322 LIFE BY THE SEASHORE. 



Islands on the coast of Norway ; (2) the Boreal, including 

 the seas bounded on the north by the line just given, and 

 on the south by a line drawn above the South Coast of 

 England ; (3) the Lusitanian, including the English Channel, 

 the Bay of Biscay, the coasts of Spain, and the Mediter- 

 ranean. Thus, except the South Coast, the whole British 

 area is within the Boreal region; but a map of the ocean 

 currents will show that certain of these sweep our western 

 shores, and crossing by the Shetland Isles sweep north- 

 wards along the coasts of Scandinavia. There is thus a 

 constant tendency for the Lusitanian types to travel up 

 along the West Coast, and such types may occur in the 

 far North in the Shetland Islands, and again on the coast 

 of Norway, while totally failing to establish themselves 

 on the East Coast. Again, as there is no sharp line of 

 demarcation between Arctic and Boreal regions, the Arctic 

 forms tend to spread southwards, and usually find it easier to 

 gain a foothold in the colder Eastern waters than in those 

 of the Western coast. Thus, except in the extreme South, 

 our marine fauna is generally of the Boreal type, but on 

 the West there is a strong admixture of Lusitanian types, 

 and on the East, especially the North-east, a strong admix- 

 ture of Arctic types. Especially curious are the conditions 

 in the Shetland Islands, where Arctic and Lusitanian forms 

 intermix. 



Further, as our whole area is small and the con- 

 ditions fairly uniform, a dominant and successful species, 

 whatever its original home, is likely to occur in varying 

 numbers in all parts of our area. Thus the Norway lobster 

 (Neplirops norvegicus), a typical Northern form, which is 

 sufficiently abundant in the Firth of Forth to be the object 

 of an important fishery, does also occur, though not in such 

 abundance, off the South and West Coasts. The common 

 hermit-crab of the Boreal region is Pagurus bernhardus, and 

 of the Lusitanian P. prideauxii ; but on the West Coast the 

 two occur together in almost equal abundance. Similarly 

 the Stenorliynchus of the Lusitanian region is S. longirostris, 

 of the Boreal S. phalangium ; but in the Firth of Clyde the 

 two occur in almost equal numbers. Perhaps the prettiest 

 example of this overlapping process, however, is the dis- 

 tribution of the common starfishes. The common starfish, 



