Evolution In the Deep Seas 



G. S. CARTER 



Department of Zoology, Cambridge University, Cambridge, England 



MY object in this paper is to compare the conditions of evolution 

 in terrestrial and deep-sea environments and to discuss how far 

 the peculiar characters of the deep-sea fauna can be ascribed to 

 the conditions of their environment. I shall restrict myself to the 

 general features of deep-sea evolution and not discuss the special 

 adaptations of members of the fauna. Before making the com- 

 parison I must first summarize the views of evolution, and espe- 

 cially microevolution, to which recent work leads. This I shall do, 

 and then pass on to discussion of evolution in the deep seas. 



One of the most striking features of almost all land and fresh- 

 water environments is the extremely local distribution of the con- 

 ditions that determine where a species is to be found. The presence 

 or absence of a species at any point is often determined by such 

 local features as the presence of a plant, the shade of a tree, or a 

 log at some special stage of decay. Even when an environment is 

 homogeneous in its general features — a pinewood, a heather moor, 

 or a desert — most species are not found everywhere in the environ- 

 ment. Only the species associated with the dominant plants are 

 generally distributed; others are found in small areas within the 

 environment where the conditions are particularly favorable to 

 them. It is not only on this small scale that the distribution of 

 animals in terrestrial environments is discontinuous. On a larger 

 scale such an environment as a grassland or a wood is usually 

 separated from other environments of the same kind by regions 

 where the conditions of life are different and therefore unsuitable 

 for the fauna of the grassland or wood. On a still larger scale an 

 island or the upper parts of a mountain may be completely isolated 

 from other similar environments. 



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