12 On Some Interactions of Organisms. 
upon any other animal or any other plant for existence. 
From the foregoing argument we conclude that, since 
the interest of a species of plant or animal and the inter- 
est of its * 1 enemies’ ’ are identical, and since the opera- 
tions of natural selection tend constantly to bring about 
an adjustment of the species and its enemies which shall 
best promote this common interest, therefore the anni- 
hilation of all the established “enemies” of a species 
would , as a rule , have no effect to increase its final aver- 
age numbers. This being a general law, applying to all 
organisms, it is plain that the real and final limits of a 
species are the inorganic features of its environment — 
soil, climate, seasonal peculiarities, and the like. 
In treating of the external forces brought to bear upon 
an oscillating species to restrain its disastrous fluctua- 
tions, I shall mention only a part of the organic checks 
to which it is subject. 
It 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 ene- 
mies, and especially as, in most cases, the backward oscil- 
lations of one set of enemies will be neutralized by the 
forward oscillations of another set. But by the opera- 
tions of natural selection, most animals are compelled to 
maintain a varied food habit — so that if one element fails 
others may be available. Tlius 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 
injurious, the most valuable and reliable protection is un- 
doubtedly afforded by those predaceous birds and insects 
which eat a mixed food, so that in the absence or diminu- 
tion 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 imme- 
diate and overwhelming attack against any threatening 
