REMARKABLE INSTANCES OF PLANT DISPERSION. 347 



The dispersion of by far the greater number of widely dispersed plants 

 has been by fruits and seeds. It would be beyond my purpose to deal 

 with many foreign plants, such as the cocoanut and Ipomoea pes-pelicanis, 

 &c, of tropical shores, which have natural facilities for dispersion by ocean 

 currents. But, confining our attention to British plants, we find, not 

 only that many are widely scattered over our islands, but have spread, 

 whenever accident has introduced them into the Colonies, with extra- 

 ordinary vigour. I will now give some illustrations. 



As remarkable instances of enormous areas now covered by introduced 

 plants, Darwin thus speaks of South America : — " Near the Guardia we 

 find the southern limit of two European plants, now become extra- 

 ordinarily common. The fennel in great profusion covers the ditch- 

 banks in the neighbourhood of Buenos Ayres, Monte Video, and other 

 towns. But the cardoon (Cynara Gardunculus) has a far wider range : 

 it occurs in these latitudes on both sides of the Cordillera, across the con- 

 tinent. I saw it in unfrequented spots in Chili, Entre Bios, and 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." Darwin then proceeds 

 to show how precisely analogous results have followed in the animal 

 world, by the introduction of the horse, cattle and sheep, of which " the 

 countless herds not only have altered the whole aspect of the vegetation, 

 but they have almost banished the guanaco, deer and ostrich. . . . While 

 packs of wild dogs may be heard howling on the wooded banks of the less 

 frequented streams, and the common cat, altered into a large and fierce 

 animal, inhabits rocky hills." * 



Speaking again of the " Thistles," Darwin when passing the small 

 towns of Luxan and Areco says : — " The estancias (private estates) are 

 here wide apart ; for there is little good pasture, owing to the land being 

 covered by beds either of an acrid clover, or of the great thistle. The 

 latter were at this time of the year two-thirds grown ; in some parts they 

 were as high as the horse's back, but in others they had not yet sprung 

 up. . . . When the thistles are full grown, the great beds are im- 

 penetrable, except by a few tracks, as intricate as those of a labyrinth. 

 These are only known to the robbers, who at this season inhabit them, 

 and sally forth at night to rob and cut throats with impunity. Upon 

 asking at a house whether robbers were numerous, I was answered, ' The 

 thistles are not up yet.' " t 



Darwin adds a more interesting feature about the thistles near 

 Mercedes on the Rio Negro : — " The geological nature of this part of the 

 province was different from the rest, and closely resembled that of the 

 Pampas. In consequence, there were immense beds of the thistles as 

 well as of the cardoon. The two sorts grow separate, each plant in 

 company with its own kind. The cardoon is as high as a horse's back, 



* Nat. Voyage, &c, p. 119. 



f Op. cit. p. 124. 



