GENERAL CONCLUSIONS. 



divisions of the animal kingdom, the Vertebrata, Annulata, and 

 the Mollusca. 



The Annulata of early geological epochs, which had pulmonary 

 respiration, belonged to the order of Arachnida. We find, in 

 the Carboniferous strata, one of the Palaeozoic group, the remains 

 of an Arachnid (281, fig. 143), already mentioned, closely allied 

 to the scorpion, which it is impossible to doubt had the same 

 organisation as scorpions of the present day, and must, therefore, 

 have lived under like conditions of animal existence. 



548. Considered in relation to their geological date, Vertebrata 

 breathing by true lungs are found during the Palaeozoic age in 

 the form of Saurians, the most perfect of reptiles. During the 

 Triassic age tortoises and birds appear, which of all animals have 

 the pulmonary system the most developed. It must, therefore, 

 be inferred, that at these remote epochs, the medium in which 

 birds and reptiles breathing the air by lungs lived, was little if 

 at all different from the medium in which similar classes now 

 live, which leads to the conclusion that, at this early period in the 

 history of the globe, the composition of the atmosphere must have 

 been nearly the same as at present. 



549. All these considerations lead to the following general con- 

 clusions : 



First. If the supposition of a gradually increasing perfection of 

 organisation were admitted, we ought to find all the animals 

 endowed with mere cutaneous respiration in the first stages of the 

 world, and the others, proceeding successively from age to age, 

 endowed with branchial, tracheal, and pulmonary respiration, 

 whereas, on the contrary, we find in the very first epoch of the 

 animalisation of the globe, all the modes of respiration manifested 

 at once a conclusion entirely at variance with the supposition of 

 progressively improving organisation. 



Secondly. Whether we compare together the increasing or de- 

 creasing development of zoological forms, or the dates of the 

 appearance of the orders of animals with the perfection of their 

 organs ; or take for the basis of our comparative researches 

 the physiological conclusion deduced from the mode of respiration 

 by animals, we uniformly arrive at the same negative results 

 relatively to the supposition of progressive improvement of animal 

 organisation. We must, therefore, accept it as proved, that no 

 such progressive improvement has existed. 



Thirdly. No appreciable modification being found in the organs 

 of respiration of animals from the most ancient epoch to the 

 present, a great number of genera having always existed with the 

 same characters since the first animalisation of the globe, it must 

 be inferred that the vital elements have not changed, and that 



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