Nature's Militia 279 



slugs, and the swallow, martin, and swift, without whom 

 the air would be so choked with flies as to be simply 

 unbreathable. 



So much, then, for the services, the incalculable and 

 indispensable services, of the ''militia." We have seen 

 something of these, and we have seen, too, how surely 

 punishment has followed where the birds have been exter- 

 minated; but there is a word or two to be said on the 

 other side of the question. 



It is very rash for man to interfere with nature by 

 exterminating any one class of the laborers employed, 

 whether in the tilling or in the protection of the fields, 

 cultivated or uncultivated; but at the same time, it is 

 hardly less rash for him to interfere in the other direction, 

 and to encourage these same laborers overmuch; or even, 

 because they are found useful in one part of the world, to 

 conclude hastily that they must be equally useful in another. 



Thistles do not overwhelm us and swamp other vege- 

 tation in Europe; but he was a very rash man who 

 imported a sack of thistle-seed into South America and 

 scattered it broadcast about Valparaiso, with an idea of 

 providing useful fodder for cattle! The thistle took to 

 the soil and climate amazingly, and having nothing to 

 check its increase, as it has at home, quickly spread over 

 large tracts of country, to the great inconvenience of the 

 cultivators. 



Then, some one may be inclined to say, why not import 

 birds to eat the seed.? But things in nature are so exactly 

 balanced that even this step would probably be found to 

 have its disadvantages, and possibly the birds might turn 

 out to be even worse than the thistles. The sparrows 

 imported into the United States, for instance, and at first 



