168 POPULATIONS OF THE SEA 



(and the reverse) of deep-floating plankton, while facilitating the 

 passage of near-surface plankton, including the pelagic larvae of 

 benthonic species. The mere fact of a pelagic existence does not 

 assure wide or instantaneous dispersal, nor does a benthonic adult 

 life exclude geologically rapid and wide distribution. 



Clearly, also, the intrinsic characteristics of migration routes and 

 obstacles to migration are highly significant. Simpson (1940a, b, 

 1952, 1953), who has discussed this question philosophically, with 

 special regard to terrestrial mammals, aptly describes the paths 

 of interchange as corridors, filters, and sweepstakes routes. Most 

 migration routes in the sea are to some degree filters. Currents 

 provide easy downstream transport but retard upstream move- 

 ment. East-west currents are likely to be temperature barriers to 

 shelf and littoral biotas, and north-south currents, like the Ameri- 

 can limb of the Gulf Stream, may abandon the hapless larva (or 

 adult) beyond the limits of its temperature tolerance or repro- 

 ductive range. Passes between land areas or into epicontinental 

 seas may be subject to fluctuations of salinity and turbidity that 

 will exclude organisms sensitive to these variables. 



Land masses that trend from north to south are likely to deflect 

 currents into areas where they would not otherwise flow, as the 

 north equatorial current of the Atlantic Ocean is turned northward 

 under the influence of Coriolis force to become the Gulf Stream 

 today. Sharp faunal distinction is common from eastern to western 

 sides of such lands. Wide oceanic depths, like those of the eastern 

 North Pacific, are barriers or sweepstakes routes to benthonic 

 biotas in the absence of strong (steady or episodic) transverse 

 surface currents. Given continued accretion of the hydrosphere 

 (Rubey, 1952) or subsidence of ocean basins through geologic time, 

 however, combined with long duration of present or antecedent 

 oceanic rises, even transoceanic "bridges" become possible at some 

 places and times (Axelrod, 1960. Figs. 6, 8, and 9). For the 

 broadly cosmopolitan abyssal communities that cover half the 

 globe (Bruun, 1957) and for the cold-loving bipolar benthos, 

 the depths themselves are corridors beneath the equatorial sur- 

 face waters and staging areas for invasions of the hadal depths 

 (Bruun, 1957, pp. 654-661; Wolff, 1960). 



