INSECTS IN RELATION TO OTHER ANIMALS 293 
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“T next point out the fact that precisely the same reasoning 
apples to predaceous and parasitic insects. Their interests, 
also are identical with the interests of the species they para- 
sitize or prey upon. <A diminution of their food reacts to de- 
crease their own numbers. They are thus vitally interested 
in confining their depredations to the excess of individuals 
produced, or to redundant or otherwise unessential structures. 
It is only by a sort of unlucky accident that a destructive spe- 
cies really injures the species preyed upon. 
“The discussion has thus far affected only such organisms 
as are confined to a single species. It remains to see how it 
applies to such as have several sources of support open to 
them,—such, for instance, as feed indifferently upon several 
plants or upon a variety of animals, or both. Let us take, 
first, the case of a predaceous beetle feeding upon a variety of 
other insects,—either indifferently, upon whatever species is 
most numerous or most accessible, or preferably upon certain 
species, resorting to others only in case of an insufficiency of 
its favorite food. 
“Tt is at once evident that, taking the group of its food- 
insects as a unit, the same reasoning applies as if it were re- 
stricted to a single species for food; that 1s, it 1s interested in 
the maintenance of these food-species at the highest number 
consistent with the general conditions of the environment,— 
interested to confine its own depredations to that surplus of 
its food which would otherwise perish 1f not eaten—interested, 
therefore, in establishing a rate of reproduction for itself 
which will not unduly lessen its food supply. Its interest in 
the numbers of each species of the group it eats will evidently 
be the same as its interest in the group as a whole, since 
the group as a whole can be kept at the highest number 
possible only by keeping each species at the highest number 
possible. 
“This argument holds for birds as well as for insects, for 
animals of all kinds, in fact, whether their food be mixed or 
simple, animal or vegetable, or both. It also applies to para- 
