MIGRATIONS OF PLANTS 
and thoroughly naturalized in a strange coun- 
try that they go a long way toward supplanting 
the original inhabitants in a very short time. 
It was Darwin who first noticed the extensive 
conquests of the Cardoon Artichoke (Cynara 
Cardunculus) in South America. In one sec- 
tion, these prickly plants covered an area of 
several hundred square miles, having entirely 
superceded the aborigines. 
It is well known that the most troublesome 
of the American weeds are of British origin. 
On the other hand, the American water weed 
Anacharis blocks up small English streams. 
The grass called Stipa Tortilis has captured 
the steppes of southern Russia. The love of 
change seems to be an inherent tendency in 
plantdom. The Pigweed and the Morning 
Glory have come north from the tropics. The 
Canada Thistle, originally a foreigner in North 
America, has spread all over Canada and New 
England. The American Erigeron Canadense 
has emigrated to all parts of the world. The 
flora of Scandinavia, like its people, are aggres- 
sive colonizers. More than one hundred and 
fifty species have reached New Zealand alone 
[49] 
