xi THE ODD-TOED UNGULATES 147 



epoch. The history of this small group alone in its bearings 

 upon evolution might occupy many lectures ; I must content 

 myself now only with one observation, borrowed from Mr. 

 Boyd Dawkins, that, in all modern rhinoceroses the molar 

 teeth have deeper crowns than in those which existed prior 

 to a certain epoch, so that the height of these teeth alone will 

 serve to distinguish a pleistocene from a pliocene form, in 

 other respects closely allied. The value of this observation 

 will be illustrated in the sequel. 



The next line of modification from Palceotherium, is that 

 which culminates in the most specialised of mammals, the 

 modern horse, an animal we are so accustomed to look at 

 that we scarcely ever notice the most remarkably adaptive 

 character of its structure for its special mode of life. If 

 we were not acquainted with the horse (and here of course I 

 include its immediate allies, the asses and zebras) we could 

 scarcely conceive of an animal whose only support was the 

 tip of a single toe on each extremity, to say nothing of the 

 singular conformation of its teeth and other organs. So 

 striking have these characters appeared to many zoologists 

 that the animals possessing them have been reckoned as an 

 order apart called Solidungula ; but palaeontology has revealed 

 that in the structure of its skull, its teeth, its limbs, the horse 

 is nothing more than a modified Palceotherium ; and though 

 still with gaps in certain places, many of the intermediate 

 stages of these modifications are already known to us, being 

 the Paloplotherium, Anchitherium, Merychippus, and Hipparion. 

 On this very interesting point, which looks more like a real 

 genealogical history than any other known, however, I need 

 not dwell, as it was so fully treated of in a lecture delivered 

 in this theatre three years ago by Professor Huxley a lecture 

 entitled the " Pedigree of the Horse." 



Lastly, there is Macrauchenia, a curiously modified Perisso- 

 dactyle found in pleistocene times in South America, appar- 

 ently another derivative of the palseotherium type, presenting 

 resemblances (though perhaps only analogical) to some of the 

 artiodactyles, especially the camels. 



Directly intermediate forms between Macrauchenia and 

 the other animals of its group are not yet known ; but 



