
are compelled to maintain a varied food habit,—so that if one 
element fails, others may be available. Thus each species 
preyed upon is likely to have a number of enemies, which will 
assist each other in keeping it properly in check. 
“ Against the uprising of inordinate numbers of insects, 
commonly harmless but capable of becoming temporarily 1n- 
jurious, the most valuable and reliable protection is un- 
doubtedly afforded by those predaceous birds and_ insects 
which eat a mived food, so that in the absence or diminution 
of any one element of their food, their own numbers are not 
seriously affected. Resorting, then, to other food supplies, 
they are found ready, on occasion, for immediate and over- 
whelming attack against any threatening foe. Especially 
does the wonderful locomotive power of birds, enabling them 
to escape scarcity in one region which might otherwise deci- 
mate them, by simply passing to another more favorable one, 
without the loss of a life, fit them, above all other animals and 
agencies, to arrest disorder at the start,—to head off aspiring 
and destructive rebellion before it has had time fairly to make 
