Evoi.rrios OF Till:' noh'si-: /\ \.\rih'h' 



spvvd <»l" the s\vin<i is r('}j;ul:il(Ml l>y t he (listMiicc of t he cent ic of ^i;i\il y 

 from \\\v \)oiu\ of at taclimcnt , as tliat of a pcnduluin is l)y the height of 

 the bob. To iiu'i('as(' the l(Mi<2;th of Unwv Ic^' and foot thcrcfoi-c (^iivcs the 

 animal j2;r(>at(M- speed; hnt it i)nts an inci-eased stiain on the ankles and 

 toe-joints, and these must he st icn^t hened corre- 

 ;]^ondinp;ly hy convert injz; them from hall-and- 

 soeket joints to "^in^lymoid" or pulley joints. 

 Additional strength, likewise at the expense 

 of flexibility, is obtained by the consolida- 

 tion of the two bones of the forearm 

 (ulna and radius) and of the leg 

 (tibia and fibula) into one, the 

 shaft of the smaller bone prac- 

 tically disappearing, while its 

 ends become fused solidh' to 

 its larger neighbor. 



The increase in 

 length of Umb ren- 

 ders it necessary 

 for the grazing 

 animal that 

 the head 

 and neck 

 >\ f \ should 



FIG. 17. EVOLUTION OF THE FORE LEG. PRINCIPAL STAGES 



Equus, Merychippus, Mesohippus, Eohippu^. 



In the four-toed horses (Eohippus) the radius and ulna are separate and their 

 shafts of about equal size. In the earher three-toed horses (Mesohippus) the ulna is 

 separate but its shaft considerably reduced. In the later three- toed horses 

 (Merychippus) the ulna is partly consolidated with the radius and its shaft is 

 reduced to a thin thread. In the one-toed horses (Equus) the ulna is more completely 

 consolidated with the radius and its shaft has entirely disappeared 



