GEOGRAPHIC DIVISIONS OF THE PELAGIAL 279 



bipolar species are not known (excepting the obviously cosmopolitan 

 species) . To these groups belong the actinians, the Gorgonacea, 33 and 

 the hydroids, 34 among the coelenterates; the Hexactinellida 35 among 

 the sponges ; less obviously the echinoderms 36 with the possible excep- 

 tion of the brittle star, Ophiocten sericeum; 37 and the brachiopods. 38 



Pfeffer 39 believes that the great similarity which often appears in 

 the organization of the Arctic and Antarctic communities, and espe- 

 cially the appearance of identical species and genera, can be explained 

 by the assumption that in early Tertiary times significant temperature 

 differences within the sea were not present, but that a homogeneous 

 warm-water fauna extended from pole to pole, from which the present 

 bipolar species survived as relicts at the poles after the appearance 

 of temperature differences. Many investigators have accepted this 

 idea, but others have opposed it. Freeh 40 doubts the palaeoclimato- 

 logical principles, but, as we have seen in Chapter X, they were accepted 

 by Chamberlin and Salisbury, though their hypothesis of cyclical 

 climatic change differs from Pfeffer's ideas. 



Ortmann 41 summarizes the differences between the two theories in 

 the following manner: according to Pfeffer's relict theory, the similar- 

 ity of both polar faunas is primary, the differences are secondary; 

 according to the migration theory, the differences are primary, the 

 similarity is secondary. The lack of bipolar species in so many animal 

 groups of little vagility seems to argue against Pfeffer's theory. It is 

 not advisable to cling too closely to any one theory in explaining the 

 similarity of animal life found at the two poles. In special cases, each 

 proposed scheme has good supporting evidence. A faunistic discussion 

 of bipolarity will be found in Ekman's Tiergeographie des Meeres 

 (1935). 



Animal communities of seas, bays, and gulfs. — As stated at the 

 beginning of the present chapter, the physical conditions of the par- 

 tially separated seas, bays, and gulfs are associated with the lessened 

 interchange of their water with those of the main ocean. Important 

 differences in temperature, salinity, and other conditions are found 

 in each such sea or bay, which tend to separate its animal commu- 

 nities, not only from those of the ocean proper, but from other, some- 

 what similar, bays or gulfs as well. 



The peculiarities of the lesser seas rest primarily on their salt con- 

 tent which is regulated by the influx of fresh water and by evapora- 

 tion. The tropical areas of the ocean show a noticeable increase in 

 salt content as a result of increased evaporation; in the polar seas, 

 on the other hand, a decrease in salinity takes place in the summer 

 time because of the small amount of evaporation and the huge amount 



