during the Formation of the Crust of the Earth. 183 



It exhibits in detail and in a decisive maimer the importance of 

 the appearance of the angiospermous Dicotyledons as a condi- 

 tion of existence for the whole terrestrial fauna. Lastly, it indi- 

 cates the importance of the synchronic relations which existed 

 between the ascertained depressions of the soil, combined with 

 the emanation of a great quantity of carbonic acid eliminated 

 immediately by the formation of coal, and the existence of the 

 singular forests of Stigmaria, connected with a vegetation com- 

 posed only of vascular Cryptogamia and gymnospermous Dico- 

 tyledons, to the exclusion of the Angiospermia*. These peculiar 

 conditions of vegetation seem to have made their appearance 

 again, although with a very local development, in the course of 

 the Jurassic period. "We arc convinced that the office of these 

 forests was to maintain the atmosphere in a respirable state at a 

 period when carbonic acid was emitted in greater abundance 

 than at the present day, and even to render it more suitable for 

 respiration, although positive proofs of this are wanting. An 

 abundant vertebrate fauna with an active respiration would in 

 course of time have acted injuriously in the opposite direction. 

 If this opinion should be confirmed, the fact of the progressive 

 development of the vegetable kingdom would enter, at least 

 partially, into dependence upon the law of adaptation of the 

 successive creations to the external conditions of existence. Ey 

 this the unity of the laws and phenomena could not but gain. 



The results at which we have arrived rest upon the actual 

 state of our knowledge of the fossil world. New discoveries 

 may therefore at any time introduce modifications of them. 

 Nevertheless the general laws which we have established repose 

 upon too numerous facts to allow any exceptions which may 

 hereafter be discovered to suffice for their complete overthrow. 

 Although, in the creation of organized beings, nature may have 

 followed the course which we have indicated, we cannot, how- 

 ever, but suppose that some exception, some deviation from the 

 rule may have taken place in consequence of causes unknown to 

 us. The pheenomena which occupy us here are by no means of 

 such a nature that we may deduce them from a fundamental 

 law with the same certainty that we can deduce the fall of a 

 body or the orbit of a planet from the law of universal attrac- 

 tion. The causes which preside over these phsenomena are too 

 manifold and too dissimilar to allow us to calculate the result 

 with certainty a priori. But even if we supposed that a perfectly 

 strict law was at the basis of all these phsenomeua, our know- 

 ledge of the remains enclosed in the strata of the earth's crust 

 can never be otherwise than fragmentary. We can never be 

 sure that certain facts do not escape us, the revelation of which 

 * See previous note at p. 177. — Ed. Anvals. 



