148 BALANCE AMONG DIFFERENT RACES. 
ignorant farmers have imagined that a neighbouring rookery 
was injurious to them, because they saw the rooks hovering 
over the newly-sown corn-fields, and seeming to pick the 
grains out of the ground; and having extirpated the rookery, 
they have found in the course of a year or two that they 
have done themselves an immense injury,—the roots of their 
corn and grasses being devoured by the grubs of cockchafers 
and other insects, the multiplication of which was before 
prevented by the rooks, whose natural food they are. 
149. On the other hand, by an intelligent application of 
this principle, the excessive multiplication of insects has been 
prevented where it had already commenced. Thus, no means 
of extirpating the larvae of the turnip-fly was found so suc¬ 
cessful, as turning into the fields a number of ducks, which 
quickly removed them from the plants. And in the island of 
Mauritius, the increase of locusts, which had been accidentally 
introduced there, and which were becoming quite a pest, was 
checked by the introduction from India of a species of bird, 
the grakle, which feeds upon them. 
150. Of the carnivorous tribes themselves, however* the 
increase might be so great as to destroy all the sources of their 
food, were it not that they are kept in check by others, larger 
and more powerful than themselves, which, not being prolific, 
are not likely ever to gain too great a power. Thus, among 
birds, the eagles, falcons, and hawks rear only two or three 
young every year, whilst many of the smaller birds produce 
and bring up four or five times that number.—The following 
is a curious instance of the system of checks and counter¬ 
checks, by which the “balance of power” is maintained 
amongst the different races. A particular species of moth 
having the fir-cone assigned to it for the deposition of its eggs, 
the young caterpillars, coming out of the shell, consume the 
cone and superfluous seed; but, lest the destruction should 
be too great, another insect of the ichneumon kind lays its eggs 
in the caterpillar, inserting its long tail in the openings of 
the cone until it touches the included insect, its own body 
being too large to enter. Thus it fixes upon the caterpillar 
its minute egg, which., when hatched, destroys it. 
151. The peculiarity of the agency of Insects, in the 
economy of nature, has been justly remarked to consist in their 
power of very rapid multiplication, in order to accomplish a 
