Upham.] 
168 
[May 21, 
in late geologic time circumpolar lands have been united, permitting 
free land communication and causing the same plants and animals 
to extend throughout the whole Arctic area. But glacial drift and 
striae show that after this a colder climate enveloped northwestern 
Europe, British America and the northern part of the United States 
wdth thick accumulations of snow and ice ; and I believe that 
this climatic change was due to the increase of the northern eleva- 
tion of the land through which the continents had become united. 
Not only vegetation but all seeds upon the glaciated areas must 
have been destroyed, though the species were mostly preserved by 
migration southward. Thus various species of the preglacial cir- 
cumpolar flora whose retreat from the European ice-sheet was cut 
off by the Pyrenees, Alps, Carpathians and Caucasus, so that they 
perished, escaped from extinction in Asia, where no general ice- 
sheet existed, and in North America, where no transverse ranges 
of mountains barred their retreat from the ice. The glacial period 
is past, but the mild Arctic climate of Tertiary times has not re- 
turned ; and species whose range was formerly continuous through 
the high northern latitudes are now found occupying widely separated 
temperate districts in Asia and on our own continent, the time since 
the Ice Age having been too brief for them to become changed 
into new forms by the influences of their different environment. 
Many plants, however, have been changed during the Ice Age and 
the postglacial epoch, and survive in slightly unlike represen- 
tative species, as they are called, each of the three grand divisions 
of northern land, North America, Asia, and Europe, possessing a 
form thus derived from a common preglacial ancestor. 
Professor Gray has conjectured that Asia may have been still 
united by a land passage with Alaska during the earlier part of the 
postglacial epoch ; but he believed, from the noticeable contrast 
between the Arctic flora of North America and that of Greenland, 
that there has been no postglacial connection of land across Baffin 
bay nor Davis strait. This contrast seems chiefly attributable to 
the glacial extinction of species in Europe which survived in North 
America, as before noted, and to the extension of the impoverished 
European flora to the Faeroe islands, Iceland, and Greenland, after 
the rigorous cold and glaciation of these lands had abated. The 
difference of the plants found on the opposite sides of Baffin bay, 
presenting a definite break while elsewhere throughout the Arctic zone 
they are intermingled as a single continuous flora, with gradual 
