17g INTRODUCTl' 
ing to lowlands and marshes ; while those which in life 
were habituated to a high temperature would be inter- 
mingled with others which had lived under the influence 
of an almost constant winter. And farther, besides the 
terrestrial shells of such different characters we might 
also find the IAmniadtB of shallow waters, the Nm 
of deeper streams, and the various species of Cyi 
Grnathodon, 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 in fact as to any point 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 living species are now known to exist. 
The preceding remarks show how erroneous this opinion 
is likely 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- 
