PALEOBIOGEOGRAPHY OF THE MARINE REALM 161 



commonly large numbers of individuals) are likely to represent 

 waters that were cold or deep, or both. Limited variety and 

 abruptly changing composition of biotas implies local salinity 

 deviations, especially if the biotas include a relatively large pro- 

 portion of internally stable or homoiosmotic organisms such as 

 fishes and some crustaceans, and exclude internally variable or 

 poikilosmotic forms, such as echinoderms, cephalopods, brachio- 

 pods, bryozoans, and corals.* 



Internal and Cobiotic Factors of Existence 



Internal variables affecting the geographic distribution of marine 

 organisms include their individual metabolism and mobility, 

 frequency and range of movement, manner of and adaptations for 

 movement or fixation, sensory responses, size and specific gravity, 

 mode and time of reproduction, and length of larval life. To these 

 should be added their adaptability to variations in the physical, 

 chemical, and gaseous conditions of the external environment; 

 their reproductive lability; and their evolutionary intensity, which 

 is a function of metabolism, rate of change, and time. 



The cobiotic factors include all facets of competition for the 

 available energy resources — light and mineral nutrient absorption 

 by the photosynthesizers, nature of and availability to the organic 

 nutrient supply by the heterotrophs, predation, symbiosis, and 

 degree of occupancy of available ecologic niches. 



The internal and cobiotic factors have important effects on the 

 numbers and individual survival prospects of the various categories 

 of organisms, as roughly indicated in Fig. 4, a scheme inspired by 

 the work of Blegvad (1915) and by Imbrie's (1959) contribution 

 to this Congress. Size, reproductive characteristics, and competi- 

 tion are not specifically included in this diagram. Number of 

 offspring, however, decreases, and size of individual, length of 



* By convention, the widely employed prefixes poikilo and homoio designate 

 marked internal \ariation vs. little or no variation, whereas eury and steno refer to 

 external tolerance. A homoiosmotic organism is also euryhaline because its ability 

 to regulate the salinity of its body fluids permits it to survive a wide range of 

 external salinities. A poikilothermal animal is characteristically stenothermal in 

 habitat tolerance, because its inability to regulate body temperature is deleterious 

 under wide external temperature variation. 



