78 



GENERAL CONSIDERATIONS ON THE 



as several types formerly wanting to a completion of the 

 series have been found in a fossil state.* 



It is scarcely less evident, from the geological record, 

 that the progress of organic life has observed some corres- 

 pondence with the progress of physical conditions on the 

 surface. We do not know for certain that the sea, at the 

 time when it supported radiated, molluscous, and articu- 

 lated families, was incapable of supporting fishes; but 

 causes for such a limitation are far from inconceivable. 

 The huge saurians appear to have been precisely adapted 

 to the low muddy coasts and sea margins of the time when 

 they flourished. Marsupials appear at the time when the 

 surface was generally in that flat, imperfectly variegated 

 state in which we find Australia, the region where they 

 now live in the greatest abundance, and one which has 

 no higher native mammalian type. Finally, it was not 

 till the land and sea had come into their present relations, 

 and the former, in its principal continents had acquired 

 the irregularity of surface necessary for man, that man 

 appeared. We have likewise seen reason for supposing 

 that land animals could not have lived before the carbon- 

 igenous era, owing to the great change of carbonic acid 

 gas, presumed to have been contained in the atmosphere 

 down to that time. The surplus of this having gone, as 

 M. Brogniart suggests, to form the vegetation, whose 

 ruins became coal, and the air being thus brought to its 

 present state, land animals immediately appeared. So, 

 also, sea-plants were at first the only specimens of vege- 

 tation, because there appears to have been no place where 

 other plants could be produced or supported. Land ve- 

 getation followed, at first simple, afterward complex, pro- 

 bably in conformity with an advance of the conditions 

 required by the higher class of plants. In short, we see 

 everywhere throughout the geological history, strong 

 traces of a parallel advance of the physical conditions and 

 the organic forms. 



In examining the fossils of the lower marine creation, 

 with a reference to the kind of rock in connexion with 

 which they are found, it is observed that some strata are 

 attended by a much greater abundance of both species and 

 individuals than others. They abound most in calcareous 



* Intervals in the series were numerous in the department of thf 

 |»achydermata ; many of these gaps are now filled up from the ex 

 tinct genera found in the tertiary formation. 



