

PROGRESSION OF ORGANIZATION . 385 



refined, on the progressive development of animals. 

 That the changes of the inorganic world have impressed 

 new conditions on the organic structures of animals, to 

 meet the necessities of their being, must be admitted. 

 Comparative anatomy has demonstrated that such sup- 

 posed differences really existed between the creatures oi 

 secondary formations those of the tertiary and the 

 present periods. It has been imagined, but upon de- 

 batable foundations, that the atmosphere, during the 

 secondary periods, was highly charged with carbonic 

 acid ; and, consequently, that though beneficial to the 

 growth of plants, and peculiarly fitted for the conditions 

 required by those which the fossil flora makes us ac- 

 quainted with, it was not adapted to support any animals 

 above the slow-breathing, cold-blooded fishes and reptiles. 

 Under the action of the super-luxuriant vegetation of 

 these periods, this carbonic acid is supposed to have been 

 removed, an addition of oxygen furnished ; and thus, 

 consequently, the earth gradually fitted for the abode of 

 warm-blooded and quick-breathing creatures. We do, 

 indeed, find a very marked line between the fossil 

 remains of the lias formations which enclose the 

 saurians, and the wealden, in which birds make their 

 appearance more numerously than in any previous for- 

 mation. 



Founded upon these facts, speculations have been put 

 forth on the gradual development of animals from the 

 lowest up to the highest orders. Between the polype 

 and man a continuous series has been imagined, every 

 link of the chain being traced into connection with the 

 one immediately succeeding it ; and, through all the 

 divisions, zoophytes, fishes, amphibia, reptiles, birds, and 

 mammalia are seen, according to this hypothesis, to be 

 derived by gradual advancement from the preceding 

 orders. The first having given rise to amphibia, the 

 amphibion gives birth to the reptile, the reptile ad- 

 vances to the bird, and from this class is developed the 



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