Marsh Collection, Peabody Museum. 433 



Bridger epochs, the general facies of the fauna and flora is that 

 of a tropical or semitropical region, as everyone who has 

 given any attention to the subject is bound to admit. Palms, 

 tropical Invertebrates, Alligators, tropical Turtles, and numer- 

 ous species of monkeys, give ample proof of the nature of a 

 climate which is to-day so necessary to their existence. In the 

 succeeding Uinta, deposits were made within less than fifty 

 miles to the southward of the typical Bridger localities, and in 

 them we find but few fragments of these tropical or semitropi- 

 cal conditions remaining. The general aspect of the fauna 

 gives evidence of a warm temperate climate, with the appear- 

 ance of extensive savannas and much open country. In the 

 Oligocene deposits of nearly the same latitude, with the excep- 

 tion of a single specimen of Alligator, no traces of any remains 

 referable to, or characteristic of, even a semitropical climate have 

 been found, notwithstanding the fact that these deposits are 

 among the richest in mammalian remains of any in the world, 

 and have been most thoroughly explored. The monkeys, 

 Palms, tropical Turtles, and Invertebrates had completely dis- 

 appeared, and there is overwhelming proof to the effect that 

 they migrated to the southward. The European Tertiaries con- 

 tain the same record, and the evidence that a general south- 

 ward retreat of both the higher plants and mammals was in pro- 

 gress throughout much of the Cretaceous and the w T hole of the 

 Tertiary is so complete, conclusive, and incontestable, that it 

 may, in my judgment, be accepted as a fully demonstrated 

 fact. 



If, therefore, the sudden introduction of these new elements 

 into the succeeding faunae and florae is to be explained upon 

 the basis of migration, we are then in a position to assign a 

 rational cause for its occurrence. We have already seen that the 

 temperature in the region of the Pole was tropical in the early 

 Cretaceous ; and that it slowly declined throughout the succeed- 

 ing epochs until finally a frigid condition was established. The 

 migration of the Puerco fauna we may readily believe to have 

 been caused by the appearance of unfavorable conditions, both 

 climatic and vegetal, which gradually supervened within the 

 limits of its ancient boreal home. In like manner, further 

 modifications of the climate and vegetation in the same region 

 caused the Wasatch exodus, and, like their Puerco predeces- 

 sors, they had no alternative other than to follow the receding 

 tropical forests to the southward. That some of them 

 remained behind and were gradually modified to fit the new 

 and changing environment, however, there can be also very 

 little question. In this manner and from this cause doubtless 

 arose those types which came finally to dwell in temperate and 

 arctic climates. When changed conditions in the environment 



