64 A SUMMER IN GREENLAND 
Grinnell Land to eastern Canada and the United 
States and Japan, two species of the widely spread 
American and Siberian genus, Cassiope, the com- 
monest of which, Cassiope tetragona, has small 
crowded leaves like green overlapping scales 
grasping the slender stems in four regular geo- 
metrical rows with here and there a yellowish white 
bell pendulous on a delicate stalk. 
The geographical distribution of many of the 
Greenland flowering plants presents interesting 
problems in relation not only to the efficiency of 
the plants as travellers, but to the position of the 
places from which they originally spread. Two 
examples may be briefly considered in this con- 
nexion. 
Phyllodoce coerulea has a wide distribution: it 
occurs on both coasts of Greenland, in Iceland, 
from northern to southern Scandinavia and the 
mountains of Scotland, in eastern Asia, where it 
extends, in Japan, south of lat. 40° N.; it flourishes 
from Labrador to the White Mountains on the 
eastern side of the United States; it is found also 
in Alaska and in the north-west district of the 
Pacific coast of North America. Phyllodoce may 
have begun its existence in the northern Pacific 
region, probably in the latter part of the Tertiary 
epoch, long before the Glacial period. As the 
climatic conditions in the north became more 
severe it gradually migrated to the south, where it 
is still represented in the central Pyrenees and in 
Japan. As conditions ameliorated, the species re- 
turned to the north or ascended to the higher 
