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 r 

 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 



