VIII. Geological Distribution of Animals. 79 



the Permian, are the first Reptiles ; iu the Trias, the first Mammals ; 

 and in the Jurassic, the earliest Birds. A study of special examples 

 gives a similar result. Amongst living Mammalia the Horse, as is 

 well-known, is in some respects, e.g., the structure of the foot, a very 

 peculiar and aberrant form. The horse itself appears late ; 

 the true horse, Bquus, with a single toe on each foot, as in the living 

 form, first occurs in the upper Pliocene, but its near relative, 

 Hipparion, having the middle toe, and also the second and fourth in a 

 reduced form, lived in the early Miocene and on into the Pliocene. 

 In the lowest strata of the Miocene, before the appearance of 

 Hipfiarian, a third genus, Anchitherium, is found, more remotely 

 related to the horse, from which the structure of its teeth shows a 

 considerable removal : it has, however, the same toes as Hippari n, 

 but better developed, and in this respect it approaches the genus 

 PalaBotherium, of the Eocene. In attemptiug to reconstruct this 

 phylogeny from anatomical considerations, it is certain that 

 Equus must be derived from Hipparion; Hipparion from 

 Anchitherium; and the latter from Palseotherium, or some nearly 

 related form : and with this the geological succession agrees closely. 



A similar genealogy may be drawn up for many other groups, 

 although the imperfect knowledge of extinct forms often renders it 

 impossible. 



Table oi' Possilipeeous Poemations. 

 Quaternary Pleistocene. 



Tertiary ( Pliocene, 

 or s Miocene. 



Cainozoic [ Eocene. 



Secondary f Cretaceous, 

 or <. Jurassic. 



Mesozoic ( Triassic. 



(Permian. 

 Carboniferous. 

 „ , Devonian. 



Palaeozoic . 



Silurian. 



\ Cambrian. 



