DR. HOOKER ON THE DISTRIBUTION OF ARCTIC PLANTS. 253 . 
dinavian plants should, under existing conditions of sea, land, and temperature, have 
not only found their way westward to Greenland, by migration across the Atlantic, but 
should have stopped short on its west coast, and not crossed to America ; or that so many 
American types should terminate as abruptly on the west coast of Baffin's Bay, and not 
cross to Greenland and Europe ; or that Greenland should contain actually much fewer 
species of European plants than have found their way eastwards from Lapland by Asia into 
Western and Eastern Arctic America ; or that the Scandinavian vegetation should in every 
longitude have migrated across the tropics of Asia and America, whilst those typical plants 
of these continents which have found their way into the arctic regions, have there remained 
restricted to their own meridians. 
It appears to me 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 Scandina- 
vian vegetation was driven southward in every longitude, and even across the tropics 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 lon- 
gitudes. To this representation, and the complexity of its character, I shall have to 
allude when indicating the sources of difficulties I have encountered, whether in limit in 
O 
the polar species, or in determining to what southern forms many are most directly re- 
ferable. Mr. Darwin's hypothesis accounts for many varieties of one plant being found 
in various alpine and arctic regions of the globe, by the competition into which their 
common ancestor was brought with the aborigines of the countries it invaded : different 
races survived the struggle for life in different longitudes ; and these races again, after- 
wards converging on the zone from which their ancestor started, present there a plexus 
of closely allied but more or less distinct varieties or even species, whose geographical 
limits overlap, and whose members very probably occasionally breed together. 
Nor is the application of this hypothesis limited to this inquiry ; for it offers a possible 
explanation of a general conclusion at which I had previously arrived f and shall have 
again to discuss here -viz. that the Scandinavian flora is present in every latitude of 
tbe globe, and is the only one that is so ; and it also helps to explain another class of 
most interesting and anomalous facts in arctic distribution, at which I have now arrived 
from an examination of the ve-etation of the several polar districts, and especially of that 
of Greenland. 
southern 
<f 
due to the cold epochs preceding and during the 
_ — * • ^L. W 
Darwi 
with 
* Origin of Species,' chap. xi. 
Flora 
2 M 2 
