450 DEPTHS OF THE OCEAN 



Vertically as well as horizontally the fauna termed by me the 

 southern one appears to exist within the same limits of tempera- 

 ture. The different species appear to be at liberty to move 

 within these limits and to be independent of depth. Thus 

 there are many observations showing that the southern species 

 occur in deeper water on the Atlantic slope than they do in 

 the North Sea. This is easy to understand, because in the 

 North Sea only the shallow upper layers are affected by high 

 summer temperatures. Nevertheless the records of such species 

 from deeper water available from the results of the "Michael 

 Sars " and other expeditions are quite surprising. Thus the 

 French deep-sea expeditions found : — 



Lophius piscatorius hoXw^tQn 2 1() 3.nd 668 „ 



Merluccius vulgaris ,, 99 „ 640 „ 



Motella tricirrhata „ 112 „ 640 „ 



Phycis albidus „ 40 „ 460 „ 



These instances are quite sufficient to show that in the 

 southern part of our area the fishes tend to migrate vertically 

 within considerable bathymetrical ranges. Evidently tempera- 

 ture here plays a dominant part, and perhaps also other factors 

 come into play, above all the deeper penetration of light in 

 southern waters. 



The Northern We have previously seen that the northern species in North 



(boreal) European waters rano^e from the Barents Sea in the north to 



west of the British Isles in the south. But within this wide area 

 we meet with many variations in detail, even though the fish 

 fauna of the whole area in a broad sense may be said to be 

 homogeneous. Thus some species belong mainly to the most 

 northerly part of the area, while others are taken in quantities 

 worth mentioning only in the far south of the region. The 

 abundance of a species does not alone depend on latitude or 

 conditions of temperature, but the extent of the area of bottom 

 suitable to the species is also of great importance. 



An analysis of this question cannot, however, be restricted to 

 a search for the geographical limits of the species. As regards 

 the northern forms, information as to their bathymetrical dis- 

 tribution is very important. The English fishery statistics 



