164 



POPULATIONS OF THE SEA 



Fig. 5. Life at sea as the paleobiogeographer sees it and as it might have been. 



ecologic conditions and optimum geography, as Axelrod (1952) 

 showed for the terrestrial angiosperms. On the other hand, the 

 more widely spaced the sites of habitable environment, the more 

 ingenious are the devices through which the successful migrant 

 attains dispersal. This is particularly true of certain euryhaline 

 estuarine species which may go through resting stages like those of 

 the nonmarine mollusks, during which they can be transported on 

 floating objects or blown about by the wind; of the cold-water 

 diatoms, whose resting spores may be carried across the inhos- 

 pitable equatorial regions by way of the upper atmosphere ; and of 

 the polychaete annelids. The polychaetes, indeed, have the largest 

 number of cosmopolitan species among the marine benthos 

 (Thorson, 1950, pp. 30-33, 38; Cloud, 1959a, pp. 395-396) simply 

 because those species, like some of the actinians and sponges, are 

 able to undergo different forms of reproduction and larval develop- 

 ment according to local conditions (Thorson, 1946, pp. 427-429, 



