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THE AMERICAN NATURALIST [Vol. XL VI 



materials had not been better digested, and much might just as 

 well have been omitted. The contrast between the over-elabora- 

 tion of many topics dealing with the Eastern flora, and the very 

 casual treatment of the far more important and divergent condi- 

 tions of the Rocky Mountain and Pacific region, is both striking 

 and regrettable. However, with all these serious faults, the 

 patient reader will be able to get a fairly comprehensive view of 

 the extremely diverse features making up the flora of the North 

 American continent. 



With the last retreat of the great ice sheet, there moved into 

 the territory thus uncovered plant migrants from various sources. 

 Keeping close to the retreating ice sheet are the various strictly 

 arctic or glacial types, and south of this northernmost flora is 

 the extensive development of bog and tundra formations, which 

 form so important a feature in the arctic and subarctic regions 

 of Alaska, Canada and Greenland. 



This northern area in America shows three sections, ranging 

 from east to west. Greenland, separated from the mainland by 

 the deep water of Baffin's Bay and Davis Strait, belongs really 

 with Europe, its scanty flora being composed almost exclusively 

 of European types like those of Scandinavia and Lapland. The 

 middle region extending from Labrador to the .Mackenzie River 

 is marked by a larger preponderance of purely American types. 

 Westward, including Alaska, the arctic flora is again charac- 

 terized by a preponderance of Old World species. Out of 364 

 species of Western Arctic America no less than 320 occur in 

 Northwestern Asia, many of them extending southward to the 

 Altai and Himalaya Mountains. Of the 379 species belonging to 

 the middle region 73 species are strictly American, the other 306 

 occur also in the arctic regions of the Old World. 



The characteristic " tundras " are plains whose subsoil is per- 

 manently frozen, but whose surface soil is covered with a dense 

 growth o^mosses or lichens, among which grow in places dwarf 



Cassiope, Andromeda, Vaccinium. Ledum, etc., as well as many 

 herbaceous plants, some with showy flowers like the Iceland 

 poppy. It is evident that the arctic flora is remarkably uniform 



