PALEOBIOGEOGRAPHY OF THE MARINE REALM 163 



preserved, and on sedimentological evidence that bears on the 

 nature of burial and amount of movement after death. 



Dispersal 



The factors already discussed define the conditions of existence. 

 Whether a given species, community, or biota actually exists at 

 the various places where it might, if it could gain a foothold and 

 displace competitors, depends also on its ability to be dispersed, 

 on the migration routes available for dispersal, and on the barriers 

 and natural filters that block dispersal or permit the passage only 

 of particular biotic elements, in a particular direction, or by 

 chance. This, in turn, depends in large degree on the global wind 

 and water circulation, and on the geographic configuration of the 

 earth, which varied in the past. The factors involved are well 

 illustrated by the phenomenon of bipolarity (e.g., Wimpenny, 

 1941); by differences and similarities between the Recent and 

 Tertiary marine faunas on opposite sides of the Isthmus of 

 Panama, on which Woodring (1959) reported at this Congress 

 (see also Woodring, 1954); by the now classic differences between 

 the Tertiary mammalian faunas of North and South America (e.g., 

 Simpson, 1953, pp. 21, 55) ; by recent Mediterranean faunal changes 

 (Peres and Picard, 1959) ; and by the well-known marsupial faunas 

 of Australia and the success with which later introduced placental 

 mammals were able to establish themselves there. Some of the 

 variables are illustrated in Fig. 5, and others will be discussed. 



The reproductive cycle is the internal control. All vertebrates 

 and ovoviviparous invertebrates disperse by moving themselves, 

 or by transport on other moving objects. Most marine inverte- 

 brates and plants, however, broadcast numbers of fertilized eggs, 

 larvae, spores, or seeds which are carried about by currents, wind, 

 or more esoteric mechanisms. High mortality is balanced by larger 

 numbers of offspring (Fig. 4), and is reduced in some bottom- 

 dwelling invertebrates by a limited ability of the individual larva 

 to select its substrate (Thorson, 1946, pp. 463-466). 



Other factors being equal, highest probabilities for dispersal are 

 naturally enjoyed by species with large breeding populations that 

 live at times of the maximum extent of favorable climatic and 



