ON BIOLOGY. 49 



age in America. In the bad lands of Nebraska, for in- 

 stance, have been discovered the fossil remains of tigers, 

 camels, rodents, hyenas, panthers, wolf, deer, horses, rhi- 

 neros, and a variety of more generalized forms. 



But among the main points to be remembered about all 

 those mammals of the Tertiary system is, that they arose 

 from small, non-placental forms which flourished during 

 the latter half of the Meozoic age, and that those forms 

 were in many ways yet intimately affined with birds and 

 reptiles. Reptile, indeed, was the main stem, and from 

 it branched off during the Triassic the bird and mammal 

 stock, and the linking types were to be found upon every 

 hand. Passing through the enormous lapse of time rep- 

 resented by the Cretaceans of Mesozoic, and successively 

 through the Eocene, Miocene and Pliocene of the Ter- 

 tiary, we find in the several parts of the United States, at 

 least, that from those small generalized types were pro- 

 duced the ancestors of the more modern mammalian 

 forms, a number of which have already been given above. 

 Among other remarkable forms there existed, for 

 example, in the Pliocene of the Niobrara Basin of our 

 Bad Land territory, several species of horses, one of 

 which was only two feet high; camels of ancient type, 

 and it is now recognized that both horse and camel orig- 

 inated upon this continent and not in the Orient as most 

 naturally suppose to be the case. As many of these early 

 mammals evolved, there was a gradual increase of the 

 brain-mass in many of the species; an elaboration of other 

 structures, as the teeth and feet; and a general tendency 

 toward the establishment of modern types. Indeed, in 

 all Nature there is no more engaging chapter in paleon- 

 tology than the great lesson taught by the specimens, and 

 specimens in abundance, showing the evolvement in time 

 of our modern horse as it has passed from the eohippus 

 of the Eocene, an animal no bigger than 

 a fox, with its four-toed feet in front and its three-toed 

 feet behind, and with its simple tooth-structure, through 

 the orohippus of the Middle Eocene; the mesohippus of the 

 Lower Miocene; the miohippus of the Miocene; and so 

 on up to recent time where we have the horses of the 

 present day. The gradual structural changes are perfect, 

 and the shad'ng from one series of fossil skeletal remains 

 to the next succeeding species is quite as imperceptible 

 as it is marvelous. And yet how widely separated are 

 the extremes, and what enormous lapses of time does 

 the entire process represent. Cope has traced, by means 

 of similar material, the development of the American 



