EVOLUTION IN THE DEEP SEAS 233 



instability of the environmental conditions is the most important 

 factor in control of the rate of evolution. 



The instability of the postglacial climate will have affected the 

 evolution of animals in other ways. Environmental changes alter 

 the distribution of species, and their frequency will determine how 

 often local populations lose their isolation by coming into contact 

 with other populations of the species. Instability of the climate will 

 therefore make the spread of variations through a species more 

 rapid and increase the rate of its evolution for this reason also. It 

 seems, then, that the recent history of land and freshwater faunas 

 should have been one of rapid evolution. This is borne out by 

 observation. To take a single example, there is the case of the 

 char {Salveliniis) in the lakes of northern Europe. These fish have 

 differentiated into numerous forms of at least subspecific grade, 

 each in a different lake, and this differentiation must have taken 

 place since the lakes were uncovered by the retreat of the ice. In 

 more stable periods similar differentiation has been found to take 

 hundreds of thousands of years (Zeuner, 1946). 



In any comparison of marine with terrestrial environments the 

 first thing to be said is that the marine environments are of almost 

 as many different kinds as the terrestrial. In the littoral regions 

 the environments may be as locally variable as those on land. So 

 far as the distribution of species is concerned, evolution of much 

 the same kind as in terrestrial environments is to be expected and, 

 I think, occurs. I shall not discuss the littoral region. Then, again, 

 it must be remembered that the sea is three-dimensional, whereas 

 terrestrial environments are for most animals two-dimensional. 

 The conditions of planktonic, pelagic, and benthic life differ in 

 the deep sea as much as in shallower waters. I shall, however, not 

 try to distinguish the conditions in these modes of life but to 

 consider whether any general conclusions can be drawn about 

 evolution in the deeper waters of the sea. 



Several features of the environment of the deep sea seem un- 

 likely to modify fundamentally the evolution of the fauna. The 

 low temperature will slow the whole tempo of life and therefore, 

 presumably, its evolution, but there is no reason to think it will 

 alter the type of the evolution. The external and restricted supply 



