AGES OF NATURE. 189 



some faint notion of the vast series of ages that must have 

 elapsed in order to allow the formation of those rocks, and 

 their regular deposition, under the water, to so great a thick- 

 ness. If, as all things combine to prove, this deposition took 

 place in a slow and gradual manner in each formation, we 

 must conclude, that the successive species of animals found 

 in them followed each other at long intervals, and are not 

 the work of a single epoch. 



466. It was once believed that animals were successively 

 created in the order of their relative perfection ; so that 

 the most ancient formations contained only animals of the 

 lowest grade, such as the Polyps, the Echincderms, to 

 which succeeded the Mollusks, then the Articulated Ani- 

 mals, and last of all, the Vertebrates. This theory, how- 

 ever, is now untenable ; since fossils belonging to each of 

 the four departments have been found in the fossiliferous de- 

 posits of every age. Indeed, we shall see that even in the 

 lower Silurian formation there exist not only Polyps and other 

 Radiata, but also numerous Mollusks, Trilobites (belonging 

 to the Articulata), and even Fishes. 



SECTION II. 



AGES OF NATURE. 



467. Each formation, as has been before stated (460), 

 contains remains peculiar to itself, which do not extend 

 into the neighboring deposits above or below it. Still there 

 is a connection between the different formations, more strong 

 in proportion to their proximity to each other. Thus, the 

 animal remains of the chalk, while they differ from those of 

 all other formations, are nevertheless much more nearly re- 

 lated to those of the oolitic formation, which immediately 



