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. 



u 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. 



u 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. 



u 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 



