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 



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 

 age." 7 



