FLIGHT OF BIRDS. 41 



not more complicated in its organisation than the 

 slowest-paced of the mammalia. But the bird, when 

 its habit is to be much on the wing, is all-over adapted 

 for flight, and the system of its mechanics, if we could 

 fully comprehend it, would certainly be the most curi- 

 ous, and far from the least instructive, in the whole 

 of the animal kingdom. 



The buoyancy, as well as the upward motion, is not 

 very difficult to understand, because the wing, from its 

 general form, and the structure of the feathers, rises 

 with much less effort than it descends. Thus the 

 constant tendency of the powerfully- winged bird is to 

 mount upwards, and on this account the firmest 

 bird, that which with the same volume of body and 

 extent of wings, has the greatest specific gravity, is 

 the best flyer, flies more steadily, and apparently with 

 less effort. This must, of course, have a limit ; be- 

 cause, leaving the incapacity of breathing out of the 

 question, no bird could fly in a vacuum, and thus 

 there must be a certain density of air which is the 

 best adapted for the flight of any given species of 

 bird. This appears, even in the case of heavy birds, 

 to be considerably less than the density of the mean 

 level of the earth's surface. Eagles are heavy birds, 

 even for their powerful wings, and yet they are high 

 fliers, even when their abodes are at great elevations 

 in the mountains. All birds which take long flights 

 fly high, whatever may be their other habits. Wild 

 geese, herons, all birds indiscriminately "take the 

 sky" when they set out upon long journeys. In some 

 this may be in part done to avoid enemies or obstacles, 

 but the habit is too general for being accounted for 

 upon any principle save that the high flight is the 

 less fatiguing. Even rooks may be observed to ad- 

 just the height of their daily excursions from the 



