meant, the missel-thrush has increased in numbers in Scotland during the last thirty years, and this has caused a decrease in the numbers of the closely allied song-thrush in the same country. The black rat (Mus rattus) was the common rat of Europe till, in the beginning of the eighteenth century, the large brown rat (Mus decumanus) appeared on the Lower Volga, and thence spread more or less rapidly till it overran all Europe, and generally drove out the black rat, which in most parts is now comparatively rare or quite extinct. This invading rat has now been carried by commerce all over the world, and in New Zealand has completely extirpated a native rat, which the Maoris allege they brought with them from their home in the Pacific; and in the same country a native fly is being supplanted by the European house-fly. In Russia the small Asiatic cockroach has driven away a larger native species; and in Australia the imported hive-bee is exterminating the small stingless native bee.
The reason why this kind of struggle goes on is apparent if we consider that the allied species fill nearly the same place in the economy of nature. They require nearly the same kind of food, are exposed to the same enemies and the same dangers. Hence, if one has ever so slight an advantage over the other in procuring food or in avoiding danger, in its rapidity of multiplication or its tenacity of life, it will increase more rapidly, and by that very fact will cause the other to decrease and often become altogether extinct. In some cases, no doubt, there is actual war between the two, the stronger killing the weaker; but this is by no means necessary, and there may be cases in which the weaker species, physically, may prevail, by its power of more rapid multiplication, its better withstanding vicissitudes of climates, or its greater cunning in escaping the attacks of the common enemies. The same principle is seen at work in the fact that certain mountain varieties of sheep will starve out other mountain varieties, so that the two cannot be kept together. In plants the same thing occurs. If several distinct varieties of wheat are sown together, and the mixed seed resown, some of the varieties which best suit the soil and climate, or are naturally the most fertile, will beat the others and so yield more seed, and will consequently in a few years supplant the other varieties.
