296 PLANT LIFE. 



There is no general agreement amongst geologists as 

 to the character and number of the Ice Ages, which, of 

 course, greatly interferes with a continuous history of 

 the successive invasions. In a broad way it may be 

 supposed that after Pliocene times, which enjoyed a 

 sort of subtropic to temperate climate, there came into 

 existence the intense cold of the first great Ice Age. 



Even then there seems to have been a series of well- 

 developed Arctic plants. They are now supposed 

 (Engler) to have originated in the Caucasian Mountains, 

 and to have very early in their history migrated north- 

 wards, so as to form a circumpolar flora stretching all 

 round the North pole, where they, with their attendant 

 mosses and lichens, still exist. 



This ice sheet seems to have advanced southwards 

 over the greater part of Central Europe. During 

 that period of ice and snow, any exposed rocks or 

 plains must have endured the severest climate of the 

 Farthest North. After a long interval, the ice sheet 

 retreated northwards, and a subtropical climate, appa- 

 rently much warmer than that of our own times, 

 reigned. Again, and probably several times over, the 

 ice sheet advanced southwards, driving before it, or 

 killing out, the subtropical inter-glacial plants, and 

 again melted. Then it finally retreated to the Arctic 

 regions, and the present distribution of animals and 

 plants came into existence. It is often supposed that 

 present conditions are as permanent as they appear to 

 us ; but this is a mere assumption. It is better to look 

 upon the flora as being still in migration. The ice cap 

 has reached a certain average Northern latitude. The 

 Arctic plants and those of the tundras form a circumpolar 

 fringe round it. South of the Arctic plants, the land 

 is occupied by forests of Pmus Sylvestris or of other 



