INSECTS IN RELATION TO OTHER ANIMALS 293 



" I next point out the fact that precisely the same reasoning 

 applies 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 ho\v 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. 



" It 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 is, it is 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 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. . . . 



" 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- 



