The persistent spreading of European weeds to the exclusion of our native plants is a fact too well known to every farmer in America, The constant movement westward of the white-weed and the Canada thistle marks the steady deterioration of our grass-fields. Especially noteworthy has been this change in Australia and New Zealand. In New Zealand the weeds of Europe, toughened by centuries of struggle, have won an easy victory over the native plants. Edward Wakefield, in his history of New Zealand, says that "many animals and birds acquire peculiarities in the new country which would indeed astonish those accustomed to them in the old. They usually run to a much larger size and breed oftener. They also take to strange kinds of food. Birds deemed granivorous at home become insectivorous here, and vice versa. Some learn the habits of the native species. Skylarks imitate the native wagtail, and may often be seen perching on fences and telegraph wires. They sing in the night-time, too, a thing unheard of in the old country, and doubtless acquired from the nocturnal habits of New Zealand birds."
The European house-fly in New Zealand has completely extirpated the large blue-bottle fly which was formerly a source of great annoyance to the settlers. An account is given of a farmer who filled a bottle with house-flies and carried them eighty miles into the country, liberating them one by one, in the vicinity of his sheep-folds, in order to let them take the place of the native flies.
It is said that red clover would not grow in New Zealand until bumble-bees were introduced to fertilize its flowers. Wakefield estimates that the introduction of these large wild bees has been worth five million dollars to the farmers in New Zealand.
Dr. Hooker states that, in New Zealand, "the cow-grass has taken possession of the road-sides; dock-and water-cress choke the rivers, the sow-thistle is spread all over the country, growing luxuriantly up to six thousand feet; white clover in the mountain districts displaces the native grasses," and the native (Maori) saying is, "As the white man's rat has driven away the native rat, as the European fly drives away our own, and the clover kills our fern, so will the Maoris disappear before the white man himself" (E. L. Youmans).