On the other hand, when once the natural conditions of a country have been disturbed, the spread and multiplication of immigrants is so rapid that it shortly becomes impossible to discover the limits of the old, indigenous flora. Take the English flora, for example. If we contrast the cultivated counties with the uncultivated, the difference of their vegetation is so great that I have often been compelled to doubt whether many of the most familiar so called wild flowers of the cultivated counties are indigenous at all; nay, more, I have been tempted to suspect that some of the more variable of them, as some species of chenopodium and fumitory, may have originated since cultivation began. In the uncultivated counties the proportion of annual plants is exceedingly small, whereas in the cultivated counties annuals are very numerous; and the further we go from cultivation, roads, and made ground, the rarer they become, till at last, in the uninhabited islets of the west coast of Scotland, and in its mountainous glens, annuals are extremely rare, and confined to the immediate vicinity of cottages. Let any one who doubts this contrast between the floras of cultivated and uncultivated regions compare the annuals in such florae as those of Suffolk or Essex, the North Riding or Cumberland, with those of the Isle of Wight and the Isle of Arran. And it is not only that annuals abound in the cultivated districts, but that so many are nearly confined to ground that is annually or frequently disturbed. The three commonest of all British plants, for example, are, perhaps, groundsel, shepherd's purse, and Poa annua. I do not remember ever having seen any of these plants established where the soil was undisturbed, or where, if undisturbed, they had not been obviously brought by man or the lower animals; and yet I have gathered one of these, the shepherd's purse, in various parts of Europe, in Syria, in the Himalayas, in Australia, New Zealand, and the Falkland Islands. Were England to be depopulated, I believe that in a very few years these plants, and a large proportion of our common annual "wild flowers," would become exceedingly rare or extinct, such as the poppies, fumitories, trefoils, fedias, various species of speed-well, polygonum, mallow, euphorbia, thlaspi, senebiera, medicago, anthemis, centaurea, linaria, lamium, etc., etc.
It is usually said of some of the above-named plants that they prefer cultivated ground, nitrogenous soil, and so forth; and this is no doubt true, but that they will flourish where no such advantages attend them, a very little observation shows; and that they do not continue to flourish elsewhere is due mainly to the fact that, being annuals, their room is taken as soon as they die, and the next year's seedling has no chance of success in the struggle with perennials.
For good instances of this rapid replacement of annuals by perennials, the new railroad embankments should be examined. Whence the plants come from which spring up like magic in the cuttings many feet below the surface of the soil, is a complete mystery, and reminds us of the so-called spontaneous generation of protozoa in newly made infusions or in distilled water. In the south of Scotland in 1840-50, and many parts of the north of England, the first plant that made its appearance was Equisetum arvense, which covered the new-formed banks, for miles and miles, with the most lovely green forest of miniature pines. In the following year comparatively few of these were to be seen, and coltsfoot, dandelions, and other biennials, especially umbelliferae, with a great number of annuals, presented themselves. For many successive years I had no opportunity of watching the struggle for life on these banks, but when I last saw them they were clothed with perennial grasses, docks, plantains, and other perennial rooted plants.
The destruction of native vegetations by introduced is a subject that has only lately attracted much attention, but it has already assumed an aspect that has startled the most careless observer. Some thirty years ago the fecundity of the horse and European cardoon in the Argentine provinces of South America, so graphically described by Sir Edmund Head, drew the attention of naturalists to the fact that animals and plants did not necessarily thrive best where found in an indigenous condition; and the spread of the common Dutch clover, Trifolium repens, in North America, where it follows the footsteps of man through the pathless forests, has long afforded an equally remarkable instance of vegetable colonization. Still more recently in South Africa, Australia, and Tasmania, the Scotch thistle, brier rose, xanthium, plantain, docks, etc., have all become noxious weeds; and this leads me to the last and most curious point to which I shall allude in this article, namely, that the same annuals and other weeds that are held so well in check by the indigenous perennial plants of our country, when transplanted to others, show themselves superior to the perennial vegetation of the latter. Of this New Zealand furnishes the most conspicuous example; it was first visited scarcely more than 100 years ago, and it is not yet fifty since the missionaries first settled in it, and scarce thirty since it received its earliest colonists. The islands contain about 1,000 species of flowering plants, amongst which no fewer than 180 European weeds have been recorded as intruding themselves and having become thoroughly naturalized; and probably double that number will yet be found, as they have never been systematically collected; but the most curious part of the history is this, that whereas of indigenous New Zealand plants scarcely any are annual, no less than half the naturalized European ones are annual.
Of the effect of these introduced European plants in destroying the native vegetation, I have given examples in an article that appeared in the Natural History Review, (January, 1864,) from which I quote the following:
In Australia and New Zealand, the noisy train of English emigration is not more surely doing its work than the stealthy tide of English weeds, which are creeping over the surface of the waste, cultivated, and virgin soil, in annually increasing numbers of genera, species, and individuals. Apropos of this subject, a correspondent, (W. T. Locke Travers, Esq., F.L.S.,) a most active New Zealand botanist, writing from Canterbury, says: "You would be surprised at the rapid spread of European and other foreign plants in this country. All along the sides of the main lines of road through the plains, a Polygonum, (aviculare,) called 'cow-grass,' grows most luxuriantly, the roots sometimes two feet in depth, and the plants spreading over an area from four to five feet in diameter. The dock (Rumex obtusifolius or R. crispus) is to be found in every river-bed, extending into the valleys of the mountain rivers until these became mere torrents. The sow-thistle is spread all over the country, growing luxuriantly nearly up to 6,000 feet. The watercress increases in our still rivers to such an extent as to threaten to choke them altogether; in fact, in the Avon, a still, deep stream running through Christ Church, the annual cost of keeping the river free for boat navigation and for purposes of drainage exceeds £300. I have measured stems twelve feet long and three quarters of an inch in diameter. In some of the mountain districts, where the soil is loose, the white clover is completely displacing the native grasses, forming a close sward. Foreign trees are also very luxuriant in growth. The gum-trees of Australia, the poplars, and willows particularly grow most rapidly. In fact, the young native vegetation appears to shrink from competition with these more vigorous intruders."
Dr. Haast, F.L.S., the eminent explorer and geologist, also writes to me as follows:
"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." It is wonderful to behold the botanical and zoological changes which have taken place since first Captain Cook set foot in New Zealand. Some pigs, which he and other navigators left with the natives, have increased and run wild in such a way that it is impossible to destroy them. There are large tracts of country where they reign supreme. The soil looks as if ploughed by their burrowing. Some station-holders of one hundred thousand acres have had to make contracts for killing them at sixpence per tail, and as many as twenty-two thousand on a single run have been killed by adventurous parties without any diminution being discernible. Not only are they obnoxious by occupying the ground which the sheep farmer needs for his flocks, but they assiduously follow the ewes when lambing, and devour the poor lambs as soon as they make their appearance. They do not exist on the western side of the Alps, and only on the lower grounds on the eastern side where snow seldom falls, so that the explorer has not the advantage of profiting by their existence, where food is scarcest. The boars are sometimes very large, covered with long black bristles, and have enormous tusks, resembling closely the wild boar of the Ardennes, and they are equally savage and courageous,
"Another interesting fact is the appearance of the Norwegian rat. It has thoroughly extirpated the native rat, and is to be found everywhere, even in the very heart of the Alps, growing to a very large size. The European mouse follows it closely, and, what is more surprising, where it makes its appearance, it drives, in a great degree, the Norway rat away. Amongst other quadrupeds, cattle, dogs, and cats are found in a wild state, but not abundantly.