THE BALANCE OF NATURE. 141 



herons and no nioor-hens, few ducks, wild-fowl, or 

 snipe. Tlie birds tliat hunted for worms and 

 insects in the soft ooze are driven elsewhere ; tlie 

 frogs, the newts, the toads, the dragon-flies are 

 deprived of the hatching-places for their tadpoles 

 or larva). The gnats and May-flies and small 

 water-haunting insects disap})ear, and with thein 

 the swifts and swallows that chased them open- 

 mouthed across the basking waters. One species 

 of butterfly, peculiar to the English fens, became 

 entirely extinct with the draining of Wliittlesea 

 Mere ; many others which still survive in conti- 

 nental Eur()[)e were driven from their last English 

 dwelling-place. In such a complex world as this 

 it is impossible to alter a single factor without 

 disturbing the whole balance of nature in a thou- 

 sand particulars. So insignificant a fact as the 

 accidental introduction of the Canadian river- 

 weed into England has cost English canal-compa- 

 nies thousands of pounds in dredging operations, 

 lias converted ponds and reservoirs into festering 

 masses of green stagnation, has killed out the 

 trout and the crayfish in innumerable streamlets, 

 and has fostered the growth of carp at the ex- 

 pense of bream, roach, and pike in hundreds of 

 rivers. It is impossible even to kill a fly or a 

 chipmunk without bringing about a whole petty 

 revolution in the world around us. Not a plant 

 but owes its safety to the friendly intervention 



