G. R. Wieland — Polar Climate in Time. 411 



[Now all the divisions of the Tertiary, the Eocene, Oligocene, 

 Miocene, and Pliocene were markedly periods of great inland 

 lakes and flood plains, which on the whole favored the deposi- 

 tion of far more extensive freshwater beds than those of 

 earlier times when the continents were rather smaller. More- 

 over, these periods witnessed the expansion of the primitive 

 mammalian stocks into the existing mammalian orders, as well 

 as the rise and extinction of many striking forms. As a con- 

 sequence, the mammals being from size, habit, and frequency 

 of preservation among the best of all horizon-markers, the 

 Tertiary record, unlike the older and more imperfect Mesozoic 

 record, is often quite complete. The North American Ter- 

 tiary strata are more than a mile in total thickness, and contain 

 imbedded at intervals, which of course depended on local con- 

 ditions, a score or more of successive faunae. When closely 

 studied these are found to possess many peculiarities of their 

 own. Many of the genera and families of each are new and 

 unheralded in preceding groups. ]^o vertebrate paleontologist 

 would consider these new elements as direct local derivatives 

 from preceding faunae. That is to say, there is throughout the 

 series an ever-recurring lack of precursor forms, if we except 

 the later stages of certain groups like the horses which evolved 

 many successive species in the great mid and late Tertiary 

 American plains. In short, most of the groups come suddenly 

 with a large proportion of new elements scarcely or not at all 

 related to preceding forms and they go as suddenly, as if by a 

 succession of " waves " or " impulses." In Europe there are 

 not such extensive Tertiary deposits as in America, but there 

 is a reasonably complete record, and it shows the same history 

 of new faunal influxes. But this is not all. The series of 

 unheralded faunal influxes forming so prominent a feature in 

 the European Tertiary was made up of much the same suc- 

 cessive elements, possessing the same peculiarities as well as 

 many of the same genera and closely related species, and 

 appearing in the same order, and at essentially the same time 

 as in America. In fact, essentially the same new complex 

 faunae appear, as explained, at about the same time on both 

 sides of the Atlantic so often and so continuously as to make 

 this mode of appearance the rule, not the exception. And 

 such differences as do present themselves are explicable on the 

 basis of the known great imperfections of the fossil record, on 

 some climatal differences, and because of the unlikelihood 

 that all the elements of faunae, including diverse orders and 

 families, could in any case whatsoever reach such widely sepa- 

 rated regions.* 



* These facts of similarity become more and more striking with exacter 

 methods of study and the multiplication of known forms with each added 

 year of exploration. Thus Charles Deperet in a recent study of the early 



