PALEOBIOGEOGRAPHY OF THE MARINE REALM 165 



476; 1950). Otherwise widespread living organisms, some narrowly 

 restricted in their adult choice of habitable conditions, are charac- 

 terized as a rule by easily dispersed, long-lasting early stages, and 

 it is a proper working hypothesis to consider that the same was 

 probably true in the past. 



The most widely distributed types of marine organisms in 

 modern times are as follows: 



1. Actively nektonic organisms such as fish, cetaceans, and some 

 cephalopods. 



2. Planktonic organisms, and those which at some stage in their 

 lives may attach themselves to, or be carried naturally within, 

 members of the migrating nekton or floating objects such as 

 driftwood or pumice. (The paleogeographer does not bother about 

 ships except in choosing analogies, or before Jurassic time about 

 birds.) 



3. Brackish-water moUusks, plants, and insects characterized 

 by viviparous or parthenogenetic reproduction; or by spores and 

 resting stages which may be transported by wind (and birds), or 

 on floating or swimming objects, and which may undergo long 

 periods of estivation. 



4. Organisms such as marine annelids which have great larval 

 and reproductive lability. 



5. Species of shallow, warm, or temperate marine waters with 

 long-lasting planktonic larval stages. 



6. Deep-water and cold-water species having abbreviated larval 

 life or viviparous (including ovoviviparous) reproduction and able 

 to move or drift with deep currents (Bruun, 1957, p. 663). 



The paleobiogeographer should have good reason for eliminating 

 the above categories, as well as the peculiarly geologic uncertain- 

 ties and the statistical probability of viable freak transport, before 

 concluding either that geographic factors require otherwise similar 

 fossil populations to belong to nomenclaturally different species, 

 or that drastic paleogeographic changes are in order. 



Wind transport deserves emphasis beyond the mere notation 

 that dispersal of spore-forming organisms can be so achieved. 



