CRITICAL PERIODS IN THE HISTORY OF THE EARTH. 483 



and larger than in the Tertiary; that, in short, this was again a continental 

 period, and probably a period of greater cold than the subsequent Tertiary. 



The change in physical geography, then, was immense, but in most places 

 by bodily upheaval, not by crumpling of the strata; and therefore the 

 usual sign of such change, namely, unconformity, is often wanting. The 

 change of climate all over the American continent was no doubt very great, 

 and the changein organic forms correspondingly great everywhere and in all 

 departments ; but this was especiall}^ true of all water-inhabiting species 

 in the region of the old Cretaceoiis interior sea, for here there was a transi- 

 tion, not only in climate but from salt to fresh water through the interme- 

 diate condition of brackish water. The Cretaceous marine species rapidly 

 disappeared, partly by extermination and partly by transmutation into fresh- 

 water species, as has been observed, recently, to take place in some crusta- 

 ceans under this change of conditions.* The Tertiarj^ fresh-water species 

 quickly ap^^eared, partly by transmutation from the previous marine species 

 and partly by transportation in various ways from other fresh-water lakes. 

 But all this occurred in some places without the slightest break in the con- 

 tinuity of the strata. 



The great change of climate and other physical conditions perhaps suf- 

 ficiently explain the change in invertebrate species, but it is impossible to 

 account for the somewhat sudden appearance of mammals in the lowest 

 Tertiary, except by migration from other regions where they had existed 

 in late Cretaceous times, having originated there by derivation in the usual 

 way. That marsupials existed" somewhere in Cretaceous times (though 

 possibly not in America or Europe) there can be no doubt ; for they lived, 

 we know, in^the preceding Jurassic and the following Tertiarj^, and they 

 exist now. It is from these rather than from Cretaceous reptiles that Ter- 

 tiary mammals were doubtless derived; and this derivation took place 

 probably at a rapid rate in the latest Cretaceous or during the lost interval, 

 in some unknown locality, whence they migrated into the Tertiary lake 

 region of the United States during the interval. This migration came most 

 probably from Northern Asia, for it must be remembered that the interval 

 was a continental period, and therefore probably a period of broad land 

 •connections between Nearctic and Palaiarctic regions. The complete examin- 

 ation of the uppermost Cretaceous of different portions of Asia will proba- 

 bly reveal the immediate progenitors of the early Tertiary mammals of 

 Europe and America. This introduces us to a most important element of 

 rapid local faunal change, especially in higher animals, namely, migrations. 

 If we do not dwell longer now on this, it is only because we shall have to 

 recur to it again. 



(to be cqntikued.) 



■'Arch, des Sciences, November, 1875, p. 284. 



