PALEOBIOGEOGRAPHY OF THE MARINE REALM 167 



inclement biogeographic realms. Natural selection, therefore, 

 accounts for the abbreviated larval life or viviparity that charac- 

 terizes cold-loving (psychrophilic) species and those of narrow 

 depth range (stenobathyal) . In the cold-temperate waters around 

 Denmark, Thorson (1946, pp. 472-479) found that about one-third 

 of the bottom invertebrates have a short or no pelagic life, whereas 

 two-thirds have a larval life little exceeding three weeks on an 

 average (longer in the same species in wintertime, presumably 

 because of metabolic retardation). It would be interesting to have 

 a similar study of the reproduction and larval development of 

 warm water species. This should show lengths of planktonic larval 

 life corresponding to the spacing of deeps and shoals, and degree 

 of endemism. 



Consider the implications of larval life and currents for dispersal. 

 In an ordinary half-knot current a floating object could travel 

 170 km in a week or 500 km in three weeks; a two- to four-knot 

 current like the Gulf Stream, if it were persistent, could travel 

 700 to 4000 km over the same time. Conceivably, therefore, a 

 species with long-lived larvae and luck could cross the narrower 

 parts of the Atlantic in one jump! If the larvae are attracted to 

 light (phototropic) and currents (rheotactic), they are more likely 

 to stay near the surface and in the main drift, and long larval life 

 with a range of substrate selectivity (Thorson, 1946, p. 479) 

 accordingly increases the possible distance of individual dispersal 

 and the prospects of continued existence of the species, assuming 

 favorable current patterns. 



If, on the other hand, planktonic larvae or adults habitually live 

 at depths greater than those of available transport channels, they 

 can be moved through such channels only as a result of upwelling 

 movements at the right time and place. Thus many pelagic 

 microorganisms are rare in shallow-water deposits because they 

 live suspended at some favored range of depth well below the 

 surface, perhaps most commonly near the bottom of the photic 

 zone where the phytoplankton have the first chance at upwelling 

 mineral nutrients and the zooplankton the richest grazing grounds. 

 An illustration of the possible paleobiogeographic significance of 

 this is provided by the Tertiary Panamanian passage, which at 

 times may have retarded the movement from Atlantic to Pacific 



