238 GENERAL ANALOGIES. 



upon their principal habit than any which can be 

 founded on a particular part of their organisation. 

 Air birds, ground birds, and those intermediate races 

 which have the double motion in the case of land 

 birds, and the triple one in the case of sea birds, 

 might form the leading divisions. 



An air bird is one which uses the wing in immedi- 

 ately obtaining its food ; thus an eagle which stoops 

 to ground prey, a falcon which captures in the air 

 but eats on the ground, a gannet which plunges in the 

 water, and a swift or other insect feeder which feeds 

 on the wing without stopping, are all equally air birds, 

 arriving at their food by the action of the wings in 

 that medium. So also a bird which immediately finds 

 its food with the wings closed, or only with a partial 

 use of them, subordinate to that of the feet, as in the 

 birds which chase their prey under water, those that 

 perch upon flexible stems and twigs, and a few others, 

 is a ground bird. No matter whether it walks the 

 bare earth, the vegetable surface, the boles or branches 

 of trees, or perches, or climbs, or swims, or dives, if 

 aerial motion does not form part of the act of capture, 

 it is not an air bird. Many ground birds range far 

 on the wing in search of places where they may feed 

 or nestle ; but the distinction between that and feed- 

 ing on the wing, that is, arriving directly at the prey 

 from the air only, is obvious enough. If the bird 

 moves anew from any support but that of the air, 

 that element has, as it were, rendered it up ; and be 

 the support rock, earth, plant, or water, there is an 

 unbroken connexion with the ground. 



The distinction between omnivorous birds and 

 either of these divisions is not so clear. But this is 

 what might be expected : the air and the ground are 



