INTRODUCTORY OBSERVATIONS. 19 



the cucurbitaceous plants, and the edible roots 

 of our gardens, as well as the horse, the dog, the 

 sheep, the swine, and our barnyard fowls, seem 

 almost exempted from subjection to climatic 

 laws. While, therefore, a degree of latitude, a 

 few hundred feet of elevation, a trifling differ- 

 ence in soil, or in the amount of atmospheric 

 humidity, oppose impassable barriers to the dif- 

 fusion of most wild plants and animals, the 

 domesticated species I have enumerated follow 

 man in his widest wanderings, and make his 

 resting-place their home, whether he dwells on 

 continent or on island, at the level of the sea or 

 on the margin of Alpine snows, beneath the 

 equator or among the frosts of the polar circle. 



Others, again, of the domesticated families of 

 the organic world seem, like the untamed tribes, 

 inexorably confined within prescribed geograph- 

 ical bounds, and incapable of propagation or 

 growth beyond their original limits ; while 

 others still, though comparatively independent 

 of climate and of soil, are nevertheless so spe- 

 cially fitted to certain conditions of surface, and 

 to certain modes of human life, to the main- 



the Cordilleras and their plateaus. It does not indeed appear 

 that maize has been found wild in North America ; but the 

 first discoverers describe it as a plant cultivated along the 

 whole Atlantic coast, from Massachusetts to the Gulf of Mex- 

 ico. 



