GENERAL HABITS 187 



incapable of flight, for functional wings are very 

 essential to the existence of most species. On 

 the other hand, there are large groups of highly 

 specialised birds that are unable to progress 

 along the ground with their legs and feet. We 

 may instance the Humming-Birds and the 

 Swifts, numbering together many hundreds of 

 species, also the Swallows, although the latter 

 birds are perhaps not quite so helpless in this 

 respect. Humming-Birds and Swifts never nor- 

 mally visit the ground ; Swallows often do so, 

 however, but they rarely attempt to make any 

 progress there, and never unless aided by their 

 wings. As we know, these birds find an abund- 

 ant compensation for their loss of terrestrial 

 motion in their remarkable volant powers. They 

 may be said to pass most of their existence in 

 the air, where legs would be a superfluity. Their 

 anterior limbs have developed at the expense 

 of their posterior ones. Then again we find 

 many species of birds that walk with difficulty 

 or with a ludicrous amount of awkwardness. 

 Among these we may mention such species as 

 Divers, Auks, Grebes, Penguins, Frigate Birds, 

 Pigeons, and Ducks. Some of these possess 

 great powers of wing (Frigate Birds), but the 

 majority, although very capable fliers, find their 



