202 HOOKER ON ARCTIC PLANTS. 



general truth of Mr. Darwin's hypothesis, that, besides harmo- 

 nizing with the distribution of Arctic plants within and beyond 

 the Polar Zone, it can also be made, without straining, to account 

 for that distribution and for many anomalies of the Greenland 

 flora, viz., 1, its identity with the Lapponian ; 2, its paucity of 

 species ; 3, the fewness of temperate plants in temperate Green- 

 land, and the still fewer plants that area adds to the entire flora of 

 Greenland ; 4, the rarity of both Asiatic and American species 

 or types in Greenland ; and 5, the presence of a few of the rarest 

 Greenland and Scandinavian species in enormously remote Alpine 

 localities of West America and the United States. 



II. — On the Local Distribution of Plants vtithin the 

 Arctic Circle. 



The greatest number of plants occurring in any given Arctic 

 District is found in the European, where 616 flowering plants 

 have been collected from the verge of the Circle to Spitzbergen. 

 From this region vegetation rapidly diminishes in proceeding 

 eastwards and westwards, especially the latter. Thus, in Arctic 

 Asia only 233 flowering plants have been collected ; in Arctic 

 Greenland, 207 species ; in the American continent east of the 

 Mackenzie River, 379 species ; and in the area westwards from 

 that river to Behring's Straits, 364 species. 



A glance at the annual and monthly Isothermal Lines shows 

 that there is little relation between the temperature and vege- 

 tation of the areas they intersect beyond the general feature of 

 the scantiness of the Siberian flora being accompanied by a great 

 southern bend of the annual isotherm of 32° in Asia, and the 

 greatest northern bend of the same isotherm occurring in the 

 longitude of West Lapland, which contains the richest flora. 

 On the other hand, the same isotherm bends northwards in 

 passing from Eastern America to Greenland, the vegetation of 

 which is the scantier of the two, and passes to the northwards 

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

 Lapland to the southward of which it passes. The June iso- 

 thermals, as indicating the most eff'ective temperatures in the 

 Arctic regions (where 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 

 54° N. lat. in the meridian of Behring's Straits, where the flora 

 is comparatively luxuriant, and the June isothermal of 32°, which 

 traverses Greenland north of Disco, passes to the north both of 

 Spitzbergen and the Parry Islands. In fact, it is neither the 

 mean annual, nor the summer (flowering), nor the autumn 

 (fruiting) temperature that determines the abundance or scarcity 

 of the vegetation in each district, but these combined with the 

 ocean-temperature and consequent prevalence of humidity, its 



