SYMPOSIUM ON PROBLEMS OF BIPOLARITY 

 AND OF PAN-TEMPERATE FAUNAS 



ANTITROPICAL DISTRIBUTION OF FISHES AND OTHER 



ORGANISMS(^) 



B\" Carl L. Hubbs, Scripps Institution of Oceanography, University 



of California 



When naturalists travel from their home lands and waters in the North 

 Temperate Zone to South Temperate regions, as many of us recently 

 have, they are struck first by the bewildering abundance of strange Hfe 

 in the tropics and are then impressed, as they enter the refreshingly 

 cooler regions still farther southward, by the reappearance of certain 

 familiar types of plants and animals. True, these forms are mixed with 

 strange organisms, but it is the familiar ones that engender the depth 

 of feeling that comes with the reunion of old friends. 



Since it first attracted the attention of travellers from Europe to far 

 southern lands this phenomenon has been called " bipolar distribution " 

 or " bipolarity," and has been variously explained. One of the theories, 

 that of separate creation, might now be modified into one of independent 

 adaptive evolution. Another idea has been that the similar forms of 

 north and of south are relicts, which stemmed from a cosmopolitan biota 

 of an early Tertiary period of relatively warm and uniform world climate. 

 On this theory the disjunct populations were forced apart by the increasing 

 heat of the Tropics and by competition with a recently evolved rich 

 tropical biota. Still another concept is that the Tropics have been crossed 

 and to some extent are still being crossed, along isothermal lines, the 

 marine forms dipping into the cold deep sea and the land forms rising 

 to cold mountain heights. Advocates of continental drift have found 

 in their aberrant hypothesis an explanation of bipolarity, as well as of 

 almost every other phenomenon of biogeography. 



A more recent and in general more plausible explanation, stressed by 

 Berg (1933), is that the Tropical waters were transgressed during the 

 Pleistocene periods of global cooling. This is the theory to which I 

 particularly subscribe, though I recognize that each alternative 

 explanation mentioned (with the exception of local creation) could be 

 supported by an impressive array of evidence and has probably been 

 valid to some degree for some groups. 



In line with this theory of Pleistocene interchange of faunas across 

 the Equator we find, on examining a large body of evidence, that the 

 phenomenon with which we are dealing is only in a very small degree one 

 of bipolarity. As Berg (1933) stressed, the vast majority of the related 

 forms of life that occur on either side of the Tropic Zone, but not within 

 it, inhabit moderate rather than high latitudes. Among marine fi-shes, 

 as Norman (1938) showed, the Antarctic contains only a small percentage 

 of groups or of species of Arctic relationship. A few forms may be cited 

 as definite examples of truly bipolar distribution ; a much larger number 

 may better be called biboreal or bitemperate, and those that are bitemper- 

 ate are warm-temperate as well as cool-temperate. Furthermore, a 



(1) Contributions from the Scripps Institution of Oceanography. 



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