140 



THE SPECIES PROBLEM WITH FOSSIL ANIMALS 



Now suppose that McKerrow had had available only two geo- 

 graphically isolated but distinctive local populations. In this case, 

 taxonomic disposition of these collections would be far more sub- 

 jective. This serves to emphasize the point that the chief taxo- 

 nomic difficulties which arise in dealing with paleontological 



10 



>^/\ 



I mm 



1.9 mm 



Fig. 9. Graph showing size and frequency distribution of growth stages 

 represented in a population of the Silurian ostracod Bexjrichia jonesi from 

 Gotland (Spjeldnaes, 1951). 



materials are due not so much to the limitations inherent in post- 

 mortem examination of skeletal anatomy, but rather to the incom- 

 pleteness of the available fossil record. 



Biased frequency distribution. The size-frequency distribu- 

 tion of a few carefully studied paleontological samples seems to 

 reflect fairly accurately the size distribution of the original living 



