THE SUIIFACE OF THE GLOBE, 47 



They might have had relation to causes still more accidental. 

 There is nothing to assure us that at the bottom of the sea certain spe- 

 cies, even certain genera, after having occupied for a longer or shorter 

 period determinate situations, have not been forced away by others. 

 Here, on the contrary, all is precise ; the appearance of the bones of 

 quadrupeds, particularly the whole carcases in the layers, betokens 

 either that the layer itself which contains them was formerly dry land 

 or that there was terra firma in its immediate vicinity. Their disap- 

 pearance renders it certain that this layer was inundated, or that this 

 dry land ceased to exist. It is then by these that we learn in a posi- 

 tive manner the important fact of the repeated irruptions of the sea, 

 with which shells and other marine productions could not have made 

 us acquainted ; it is by studying them profoundly that we may hope 

 to ascertain the numbers and periods of these irruptions. 



Secondly, the nature of the revolutions which have altered the sur- 

 face of the globe must have exercised a more entire action over terres- 

 trial quadrupeds than marine animals. As these revolutions have in a 

 great measure consisted in changes of the bed of the sea, and the 

 waters must have destroyed all the quadrupeds which they reached, if 

 the irruption were general, the whole class must have perished ; or, if 

 only operating on certain continents, it must have destroyed at least 

 the species peculiar to these continents, without exercising the same 

 influence upon marine animals. On the contrary, millions of aquatic 

 individuals might have been left on dry land, or buried under new 

 layers, or thrown with violence on the shore, and their race be still 

 preserved in some places more tranquil, where it might again be pro- 

 pagated after the disturbance of the waters had ceased. 



Thirdly, this action, as more complete, is more easily seized on ; it 

 is more easy to demonstrate its effects, because, the number of quad- 

 rupeds being limited, the greater part of their species, at least of the 

 larger kind, being known, we have still further means afforded us of 

 ascertaining whether the fossil bones belong to one of them, or if they 

 formed a part of the species now extinct. As we are, on the contrary, 

 very far from knowing all the marine testacea and sea fish ; (and are pro- 

 bably ignorant yet of the greater part which are in the depths of the 

 ocean), it is impossible to know with eertainty if a species found fos- 

 silized be or be not extinct. Thus we observe learmed men obsti- 

 nately bent on giving the name of pelagian shells, that is, shells of the 

 deep sea, to belemnites, to cornua-ammonis, and other shelly relics, 

 which have as yet only been observed in ancient layers ; meaning by 

 that, that if they have not been yet found in a living state, it is be- 

 cause they inhabit depths beyond the reach of our nets. 



