1868.] HOOKER — ARCTIC FLORA. 343 



generally diffused arctic plants, or natives of any other parts 

 of Temperate America but the Rocky Mountains. 



The arctic circle at Kotzbue Sound is crossed by the isotherm 

 of 23°, and at the longitude of the Mackenzie by that of 12° 5' ; 

 whilst the June isotherm of 41° ascends obliquely from S.W. to 

 N.E., from the Aleutian Island to the mouth of the Mackenzie, 

 and passes south of this province; the June and the September 

 isotherms of 41° and 32° both traverse it obliquely, ascending to 

 the N.E. 



The vast extent of the Pacific Ocean and its warm northerly 

 currents greatly modify the climate of West Arctic America, 

 causing dense fogs to prevail, especially throughout the summer 

 months, whilst the currents keep the ice to the north of Behring's 

 Straits. The shallowness of the ocean between America and Asia, 

 north of lat. 60°, together with the identity of the vegetation in 

 the higher latitudes of these continents, suggests the probability of 

 the land having been continuous at no remote epoch. 



The number of phaenogamic plants hitherto found in Arctic 

 West America is 364 (Monocotyledons, 76; Dicotyledons, 288 

 = 1 : 3-7.) The proportion of genera to species is 1 : 1-7. Of 

 these 364 species, almost all but the littoral and purely arctic 

 species are found in West Temperate North America, or in the 

 Rocky Mountains, 26 in the Andes of Tropical or Subtropical 

 America, and 37 in Temperate or Antarctic South America. Com- 

 paring this flora with that of Temperate and Arctic Asia, I find 

 that no less than 320 species are found on the north-western shores 

 and Islands of that continent, or in Siberia, many extending to 

 the Altai and the Himalaya. A comparison with Eastern Arctic 

 America shows that 281 are common to it, and the following 38 

 are found in Temperate, but not Arctic East America: — 



Anemone alpina. Saxifraga bronchialis (Eu., As., & R. M.) 



Pennsylvanica. Archangelica officinalis (Eu., As., & A.). 



Hutchinsiacalycina (Rocky Mountains only Ligusticum Scoticum (Eu., Asia, Am.). 



and Asia.) Cornus Suecica (Europe, Asia, Am.). 



Sisymbrium humile (Rky. Mrs. and As.). Galium rubioides (Europe, Asia, Am.). 



Draba oligosperma (Rocky Mounts, only). Senecio resedaefolius (Europe, Asia, Am.). 



Lathyrus palustri» (Europe, Asia, East Pseudo- Arnica ( Asia and America.) 



and West America). Cassandra calyculata (Europe, Asia, Am.). 



Spirasa salicifolia (Eu., As., E. & W. Am.). Gentiana arctophila (Rocky Mounts, only). 



Potentillafruticosa(Eu., As., E. & W. Am.). prostrata (Europe, Asia. Am.). 



Pennsylvanica (E., A., E.&W.Am.). tenella (Europe, Asia, Am.). 



Comarum palustre (Eu., A., E. & W. Am.). Veronica scutellata (Europe, Asia, Am.). 



Montia fontana (Eur., As., and W. Am.). Pedicularis palustris (Europe, Asia, Am.). 



Saxifraga Sibirica (Asia and Labrador only). Atrip'lex patula (Europe, Asia, Am.). 



— Dahurica (Asia and Rky. Mts. only). Corispermum hyssopifolium (E., A., Am.). 



