292 ENTOMOLOGY 



" 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.' " 



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 num- 

 bers 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, air, light and 

 food. The interest of the insect is therefore identical with 

 the interest of the plant it feeds upon. \Yhatever 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 num- 

 bers 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 superfluous individuals which must perish 

 otherwise, if not eaten, or, surviving, 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 in- 

 sect below the normal number, will evidently injure the food 

 plant by preventing a reduction of its excess of growth or 

 numbers, and will also set up an oscillation like the preceding, 

 except that the steps will be taken in reverse order. 



