124 HISTORY OF THE EUROPEAN FAUNA. 



water lake between England and Ireland is indicated 

 by the distribution of the Charrs and also by the 

 various species of British Coregonus. There are 

 three British species of Coregonus, viz., C. clupeoides, 

 C. vandesius, and C. pollan. These are confined to 

 the lakes of North Wales, North-western England, 

 South-western Scotland, and Ireland. All but the 

 latter communicate at present directly with the Irish 

 Sea. The lakes of the latter country, however, must 

 have done so at a time when the west of Ireland 

 stood at a higher level than it does now. The 

 ancestors of the three Coregonus species, and also 

 those of the Charrs, then lived in the large freshwater 

 lake indicated on the map (p. 60), and when the sea 

 gradually crept up the river valley and finally con- 

 verted the lake into a gulf, the freshwater fish took 

 refuge^ in the rivers which supplied it with water. 

 ^NQW as for the continuous sea-shore between the 

 coast of Brittany and the south-west of Ireland, 

 zoological distribution again aids us in proving that 

 such must have actually existed at no very distant 

 geological date. Most of our common shore forms 

 of life migrate along the coast exactly as land 

 animals do step by step. Their eggs are care- 

 fully attached to fixed objects, so as not to be 

 carried away by the waves, whilst the young often 

 remain and grow old in some particular little pool, 

 rarely venturing farther than a few yards from the spot 

 where they first saw the light of day. A number of 

 such shore forms are found on the west coast of France, 



