148 VERTEBRATE PALAEONTOLOGY 



U.S.A. and probably a total of 100 in the world. There are not 

 any mounted skeletons of Eohippus> Archaeohippus, Megahippus, 

 Stylohipparion, Nannippus Calippus, Onohippidium or Parahippus, 

 and none in the United States of Anchitherium or Hipparion. 



There are, however, several thousand of horse fragments 

 collected in the various museums of the world. It is expecting a 

 great deal to have fully prepared specimens of all the major genera 

 of fossil horses. But since the horse is such a key example in the 

 evolutionary doctrine it is important that our knowledge of the 

 fragments be collected, possibly in the first place as a card index 

 system and then later published as a catalogue, so that the results 

 can be made available in synoptic form to all those interested. 



A third problem concerns the validity of the various genera and 

 generic differences. The number of genera described has in- 

 creased considerably. Thus Kowalevsky in 1874 knew of three; 

 Lull in 1917 described fifteen; Simpson in 1945 lists twenty-six 

 genera. To some extent this is due to the discovery and description 

 of new material but one wonders how valid these genera really 

 are. 



Another problem concerns the dating of these genera. When 

 Matthew worked out the time taken for the evolution of the horse, 

 he came to the conclusion it would take some 45 million years. 

 His calculation was a rough one but it provided a useful guide. 

 Since then this calculation has been modified by a uranium dating 

 which places the Eocene back to 60 million years. Over this 

 60 million years we have had some twenty-six genera and a large 

 number of species of fossil horse evolving and it would be of the 

 greatest interest to know the relative positions of these animals 

 to one another, together with some indication as to the accuracy 

 of the relative dating. Thus if we could know the parts of, say, 

 Mesohippus skeleton that have been found in perfect condition, 

 the number of specimens, fragments and so on of Mesohippus 

 that are available, the strata from which each of these was derived, 

 the degree of contemporaneity of the strata plus or minus so 

 many million years, then we should have no qualms in accepting 

 the evidence presented to us. At present, however, it is a matter of 

 faith that the textbook pictures are true, or even that they are the 

 best representations of the truth that are available to us at the 

 present time. 



