54 THEORY OF THE EARTH. 



In the first place, they indicate much more 

 clearly the nature of the revolutions to which 

 they have already been subjected. Shells certain- 

 ly announce the fact, that the sea has once exist- 

 ed in the places where they have been formed ; 

 but the changes which have taken place in their 

 species, when rigorously inquired into, may have 

 arisen from slight changes in the nature of the 

 fluid in which they lived, or merely in its tempe- 

 rature. They may even have been produced by 

 causes still more accidental. We can never be 

 perfectly assured that certain species, and even 

 genera, inhabiting the bottom of the sea, and oc- 

 cupying certain fixed spaces, for a longer or shor- 

 ter time, may not have been driven away and 

 supplanted by other species or genera. 



In regard to quadrupeds, on the contrary, every 

 thing is precise. The appearance of their bones 

 in strata, and still more of their entire car- 

 cases, announces, either that the stratum itself 

 which contains them has, at a former period, been 

 laid dry, or, at least, that dry land must have exist- 

 ed in its neighbourhood. Their disappearance ren- 

 ders it certain, that this stratum has been inun- 

 dated, or that the dry land in question has ceased 

 to exist. It is from them, therefore, that we learn 

 with perfect certainty the important fact of re- 

 peated irruptions of the sea, which the shells 

 and other marine productions could not of them- 



