Srno/I 4-Toed //orses 



Sma/1 3-Totd Larqt 3'Tofd Lar^t /-Toed 



ACE OF MAMMALS 



ACE or MAN 



Eocene Efiock \Oligocerit M ioc e nt. Plioct nt Pleistocene Rerent 



Eo^ippui- 



Orohippus 

 Eptfuppui 

 Mtsokippui 

 Ancnilnenum 

 Hypo/iippus 

 ParaJiippus 

 Merifcnippus 

 ProCoAippus 

 Pliofiippui 

 Hippidium. 



( Ed rope 



I 



[ 



U No,rM A. 



I 



( Nurfh Ainftita) 



ffi Aiir e r , c a ) \ 



{Nor Ik 



-{Eu rope a/.i NortA Aincnca)- -. - -"-m^^ 



^ \ 



■(Nortfi A rnerica , Asia and \Earope)-^^ 



. (Norlh America) 



(/Vort\ America.)- \ -'-^ 



. - _ ( NortA America) 



(l\loiith America)- '- - , 



( S\>uth America} N^ 



Onohippidiu^ (S^out/i Americ 



Hipparion. 

 Eguus 



-(Morlh America , Asia. Eur\)pe and A/ort/t A/nca)- 

 {Norlh and Souf/r An.cnca Asia , Europe and Afr/caJ.x 



Fig. 16. CJeological and geop;raphi(*al range of ancestors of the horse. The 

 l)ositi()n and length of the heavy black Hnes show the occurrence and range of each 

 genus in the successive geologic epochs, while the thickness of the lines indicates 

 relative abundance. The dotted connecting lines with arrows indicate the genealogy 



MEANING OF THE CHANGE IN FEET AND TEETH 



A LONG with the disappearance of the .^ide toes in the evolution of 

 Z_\ the horse there is a considerable increase in the proportionate 

 length of the limbs, and especially of the lower part of the le^ and 

 foot. The surfaces of the joints, at first more or less of the ball-and- 

 .^ocket kind which allows free motion of the limb in all directions, become 

 keeled and grooved like a puUe^'-wheel, permitting free motion forward 

 and backward, but limiting the motion in all other directions and increas- 

 ing considerably the strength of the joint. By this means the foot is made 

 more efficient for locomotion over a smooth regular surface, but less so for 

 traveling over very rough ground, and it becomes of little use for strik- 

 ing or grasping or the varied purposes for which the feet of poly dactyl 

 animals are used. 



The increased length in the lower leg and foot increases the length of 

 the stride without decreasing its quickness. The heavy muscles of the 

 leg are chiefly in the upper part, and to increase the length of the lower 

 part changes the centre of gravity of the limb very little. Consequenth' 

 the leg swings to and from the socket nearly as fast as l)efore, .*5ince in 

 an ordinary st(^p the action of the leg is like that of a pendulum, and the 



