90 THE HARMONIES OF NATURE. 



to the period of our observation, but in reference to the duration 

 of species in general. 



Let us trace the operation of this cause in connexion with 

 others. A tempestuous wind bears the seeds of a plant many 

 miles through the air, and then delivers them to the ocean; the 

 marine current drives them to a distant continent ; by the fall 

 of the tide they become the food of numerous birds, and one 

 of these is seized by a hawk or eagle, which, soaring across hill 

 and dale to a place of retreat, leaves, after devouring its prey, 

 the unpalatable seeds to spring up and flourish in a new soil. 



But no bird or four-footed animal is so instrumental in 

 diffusing plants over the surface of the globe as man, that 

 restless wanderer who claims the whole of it as his inheritance. 

 He transports with him into every region the vegetables which 

 he cultivates for his wants ; through him the potato has been 

 conveyed from the New World to Europe, and the Cinnamon- 

 tree of Ceylon made to flourish in the Western Indies. 



'When the introduction of cultivated plants is of recent date,' 

 says De Candolle, ' 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, nor the origin in the old 

 world of the coffee-tree and of wheat. But there are certain 

 objects of culture of very ancient date between the tropics, 

 such, 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 expedi- 

 tions of the Eomans, 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, or introduced 

 from foreign countries for the embellishment of our gardens, 

 the number which have been naturalised by accident, or which 

 man has spread unintentionally, is considerable. 



' We have introduced everywhere,' observes De Candolle, 

 ' some weeds which grow among our various kinds of wheat, and 

 which have been received perhaps originally from Asia along 

 with them. Thus, together with the Barbary wheat, the in- 

 habitants of the south of Europe have sown, for many ages, the 



