18 On Some Interactions of Organisms. 



insects and insectivorous birds which eat a varied food, 

 using most freely those elements of their dietary which 

 are, for the time being, most abundant. 



When we compare the results of the primitive natural 

 order with the interests of man, we see that, with much 

 coincidence, there is also considerable conflict. While the 

 natural order is directed to the mere maintenance of the 

 species, the necessities of man usually require much more. 

 They require that the plant or animal should be urged to 

 excessive and superfluous growth and increase, and that 

 all the surplus, variously and widely distributed in nature, 

 should now be appropriated to the supply of human wants. 

 From the consequent human interferences with the estab- 

 lished system of things, numerous disturbances arise, — 

 many of them full of danger, others fruitful of positive 

 evil. Oscillations of species appear, not less injurious to 

 man than to the plants and animals more directly involved. 

 Indeed, most of the serious insect injuries, for example, 

 are due to species whose injurious oscillations have result- 

 ed from changes of the organic balance initiated by man. 



To avoid or mitigate the evils likely to arise, and to 

 adapt the life of his region more exactly to his purposes, 

 man must study the natural order as a whole, and must 

 understand the disturbances to which it has been subject. 

 Especially he must know the forces which tend to the re- 

 duction of these disturbances and those which tend to per- 

 petuate or aggravate them, in order that he may reinforce 

 the first and weaken or divert the second. 



The main lesson of conduct taught us by these facts and 

 reasonings is that of conservative action and exhaustive 

 inquiry. Reasoning unwarranted by facts, and facts not 

 correctly and sufiiciently reasoned out, are equally worth- 

 less and dangerous for practical use. 



