396 GEOLOGICAL SUCCESSION OF ANIMALS. 



succeeded the mollusca, then the articulated animals, and 

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

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

 partments have been found in the fossiliferous deposits of 

 every age. Indeed, we shall see that even in the lower Silu- 

 rian formation there exist not only polyps and other radiata, 

 but also numerous mollusca, trilobites (belonging to the arti- 

 culata), and even fishes and reptiles.* 



SECTION II. 



AGES OF NATTJBE. 



656. EACH formation, as has been before stated ( 649), 

 contains remains peculiar to itself, which do not extend into 

 the neighbouring 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 

 precedes, than to those of the carboniferous formation, which 

 is much more ancient ; and in the same manner, the fossils of 

 the carboniferous group approach more nearly to those of the 

 Silurian formation than to those of the Tertiary. 



657. These relations could not escape the observation 

 of naturalists, and indeed they are of great importance for 

 the true understanding of the development of life at the sur- 

 face of our earth. And, as in the history of man, several 

 grand periods have been established, under the name of Ages, 

 marked by peculiarities in his social and intellectual. condition, 

 and illustrated by contemporaneous monuments, so, in the 

 history of the earth also, are distinguished [several great pe- 

 riods, which may be designated as the various Ages of Nature, 

 illustrated in like manner by their monuments, the fossil re- 

 mains, which, by certain general traits stamped upon them, 

 clearly indicate the eras to which they belong. 



658. We distinguish four Ages of Nature, correspond- 

 ing to the great geological divisions, namely : 



1st. The Primary or Paleozoic Age, comprising the lower 



* See an important communication, by Mr. Logan, on the Footprints of 

 Reptiles in the Potsdam sandstone of Lower Canada, Quart. Jour. GeoL 

 Soc. vol. vii. p. 247. ED. 



