ARRANGEMENT AND CLASSIFICATION 67 



The above represents the true arrangement of animals, and also 

 the classes established among them. 



We now have to examine a very important problem, which appears 

 never to have been fathomed nor discussed ; but the solution of which 

 is necessary ; it is this : 



All the classes, into which the animal kingdom is divided, necessarily 

 form a series of groups arranged according to the increasing or decreas- 

 ing complexity of their organisation. In drawing up this series, ought 

 we to proceed from the most complex to the simplest, or from the 

 simplest to the most complex ? 



We shall endeavour to give the solution of this problem in Chap. 

 VIII. which concludes this part ; but we must first examine a very 

 remarkable fact, most worthy of our attention, which may lead us 

 to a perception of nature's procedure, when bringing her diverse 

 productions into existence. I refer to that remarkable degradation of 

 organisation which is found on traversing the natural series of animals, 

 starting from the most perfect or the most complex towards the simplest 

 and most imperfect. 



Although this degradation neither is nor can be finely graduated as 

 I shall show, it so obviously and universally exists in the main groups, 

 including even the variations, that it doubtless depends on some 

 general law which it behoves us to discover and consequently to 

 search for. 



