GENERAL OBSERVATIONS ON THE TERTIARY PERIOD. 543 



from the Pantolestes of the Early Eocene, through the Poebrotherium 

 of the Miocene, and the Procamelus of the Pliocene, to the modern 

 camel.* Similarly also the modern deer, with its branching antlers, 

 may be traced from the Lower Pliocene, where they had antlers of one 

 or two points through the Upper Pliocene, where the antlers are more 

 complex, to the magnificent, many-branched antlers of the Quaternary 

 and modern times. 



Erom the earliest and most generalized types, therefore, to the pres- 

 ent specialized types, the principal changes have been, first, from planti- 

 grade to digitigrade ; second, from short-footed digitigrade to long- 

 footed digitigrade, i. e., increasing elevation of tlie heel ; third, from 

 five toes to one toe in the Horse, or two toes in Kuminants; and, 

 fourth, from simple omnivorous molars to the complex herbivorous 

 millstones of the Horse and the Ox. 



The change from plantigrade to digitigrade, with increasing eleva- 

 tion of the heel, when taken in connection with increasing size of the 

 brain, and therefore presumably with increasing brain-power, shows a 

 gradual improvement of structure adapted for speed and activity, and a 

 pari-passu increase of nervous and muscular energy, necessary to work 

 the improved structure. 



4. JS'ot only does the mammalian fauna of the Miocene differ com- 

 pletely from that of the Eocene, which precedes, and from the Pliocene, 

 which succeeds it, but there seem to have been at least three distinct 

 Eocene and two distinct Miocene faunas. Thus there have been many 

 complete changes in the mammalian fauna in Tertiary times. 



General Observations on the Tertiary Period. 



"We have already seen (p. 470) that during Cretaceous times a wide 

 sea occupying the position of the Western Plains and Plateau region, 

 divided America into two Continents, an Eastern and a Western. We 

 have also seen (p. 495) that at the end of the Cretaceous, this sea was 

 obliterated by continental upheaval, and the continent became one. 

 During the Eocene, the eastern portion of the place formerly occupied 

 by this sea was probably dry land, but in the Plateau region there 

 were great fresh-water lakes, one north of the Uintah Mountains, 

 Green Elver Basin, and one south of the same, and probably one in 

 Oregon. There were possibly others yet unknown. At the end of 

 the Eocene, there was a rise in the Plateau region, which drained the 

 Eocene lakes, through the Colorado Eiver, and a corresponding depres- 

 sion in the Plains region on the one side, and the Basin region on the 

 other, not sufficient to form a sea again, but sufficient to form great 

 Miocene lakes there. During the Miocene, the bared bottoms of the 



* American Naturalist, vol. xx, p. 611, 1886. 



