14 A HISTORY OF BIRDS 



fairly well spaced on the former, crowded together at their 

 bases on the latter (111. 5). The quills of the fore-arm, or 

 " secondaries," vary in number according to its length, never 

 falling below six or exceeding thirty-seven . But the quills of the 

 hand— the " primaries "—never exceed twelve in number among 

 Neognathine birds, and never fall below ten, though the tenth 

 may be reduced to a mere vestige, known as a " remicle". 



The bases of these " quills," and the surface of the arm 

 generally, are covered by feathers arranged in definite order. 

 These are known as the coverts, whereof four distinct groups 

 are distinguished. The first of these are the major coverts. 

 These are always large and strong feathers, and are closely 

 attached to the bases of the quill feathers ; the next row are 

 known as the median coverts ; beyond these run from one to five 

 -rows of minor coverts, while beyond these come the marginal 

 coverts. The same rows are represented on the under surface 

 of the wing. 



Since these coverts present characteristic features in each of 

 the great groups of birds, they are useful for taxonomic pur- 

 poses ; and this is more especially true of the manner in which 

 those of the upper surface overlap one another. Thus, the 

 major coverts always lie with their free edges turned towards 

 the tip of the outstretched wing, and so also, very commonly, 

 do the median series. The overlap is then said to be " distal ". 

 But the outermost median and minor coverts frequently have 

 an opposite, or proximal overlap, that is to say, the free edges 

 ""of the feathers face towards the body. The marginal coverts 

 always overlap distally. But the details of this matter should 

 be sought for in the special treatises on this subject rather than 

 in these pages. 



While it seems impossible to discover any significance in 

 the matter of this overlap in the smaller coverts, this is by no 

 means the case with the great flight feathers and their coverts. 

 With a distal overlap, as the broad surface of the wing is raised 

 during the up-stroke in flight, the resistance of the air is reduced 

 to a minimum, since it forces down the inner webs of these 

 feathers and so escapes ; but during the down-stroke, this re- 

 sistance forces these inner webs upwards, driving one against 

 another, and so forming a continuous, unyielding surface, as 

 perfect as that afforded by the membrane of the wing of the bat. 



