110 PICTURESQUE SKETCHES 



of that law of universal dissolution which appears to 

 influence species, as well as individuals, allotting to 

 each its appointed time, and causing each to pass 

 through the different phases of imperfect development, 

 full growth and vigour, and then gradual but certain 

 decay and death. 



Little as the transition from the rocks of the older 

 to those of the secondary period is marked by mineral 

 changes in the strata, this total difference in the nature 

 of the organic remains is far too important to be 

 passed by without notice. Both the upper part of 

 the magnesian limestone series, and the strata that 

 are superimposed, consist of sands often loosely aggre- 

 gated, but sometimes hardened into stone by the infil- 

 tration of oxide of iron or some other cementing 

 medium. Neither of these beds is prolific in fossils, 

 but each contains a few; and this is the case as 

 well in England as on the Continent, where the 

 development is much more remarkable, and where the 

 beds contain many more fossils. The difference be- 

 tween the organic remains of the two beds is, however, 

 total, and in fishes is carried even into a point of struc- 

 ture which seems to be connected, though in an obscure 

 way, with the whole organization of the class.* The 

 vegetation, too, of the newer period is distinct ; and 

 the introduction of reptilian animals in great abund- 



* In the fishes met with in the older rocks the vertebral column is in- 

 variably continued to the extremity of the tail (fig. 42) ; and the upper lobe 

 of the tail-fin, into which the back-bone extends, is larger than the other. 

 In the rocks of the secondary period, the vertebral column does not extend 

 into the tail, but the tail-fin is generally unsymmetrical, the upper lobe 

 being the largest. In more modern fishes the tail-fin is perfectly symme- 

 trical in every respect (fig. 43). 



