I TO FOUKTH REPORT 1831. 



cognise three primary groups into which the class Aves is divisi- 

 ble. To these he does not affix names, hut merely designates 

 them as the typical, the siibtypical, and the aberrant. His 

 secondary divisions, at least those adopted in the above work, 

 which are equivalent to the orders of other authors, are the 

 same as those of Mr. Vigors. In the details of the arrangement 

 the systems of these two authors are in many respects different. 

 The latest arrangement of this class with which I am acquainted 

 is that of C.L. Bonaparte, in his Saggio di una Distribuzione,hc. 

 He divides it into the two subclasses of Insessores and Gralla- 

 tores : the former containing the orders Accipitres and Passeres, 

 Cuv. ; the latter those of GalUncE, Grallce, and Anseres. 



The above are the principal authors who have treated of the 

 systematic arrangement of this class of late years. The general 

 leaning seems to be towards the adoption of the same orders as 

 those just mentioned*. The group which presents most diffi- 

 culties in the way of a natural classification is undoubtedly that 

 of Scansores, on the value of which naturalists are not agreed. 

 Latreille considers it as forming a parallel order to that of the 

 Passeres. It will probably, however, be allowed ultimately to be 

 only a subordinate group in this last order, as is already the 

 opinion of Vigors, Lesson, and others. In the details of the 

 system there is still much inicertainty, though more in some 

 groups than others. And this uncertainty can only be cleared 

 up by a more rigorous analysis of external characters, combined 

 with anatomical investigation. This last has already been suc- 

 cessfully resorted to in some families, for the detei-mination of 

 true affinities. Thus, Mr. Yarrell, by studying the internal struc- 

 ture of the Anatkke, has sketched out an arrangement of this 

 group t, which Mr. Swainson finds in accordance with his own 

 views on the subject J derived from the external characters and 

 habits. The same gentleman has recoi'ded some important notes§ 

 respecting the internal organization of Cereopsis and some allied 

 species, serving in like manner to confirm the notions previously 

 entertained respecting the affinities of these birds. There can 

 be no doubt also that we may derive much assistance from study- 

 ing the systems of those authors who, like Blainville and 

 L'Herminier, have taken some one of the internal organs as the 

 basis of their arrangement. For however it may be true that 

 no such arrangement can be natural in itself, founded upon "cha- 

 racters derived from one organ exclusively, yet it affords an in- 



* I speak of the groups themselves without reference to any particular names 

 for them. 



t Linn. Trans., vol. xv. p. 378. J Fn. Bor. Am., vol. ii. p. 430. 



§ Proceed, of Zool. Soc. 1831, p. 25. 



