122 LECTURES ON EVOLUTION m 



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 hypothesis 

 of evolution lead to the conclusion that the horse 

 must have been derived from some quadruped 

 which possessed five complete digits on each foot ; 

 which had the bones of the fore-arm and of the 

 leg complete and separate ; and which possessed 

 forty-four teeth, among which the crowns of the 

 incisors and grinders had a simple structure ; 

 while the latter gradually increased in size from 

 before backwards, at any rate in the anterior part 

 of the series, and had short crowns. 



And if the horse has been thus evolved, and 

 tin remains of the different stages of its evolution 

 have been preserved, they ought to present us 

 with ,i teriei of forms in which the number of the 

 digits becomes reduced ; the bones of the fore-arm 

 :ml l<-_, r gradually take on the equine condition; 

 ami the t'nnii and aiTaii-rninit of the teeth 



