378 A HISTORY OF BIRDS 



the arm (humerus) has attained a relatively enormous length 

 while the hand is unusually short : in the latter these relative 

 lengths are reversed. Yet the form of the wings in the living 

 birds closely resemble one another, both having a very extra- 

 ordinary ribbon-like contour. Although much has been written 

 on the flight of the Albatross, and though the remarkable 

 powers of the wing which the Swift possesses are familiar 

 enough to us all, no close comparison or analysis of the mechanics 

 of the flight of these two birds has yet been made. Till this 

 is done the differences of the form of these wings must remain 

 in the province of curious facts requiring explanation. 



In some species at least, the several segments of the wing 

 vary in their relative lengths at different stages of development, 

 a fact which has already been demonstrated in describing the 

 wings of the nestling Hoatzin and Common Fowl. These varia- 

 tions are undoubted adaptations, fulfilling different purposes 

 during the phases of growth therein described. 



The innumerable differences discernible in the wings of 

 different groups of birds in the matter of the relative lengths of 

 the several segments, the development of crests for muscle attach- 

 ment, and so on, are all instances of adaptation, but too intricate 

 for profitable discussion. 



A brief reference may, however, be made to the difl"erence 

 in the contour of the wing as a whole. In birds which spend 

 much, if not most, of their time on the wing, this organ when 

 extended presents the form of a long, narrow, pointed blade, as 

 in the Swift, Albatross and Frigate-bird. In those which fly but 

 little the contour is that of a broad, much rounded, almost fan- 

 shaped plate. That this shape is correlated with the nature of 

 the flight is demonstrated in one or two striking cases. In the 

 Tawny Owl or Wood Owl, for example, the wing is of the short 

 rounded type ; in the Barn Owl, on the other hand, a bird which 

 forages in the open, the wing is long. In the diurnal birds of 

 prey similar relations are found. One might multiply instances 

 by the score. Yet, in spite of this, systematists have unhesitat- 

 ingly used the form of the wing as a factor in classification ! 

 The celebrated order Timcliidce is founded upon this character 

 alone. " Wings short and rounded, fitting close to the body ! " 

 No wonder that this " Order " contains a chaotic medley of 

 forms, 



