Geographical Distribution of British Plants. 25 
great number of species common to this district and the op- 
posite coasts of France. The characters of the entomological 
fauna bear a relation to this flora; and such is likewise the 
case with the land-shells either confined to this district, or 
very rarely extending beyond it. 
The plants of the Scottish mountains, which compose the 
fourth flora, are few in number to the south, in Northumber- 
land and Wales, but they are all identical with those of the 
northern chains, such as the Scandinavian Alps, where we like- 
wise find associated with them certain species not occurring 
in the British Islands. The Alpine forms diminish progres- 
sively from the north to the south, and the same distribution 
seems to exist in regard to the fauna of the mountainous re- 
gion. 
Lastly, the fifth flora, whether viewed alone, or associated 
with the rest, is identical with that of Central and Western 
Europe, or the German flora, and the accompanying fauna 
diminishes as we advance northwards and westwards. 
It was not till after the deposition of the London clay, or 
lower tertiary formation, that the migrations of the plants and 
animals in question could have commenced, the temperature 
before that period having favoured the development of organ- 
ized beings of a very different kind. These migrations must 
likewise have taken place before the appearance of man, for the 
peat deposits, composed of the remains of vast forests, which 
occupied a great part of the existing surface of the British 
Islands during the most remote historical times, contain fresh 
water marls with Cervus megaceros, Sc., which, in their 
turn, lie above the tertiary pleistocene deposits, forming the 
elevated bed of the sea at the time of the glacial period.* 
During the quaternary (post-pliocene) period, the greater 
part of the flora and fauna of the British Islands migrated 
from the Continent on this elevated bed of the glacial sea. 
The animals, as well as vegetables of Germanic type, shew, 
by their distribution in the east of England, not less than 
by their rarity in proportion as we advance westward, and 
* Throughout his whole work, Mr KH. Forbes regards the existence of the 
glacial period as a fact that has been demonstrated, 
