Nature did not sweep off the superfluous young; for once in a way they were not superfluous. Kestrels passed the message on, rocks 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 themselves 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 Agriculture gave advice, and some effort was made to reduce the plague, but it was the rocks 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 nourishment 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 spreading 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
