EVOLUTION IN THE DEEP SEAS 235 



since the interbreeding populations will be wider, and probably 

 contain more individuals, there would seem to be less possibility 

 of neutral characters being evolved. I do not think our knowledge 

 of the deep-sea fauna is detailed enough to allow us to say whether 

 this is so or not. Even in terrestrial faunas it is always difificult to 

 say whether a character is really neutral or has some value in the 

 biology of the animal. 



Deep-sea environments differ from the terrestrial in another 

 respect: the recent Ice Age will have had much less effect upon 

 them. Extension of the polar ice cap must have caused some 

 latitudinal change in the distribution or temperature and perhaps 

 of currents, but it can hardly have altered conditions greatly in 

 other ways. It would seem that the deep-sea fauna cannot have 

 been subjected to the large environmental changes and consequent 

 rapid evolution to which the terrestrial fauna has been subjected 

 in its recent history. In the deep seas there must have been long 

 periods of more stable conditions, stretching back at least into the 

 Tertiary, in which the fauna evolved to its present condition. In 

 this period most of the specific characters of the fauna will have 

 been evolved. This conclusion is not negatived by the fact (Menzies 

 and Imbrie, 1958) that archaic forms are not numerous in the 

 deep seas. That they are not more numerous is perhaps surprising 

 for one might have expected many such forms to find there refuges 

 in which they could survive. The hope that this might be so was 

 one of the reasons why the original Challenger expedition was 

 organized. But the reason must lie in the earlier history of the 

 oceans, a subject I cannot discuss. 



The recent stability of the deep-sea environment may explain 

 another striking feature of the fauna. In some groups of which 

 many of the fishes are the most obvious example, but some of the 

 cephalopods and holothurians should also be included, we find 

 fantastic modifications of the form of the body and great dif- 

 ferences between closely related species. The great development of 

 the mouth and stomach in some of the gulper eels (Sacco pharynx, 

 etc.) and the forms of the body in some ceratioid angler fishes are 

 well-known examples, but some of the forms of the holothurian 

 group of the Elasipoda might equally be quoted. All these are 



