DISTRIBUTION OF ARCTIC PLANTS. 127 



great southern bend in Asia, and its richness in Lapland, with 

 an equally great northern bend there, of the annual isotherm 

 of 32°. Yet " the same isotherm bends northwards in passing 

 from eastern America to Greenland, the vegetation of which 

 is the scantier of the two ; and it passed to the northward of 

 Iceland, which is much poorer in species than those parts of 

 Lapland to the southward of which it passes." A glance at 

 the supposed former state of things would suggest the ex- 

 planation of all that is anomalous here. 



" The June isothermals, as indicating the most effective 

 temperatures in the arctic regions (when all vegetation is 

 torpid for nine months, and excessively stimulated during the 

 three others), might have been expected to indicate better the 

 positions of the most luxuriant vegetation. But neither is 

 this the case ; for the June isothermal of 41°, which lies 

 within the arctic zone in Asia, where the vegetation is scanty 

 in the extreme, descends* to lat. 54° in the meridian of Behr- 

 ing's Straits, where the flora is comparatively luxuriant." The 

 aridity of the former and the humidity of the latter district 

 here offers an obvious explanation ; also the great severity of 

 the winter in the former, and its mildness in the latter. And 

 Great Britain, in which a far greater diversity of species are 

 capable of surviving without protection than in the eastern 

 United States under the same annual isotherms, indicates the 

 advantage of a mean over an extreme climate in this respect, 

 if only there be a certain amount of summer heat. For lack 

 of that, doubtless, very many of the introduced denizens of 

 Britain would soon disappear, if deprived of human care. 



" The northern limit to which vegetation extends varies in 

 every longitude ; the extreme is still unknown ; it may, in- 

 deed, reach to the pole itself. Phsenogamic plants, however, 

 are probably nowhere found far north of lat. 81°. Seventy 

 flowering plants are found in Spitzbergen ; and Sabine and 

 Ross collected nine on Walden Island, towards its northern 

 extreme, but none on Ross's Islet, fifteen miles further to the 

 north. 



" Saxifraga ojypositifoUa is probably the most ubiquitous, 

 and may be considered the commonest and most arctic flower- 



