ITS NEAREST EXISTING RELATIONS 



67 



to be explained in the next chapter, are known; but 

 they are sufficient to enable any one conversant with 

 them to recognise a single molar of an hipparion 

 from that of any of the existing species, and to show 

 that the horse-like teeth found occasionally among the 

 debris of former Miocene or Pliocene formations in the 

 Red Crag of Suffolk belong to animals of this group. 



These dental characters, and also details in the struc- 

 ture of the bones of the feet, have led even more conclu- 

 sively than the presence of the suborbital depression to 

 the view that the hipparion, or, at all events, the Euro- 

 pean Hippotherium gracile, was not on the direct line of 

 descent of the modern horses, but that it was a form 

 which, having attained a considerable degree of special- 

 isation in some particulars, a wide geographical distribu- 

 tion and great abundance of individuals, became, as has 

 so often happened in similar cases, extinct without direct 

 descendants from causes which we at present cannot 

 divine. Perhaps an inability to lose the useless outer 

 toes may have given it a disadvantage in a severe 

 competition for existence with otherwise closely allied 

 forms, which had already adopted the style of foot which 

 clearly shows itself the best for the existing requirements 

 of the race. 



Judging from tooth-structure alone, a very perfect 

 series of modifications from Anchitherium to the modern 

 horses can be shown through various species of the 



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