FAUNA AND FLORA OF YORKSHIRE. 189 



With such a character of surface long continued, there would 

 be no difficulty in admitting the gradual distribution over the 

 British Isles of a large proportion of the terrestrial forms of 

 Europe, quadrupeds, birds, reptiles, insects, land mollusca, *and 

 plants ; the difficulty would be greater in regard to the denizens 

 of rivers and lakes; and in fact there are some considerable local 

 distinctions in respect of these, not only in contrasting Britain 

 with the continent, but also in comparing one of our rivers with 

 another. 



One such migration must be supposed to have happened in 

 the preglacial period : were all the animals and plants of this 

 colony destroyed by the glacial ocean; and is it necessary to admit 

 another migration after that ocean had been withdrawn ? If 

 the glacial ocean covered all our islands, the second migration 

 must be admitted; but there is no proof that that ocean did 

 cover all our mountains. On the contrary, there seems reason 

 to limit its height, as a general rule in the north of England and 

 Wales, to something less than 1500 feet. This indeed would 

 reduce everywhere to a series of islands what is now the land of 

 Britain, a condition under which some races of animals and 

 plants must perish ; yet the islands might preserve many spe- 

 cies, which on the retirement of the sea would spread down- 

 ward from the mountains as far as climatal conditions allowed, 

 according to the notion long since put forth by Linnaeus in his 

 treatise ' De Telluris Orbis Incremento/ Some species, however, 

 would remain confined to the mountains. 



This is the fact in regard to some species of plants, which oc- 

 cur on the mountains of Scotland and Cumberland, and the 

 most elevated regions of Yorkshire, and which appear to be parts 

 of a Scandinavian flora, communicated to Britain before the 

 glacial period, and now preserved on certain elevated tracts 

 which, during that period, stood above the water. 



As examples of the plants here alluded to, we may quote 

 from Baines's 'Flora of Yorkshire' the following well-known 

 species : 



