378 DISTRIBUTION OP PLANTS. 



centre to the conditions of the creation, and in like man- 

 ner the migration of tribes is solely due to these 

 physical forces we have been considering. In every 

 zone we find that vegetable organization is peculiarly 

 fitted for the considerations by which it is surrounded. 

 Under the equator we have the spice-bearing trees, the 

 aiutmeg, the clove, the cinnamon, and the pepper-tree ; 

 there we have also the odoriferous sandal, the ebony, 

 the banyan, and the teak : we have frankincense, and 

 myrrh, and other incense-bearing plants; the coffee- 

 tree, the tea-plant, and the tamarind. 



A little further north we have the apricot, the citron, 

 the peach, and the walnut. In Spain, Sicily, and Italy, 

 we have the orange and lemon-tree blooming rich with 

 perfume, and the pomegranate and the myrtle growing 

 wild upon the rocks. Beyond the Alps the vegetation 

 again changes ; instead of the cypress, the chesnut, and 

 the cork-tree, which prevail to the south of them, we 

 have the hardier oak, the beech, and the elm. Still further 

 north, we have the Scotch and spruce fir and larch. 

 On the northern shores of the Baltic, and in that line of 

 latitude, the hazel alone appears ; and beyond this the 

 iioary alder, the sycamore, and the mountain ash. 

 Within the Arctic circle we find the mezereum, the 

 water-lilies, and the globe-flowers; and, when the 

 weakness of the solar radiations becomes too great even 

 for these, the reindeer moss still lends an aspect of 

 gladness to the otherwise sterile soil. 



The cultivation of vegetables depends on the tem- 

 perature of the clime. The vine flourishes where the 

 mean annual temperature ranges between 50 and 73, 

 .and it is only cultivated profitably within 30 S. and 50 

 N. of the equator. To the same limits is confined 

 the cultivation of maize and of olives. Cotton is grown 

 profitably up to latitude 46 in the Old World, but only 

 up to 40 in the New. We have evidence derived from 

 photographic phenomena, that the constitution of the 



