92 The Food of Birds. 



where the ultimate good is a certain and liberal compen- 

 sation. 



Again, it is becoming evident that carnivorous insects 

 and insectivorous birds all have their food-preferences- 

 Probably no one species — certainly no one family — of birds 

 or insects would quite take the place of another. Suppos- 

 ing, then, that some birds eat predaceous insects, in part, 

 as well as phytophagous ones, — eat the former, perhaps, 

 in undue ratio — still, as the chances are practically infinite 

 that the predaceous insects it eats would not, if allowed 

 to live, eat precisely the same amount and kind of injuri- 

 ous insects as the bird itself, by destroying the bird we 

 should merely liberate a second cause of numerous oscilla- 

 tions. Those species neglected by the carnivorous insects 

 would increase beyond their bounds, and those eaten by 

 them would be unduly diminished. It follows from the 

 foregoing reasoning that, as a general rule, a hird should 

 not he discredited for the regular and established hahit of 

 destroying predaceous or parasitic insects., unless it can be 

 shown that those insects would, if left to themselves, check 

 the fluctuations of some injurious species, or afford a bet- 

 ter safeguard against the possible fluctuations of others. It 

 must also be shown that this prospective good will not be 

 overbalanced by some greater evil. In short, the whole 

 burden of proof is on the side of those who would disturb 

 the fixed order of Nature.* 



The most important question respecting the relations 

 of birds to insects is, thereforie, the determination of those 

 species of birds which serve the most useful purpose as a 

 constant check upon those insects which are either injuri- 

 ous or capable of becoming so if they appear in largely in- 

 creased numbers. Fortunately, whatever oscillations or 

 irregularities may arise, and whatever may be their cause, 

 the general tendency of things is towards their correction. 

 In course of time, if new disturbances do not continually 

 unsettle even the newest arrangements, they will usually 

 right themselves more or less completely. The methods 



*For a discussion of the general subject, see Herbert Spencer's Prin- 

 ciples of Biology, Vol. 2, Pt. VI., Chap. II., p. 397; and the preceding pa- 

 per, "On Some Interactions of Organisms." 



