PROGRESSIVE AMMALISATION". 



orders of Vertebrata appeared. The orders of birds which, 

 appeared at this most recent epoch were not more perfect in their 

 organisation than the waders of the Triassic age, while the orders 

 of reptiles, including the serpents and Amphibia, were certainly the 

 lowest in organisation of this class. 



539. It appears, therefore, that of the four classes of Vertebrata, 

 which appeared from the earliest geological age to the present 

 time, two, the reptiles and fishes, instead of proceeding according 

 to the supposition of progressively improving organisation, 

 showed a series of changes of a directly opposite character, the 

 most perfect being the earliest, and the least perfect the most 

 recent ; while the class of birds, with reference to their organi- 

 sation only, and without reference to their number, remained 

 stationary from the Triassic to the actual period. 



540. A question of high philosophical interest arises in reference 

 to the progressive animalisation of the globe, which admits of solu- 

 tion by a due examination of the organic remains deposited in 

 the strata of the earth. This question is that of determining if 

 the various organs of the most ancient animals have remained the 

 same from the commencement of the world, or if they have been 

 modified in consequence of the changes which may have taken 

 place in the external conditions of their existence. 



541. Of all the organs, those of respiration are most intimately 

 connected with this question. The several modes of animal 

 respiration are as follows : 



1. Cutaneous respiration, made by the whole surface of the body, 



and not by any special organ appropriated to that function. 



2. Aquatic respiration, made by means of branchia, or gills ; a 



special organ adapted to disengage the oxygen of the air 

 from the water and appropriate it to the vital functions. 



3. Tracheal respiration, with which animals are endowed which 



live in the atmosphere, and appropriate its oxygen to the vital 

 functions by means of trachea). 



4. Pulmonary respiration, which is performed by lungs, as in the 



case of Mammalia generally. 



Marine animals respire either without any special organ and 

 by means of the whole surface of their bodies, or by branchia 

 (gills). 



542. The animals which respire without any special organ, are 

 those of the lowest degree of organisation. Of these all the four 

 classes of Eehinoderinata, Polyparia, Foraminifera, and Amor- 

 phozoa, appeared on the earth in the Palaeozoic age in various 

 degrees of perfection. We have therefore here, in the first epoch 

 of creation, all the forms of cutaneous respiration. 



543. The animals which have aquatic respiration by means of 



149 



