J. LeConte—Critical Periods in the History of the Earth. 105 
change in organic forms correspondingly great everywhere and 
in all departments; but this was especially true of all water- 
inhabiting species, in the region of the old Cretaceous interior 
sea; for here there was a change not only in climate but from 
salt to fresh water through the intermediate condition of brack- 
ish 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 crustaceans under this change of conditions.* The Ter- 
tiary fresh-water species rapidly appeared partly by transmuta- 
tion from the previous marine and partly by transportation in 
various ways from other fresh lakes. But all this occurred in 
some places without the slightest break in the continuity of the 
strata. 
perhaps sufficiently explain the change in envertebrate species, 
ut it is impossible to account for the somewhat sudden appear- 
Interval in some unknown locality whence they migrated into 
the Tertiary lake region of the United States during the inter- 
period and therefore probably a period of broad land connec: 
fons between Nearctic and Palearctic re ions. The complete 
early Tertiary mammals of Europe and America. This intro- 
duces us to a most important element of rapid local faunal 
change especially in higher animals, viz: migrations. If we 
do not dwell longer now on this, it is only because we shall have 
to recur to it again, 
I have pretorred, thus far, to speak of general evolution- 
organisms, whether slow or rapid, as produ 
* Arch. des Sciences, Nov., 1875, p. 284. 
Am. Jour. Sct.—Turrp Serres, Vo. XIV, No. 80.—Aueust, 1877. 
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