74 (hi 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 fi-om 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. ^® 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,^^ 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.^'^ 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 

 IDlace 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.^^ The period of elevation was 



18 Wallace, op. cit., pp. 171, 172. 



i'-" Eev. Gr. 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 reaiopearing 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 East China, and Japan." — Sir Joseph Hooker's Addi'ess to the 

 Geographical Section of the British Association (York, 1881). 



