nal defeat that native plants often incur at the hands of invading strangers.
Why does the water-cress, harmless enough in our ditches, block up the water-courses in New Zealand to such an extent as to become a costly nuisance? What can there be in English ditches and canals so propitious to the growth of the American water-weed (Anacharis) as to have caused it to obstruct even our navigable rivers? In America, whence it came, it is no more of an inconvenience than any other water-weed. Why in other places does the white clover (Trifolium repens) overcome the native grasses, and dispossess them of their territory? Why has a particular grass, the Stipa tortilus, invaded the South-Russian steppes to such an extent as to displace almost every other plant?
There are numberless such instances—from that afforded by the island of St. Helena, in which the original vegetation is almost completely dispossessed, and its room occupied by foreign importations, to the banks of a Surrey river, yellow with the flowers of an American balsam—and the reason is not obvious. The fact is patent, and is not without analogies in the virulence with which epidemic diseases spread when introduced for the first time among a population not heretofore subjected to them.
Such cases as these recall the opinions of Humboldt and others on the antipathies of plants. According to this notion, certain plants are positively injurious to others, not so much by any peculiarity of structural organization as by the excretion of matters hurtful to other plants. It has been asserted, for instance, that the darnel (Lolium temulentum) is injurious to wheat; that a species of thistle (Serratula arvensis) is obnoxious to oats; that a spurge (Euphorbia Peplus) and a scabious (Knautia arvensis) are detrimental to flax; and spurrey (Spergula arvensis) similarly prejudicial to buckwheat.
In so far as this detrimental influence is due to any excrementitious product from the plant, the verdict given by modern physiologists amounts to "not proven." Some would even say "not guilty;" but we do not see clearly how those who take this view can reconcile it entirely with the existence of that natural alteration of which Dureau de la Malle speaks, and which is admitted by all subsequent observers.
Mere exhaustion of the soil will not account for the phenomena in all cases, because a crop will fail on a particular soil after a while, and yet chemical analysis of that soil will reveal the fact that the particular elements required by a given plant are still contained in sufficient abundance in it. Land, for instance, that is "clover-sick"—on which, that is, good crops of clover cannot be grown—is by no means necessarily deficient in the constituent required for the growth of the plant; and, indeed, in the Rothamsted experiments the constituents in question have been supplied as manure, but without any good result.
