The Evolution of the Horse 



most tooth is rather larger than those which 

 follow it. 



I have now enumerated those characteristic 

 structures of the horse which are of most im- 

 portance for the purpose we have in view. 



To any one who is acquainted with the mor- 

 phology [comparative forms!] of vertebrated 

 animals, they show that the horse deviates 

 widely from the general structure of mammals; 

 and that the horse type is, in many respects, 

 an extreme modification of the general mam- 

 malian plan. The least modified mammals, 

 in fact, have the radius and ulna, the tibia 

 and fibula, distinct and separate. They have 

 five distinct and complete digits on each foot, 

 and no one of these digits is very much larger 

 than the rest. Moreover, in the least modified 

 mammals the total number of the teeth is very 

 generally forty-four, while in horses the usual 

 number is forty, and in the absence of the 

 canines it may be reduced to thirty-six; the 

 incisor teeth are devoid of the fold seen in those 

 of the horse: the grinders regularly diminish 

 in size from the middle of the series to its front 

 end; while their crowns are short, early attain 

 their full length, and exhibit simple ridges or 

 tubercles, in place of the complex foldings of 

 the horse's grinders. 



Hence the general principles of the hypothe- 

 sis of evolution lead to the conclusion that the 

 horse must have been derived from some quadru- 

 ped which possessed five complete digits on 

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