292 ENTOMOLOGY 
‘A correct idea of the economic role of the feathered tribes 
may be cbtained 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 equilbrium 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, hght 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 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 fohage 
or fruit, or else superfluous individuals which must perish 
otherwise, 1f 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. 
