THE RELATIONS OF THE ABYSMAL ZONE 47 



nevertheless each showing a number of characteristic 

 features. This idea is expressed, for example, when 

 we speak of the Alpine fauna, the Himalayan fauna, 

 or the fauna of the great Andes. 



In the study of the marine fauna and flora we 

 must notice, it is the depth of the water, or in other 

 words the depression of the habitats below the sea- 

 level, that forms the most important consideration. 

 Geographical sub-regions may be recognised and 

 defined with a certain amount of accuracy, especially 

 in the case of the fauna of the shallow waters, but by far 

 the most important changes in the general characters 

 of the fauna are found when we pass from one ' zone ' 

 of depression to another. Thus in describing any 

 particular marine fauna we should mention first of 

 all its zone or sub-zone of depression and then its 

 geographical region and sub-region. For example, 

 we may speak of the fauna of the pelagic zone of 

 the British sub-region of the European region, or the 

 fauna of the abysmal zone of the Northern sub-region 

 of the Atlantic region. 



We can recognise three primary zones of the 

 marine fauna which we may call the ' Pelagic,' the 

 4 Neritic,' and the ' Abysmal ' zones. 



The Pelagic zone includes the superficial waters 



