152 Rhodora [SEPTEMBER 
north shore of Lake Superior, and at other similar points) is of course 
due in part to the fact that we have in this large district only a limited 
number of regions in which the climatic conditions are comparable 
with those of the Arctic. And the occurrence in exposed or frigid 
situations of Mt. Washington or along the Gaspé rivers of such isolated 
species as Empetrum nigrum, Rubus Chamaemorus and Dryas Drum- 
mondii is one of the strong links of evidence which has convinced 
botanists of the inevitableness of the conclusions long ago reached 
by Hooker, Gray, and others, that it is “difficult to account for these 
facts, unless we admit Mr. Darwin's hypotheses, first, that the existing 
Scandinavian flora is of great antiquity, and that previous to the glacial 
epoch it was more uniformly distributed over the polar zone than it 
is now; secondly, that during the advent of the glacial period this 
Scandinavian vegetation was driven southward in every longitude, 
and even across the tropies into the south temperate zone; and that 
on the succeeding warmth of the present epoch, those species that 
survived both ascended the mountains of the warmer zones, and also 
returned northward accompanied by aborigines of the countries they 
had invaded during their southern migration. Mr. Darwin shows 
how aptly such an explanation meets the difficulty of accounting for 
the restriction of so many American and Asiatic arctic types to their 
own peculiar longitudinal zones, and for what is a far greater difficulty, 
the representation of the same arctic genera by most closely allied 
species in different longitudes."'! 
Practically every newly explored alpine district or cliff-region of 
New England and eastern Canada furnishes its addition to the already 
extensive list of polar and high-northern species which are isolated 
south of the St. Lawrence; and to-day we know in this area, between 
Long Island Sound and the mouth of the River St. Lawrence, more 
than four hundred such Pteridophyta and Spermatophyta, a list which 
would be significantly increased by the addition of the lower groups 
of plants. But the discovery of a few additions to this very long list 
of arctic-alpine and high-northern plants south of the St. Lawrence, 
however interesting it always proves, is, in view of the extensive data 
already amassed, only of minor importance compared with the greater 
problem of determining the reasons for this isolated distribution. 
1J. D. Hooker, Outlines of the Distribution of Arctic Plants: Trans. Linn. Soc, 
xxiii. pt. 2, 253 (1861). See also Darwin, Origin of Species, Chap. XI; Gray, Am, 
Journ. Sci. Ser. 2, xxxiv. 144 (1802). 
