836 
THE PRINCIPLES OF BIOLOGY. 
be subjected to new conditions of various kinds. The hardier 
species would remain behind, but their interactions would be 
changed and they would be affected by migrations from the 
zone above. The effect would be a migration southwards 
of a vast numbers of species. When the European stream 
reached the foot of the Alps it would divide into two branches, 
the one travelling into France and Spain, the other into 
Hungary, Turkey, and the Levant. At the same time with 
these migrations the Alpine species would descend from the 
mountains into the plains. As the climate grew warmer the 
migrations would continue but their direction would be 
reversed. The southern species would return northward, the 
Alpine species would travel partly to the mountain summits, 
partly to the Arctic zone. During the glacial epoch there 
was a great submergence, by which many of the plants of 
northern Europe must have been destroyed. These changes 
must have produced large specific modifications in the 
European Flora, and account for many of the singular 
phenomena of distribution which it presents. 
We observe, for instance, striking differences between 
the Floras of the east and west end of the Alps, small groups 
of remarkable species restricted to areas of very limited 
dimensions, single species appearing in a few spots widely 
distant from each other, such as the Welsh Poppy in Wales, 
Central France and the Pyrenees, or restricted even to a 
single locality, as Wulfenia carinthiaca to one mountain in 
the eastern Alps. On the other hand, we find plants 
restricted to a few localities, but very widely distributed, 
strangely incapable of colonising the intervening spaces, even 
when provided with the feathery pappus of composites, such 
as Hypocliccris metadata, and some of the Hawkweeds. Finally 
we have the Alpine Flora, characteristic of the higher 
summits of the Pyrenees, Alps, Carpathians, Altai, and 
Caucasus, and reappearing to a great extent both in the Old 
and New World, within the Arctic zone. 
The origin of this Flora and the nature of its connec¬ 
tion with that of the Arctic zone is a question of great 
interest which has been discussed with signal ability by Mr. 
John Ball.* Sir J. D. Hooker was of opinion that the Alps 
had been colonised from the Arctic regions ; Mr. Ball, with 
more justice, as it seems to me, holds the view that the Arctic 
regions were colonised from the Alps. 
The genus most characteristic both of the Alps and Arctic 
regions is certainly the saxifrage. Its distribution in Europe 
* Proceedings Royal Geog. Soc., New series, vol. 1, p. 564. 
