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, 
