138 THE MIGRATION OF BRITISH BIRDS 



rupeds. So it is also with the birds of short flight ; so 

 most remarkably, no less than half the species being 

 different, with the reptiles ; so also with the insects and 

 the pulmoniferous mollusca." Professor Geikie, in criti- 

 cizing Forbes's remarks, continues : " 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 dif- 

 fusion, 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 [emi- 

 gration] was due to submergence. Were the whole 

 British area to be elevated for six hundred feet or 

 thereabout, the Irish Sea would disappear, but Ireland 

 would still be separated from England by a great and 

 deep lake, averaging twenty-five miles at least in 

 breadth, and extending from what is now the Sound 

 of Jura in Scotland down through the basin of the 

 Irish Sea to a point between Braich-y-pwll in Car- 

 narvon and Greenore Point in We.xford. This lake 

 receiving the tribute of many Scottish, Irish, and Eng- 

 lish streams, would discharge a broad river from its 

 lower end, which might well be impassable by many of 

 the smaller vertebrates. That it was rather the pre- 

 sence of this lake and the obstacle of the Welsh 

 mountains than the premature appearance of the Irish 

 Sea which arrested the westward migration [emigra- 

 tion] of plants and animals, is shown by the remarkable 

 fact pointed out by Professor Leith Adams that the 



