THE LOCAL DISTRIBUTION OF PLANTS. 679 



mum vulg are), the iniquitous Canada thistle (Cirsium arvense), and 

 the obnoxious burdock {Lappa officinalis), as examples of species which 

 shae 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 spe- 

 cies. Some of the species last mentioned belong to this class, particu- 

 larly 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 trou- 

 blesome. The same is true of many plants of the mint family, partic- 

 ularly Nepeta glechoma (ground-ivy). On the other hand, some Ameri- 

 can species, like Erigeron Canadense, have migrated by the aid of man 

 into almost every country on the globe, always thriving best where civ- 

 ilization 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 introdxiction into South 

 America of the cardoon (Cgnara carduncuhis) , 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 con- 

 ditions favorable to their migration. Conspicuous among these are 

 Chenopodium (pigweed), Amarantus (amaranth), Ipomoea (morning- 

 glory), and others. 



If we take a wider view of this class of phenomena, we may per- 

 ceive that it is only by an extension of the same principle that all the 

 beneficial changes made by man in the vegetable kingdom have be- 

 come possible. Every plant he has improved and rendered subservi- 



1 " Naturalist's Voyage round the World," p. 119. 



