36o POLAR PROBLEMS 



spheres and absent from the tropics represented hardy coeval sur- 

 vivors of an almost universally distributed Mesozoic or early Tertiary 

 fauna which had withstood a subsequent cooling of the seas. Under 

 the latter influence a process of separating out and selection had taken 

 place, and the similarity of the causes had resulted in the same com- 

 ponents of the old fauna remaining behind in both the north and the 

 south. In this manner, according to these supporters, arose the still 

 well-marked similarity of two faunas of completely discontinuous 

 range. 



The theory was promptly combated by Thompson,^" Ortmann," 

 and other investigators. Thompson showed that the evidence ad- 

 vanced by Murray was inconclusive, much of it being based upon 

 doubtful cases of identity due to poor material or to insufificiently 

 critical taxonomy. He held the distributional facts to be proved only 

 in the case of certain demersal organisms and a few others which 

 inhabit the surface in higher latitudes but also the deeper and colder 

 waters of tropical seas. Further search in temperate and warm oceans 

 would reveal, he believed, the presence of many other "bipolar" 

 species in intermediate zones. 



Chun^' also regarded the known facts as sufficiently accounted 

 for by the continuous distribution of "bipolar" forms via the deeper 

 water, of which he cited examples among the plankton. 



The testimony of more recent students has been variously pro 

 and con. Pratt^^ pointed out 32 cases of alleged bipolarity among 

 crustaceans, annelids, and other littoral invertebrates. None of these 

 species, according to the author, has been found within the tropics 

 in either deep or shallow water, and only two of them in temperate 

 seas. In only two instances is there any evidence of passage of these 

 forms along the prevailingly cool western coast of America, while 

 evidence of similar interchange by way of the west coast of Africa 

 is entirely lacking. 



Still more recently Meisenheimer^"* has recorded practical identity 

 in two or more species of northern and sub-Antarctic pteropods. 

 He calls attention to the fact that one of the northern species which 

 he cites has broken up into several varieties, whereas the respective 

 Antarctic representative exists as a single circumpolar form. Meisen- 

 heimer suggests the tropical origin of the pteropods and their pelagic 



1° D'A. W. Thompson: On a Supposed Resemblance Between the Marine Faunas of the Arctic 

 and Antarctic Regions, Proc. Royal Soc. of Edinburgh, Vol. 22, 1897-99. PP- 3II-349 (read in 1898). 



11 A. E. Ortmann: On New Facts Lately Presented in Opposition to the Hypothesis of Bipolarity 

 of Marine Faunas, Amer. Naturalist, Vol. 33, 1899. PP- 583-591- 



" Carl Chun: Die Beziehungen zwischen dem arktischen und antarktischen Plankton, Stuttgart, 

 1897. 



WE. M. Pratt: Some Notes on the Bipolar Theory of the Distribution of Marine Organisms, 

 Memoirs and Proc. Manchester Lit. and Philos. Soc, Vol. 45, 1901, No. 14, pp. 1-21. 



" Johannes Meisenheimer: Die Pteropoden der Deutschen Sudpolar Expedition (Deutsche 

 Sudpolar Expedition 1901-1903, herausg. von Erich von Drygalski, Vol. 9: ZobXogy, Part I, pp. 93- 

 153), Berlin, 1906. 



