would adjust its dislocated balance. And what would happen, any time might happen, to the guillemot if the carrion crow, jackdaw or other egg-snatching species should for a few years increase abnormally? Certainly there would be a rapid depopulation of the crowded ledges of our steepest maritime cliffs.
Fluctuations do occur, sometimes through climatic variation, often directly due to human stupidity, or from reasons which we cannot fathom. Suddenly we awake to the fact that the field vole is swarming in some hilly area, that the concourse of starlings is beyond all calculation, that the gamma moth is on every plant, that an army of caterpillars of the antler moth are eating all before them. For two or three seasons the overabundance continues, and we are threatened by a new plague of Egypt. The last serious vole plague happened in Lowland Scotland in 1889-90; grass and herbage were devoured, sheep were starved for want of nourishment. Shepherds and farmers, unable to stop the increase of the little mammal with dog, trap, and poison, appealed pitifully for help, and a Parliamentary Committee was appointed to enquire into the trouble. Some very interesting zoological facts were ascertained; man rose in arms against the rodent; many useless suggestions were mooted, and still the voles increased. The Committee laboured, sent a commission out to Greece to learn what they did when voles troubled them, wrote a very instructive Blue book, and drew fees. But Nature could not wait for Mediterranean steamers to return, and took the matter in hand. Who can explain what happened? Short-eared owls came over in the autumn in greater numbers than had ever previously been known, and fewer returned across the North Sea in spring; they had hit on a land of plenty, and they stopped. They nested and reared double broods, laid larger clutches than usual, and
