BIRDS IN RELATION TO PARASITIC INSECTS. 83 



by overcrowding. If the plant-feeder multiply beyond the 

 above limit, evidently the diminution of the food supply will 

 soon react to diminish its own numbers, a counter reaction 

 will then take place in favor of the plant, and so on through 

 an oscillation of indefinite continuance. 



" On the other hand, the reduction of the phytophagous 

 insect below the normal number will evidently injure the 

 food plant by preventing a reduction of its excess of growth 

 or numbers, and will also set up an oscillation like the pre- 

 ceding except that the steps will be taken in reverse order. 



" I next point out the fact that precisely the same reason- 

 ing applies to predaceous and parasitic insects. Their inter- 

 ests also are identical with the interests of the species they 

 parasitize or prey upon. A diminution of their food reacts to 

 diminish their own numbers. They are thus vitally inter- 

 ested in confining their depredations to the excess of indi- 

 viduals produced or to redundant or otherwise unessential 

 structures. It is only by a sort of unlucky accident that a 

 destructive species really injures the species preyed upon. 



" The discussion thus far has 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 cer- 

 tain species, resorting to others only in case of an insufficiency 

 of its favorite food. 



" It is at once evident that, taking 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 interested in the maintenance 

 of these food-species at the highest number consistent with 

 the general conditions of the environment, interested to con- 

 fine its own depredations to that surplus of its food which 



