TRAGEDY IN NATURE 153 



Nature did not sweep off the superfluous young; for once 

 in a way they were not superfluous. Kestrels passed the 

 message on, rooks swarmed to the field of action and 

 became bloodthirsty mouse eaters. For generations 

 game-preserving man had been fighting against these 

 vermin, and vermin came in hordes, returning good for 

 evil, to feed on vermin. The voles declined, the voles 

 vanished, the tainted fields recovered and clothed them- 

 selves with grass, and the enemies of voles, replete, either 

 went back to their own place or in their turn perished. 



But a few years ago the papers were full of the ravages 

 of the antler moth; from the Cheviots to the Peak all the 

 uplands swarmed with hungry caterpillars; they devoured 

 the grass on the hillsides and descended in solid, squirming 

 armies to hunt for food. It was a wonderful sight to see 

 the travellers striving to top the rough grit walls, and to 

 note the streams and roadside horse troughs full of their 

 drowned, bloated bodies. Again the Board of Agricul- 

 ture gave advice, and some effort was made to reduce the 

 plague, but it was the rooks and daws, the partridges and, 

 most of all, the ichneumon flies, which really tackled the 

 problem; the grass came up again, doubtless fertilised 

 by the parasitised corpses of its late enemies; once more 

 Nature was responsible for righting the wrong. 



Well known is the American pond weed, choking 

 canals, rivers, and lakes; steady and deadly its increase, 

 sudden its death when it has devoured all the nourish- 

 ment in the mud and water. Watercress, an introduced 

 plant in New Zealand, has blocked rivers so as to cause 

 floods, and willows have been planted so that their spread- 

 ing roots may rob the cress of nourishment. By no means 

 can man always call in the correct natural antagonist 

 to stem the torrent of increase which unwise introduction 

 of an alien species often causes. Stoats and weasels sent 

 to Australia to tackle the rabbit problem found the 



