1 8 THE HORSE AND ITS RELATIVES 



high speed. Such a type of limb is the one evi- 

 dently best suited to this end, since if the short, many- 

 toed, and many-boned limb had been elongated 

 without special modification to the extent of that of 

 the horse, it is perfectly certain that it would have 

 been unequal to the strain of carrying the body of 

 such a heavy animal at a high rate of speed over 

 hard ground. 



In their limb-specialisation the horse and its 

 relatives have attained practically the same evolu- 

 tionary platform as the ruminant ungulates, only by 

 a different line of development. In the horse group, 

 as we have just seen, the development of a long 

 cannon-bone in the lower segment of each limb has 

 been brought about by the lengthening and strength- 

 ening of the middle element of the primitive five-toed 

 foot. In the ruminants, on the other hand, the same 

 end has been attained by the lengthening and fusion 

 of two adjacent elements, so as to form a compound, 

 in place of a simple, cannon-bone. At the present 

 day the members of the horse family are absolutely 

 unique in the matter of limb-structure, no other 

 living mammal (or, for that matter, no other living 

 animal) having a single-toed, or monodactyle, foot. 

 It is, however, not a little remarkable that during 

 the middle, or Miocene, portion of the Tertiary 

 period South America was the home of a genus 

 of hoofed mammals known as Thoatheriuin, 

 in which a monodactyle type of foot had likewise 



