84 THE BOOK OF NATURE STUDY 



Manx-shearwater, may be taken as examples, are commonly 

 supposed to be related to the gulls. This is not so, however ; 

 for the gulls are closely related to the plovers, of which the green 

 plover or lapwing may be taken as an example. Externally the 

 petrel may always be distinguished by the fact that the nostrils 

 open in the form of a pair of tubes, while in the gulls they are 

 represented by long slits in the middle of the beak. These super- 

 ficial resemblances, then, are further instances of " adaptation " 

 to which reference has already been made (p. 79). 



On Flight. This chapter must needs contain a brief summary 

 of the more important facts concerning flight, at least in so far as 

 its simpler problems are concerned. 



But to appreciate the nature of the flight of birds a clear 

 understanding of the main structural characters of the wing is 

 necessary. This is a compound organ consisting of (i) a bony, 

 jointed rod the " skeleton " of the wing ; and (2) the membranes 

 supported thereby. 



This skeleton has already been described, and the " mem- 

 branes " which it supports have also been described, in so far 

 as their general structural characters are concerned, for these 

 membranes are formed by feathers, albeit specially modified 

 feathers. 



We say " specially modified " feathers advisedly, because, 

 in the matter of their arrangement, their length, shape and 

 size, and in their great strength, they show that they have been 

 moulded, or modified, to serve the mechanical requirements of 

 flight. 



Let us examine their feathers then, both in their relation to the 

 bony, jointed rod which supports them, and to one another. 



In Fig. 23 (p. 74) it will be noted that these feathers are dis- 

 tributed along the fore-arm and hand, and that while those of the 

 fore-arm are separated by fairly wide spaces, on the hand they 

 are closely crowded together at least at their attachment to the 

 skeleton. Then it will be noted that they overlap one another, 

 and this so that the outer edge of each overlaps the inner border 

 of the feather next in front, and this, as will be shown presently, 

 is a most important point. Next, as to number and shape. In the 

 first particular there is a wide range ; the smallest wings having as 



