74 On the Origin and Distribution of the British Flora.
Biscay to the British Isles, to the importance of which I
shall presently again allude.
Mr. Wallace has, I think, brought forward sufficient argu-
ments to enable us to conclude that these geographical
changes would prevent the recurrent glacial periods neces-
sitated by Dr. Croll's hypothesis from having more than the
local effect in the Flysch deposits between Switzerland and
Vienna, and the ice-scratched boulders in the upper Miocene
of Turin.18 Directly, however, we pass to the Pliocene a
cooling of the climate seems to have taken place, as seen in
the pines and alders of the Cromer forest bed,19 and the
presence of the Arctic willow (Salix polaris) and the dwarf
birch (Betula nana) in the clay deposit overlying the sub-
tropical miocene lignite at Bovey Tracey.20 The glacial
periods probably then commenced with elevation which would
cause a southerly extension of the ice and cold, driving the
Miocene flora southwards. The more temperate species could
then cross the Tropics along the chain of the Andes, and
from the Caucasus through the Himalayas and the moun-
tains of Aracan and Java to the north of Queensland.
Probably, at a still earlier period, a migration had taken
place along this last line, not only to Tasmania, but to New
Zealand, the more modern flora of West Australia being
then shut off by a central sea.21 The period of elevation was
18 Wallace, op. cit., pp. 171, 172.
19 Rev. G. Henslow, in "The Origin and Present Distribution of the
British Flora," Trans. Watford Nat. Hist. Soc., vol. ii. (1879), p. 138.
20 Henslow, loc. cit., and Lyell, op. cit., p. 221.
21 " Thus the plants of Fuegia extend northward along the Andes,
ascending as they advance. Australian genera reappear on the lofty
mountain of Kinibalu in Borneo; New Zealand ones on the mountains
of New Caledonia; and the most interesting herbarium ever brought
from Central Africa, that of Mr. Joseph Thomson, from the highlands of
the lake districts, contains many of the endemic genera, and even species
of the Cape of Good Hope. . Nor does the northern representation of the
south temperate flora cease within the tropics ; it extends to the middle
north temperate zone; Chilian genera reappearing in Mexico and Cali-
fornia ; South African in North Africa, in the Canary Islands, and even
in Asia Minor ; and Australian in the Khasia Mountains of East Bengal,
in Bast China, and Japan."—Sir Joseph Hooker's Address to the
Geographical Section of the British Association (York, 1881).