mum vulgare), the iniquitous Canada thistle (Cirsium arvense), and the obnoxious burdock (Lappa officinalis), as examples of species which share with man, not only his cosmopolitan character, but also some of his vices. But these foreign immigrants often furnish us with one of the most striking exemplifications of the anomaly, if such it may still be called, which I am endeavoring to illustrate.
It frequently happens that a plant, taken from one country into another having an entirely different flora, thrives more vigorously than it did at home, and even threatens to drive out indigenous species. Some of the species last mentioned belong to this class, particularly the Canada thistle, which, notwithstanding its popular name, has been introduced into this country from Europe, and has spread not only over Canada and New England, but far south and west. Cnicus lanceolatus (common thistle) is only less prominent because less troublesome. The same is true of many plants of the mint family, particularly Nepeta glechoma (ground-ivy). On the other hand, some American species, like Erigeron Canadense, have migrated by the aid of man into almost every country on the globe, always thriving best where civilization is highest. But some of these do not confine themselves to the circle of man's protective influence. Sometimes they strike out into the forest or spread over the plains, carrying dismay to the native vegetation. Mr. Darwin, speaking of the introduction into South America of the cardoon (Cynara cardunculus), a congener of the artichoke, says: "It occurs in these latitudes on both sides of the Cordillera, across the continent. I saw it in unfrequented spots in Chili, Entre Bios, and the Banda Oriental. In the latter country alone, very many (probably several hundred) square miles are covered by one mass of these prickly plants, and are impenetrable by man or beast. Over the undulating plains, where these great beds occur, nothing else can now live. Before their introduction, however, the surface must have supported, as in other parts, a rank herbage. I doubt whether any case is on record of an invasion on so grand a scale of one plant over the aborigines."[1] He also mentions other analogous cases, though of a character less marked.
This love of change, if I may so characterize it, seems to inhere in the entire vegetable kingdom. Not even climate avails to overcome it, as is evidenced by the rapid invasion from the tropics of many plants whenever the presence of man in any manner creates the conditions favorable to their migration. Conspicuous among these are Chenopodium (pigweed), Amarantus (amaranth), Ipœma (morning glory), and others.
If we take a wider view of this class of phenomena, we may perceive that it is only by an extension of the same principle that all the beneficial changes made by man in the vegetable kingdom have become possible. Every plant he has improved and rendered subservi-
- ↑ "Naturalist's Voyage round the World," p. 119.
