46 THE FAUNA OF THE DEEP SEA 



debateable grounds in the world where the fauna 

 characteristic of one region is mixed with that 

 characteristic of another. 



But this difficulty in defining the exact limits 

 of the terrestrial faunistic regions is even more pro- 

 nounced in the case of the regions and zones of the 

 marine fauna. 



On the dry land we find mountain ranges, forests, 

 deserts, and other barriers, that to a very considerable 

 extent prevent the mixing of one fauna with another, 

 but in the sea there are no barriers of anything like 

 the same importance, but one fauna gradually merges 

 into the neighbouring fauna according to the temper- 

 ature, the pressure, the amount of light, the salinity 

 of the water or the food supply. This then is one of 

 the difficulties met with in the study of the geo- 

 graphical distribution of the marine fauna. 



But there is another that leads to almost greater 

 complications. In considering terrestrial life it is 

 customary to refer only to regions of geographical, 

 or perhaps it would be more correct to call it 

 superficial distribution. It would be quite possible, 

 however, to subdivide the geographical areas into 

 zones of elevation above the sea-level, not very 

 clearly marked off from one another, it is true, but 



