144 VERTEBRATE PALAEONTOLOGY 



unchanged since the Early Cambrian and which Simpson classifies 

 as a slowly evolving group, it becomes clear that different views 

 concerning the survivorship of the genera can be obtained by 

 examining the genera described in 1894, 1929 or 1956; the mean 

 life of a genus being 64, 56 or 53 million years respectively. This 

 would still mean little when compared to the 6J million years 

 of the carnivore genus, i.e. the brachiopods would appear to have 

 evolved some seven times more slowly. 



Williams then points out that the figure for the carnivores is 

 that of a small group taken over the climax of their evolutionary 

 history, i.e. when the carnivores show the greatest variation and 

 formation of new genera. On the other hand the figures for the 

 lamellibranches and the brachiopods are taken over the whole 

 range of the animals' geological record and both groups are 

 found from almost the earliest geological time. Williams therefore 

 suggests that it is more logical to consider the climax of evolution 

 of such group as the brachiopods. This would be the 231 genera 

 of Ordovician times. The average duration of these genera is 16 

 million years and some have as short a duration as 10 million years. 

 Thus if the climax of evolution is taken for both the Carnivora and 

 Brachiopoda, the difference of the mean duration of each genus 

 changes from one of 6| million years and 53 million years to one of 

 6J million years and 16 million years. It would be interesting to 

 have the similar calculations applied to the lamellibranches and, 

 say, the early reptiles, the latter showing an explosive type of 

 evolution that took place some time ago, thus allowing the post- 

 climax period to be analysed. The example just quoted concerning 

 the rate of evolution of the Brachiopoda shows how careful one 

 must be in assuming that conclusions are valid unless one makes 

 a careful consideration and analysis of the data supporting 

 these conclusions. 



The Evolution of the Horse 



It would not be fitting in discussing the implications of Evolu- 

 tion to leave the evolution of the horse out of the discussion. The 

 evolution of the horse provides one of the keystones in the teach- 

 ing of evolutionary doctrine, though the actual story depends to a 

 large extent upon who is telling it and when the story is being 



