INSECTS IN RELATION TO OTHER ANIMALS 227 



"4. The secondary parasite of an insect feeding on a noxious plant. 



"5. The primary parasite of an insect feeding on a wild plant of no 

 economic value. 



"6. The secondary parasite of an insect feeding on a wild plant of no 

 economic value. 



"7. The primary parasite of a predaceous insect. 



"8. The primary parasite of a spider or a 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 balance of nature." 5 



This broader view was first elaborated by Professor Forbes, in his 

 masterly paper, "On Some Interactions of Organisms," the substance of 

 which is given below. 



"Evidently a species can not 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 dis- 

 advantage in the struggle with other plants for foothold, air, light 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, therefore, be regarded as the extreme normal limit 

 of the numbers of a phytophagous species, a limit such that its depre- 

 dations shall do no especial harm to the plants upon which it depends for 

 food, but shall remove only the excess of foliage or fruit, or else super- 

 fluous individuals which must perish otherwise, if not eaten, or, surviv- 

 ing, must injure their species by over-crowding. If the plant-feeder 

 multiply beyond the above limit, evidently the diminution of its food 

 supply will soon react to diminish its own numbers; a counter reaction 

 will then take place in favor of the plant, and so on through an oscillation 

 of indefinite continuance. 



"On the other hand, the reduction of the phytophagous insect below 

 the normal number will evidently injure the food plant by preventing a 



