ON THE DISTRIBUTION OF MOSSES, 955 
a height of some hundreds of feet above our present elevation. For 
there must have been surface and elevation for a meridional, temperate, 
and alpine flora. 
The land began to subside again as the temperature lowered ; the 
more southern forms retreated, but a few remained in stray nooks. 
From the peculiarities of temperature in Britain, through the Gulf 
Stream, our climate has been always, since the glacial epoch, less ri- 
gorous than corresponding latitudes. Hence southern plants have 
remained with us, while they have altogether vanished from the rest of 
Northern Europe. A goodly number have clung, in all subsequent 
vieissitudes to the south of England, but especially to the south-west 
of Ireland. 
The career of invasion and extension was stopped when Britain was 
again isolated. The Gulf Stream became more thoroughly a modifying 
agency in the climate of our islands, keeping our latitudes tolerable 
to the delicate southerners, but crippling at the same time our alpines. 
There yet remains one inquiry to be investigated. When was the 
community of species between America and Europe brought about ? 
It must have been anterior to the entrance of the various floras into 
Britain, for subsequent to the glacial epoch there was no period cold 
enough to admit of the transmission of alpine and subalpine species 
over the plains to the mountains. 
Similar phenomena of community of species in the two continents 
happened during previous geological ages. The miocene floras of 
Greenland, Iceland, and North America have many species in common 
with the same floras of Northern and Central Europe. That age, 
which allowed of the same plants which occurred in Central Europe— 
trees of considerable dimensions and a vegetation of some luxuriance 
—to penetrate into Greenland and Iceland, must have been one of 
considerable temperature. 
The earliest traces of the present assemblage of plants in our islands 
are found in the celebrated Cromer Forest, which overlies the Tertiaries 
of Norfolk. The prevailing tree is the Pinus sylvestris, which is found 
now in the more northern latitudes. The age of the Cromer hse 
immediately preceded the glacial. We have, y'en sem as is adm 
deduced by Lyell, evidence of a gradual refrigeration from the miocene 
period to the glacial. 
This course of argument restricts us to the conclusion that the pre- 
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