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Ch. XL.] 



MIGKATION OF SPECIES. 



397 



greater 

 being. 



num'ber of tlie living species of plants came into 

 And we sliall see in the Forty- second Chapter how 



much the rarity, or even the entire extinction, of sj)ecies is 

 promoted by these changes. 



Agency of man in the dispersion of plants. — But in addition 

 to all the agents already enumerated as instrumental in 

 diffusing plants over the globe, we have still to consider man 



one of the most important of all. He transports with him, 

 into every region, the vegetables which he cultivates for his 

 wants, and is the involuntary means of spreading a still 

 greater number which are useless to him, or even noxious. 

 ' When the introduction of cultivated plants,' says De Candolle, 

 ' is of recent date, there is no difficulty in tracing their origin ; 

 but when it is of high antiquity, we are often ignorant of 

 the true country of the plants on which we feed. No one 

 contests the American origin of the maize or the potato ; nor 

 the origin^ in the Old Worlds of the coffee-tree^ and of wheat. 

 But there are certain objects of cnlture, of very ancient 

 date, between the tropics, snch for example as the banana, 

 of which the origin cannot be verified. Armies, in modern 

 times, have been known to carry, in all directions, grain and 

 cultivated vegetables from one extremity of Europe to the 

 other; and thus have shown us how, in more ancient times, 

 the conquests of Alexander, the distant expeditions of the 

 Romans, and afterwards the crusades, may have transported 

 many plants from one part of the world to the other. '"^ 



But, besides the plants used in agriculture, the number 

 which have been naturalised by accident, or which man 

 has spread unintentionally, is considerable. One of om- old 

 authors, Josselyn, gives a catalogue of such plants as had, in 

 his time, sprung up in the colony since the English planted 

 and kept cattle in New England. They were two-and-twenty 

 in number. The common nettle was the first which the 

 settlers noticed ; and the plantain was called by the Indians 

 ' Englishman's foot,' as if it sprung from their footsteps. f 



' We have introduced every where,' observes De Candolle, 

 ' some weeds which grow among our various kinds of wheat. 



* De Candolle, Essai Elemen. &c. 



p. 50. 



t Quarterly Eevlcw, vol. xxx. p. 8 



