J. IMBRIE 129 



variation norms and to place taxonomic discrimination on a quan- 

 titative basis. But such computations are far too laborious to en- 

 able the large number of population samples at hand to be proc- 

 essed. Another complicating factor is the labyrinthine course of 

 evolution followed by these British clams: with one exception, 

 rectilinear evolutionary trends have not been identified. 



Faced with these complexities, Trueman and his co-workers 

 have evolved an ingenious and workable taxonomic procedure. 

 Each population sample is analyzed into variants which are illus- 

 trated and arranged in a systematic (and subjective) manner as 

 in Fig. 1. Subsequent analyses of similar populations can then 

 be abbreviated by means of appropriately placed dots on dis- 

 tribution diagrams. One or more modal or characteristic variants 

 in any population may then be described as a species. As a con- 

 sequence, an assemblage of shells from a single horizon and 

 locality may be described as a group of species, even though 

 there is every evidence that variation between the named species 

 is continuous. 



The students who employ the system described above recog- 

 nize that their use of the term species violates the modern bio- 

 logical species concept, but the claim is made that this system 

 of nomenclature has proved to be the only practical one for 

 their peculiar biostratigraphic problems (Eagar, 1952a). Other 

 workers have argued that the same practical results could be 

 obtained within the framework of the biological species concept 

 (Sylvester-Bradley, 1952) and without burdening international 

 nomenclature with names of purely local application (Newell, 

 1956). 



Biological Species in Paleontology. A majority of articulate 

 paleontologists working today employ a biological species con- 

 cept. Although this consensus reflects prevailing neontological 

 views, the paleontologists concept of species, which must take 

 into account important segments of geological time, is inevitably 

 more complex than the corresponding concept of zoologists. In 

 order to clarify this point it will be helpful to examine the sim- 

 plest possible phylogenetic model which includes the time di- 

 mension. Such a model, representing a small fragment of a 



