28 



.1 UAViVr.t.V MCSKIM criDE Lh'AFLh'TS 



increase in length in order to enable tlie mouth to reach tlie *2;roun(l. 

 An exanij)le of these clian^es is the modern horse, in which we find 

 the neck and head much elonji;ated when compared with the little 

 Hyracotherium and this olon^^ation has taken place pari passu with the 

 elonjjjation of the le^s. The reduction and disa})pearance of the side 

 toes and the concentration of the step on the single central toe serve 

 likewise to increase the speed over smooth ground. The soft yielding 

 surface of the })olydactyl (several-toed) foot is able to accommodate 

 itself to a rough irregular surface, hut on smooth ground the yielding 



step entails a certain 

 loss of speed. A some- 

 what similar case is seen 

 in the pneumatic tire of 

 a bicycle; a ''soft " tire 

 accommodates itself to a 

 rough road and makes 

 easier riding, but a 

 ''hard" tire is faster, 

 especially on a smooth 

 road. Similarly, the 



Fig. 18. EVOLUTION OF THE HIND LEG. PRINCIPAL STAGES 



Equns, Mcri/clii i>i)us. Mesohippus, Eohippus. 



In the four-tood \u)rs^'s (Eohippn.^) thv fibula i.>^ coinploto and separate from 

 the tibia. In the earlier three-toed lior.se (.Uc.so////)/)(/.s) it is very .slender, and fu.sed 

 with the tibia from about the middle of the shaft down. In the later three-toed 

 horses (Merychippui^) the shaft of the fil)iila is ineomplete and both ends are fu.sed 

 with tlie tibia. In the one-toed horses (EqUus) the lower end is more thoroughly 

 united so as to he in(hstinguishal)le from the til)ia. 



