INSECTS IN RELATION TO OTHER ANIMALS 295 



" We notice incidentally that it is thus made unlikely that an 

 injurious species can be exterminated, can even be permanently 

 lessened in numbers, by a parasite strictly dependent upon it, 

 a .conclusion which remarkably diminishes the economical role 

 of parasitism. The same line of argument will, of course, 

 apply, with slight modifications, to any animal, or even to any 

 plant dependent upon any other animal or any other plant for 

 existence. 



" It is a general truth, that those animals and plants are 

 least likely to oscillate widely which are preyed upon by the 

 greatest number of species, of the most varied habit. Then 

 the occasional diminution of a single enemy will not greatly 

 affect them, as any consequent excess of their own numbers 

 will be largely cut down by their other enemies, and especially 

 as, in most cases, the backward oscillations of one set of ene- 

 mies will be neutralized by the forward oscillations of another 

 set. But by the operations of natural selection, most animals 

 are compelled to maintain a varied food habit, so that if one 

 element fails, others may be available. Thus each species 

 preyed upon is likely to have a number of enemies, which will 

 assist each other in keeping it properly in check. 



" Against the uprising of inordinate numbers of insects, 

 commonly harmless but capable of becoming temporarily in- 

 jurious, the most valuable and reliable protection is un- 

 doubtedly afforded by those predaceous birds and insects 

 which eat a mixed food, so that in the absence or diminution 

 of any one element of their food, their own numbers are not 

 seriously affected. Resorting, then, to other food supplies, 

 they are found ready, on occasion, for immediate and over- 

 whelming attack against any threatening foe. Especially 

 does the wonderful locomotive power of birds, enabling them 

 to escape scarcity in one region which might otherwise deci- 

 mate them, by simply passing to another more favorable one, 

 without the loss of a life, fit them, above all other animals and 

 agencies, to arrest disorder at the start, to head off aspiring 

 and destructive rebellion before it has had time fairly to make 



