234 POPULATIONS OF THE SEA 



of food will control the density of the fauna but should not modify 

 the type of evolution except by encouraging the development of 

 food -gathering adaptations. The high hydrostatic pressure should 

 have still less effect. The absence of light reduces the food supply 

 by exclusion of plants and will lead to adaptations for life in a 

 dark environment, with emphasis on olfaction and biolumines- 

 cence, but, again, it will not modify the evolution of the fauna in 

 any fundamental way. 



A factor which has probably had a much more general effect 

 on evolution in the deep sea is the greater homogeneity of the 

 environment over wide areas than in terrestrial or littoral environ- 

 ments. Recent work may have shown that the abyssal floor is not 

 so generally composed of featureless plains as we formerly thought, 

 but it is surely true that local differentiation of conditions is much 

 less on the abyssal floor than in en\"ironments where plants are 

 plentiful. In the benthic fauna there will be less general separation 

 within species of local and isolated denies, and the presence of 

 currents in the water above the bottom will lead to the same 

 result by increasing the distribution of the fauna. In fact it is 

 difficult to see how any of the causes of isolation effective on land 

 will be operative on the sea floor except distance isolation. For 

 pelagic animals local differentiation of conditions will be still less. 



We should therefore expect that the recent e\'olution in the 

 deep sea should be less rapid than on land, and perhaps that the 

 number of species formed should be fewer. This is what we find. 

 Ekman (1953), for instance, emphasizes the wide distribution of 

 abyssal species and the absence of local forms. On land, distance 

 isolation often results in formation of clines and they might be 

 expected to occur also in the deep seas. But clines will not always 

 be formed in widely distributed species. Even though the isolation 

 is mainly by distance, it need not be wholly so, and specific 

 differences may be evolved in isolated parts of the species range. 

 Owing to further environmental change species so formed may 

 later come into contact as sympatric sibling species. In fact such 

 closely related sympatric species do occur in the deep seas (e.g., 

 species of Stomias, cf. Marshall, 1954, p. 343), though they seem 

 to be less frequent than on land, perhaps not surprisingly. Finally, 



