On Some Interactions of Organisms. t 



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 whatev- 

 er 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. 



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 high- 

 est 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 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 inter- 

 est 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 being, accord- 

 ing to the doctrine of evolution, like their instincts, in- 

 herited 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 

 parasitic plants. The ideal adjustment is one in which 

 the reproductive rate of each species should be so exactly 



