MANY-TOED HORSES. 



259 



It was about as large as a fox, and 

 the base of the Tertiary formation. 

 Huxley's lectures were given, and 

 since the diagrams we follow were 

 made, and we therefore have no 

 figure of it. The Orohippus^ in the 

 next higher division of the Eocene, 

 resembled its predecessor in size, 

 but had only four toes in front, as 

 the diagram shows. The Mesohip- 

 pus came later, was about as large 

 as a sheep, and had three usable 

 toes, and the splint of another, on 

 each fore-foot. In the later Mio- 

 hippus, the splint-bone is reduced 

 to a short remnant. In the Plio- 

 cene above, a three-toed horse {Pro- 

 tohippus), about as large as a don- 

 key, was abundant ; and, still higher 

 up, a near ally of the modem horse 

 (Pliohippus) makes his appearance. 

 The series is completed in the sub- 

 sequent appearance of a true Equus, 

 as large as the existing horse. 



The horse has thus advanced in 

 his development by getting rid of 

 superfluous toes or digits ; but, un- 

 der the principle of reversion to an 

 early ancestral type, to which it is 

 now well understood that animals 

 are liable in various ways, these sup- 

 pressed splints or digits break out 

 as extra hoofs. Professor Marsh 

 says : " In addition to each main 

 digit of the ordinary horse, the anat- 

 omist finds concealed beneath the 

 skin two slender metapodial ' splint- 

 bones,' which are evidently the rem- 

 nants of two other toes originally 

 possessed by the ancestor of the 

 horse. It is an interesting fact that 

 these splint-bones are sometimes 

 quite fully developed, and may 

 even support extra digits which are 

 much shorter and smaller than the 

 main foot. As these small hooflets 



appears in the lower Eocene or at 

 It was discovered since Professor 



puonsppcs. 



PKOTOKIPPCS 



iUipparioii). 



MTOnrPPTTS 



{^Andiitherium). 



IIESOHCFPITB. 



PLIOCENE. 



MIOCENE 



{NeartJiS base\ 



OEOHipprs. 



Eonippus. z 



{Four toes and the 'fi EOCENE 



ritdime,nt of another g {_2i'ear the base), 



on each fore-foot.) « 



(Original f/te-toed 

 ancestor of the 

 horse.) 



