58 JOURNAL OF THE EOYAL HORTICULTURAL SOCIETY, 
At the end of Tertiary times, before the coming of the Ice Age, the 
configuration of Europe was in its main outHnes much the same as 
to-day. There were differences in detail, however. At that time Britain 
was still connected with the Continent, and, as far as our knowledge 
extends, the fauna and flora of both were much alike. We can thus 
understand how it is that the floras of the British Isles and of Switzer- 
land present so many points of close relationship, as we have seen to be 
the case from a comparative study of the two floras. For there is little 
doubt that our present flora is directly descended from that of Tertiary 
times. 
The close of the Tertiary period was marked by a gradual lowering of 
the temperature throughout the Northern Hemisphere, the exact causes 
of which are still disputed. Ice advanced slowly from the direction of 
Scandinavia over Central and Western Europe. The greater part of 
Britain became covered with an ice sheet, just as Greenland is to-day. 
On the Alps extensive glaciers were formed, owing to the intense cold, 
and these filled all the valleys with an almost unbroken covering of ice. 
There is evidence, however, that this Glacial period did not come quite 
without warning. As the temperature gradually sank the flora retreated 
further and further southward. In Britain the main glaciation did not 
reach to the southern counties, and there the flora took refuge, or migrated 
to France, which was then joined to Britain, and which for the most part 
remained free from ice. In Switzerland the flora was either driven 
across the Alps into Italy, or across the Jura into France. How long 
the Ice Age lasted is still disputed, but in all probability it extended over 
a considerable period. After a time, however, the cold became less in- 
tense and the ice began to retreat, and then the flora gradually returned. 
The precise influence of the Glacial period in regard to the origin of 
the present alpine flora of Europe is, however, still in dispute. All are 
agreed that this flora is ancient, and that part of it at least existed before 
the Glacial period, and that its present distribution was in the main 
determined by that period. 
Some twenty years ago this point was much discussed by both British 
and Continental botanists. In 1875 Alphonse de Candolle * published 
his researches on the causes of the inequality in the distribution of rare 
plants in the Alps. It has been long known that the south side of the 
Alpine chain is much richer in species than the northern, just as certain 
districts in Switzerland are richer than others. According to de 
Candolle the glaciers on the south side of the Alps were much less exten- 
sive than on the north, and consequently, when the cold became less 
intense, they were among the first to disappear. The pre-Glacial alpine 
flora found a refuge on the south side of the Alps, and de Candolle 
thought that when the ice retreated, part at least of this flora returned, 
those mountains and valleys which first became free from ice having 
to-day the greatest number of rare species and the most varied flora. 
An altogether different origin of the Swiss alpine flora was suggested 
by Sir J. Hooker about the same time. From a study of the Arctic 
flora Hooker found that a large percentage of that flora occurs in the 
Alpine regions of Southern Europe, as well as in many other parts of 
• * De Candolle, A. Vide Nature, April 1876. 
