Remarhs on the Descent of Animals. 145 



and another in such order as they must have followed if they had 

 constituted a succession of stages, in time of the development of 

 the form at which they ultimately arrive." In short, it would 

 have to be shown, that, with reference to birds and reptiles, for 

 instance, " that in some ancient formation reptiles alone should 

 be found ; in some later formations birds should first be met with; 

 and in the intermediate strata we should discover in regular suc- 

 cession the forms which are intermediate between reptiles and 

 birds." 



Precisely this kind of evidence has of late years been accumu- 

 lating rapidly respecting many groups of the animal kingdom. 

 The development of the horse offers us, perhaps, the best illustra- 

 tion of this kind of evidence, and T give the substance of " these 

 thoroughly and patiently worked-out investigations of Prof. 

 Marsh," in his own words. He says : "I have unearthed with 

 my own hands not less than thirty distinct species of the horse 

 tribe, in the tertiary deposits of the west alone. 



" The oldest representation of the horse at present known is the 

 diminutive Eohi2:)pus. from the lower Eocene. Several species 

 have been found, all about the size of a fox. Like most of the 

 early mammals, the ungulates had forty-four teeth, the molars with 

 short crowns, and quite distinct in form from the premolars. The 

 ulna and the fibula were entire and distinct, and there were four 

 well-developed toes, and the rudiment of another on the fore-feet, 

 and three toes behind. In the structure of the feet and in the 

 teeth, the Eohippus indicates unmistakably that the direct ances- 

 tral line to the modern horse has already separated from the other 

 perissodactyles. In the next higher division of the Eocene, an- 

 other genus (Orohippus) makes its appearance, replacing Eohip- 

 pus, and showing a greater, although still distant, resemblance to 

 the equine type. The rudimentary first digit of the fore-foot has 

 disappeared, and the last premolar has gone over to the molar 

 series. Orohippus was but little larger than Eohippus ; in most 

 other respects very similar. Near the base of the Miocene, we 

 find a third closely allied species, Mesohippus, which is about 

 as large as a sheep, and one stage nearer the horse. There are 

 only three toes and a rudimentary splint bone on the fore-leg, and 

 10 



