INTRODUCTION xxxix 



surface of the humerus, or upper arm-bone, and their 

 purpose is to close the gap which would otherwise be 

 left between the wing and the body during flight — a 

 gap which, indeed, would make flight well-nigh, if not 

 quite, impossible in birds which, like the Albatross, 

 have a long humerus. Above the "quill" or "flight" 

 feathers — the primaries and secondaries — will be 

 found a number of rows of smaller feathers, which 

 can be divided into several distinct series. These are 

 the "coverts," and are known respectively as the 

 major, median, minor, and marginal coverts. Of the 

 first and second series — the major and median coverts 

 — there is but a single series; while the minor coverts 

 and marginal coverts number several rows each. 

 Some of these smaller coverts have, for clearness's 

 sake, been removed in the accompanying diagram. 

 The row marked t. m. is formed by the major coverts, 

 or tectrices ma j ores. 



The manner in which the feathers of the wing over- 

 lap one another is a matter of some importance to 

 those who are engaged in the work of classifying 

 birds, and in the mechanism of flight is even more im- 

 portant. If the wing of any bird be examined, it will 

 be found that the free edges of the "quill" feathers and 

 of the major coverts are turned outwards — 'towards 

 the tip of the wing; while more or fewer of the other 

 coverts have the free edges turned towards the body. 

 Consequently, when the wing is raised the wind forces 

 its way easily between the feathers, and so offers the 

 least possible resistance to the upstroke; but during 

 the downstroke the wing affords an unyielding sur- 

 face, and so forces the body upwards and forwards 



