VERTEBRATE PALAEONTOLOGY 149 



One thing concerning the evolution of the horse has become clear. 

 The story of the evolution of the horse has become more and 

 more complex as further material is collected, and instead of a 

 simple family tree the branches of the tree have increased in size 

 and complexity till the shape is now more like a bush than a tree. 

 In some ways it looks as if the pattern of horse evolution might 

 be even as chaotic as that proposed by Osborn (1937, 1943) for 

 the evolution of the Proboscidea, where, " in almost no instance 

 is any known form considered to be a descendant from any other 

 known form; every subordinate grouping is assumed to have 

 sprung, quite separately and usually without any known inter- 

 mediate stage, from hypothetical common ancestors in the Early 

 Eocene or Late Cretaceous " (Romer 1949). We now know that 

 the evolution of the horse did not always take a simple path. In 

 the first place it is not clear that Hyracotherium was the ancestral 

 horse. Thus Simpson (1945) states, " Matthew has shown and 

 insisted that Hyracotherium (including Eohippus) is so primitive 

 that it is not much more definitely equid than tapirid, rhinocerotid, 

 etc, but it is customary to place it at the root of the equid group." 



Similarly it is clear that though in general the horses did 

 increase in size, certain genera such as Orohippus, Archaeohippus 

 and Nannippus appear to have been smaller than their ancestors. 

 Edinger (1948) from her studies of the casts of the skull and the 

 brains of fossil horses has concluded that the brain surface of the 

 early fossil horses was perfectly smooth and that the sulci have 

 developed at a later date. This would indicate that any resem- 

 blances that have been drawn between the sulci on the brain of 

 the modern horse and those of other mammals are either due to 

 convergent evolution or to homoiology. 



It is quite likely that further studies will show that the complex- 

 ity of horse evolution will prove to be as great as that found in the 

 Proboscidea, Rhinocerotidea or Camelidae. 



