. The horse and its relatives . s have attained practically the same evolu-tionary platform as the ruminant ungulates, only bya different line of development. In the horse group,as we have just seen, the development of a longcannon-bone in the lower segment of each limb hasbeen brought about by the lengthening and strength-ening of the middle element of the primitive five-toedfoot. In the ruminants, on the other hand, the sameend has been attained by the lengthening and fusionof two adjacent elements, so as to form a compound,in place of a simple, cannon-bone. At the presentday the members of
. The horse and its relatives . s have attained practically the same evolu-tionary platform as the ruminant ungulates, only bya different line of development. In the horse group,as we have just seen, the development of a longcannon-bone in the lower segment of each limb hasbeen brought about by the lengthening and strength-ening of the middle element of the primitive five-toedfoot. In the ruminants, on the other hand, the sameend has been attained by the lengthening and fusionof two adjacent elements, so as to form a compound,in place of a simple, cannon-bone. At the presentday the members of the horse family are absolutelyunique in the matter of limb-structure, no otherliving mammal (or, for that matter, no other livinganimal) having a single-toed, or monodactyle, is, however, not a little remarkable that duringthe middle, or Miocene, portion of the Tertiaryperiod South America was the home of a genusof hoofed mammals known as Thoatheriuin,in which a monodactyle type of foot had likewise III Fi( Skeleton of front (Fig. i) and hind (Fig. 2) limbs of Eclipse/ s, scapula ;^ humerus; u, ulna; r, radius; c, carpus or wrist (knee); caK,Annon-bone ; ///, phalanges or toe-bones ; />, pelvis ; /, femur ; /tibia ; ca, cakaneum or upper bone of tarsus or ankle (hock). POSITION AND STRUCTURE OF HORSE 19 been developed by the reduction of the lateral toesof a nearly allied tridactyle relative. In this casethe specialisation was even greater than in thehorse group, as the splint-bones were reduced tomere nodules of bone on either side of each cannon-bone. If TJioatheriu7ii had been a near relative of thehorse, there would be no cause for surprise in itshaving attained the same remarkable and final stageof foot-development. As a matter of fact, itbelongs, however, to a totally different and muchmore primitive group of ungulates, which appearsto have been always restricted to South America,and although presenting certain structural resem-blances to the Peri
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Keywords: ., bookcentury1900, bookdecade1910, booksubjecthorses, bookyear1912
