168 On the Classification of Mammalia. 



Towards the end of that very century, however, compara- 

 tive anatomy started as a science ; and at the beginning of 

 the nineteenth, it introduced an entirely new method of 

 classification. Systematic zoology underwent a metamor- 

 phosis. 



The first half of the present century had not yet elapsed, 

 when another science grew up with rapid steps, claiming her 

 share in the question of the natural classification of the ani- 

 mal kingdom : we allude to embryology. The formation of 

 the young mammal, its genesis, its development prior to the 

 period when it makes its first appearance in the world, if not 

 entirely unveiled yet, are no longer mysterious, and their 

 bearing upon systematic zoology is universally felt. 



Palseontological data are not less important in arriving at 

 a natural classification, than those derived from either com- 

 parative anatomy or embryology ; and indeed palaeontology, 

 comparative anatomy, and embryology, hold an equal rank in 

 respect to zoology. 



As investigations progress in these fields of researches, 

 new light is daily thrown on some obscure points, and diffi- 

 cult questions are thus elucidated ; but as yet, no methodical 

 arrangement of the class of Mammalia has been universally 

 adopted: there is still as much diversity of opinion, and 

 perhaps even more at the present time than in the two past 

 centuries, although, as a whole, our views on the subject have 

 been improved upon those of our ancestors. 



II. In order to render more tangible our thoughts on the 

 subordination of the various groups which constitute the class 

 of Mammalia, we have prepared the accompanying plate, 

 which we shall now examine. 



The orders Edentata and Marsupialia are considered as 

 the trunks of the class : these two groups, we place on the 

 same level. They constitute the foundation, the bottom of 

 the class, and accordingly are the lowest of all. 



