AGES OF NATURE. 221 



ness with which corals and shells are formed, it will give us 

 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 low- 

 est grade, such as the Polyps, the Echinoderms, to which 

 succeeded the Mollusks, then the Articulated Animals, and, 

 last of all, the Vertebrates. This theory, however, is now 

 untenable ; since fossils belonging to each of the four depart- 

 ments have been found in the fossiliferous deposits 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 Articu- 

 lata,) 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 ani- 

 mal remains of the Chalk, while they differ from those of all 

 other formations, are, nevertheless, much more nearly related 



19* 



