0 On Some Interactions of Organisms. 
tie.” 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 lat- 
ter, 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 depreda- 
tions 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 either perish otherwise, if not eaten, or, surviving, 
must injure their species by overcrowding. 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 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. * 
I next point out the fact that precisely the same reason- 
ing applies to predaceous and parasitic insects. Their in- 
terests, also, are identical with the interests of the species 
they parasitize or prey upon. A diminution of their food 
reacts to decrease their own numbers. They are thus vi- 
tally interested in confining their depredations to the ex- 
cess of individuals produced, or to redundant or other- 
wise unessential structures. It is only by a sort of un- 
* See “Principles of Biology,” by Herbert Spencer, Vol II, pp. 397-478. 
