The importance of natural checks to over-multiplication, and especially their bearing upon the competition hypothesis, seems never to have been taken into due account The checks, or rather some of them, are mentioned, but their action is seldom studied in detail.  However, if we compare the action of the natural checks with that of competition, we must recognize at once that the latter sustains no comparison whatever with the other checks.  Thus, Mr. Bates mentions the really astounding numbers of winged ants which are destroyed during their exodus.  The dead or half-dead bodies of the formica de fuego (Myrmica saevissima) which had been blown into the river during a gale “were heaped in a line an inch or two in height and breadth, the line continuing without interruption for miles at the edge of the water."(35) Myriads of ants are thus destroyed amidst a nature which might support a hundred times as many ants as are actually living.  Dr. Altum, a German forester, who wrote a very interesting book about animals injurious to our forests, also gives many facts showing the immense importance of natural checks.  He says, that a succession of gales or cold and damp weather during the exodus of the pine-moth (Bombyx pini) destroy it to incredible amounts, and during the spring of 1871 all these moths disappeared at once, probably killed by a succession of cold nights.(36) Many like examples relative to various insects could be quoted from various parts of Europe.  Dr. Altum also mentions the bird-enemies of the pine-moth, and the immense amount of its eggs destroyed by foxes; but he adds that the parasitic fungi which periodically infest it are a far more terrible enemy than any bird, because they destroy the moth over very large areas at once.  As to various species of mice (Mus sylvaticus, Arvicola arvalis, and A. agrestis), the same author gives a long list of their enemies, but he remarks:  “However, the most terrible enemies of mice are not other animals, but such sudden changes of weather as occur almost every year.”  Alternations of frost and warm weather destroy them in numberless quantities; “one single sudden change can reduce thousands of mice to the number of a few individuals.”  On the other side, a warm winter, or a winter which gradually steps in, make them multiply in menacing proportions, notwithstanding every enemy; such was the case in 1876 and 1877.(37) Competition, in the case of mice, thus appears a quite trifling factor when compared with weather.  Other facts to the same effect are also given as regards squirrels.

As to birds, it is well known how they suffer from sudden changes of weather.  Late snow-storms are as destructive of bird-life on the English moors, as they are in Siberia; and Ch.  Dixon saw the red grouse so pressed during some exceptionally severe winters, that they quitted the moors in numbers, “and we have then known them actually to be taken in the streets of Sheffield.  Persistent wet,” he adds, “is almost as fatal to them.”