156 POPULATIONS OF THE SEA 



keystone of the pyramid described by the long- and wide-ranged, 

 ecologically restricted biotic elements that make the best paleo- 

 ecologic indicators, and the wide-ranged, facies-crossing forms of 

 short time span that are favored for stratigraphic markers. 

 Paleontology in practice is not so simple as that, but it is a fact 

 that a wide range of intermediate geographic and habitat charac- 

 teristics is of interest for paleobiogeography, provided the species 

 are not too ubiquitous; and provided the ecology, evolutionary 

 sequence, and time dimension can be supplied by other forms. 



No biogeographic or paleobiogeographic conclusions are any 

 better than the data and interpretive principles on which they are 

 based, so an accurate and refined systematics and a knowledge of 

 the geographical distribution of modern organisms are at the base 

 of the pyramid. Without this foundation the structure floats in 

 thin air. To assume, for instance, as some have done, that wide 

 geographic separation excludes the prospect of systematic identity 

 is to bias the data. It is only by critical analysis of objective 

 similarities and differences between biotas, in context with the 

 inferred relations of land and sea and the paleoecologic implications 

 of the enclosing sediments, that useful concepts of migration routes 

 and dispersal mechanisms can be evolved for comparison with the 

 land connections, current systems, and regional climatologic 

 variation demanded by given earth models. 



Distribution of Marine Organisms 



According to the basic geological principle of uniformitarianism,* 

 which uses observable and testable processes and dynamic relations 

 as the keys to past events, the interpretation of marine paleobio- 

 geographic data is based on analysis of the mechanisms that affect 

 the survival and dispersal of organisms and organic remains in 

 the present seas (Fig. 3). Given a particular set of geographic, 

 climatologic, and historic conditions, the variables involved are of 



* Uniformitarianism is not to be confused with gradualism, or thought of as 

 properly incorporating purely static analogy, a misconception that has led to 

 uncritical rejection of this fundamental operational principle, without which 

 geology cannot be thought of in scientific terms. It does not exclude catastrophic 

 processes or unusual events, but onh- ad hoc reasoning. 



