CHAP. XVI 



THE BRITISH ISLES 



343 



Ireland ; and the latter country has been longer isolated, for 

 the Irish Sea with its northern and southern channels is 

 considerably deeper than the German Ocean and the 

 Eastern half of the English Channel, so that, when the 

 last subsidence occurred, Ireland would have been an 

 island for some length of time while England and Scotland 

 still formed part of the continent. Again, whatever 

 differences have been produced by the exceptional climate 

 of our islands will have been greater in Ireland, where 

 insular conditions are at a maximum, the abundance of 

 moisture and the equability of temperature being more 

 pronounced than in any other part of Europe. 



Among the remarkable instances of limited distribution 

 afforded by these fishes, we have the Loch Stennis trout 

 confined to the little group of lakes in the mainland of 

 Orkney, occupying altogether an area of about ten miles by 

 three; the Welsh charr confined totheLlanberis lakes, about 

 three miles in length ; Gray's charr confined to Lough Mel- 

 vin, about seven miles long ; and the Loch Killin charr, 

 known only from a small mountain lake in Inverness-shire. 



Cause of Great Speciality i7i Fishes. — The reason why 

 fishes alone should exhibit such remarkable local modifica- 

 tions in lakes and islands is sufficiently obvious. It is due 

 to the extreme rarity of their transmission from one lake 

 to another. Just as we found to be the case in Oceanic 

 Islands, where the means of transmission were ample 

 hardly any modification of species occurred, while where 

 these means were deficient and individuals once transported 

 remained isolated during a long succession of ages, their 

 forms and characters became so much changed as to bring 

 about what we term distinct species or even distinct genera, 

 — so these lake fishes have become modified because the 

 means by which they are enabled to migrate so rarely 

 occur. It is quite in accordance with this view that some 

 of the smaller lakes contain no fishes, because none have 

 ever been conveyed to them. Others contain several ; and 

 some fishes which have peculiarities of constitution or habits 

 which render their transmission somewhat less difficult 

 occur in several lakes over a wide area of country, though only 

 one appears to be common to the British and Irish lakes. 



