VERTEBRATE PALAEONTOLOGY 143 



genus disappeared. From this information it is possible to draw 

 a curve showing the percentage of Devonian genera that are alive 

 at more recent times. From these curves one can see that the 

 lamellibranches differs from the Carnivora in that the mean 

 survivorship for a lamellibranch genus is some 78 million years 

 whilst that for the Carnivora is 6J million years. Simpson con- 

 cludes, " The data undoubtedly exaggerate the difference for 

 various reasons, but it is safe to say that the carnivores have 

 evolved, on the average, some ten times as fast as pelecypods 

 (lamellibranches) " (1944). This does not mean that all lamelli- 

 branch genera lasted for 78 million years and hence ten times 

 as long as each carnivore genus; the values referred to are mean 

 values. 



Simpson's views concerning the mean length of genera have 

 not gone unchallenged. In particular Williams (1957) has made 

 some interesting objections to the techniques employed by 

 Simpson. Williams states, " An ever-increasing number of papers 

 dealing with the development of fossil groups contains a host of 

 graphical and numerical devices designed to provide a sober tone 

 of objectivity to the accompanying text. On the whole they appear 

 to be extremely useful but there is a real danger that the student 

 will lose sight of the tenuous and arbitrary nature of most of the 

 data used in the compilation of such charts, for there is always a 

 tendency to accept numbers as the only worthwhile facts in papers 

 of this kind." 



Williams goes on to point out that a great deal in such calcula- 

 tions depends on the nature of the systematics of the groups studied 

 and whether the systematists working on the groups were 

 " lumpers " or " splitters." The former group as many species 

 as possible together into one genus, the latter separate each species 

 into a separate genus! If a lumper has been at work on a group, 

 there would be few genera and each would exist for a long period 

 of geological time. There are also comparatively few genera if a 

 group has not been " monographed " for some time. (There 

 appears to be a good correlation between the number of mono- 

 graphs that has been published on a group and the number of 

 genera described for such a group (Cooper and Williams 1952).) 

 When the concept of " lumpers and splitters " is applied to the 

 Brachiopods, a group that includes Lingida which has remained 



