Miscellaneous. 



399 



stxccession from each of the geological periods in which this group 

 of mammals is known to have lived : — 



a. Orohippits (Eocene) ; b. 3Iiohippus (Miocene) ; c. Hipparion (Pliocene) ; 

 d. EquHS (Quaternary). 



The ancient OroMppus had all four digits of the manus well 

 developed. In Miohippus, of the next period, the fifth toe has 

 disappeared, or is only represented by a rudiment, and the limb is 

 supported by the second, third, and fourth, the middle one being 

 the largest. Hipjparion, of the later Tertiary, still has three digits ; 

 but the third is much stouter, and the outer ones have ceased to be 

 of use, as they do not touch the ground. In Equus, the last of the 

 series, the lateral hoofs are gone, and the digits themselves are 

 represented only by the rudimentary splint bones*. The middle, 

 or third, digit supports the limb ; and its size has increased accord- 

 ingly. The corresponding changes in the posterior limb of these 

 genera are very similar, but not so manifest, as the oldest type 

 (^Ofohippus) had but three toes behind. An earlier ancestor of the 

 group, perhaps in the lowest Eocene, probably had four toes on this 

 foot, and five in front. Such a predecessor is as clearly indicated 

 by the feet of OroMppus, as the latter is by its Miocene relative. 

 A still older ancestor, possibly in the Cretaceous, doubtless had 

 five toes in each foot, the typical number in mammals. This 

 reduction in the number of toes may perhaps have been due to 

 elevation of the region inhabited, which gradually led the animals 

 to live on higher grounds, instead of the soft lowlands where a 

 polydactyle foot would be an advantage. 



The gradual elongation of the head and neck, which took place 

 in the successive genera of this group during the Tertiary period, 

 was a less fundamental change than that which resulted in the 

 reduction of the limbs. The process may be said to have already 

 begun in Orohippus, if we compare that form with other most 

 nearly allied mammals. The diastema, or "place for the bit," 

 was well developed in both jaws even then, but increased materially 



* The modem horse occasionally has one of the ancestral hooflets de- 

 veloped, usually on the fore foot. 



28* 



