552 DEPTHS OF THE OCEAN 



Norwegian Sea. Certainly, many species are common to 

 both, but there are far more pecuHar species, the difference 

 becoming more pronounced the farther south we go. The 

 British Isles and the English Channel, the shallow-water fauna 

 of which has been thoroughly studied, may be taken as the 

 boundary where the northern and southern forms meet, both 

 categories having reached their respective southern and 

 northern limits of distribution. Along the British coasts and 

 the Channel we get, accordingly, a kind of coalition territory, 

 which has often been considered a separate faunal "province," 

 and has actually been termed Lusitanian, though in my opinion 

 without sufficient justification. The shallow - water faunas of 

 Iceland and the Faroe Islands are so little known that it is 

 impossible to say whether they are coalition territories or not. 

 We must remember that it is much more difficult for shallow- 

 water forms to find access to insulated areas like these, cut off 

 as they are by profound depths and special conditions of 

 temperature, than to the British coasts. 



It is now admitted that faunal resemblances and dis- 

 similarities between different marine areas are chiefly due to 

 the physical conditions of the sea-water, but we must not 

 regard them as the sole factors that regulate distribution. Two 

 marine areas may have similar physical conditions and yet 

 differ greatly faunistically. The Northern Pacific and Northern 

 Atlantic have in many cases similar hydrographical con- 

 ditions, but their faunas are on the whole quite distinct. 

 There are other factors at work, and isolation probably 

 does more than anything else to cause faunal differences. 

 Two areas may be isolated from each other owing to 

 the topographical character of the bottom, or because the 

 physical properties of the water prevent any faunal connection, 

 and consequently their faunas develop in different directions. 

 Temperature is another of the chief physical conditions 

 affecting distribution, and this explains why the British coasts, 

 the Mediterranean, the Azores, and the Canary Islands, not to 

 mention tropical coastal areas, shelter many forms which do not 

 occur in the Norwegian Sea, although there do not seem to 

 be any obstacles of a topographical character in the long 

 connected coast of western and northern Europe. 



We often see the limit of the arctic fauna in the Norwegian 

 Sea put at about lat. 67" N., it being apparently forgotten 

 that, owing to the hydrographical conditions, a large arctic area 

 (part of the arctic-abyssal) extends as far south as lat. 60" N., 



