THE EUROPEAN SEAS. 



221 



which have their limits there. When, as on our 

 western coasts, or on those of France or the Chan- 

 nel Islands, hard rocKS face the sea, what rich 

 gathering grounds does the sea-side naturalist there 

 meet with, over miles of broad, horizontal ledges ! 

 A few successive winters, with low temperatures, 

 would destroy the whole of the fauna of this broad 

 intertidal zone, would alter the present relative pro- 

 portions of littoral species, and give another aspect 

 to our marine testacea, viewed collectively. 



A mean winter temperature of 54° F. includes 

 the north, west, and east coasts of Spain, together 

 with Sicily and Greece. Gibraltar, and the south- 

 ern shores of the Mediterranean, are warmer by 

 several degrees. From the Arctic Ocean to the 

 mid-Mediterranean there is a difference of mean 

 winter temperature of more than 50° F. 



The influence of summer temperature is best 

 indicated by the range of southern forms. That 

 part of our own coast which just comes within the 

 July mean temperature of 64° F., namely, the ex- 

 treme south-west parts of Devon and Cornwall, 

 is also that from which our rarer southern forms of 

 fishes and Mollusca are taken. The researches of 

 Mr. M Andrew have shown that Yigo Bay has an 

 isolated assemblage of testacea of a somewhat north- 

 ern character ; it may be an outlier of the marine 

 fauna of a former period, but its present distinct- 

 ness may have been maintained by the somewhat 

 lower summer temperature of Gallicia, compared 



