82 BIRDS IN* THEIR RELATIONS TO MAN. 



(8) The primary parasite of a spider or spider's egg. 



This list might easily be extended still farther, and the 

 assumption that the parasite belongs to the first of these 

 categories is unwarranted by the facts and does violence to 

 the probabilities of the case. 



A correct idea of the economic role of the feathered tribes 

 may be obtained only by a broader view of nature's methods, 

 a view in which we must ever keep before the mind's eye 

 the fact that all the parts of the organic world, from monad 

 to man, are linked together in a thousand ways, the net result 

 being that unstable equilibrium commonly called " the bal- 

 ance of nature." 



The fact that in eating insect parasites birds do not neces- 

 sarily cause an economic loss was first pointed out by Pro- 

 fessor S. A. Forbes in an admirable essay entitled " On some 

 Interactions of Organisms." As we find it impossible to im- 

 prove and difficult to condense the argument there printed, 

 we quote the following extended extract. 



" Evidently a species cannot long maintain itself in numbers 

 greater than can find sufficient food year after year. If it is 

 a phytophagous insect, for example, it will soon dwindle if it 

 seriously lessens the numbers of the plants upon which it 

 feeds, either directly by eating them up, or indirectly by so 

 weakening them that they labor under a marked disadvantage 

 in the struggle with other plants for foothold, light, air, and 

 food. The interest of the insect is therefore identical with 

 the interest of the plant it feeds upon. Whatever injuriously 

 affects the latter, equally injures the former; and whatever 

 favors the latter, equally favors the former. This must there- 

 fore be regarded as the extreme normal limit of the members 

 of a phytophagous species, a limit such that its depredations 

 shall do no especial harm to the plants upon which it de- 

 pends for food, but shall remove only the excess of foliage or 

 fruit, or else superfluous individuals which must either perish 

 otherwise if not eaten or, surviving, must injure their species 



