7:; 



LECTURE V. 



ON THE CLASSIFICATION OF ANIMALS. 



ON THE ARRANGEMENT OF THE CLASSES INTO LARGER 



GROUPS. 



Having now arrived at the end of the list of classes, and having 

 endeavoured to furnish you with a statement of the structural 

 features common to, and characteristic of, each class, it will be 

 my next object to discuss the relations of these classes one to 

 another, and to inquire how far they present such common 

 characters as will enable us to group them into larger divisions. 



And, to commence with the highest classes, it is clear that 

 the Mammalia, Aves, and Reptilia are united together by certain 

 very striking features of their development. All possess an 

 amnion and an allantois, and are devoid, throughout life, of any 

 apparatus for breathing the air which is dissolved in water. In 

 other words, they constitute what has been termed the "pro- 

 vince " of Abranchiate Vertebrata, in contradistinction to 

 Pisces and Amphibia, which possess no amnion, nor allantois 

 (or at most a rudimentary one), and, being always provided at a 

 certain period, if not throughout life, with branchiae, have been 

 called Branchiate Yertebbata. 



The abranchiate, however, form a far less homogeneous as- 

 semblage than the branchiate Vertebrata — Mammals being so 

 stroDgly separated from Reptiles and Birds that I am disposed 

 to regard them as constituting one of three primary divisions, or 

 provinces, of the Vertebrata. The structure of the occipital 

 condyles, the structure and mode of articulation of the mandi- 



