Paleontology. 157 



record of the chronological succession of the forms of 

 life which from time to time have peopled the globe. 

 Now in one sense this notion is partly true, but in 

 another sense it is profoundly false. It is partly true 

 if we have regard only to those larger divisions of 

 the vegetable or animal kingdoms which naturalists 

 designate by the terms classes and orders. But the 

 notion becomes progressively more untrue when it is 

 applied to families and genera, while it is most of all 

 untrue when applied to species. That this must be so 

 may be rendered apparent by two considerations. 



In the first place, it does not follow that because 

 we have a tolerably complete record of the succession 

 of geological formations, we have therefore any 

 correspondingly complete record of their fossiliferous 

 contents. The work of determining the relative ages 

 of the rocks does not require that every cubic mile of 

 the earth's surface should be separately examined, in 

 order to find all the different fossils which it may 

 contain. Were this the case, we should hitherto have 

 made but very small progress in our reading of the 

 testimony of the rocks. The relative ages of the 

 rocks are determined by broad comparative surveys 

 over extensive areas ; and although the identification 

 of widely separated deposits is often greatly assisted 

 by a study of their fossiliferous contents, the mere 

 pricking of a continent here and there is all that is 

 required for this purpose. Hence, the accuracy of 

 our information touching the relative ages of geo- 

 logical strata does not depend upon and, therefore, 

 does not betoken any equivalent accuracy of know^ 

 ledge touching the fossiliferous material which these 

 strata may at the present time actually contain. And, 



