CIIAKACTErtS OF IliKMATOTIIERMA. 7 



by means of the mouth, which consists of two lipless and tooth- 

 less jaws, sheathed with horn. To facilitate the prehensile and 

 other actions thus transferred to the head, the neck is elono-ated, 

 and the body generally inclined forward and downward from the 

 hip-joints. The thighs are accordingly extended forward at an 

 acute angle from the pehis toward the centre of the trunk, and 

 the toes are lengthened and spread out to form an adequate base 

 of support. The actions of perching, walking, hopping, running, 

 scratching, burrowing, wading, and swimming, require for their 

 perfect performance different modifications of the posterior extre- 

 mities. The mandibles, again, present as many varieties of form, 

 each corresponding to the nature of the food, and in some degree 

 indicative of the organisation necessary for its due assimilation. 

 Ornithologists have, therefore, founded their divisions of the class 

 chiefly on the modifications of the bill and feet. Since, however. 

 Birds in general are associated together by characters so peculiar, 

 definite, and unvarying, it becomes in consequence more difficult 

 to separate them into subordinate groups, and these are neces- 

 sarily more arbitrary and artificial than are those of the other 

 vertebrate classes. 



A binary division of the class ^ may be founded on the condition 

 of the newly-hatched young, which in some orders are able to 

 run about and provide food for themselves the moment they quit 

 the shell (Aves prcecoces) ; while in others the young are excluded 

 feeble, naked, blind, and dependent on their parents for support 

 (^Aves altrices). 



Nitzsch^ grouped together the feathered tribes under three 

 series, according to the great divisions of the terraqueous globe 

 which form respectively the principal theatres of their actions. 

 The first order consists of the birds of the air, Aves aerecB (Luft- 

 vogeln) ; the second embraces the birds of the land, Aves terrestres 

 (Erdvogeln); the third includes the birds which frequent the 

 waters, Aves aquaticcB (Wasser-vogeln). The eagle and lark 

 exemplify the first ; the ostrich and common foAvl the second ; 

 the heron and the gull the third, of these extensive divisions of 

 the class. 



Vigors ^ proposed a more definite system upon a similar prin- 

 ciple, distributing Birds into five orders. The first includes 

 those which soar in the upper regions of the air, which build their 

 nests and rear their young on high cliffs or lofty trees ; they are 

 the chief of aerial birds and form the order termed Raptorcs, 



' vir. p. 2G5. 2 yj,i._ s ^•^. 



