160 POPULATIONS OF THE SEA 



characteristics, regional distribution, and evolution of his fossils, 

 and from the geochemical, mineralogical, textural, and structural 

 characteristics of his sediments. 



It is gratifying to know, therefore, from contemporaneous 

 biogeographic and ecologic research that there are distinctive 

 patterns in the variety, numerical abundance, sizes, and even 

 shapes of organisms that denote trends, if not specific points, in 

 the external environment of the life assemblages concerned 

 (Clements and Shelford, 1939; MacGinitie and MacGinitie, 1949; 

 AUee et al., 1949; Jones, 1950; Ekman, 1953; Hedgpeth, 1957a; 

 Fischer, 1960), The curves shown in Fig. 3, to be sure, are only a 

 caricature of the changes in relative taxonomic variety (including 

 variety but not bulk of total biomass) that accompany changes in 

 the external variables, but they do suggest their general shape. 

 We owe to Petersen (1914, 1915a, b) and Thorson (e.g., 1957) the 

 emphasis on the great variation displayed by the epifauna, and 

 to some extent by the nekton and plankton, as contrasted to the 

 intrinsically less varied infauna (m the substrate, not the endo- 

 fauna, which includes internal symbionts). This difference reflects 

 the closer interrelations of the epifauna with external climate and 

 associated plant and animal communities. If it were not known, it 

 would have been logical to deduce that the infauna occupies the 

 ecologic realm of least variation and greatest stability. 



I have summarized elsewhere (Cloud, 1959b, p. 931) the paleo- 

 biogeographically useful and well-known gradients in variety, 

 abundance, and size with temperature, salinity, and depth, and 

 these are discussed in articles by several authors in Hedgpeth 

 (1957a) and by Jones (1950). It will suffice here to underscore that 

 these gradients are broadly similar in kind and are to be dis- 

 tinguished primarily on the basis of abruptness of variation, 

 associated changes in morphology and composition of the biotas, 

 and sedimentological evidence, including that from isotopic and 

 geochemical methods. 



In general, however, the paleobiogeographer is safe in concluding 

 (Fig. 3) that great biotal variation is presumptive evidence of 

 warm, shallow seas of normal salinity and that widespread faunas 

 of limited variety and relatively persistent composition (and 



