PHYSICAL CONDITIONS— POSTGLACIAL. 511 



with the reptiles ; so also with the insects 1 and the pulmoni- 

 ferous mollusca." These peculiarities of distribution Forbes has 

 accounted for by supposing that Ireland was separated from 

 England by the influx of the Irish Sea before the species, less 

 speedy of diffusion, could make their way into the sister island, 

 and this view has been repeated by every writer who has touched 

 upon the question since the appearance of Forbes's famous essay. 

 But a glance at the Admiralty's chart of the Irish Sea shows us 

 that there is no necessity for inferring that the arrestment of the 

 migration was due to submergence. Were the whole British 

 area to be elevated for 600 feet or thereabout the Irish Sea 

 would disappear, but Ireland would still be separated from Eng- 

 land by a great and deep lake, averaging 25 miles at least in 

 breadth, and extending from the head of what is now the Sound 

 of Jura in Scotland down through the basin of the Irish Sea to a 



1 My friend Dr. Buchanan White has recently discussed the distribution of 

 the mountain -lepidoptera of Britain and its causes {Scottish Naturalist, July 

 1879). In this paper he gives an admirable resume of the evidence relating to 

 the introduction of the alpine or Scandinavian flora of our islands, and has traced 

 in a very suggestive manner the route followed by the lepidoptera, which are now 

 restricted to our mountain-regions. The facts and suggestions put forward in his 

 paper are thus summed up : — 



"1. The British Isles, being at one time subject to extreme arctic conditions, 

 had no fauna or flora. 



"2. At the close of the last glacial period they were peopled by plants and 

 animals from Continental Europe. 



" 3. Most of these plants and animals reached Britain across the dry or nearly 

 dry bed of the German Ocean. 



"4. Plants necessarily arrived before animals; and of the former certain 

 classes of cryptogamic plants, and the maritime and wind-fertilised species of the 

 higher plants, were the first comers. 



" 5. The arctic and arctic-alpine plants and animals, being those that followed 

 closest on the retreating ice, were amongst the earliest arrivals, and had a wide 

 range through the country. 



" 6. From their present distribution in Britain it is probable that all the 

 species (in question) did not enter Britain at the parts nearest Continental 

 Europe, but that they reached it at various points on the present east coast. 



11 7. The distribution of the species [of lepidoptera] (treated of in this paper) 

 is not co-extensive with that of their food-plants. 



" 8. Climate has been a chief factor in producing the present distribution. 



"9. Ireland derived some of its insects from Scotland. 



"10. At least some of the British mountain lepidoptera existed as species 

 previous to the last glacial period. " 



