FLORA. 235 
of a warm polar climate ; hitherto, however, they have been, 
and to some extent still are, divided on the question of 
accounting for it. Some have attributed it to the passage of 
the solar system through a warmer region of stellar space, 
others to alteration in the position of the poles. fe 
Abundant evidence of the occurrence in Arctic and Sub- 
arctic regions of a series of warm periods extending as far 
back as the Silurian times, is found in the fossils of the 
various formations represented in their strata; and the 
remarkably complete succession of fossil floras there met 
with, and their marked resemblance to those of lower lati- 
tudes, forms one of Saporta’s arguments in favour of his 
view that the circumpolar area has been the birthplace of 
plants and the centre of their dispersal or migration, a 
theory which, in its main features, has received remark- 
able corroboration from the recent investigations of Daw- 
son, Dyer, and Gardner. The rich vegetation of circum- 
polar lands in eocene times, migrated southward as the 
climate gradually grew colder, giving place to the modern 
Arctic flora, which in turn crept slowly southward as the 
cold of the glacial epoch became gradually more intense, 
until at length a truly Arctic flora abounded in Central 
Europe. As the climate slowly ameliorated, the Arctic 
plant, in order to find suitable conditions, migrated north- 
wards unless where the presence of mountains enabled. 
them to obtain the necessary cold by climbing upwards 
instead of polewards, and the present alpine flora of the 
Pyrenees, the Alps, Britain, and Scandinavia, chiefly 
resembling as it does the vegetation of the Arctic regions, 
is, as Professor Geikie recently expressed it in his lecture 
on Geographical Evolution, “a living record of the ice 
7 
age. 
