PROCEEDINGS-PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
Cl 
the 24th degree of longitude east of Greenwich, beyond which it 
gradually withdrew to the north. At the same time the alpine 
regions south of the great ice sheet had more or less extensive local 
ice coverings. 
Now, though the whole of Britain was not actually covered—so 
far as we know—by the ice sheet, yet the climatic conditions of the 
narrow strip which remained uncovered must probably have been 
such as to exclude the idea that any plants—unless it were those of 
the most arctic nature, and even that is doubtful—were able to 
survive. Consequently the whole British flora was practically de¬ 
stroyed, and when the ice sheet disappeared the country was, as it 
were, a bare field ready to receive and welcome all comers. 
It must be remembered that in South Central and South Europe 
during the climax of the ice age the more tender temperate, as well 
as the strictly southern, plants could not have survived, but were 
driven southwards, and their places taken by an alpine, northern, 
and arctic flora. 
These cold and icy conditions were not to last for ever. The 
climate became warmer, and the ice retreated northwards or up the 
hills, and as it retreated the plants pursued, following in the order 
in which their ability to resist cold permitted. 
The fluctuations in the retreat of the ice, in the climatic con¬ 
ditions, and in the relative proportions of land and water, must all 
have exercised a powerful influence on the flora, but need not be 
considered, even'if it were possible, in detail just now. The period 
with which we are concerned is that one in early postglacial times, 
which Professor Geikie calls the “First age of Forests.” In this 
period the climate was mild and genial, and Britain had a land 
connection with continental Europe. All the North Sea was then 
dry land, which, beginning in the Arctic Ocean, extended far to the 
west of the present coast of Norway, had its coast-line to the north 
of the Shetlands, and stretched away southwards outside the west of 
Ireland and the south-west coast of France, and just reached the 
north of Spain. From the west of the Shetlands an irregular strip 
of land probably extended north-westwards, and included the Foeroes 
and Iceland, and possibly even reached Greenland. In this great 
land extension were several great lakes of fresh water, such as the 
Baltic, a deep trough between Denmark and Norway, and a long 
hollow in the Irish Sea. 
It was during this land connection that the great bulk of our 
flora reached this country, crossing over the dry bed of the English 
Channel and North Sea. There is no difficulty in knowing which 
were the plants to arrive first. These were the northern and alpine 
species, and the reason for their being in the vanguard is to be 
looked for in the fact of their fitness to contend with the cold of the 
regions nearest the retreating ice. But there is another reason for 
supposing that they were amongst the first plants to reach this 
country. To attain the mountain ranges, where alone they now 
survive, they had to cross the lowland plains and valleys, and had 
these been clothed with their present vegetation the alpine species 
could never have reached the hills. This arises not so much from 
