The Scottish Naturalist. 89 



of the land seems absolutely covered with their vast hordes, not 

 even rivers or lakes checking their onward course, and few, if 

 any, ever returning to their native homes. A wonderful invasion 

 or uTuption appointed to serve some mysterious purpose of 

 nature ! Similar phenomena may be seen in insect life, the 

 locust, the butterfly, cockchafer, and many others, advancing in 

 vast hosts, some in the embryo state, others in that of the 

 fully developed insect, devastating whole districts in which they 

 may chance to appear. 



I shall not, however, dwell further on this subject, but revert 

 to migration as we see it so truly depicted and forcibly brought 

 before us in the study of ornithology ; not a season, scarcely a 

 month passes by, but what the most superficial of observers 

 may detect some change in the bird-life around him. The 

 anatomy and structure of the bird, so wonderfully adapted to 

 his requirements in the medium through which he travels, 

 enables him to traverse with comparative ease over regions 

 of the globe, with a speed not to be excelled by any other 

 creature. Every part of the frame is formed for lightness and 

 buoyancy. The bones are all hollow, and so formed as to assure 

 the maximum of strength combined with the minimum of 

 weight ; the body clothed with the softest plumage, the wings 

 of the lightest material, and yet endowed with strength to strike 

 with a force sufficient to impel a body of many pounds' weight 

 through the air with astonishing rapidity. The internal 

 structure also is wonderfully adapted for the same purpose ; 

 " the lungs placed close to the back bone and ribs, the air 

 entering in by a communication through the wind-pipe, (which 

 in many cases is elaborately formed) passes through and is 

 conveyed into a number of membraneous cells, which lie upon 

 the sides of the pericardium, and communicate with those of 

 the sternum, these cells continuing down the wings extend even 

 to the pinions, thigh bones, and other parts of the body, which 

 can be filled and distended at pleasure. The feathers even, 

 and especially those of the wings and tail, contain quantities of 

 air."t This wonderful difi"usion of air through the body in 

 birds, is not only essential to buoyancy and floating power, but 

 also to the supply of the extra heat necessary for the high 

 elevations and amount of speed to be attained ; birds being 

 endued, as it is well known, with a larger supply of caloric 



f Bewick ; Introduction, vol. I. 



