On Some Interactions of Organisms . 7 
lucky accident that a destructive species really injures 
the species preyed upon. 
The discussion has thus far affected only such organ- 
isms 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 acces- 
sible, or preferably upon certain species, resorting to 
others only in case of an insufficiency of its favorite food. 
It is at once evident that, taking the group of its food- 
insects as a unit, the same reasoning applies as if it were 
restricted to a single species for food; that is, it is inter- 
ested in the maintenance of these food-species at the 
highest number consistent with the general conditions of 
the environment — interested to confine its own depreda- 
tions to that surplus of its food which would otherwise 
perish if 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. 
If the predatory insect prefer some species of the group 
to others, we need only say that whatever interest it has 
in any species of the group, will be an interest in keeping 
up its numbers to the highest limit; and any failure in 
this respect will injure it in precisely the ratio of the 
value of that species as an element of its food. It would 
be most injured by anything injuriously affecting the 
species it most preferred — the preferences of animals be- 
ing, according to the doctrine of evolution, like their in- 
stincts, inherited tendencies toward the things which 
have proved beneficial to their progenitors. 
This argument holds for birds as well as for insects, for 
animals of all kinds, in fact, whether their food be simple 
or mixed, animal or vegetable, or both. It also applies to 
