MANKIND AND THE INSECT 335 



the foundation of the whole subject of economic en- 

 tomology ; and constitute, in fact, the principal part of 

 the science ; for until these are clearly and completely- 

 made out for any given injurious species, we cannot 

 possibly tell when, where, or how to strike it at its weakest 

 point. But besides this, we must also know the con- 

 ditions favourable and unfavourable to it; the enemies 

 which prey upon it, whether bird or insect or plant 

 parasite ; the diseases to which it is subject, and the 

 effects of the various changes of weather and season. 

 We should make, in fact, a thorough study of it in 

 relation to the whole system of things by which it is 

 affected. Without this we shall often be exposed to 

 needless alarm and expense, perhaps, in fighting by 

 artificial remedies, an insect already in process of rapid 

 extinction by natural causes ; perhaps giving up in despair 

 just at the time w r hen the natural checks upon its career 

 are about to lend their powerful aid to its suppression. 

 We may even, for lack of this knowledge, destroy our 

 best friends under the supposition that they are the 

 authors of the mischief which they are really exerting 

 themselves to prevent." 



