178 INTRODUCTION. 



ing to lowlands and marshes ; while those which in life 

 were habituated to a high temperature would be inter 

 mingled with others which had Uved under the influence 

 of an almost constant winter. And farther, besides the 

 terrestrial shells of such different characters we might 

 also find the Limniadce of shallow waters, the Naiades 

 of deeper streams, and the various species of Cyrena, 

 Crnathodon, and Neritina of the mouths of rivers, all 

 mingled together with the truly marine genera. It is 

 manifest that in the confusion of species which such a 

 deposit would present, but few legitimate inferences 

 could be drawn as to the former climate and condition of 

 that part of the earth's surface where they occur, or as 

 to the changes of habit and locality of the species them- 

 selves, or ui fact as to any pomt except their contem- 

 poraneous existence, and their affinities to the species 

 which are now extant. 



It has been held that as the presence of certain spe- 

 cies coincides in general with temperature, the occur- 

 rence of certain fossil forms in a particular geological 

 formation indicates that the climate of the locality was, 

 at the period of deposition, similar to that in which the 

 same or analogous Hving species are now known to exist. 

 The preceding remarks show how erroneous this opinion 

 is hkely to be when founded upon the occurrence of the 

 terrestrial and fresh-water species ; for the place of their 

 origin and its climate must be in a great degree uncer- 

 tain, and while it may coincide with their present condi- 



