57o 
Sinnott and Bailey . — - The Origin and 
Rocky Mountains, the Faroes and Ellesmereland, and have shown that in 
all these woody plants constitute only about io per cent, of the whole. 
This io per cent, includes mainly such genera as Salix and Betiria , which are 
very hardy. Many families are represented here as herbs, however, which 
are quite unable to maintain in such regions representatives with aerial 
stems which persist through the winter. 
The predominance of herbaceous plants in arctic and alpine regions 
suggests an explanation for the very high percentage of herbs in the flora 
of northern Europe, and provides important evidence as to the climate of 
that region during the glacial period. We know that the ice-sheet did not 
extend over the extreme south of England or over much of northern France 
and Germany, and it is highly probable that the present flora of countries 
north of the Alps has been derived from the remnant of the ancient flora 
which was able to exist on these unglaciated areas. The fact that the per- 
centage of herbaceous plants on the lowlands of northern Europe to-day is 
so very high (really an arctic or alpine percentage) seems to indicate that 
the climate during the height of the glacial period was cold enough to kill 
off all but those hardy trees and shrubs which are now in the flora of 
northern Europe, but was not cold enough to exterminate the herbaceous 
vegetation to nearly as great an extent. It was probably very much like 
that of Switzerland or any similar mountainous region of the present day, 
and was neither extremely rigorous nor very mild. 
This conclusion is strengthened by the remarkable similarity which 
exists between the flora of northern Europe and that of the Rocky 
Mountains. The two are composed of almost exactly the same families, 
and in these are hosts of identical genera and a large number of identical 
species. Many of these genera and species are found nowhere else in 
America ; in fact, the flora of the Rockies presents a much closer resemblance 
to that of Europe than does the flora of the eastern part of the continent. 
This suggests that both in the Rocky Mountains and in northern Europe 
we have a flora essentially like that which flourished near the ice-front 
during the glacial invasion, and that it is composed of the survivors (mainly 
herbs) of the widespread northern preglacial flora which were able to with- 
stand the rigours of a much colder climate. In temperate America the 
percentage of woody plants has been greatly increased since glacial times 
by migration from the south, but in Europe this has probably occurred only 
to a very slight extent owing to mountain ranges ; for the climate of 
northern Europe at present is such as would support a much larger per- 
centage of woody plants than does that of eastern North America, but its 
actual percentage is only about half as great. Such facts as these point 
again to the important connexion between the past and present climate and 
geography of a region, &nd the size of its herbaceous flora. 
It is almost certain, therefore, that the percentage of woody plants in 
