1 91 8.] Under Wing-Coverts of Birds. 579 



of coverts. The function of coverts to the remiges is to 

 close the insterstices between the quills and prevent the air 

 from passing through, so causing leakage and loss of force 

 in the wing-stroke in flying. This is accomplished more 

 perfectly by two continuous rows of feathers overlapped in 

 the opposite ways, than by two overlapped in the same way, 

 since any accidental opening or separation, allowing air to 

 pass, between two feathers in the one row, would tend 

 to be continued in the same direction in the other row, 

 were it overlapped in the same way ; but would tend to be 

 stopped in the otlier row, were it overlapped in the contrary 

 way. Thus it is essential to the best performance of their 

 office that the feathers of these two rows be overlapped in 

 opposite ways ; accordingly such a condition is found in all 

 birds that make constant use of their wings in flight. In the 

 Rails, both rows have the contrary overlap and are almost 

 mixed together in one row ; in the Fowls, the small and 

 almost functionless median under coverts were often found 

 to have the conforming overlap : but birds of these groups 

 fly comparatively little. 



The next step or modification, leading further away from 

 the primitive type, is the assumption of the contrary overlap 

 by some of the major under coverts — generally those on the 

 proximal part of the hand. This is very common among 

 birds of many, or most, orders, and in some orders it is 

 universal. It takes place, it will be noticed, on the central 

 part of the wing where the resistance of the air in the wing- 

 stroke is great, and at or near the place where the median 

 coverts cease. Thus it is evidently to be accounted for 

 in much the same way as is the contrary overlap of the 

 median coverts, by the greater efficiency as coverts of 

 feathers with the overlap in the opposite way to the remiges. 

 That the contrary overlap in the major under coverts is not 

 carried on out to the tip of the wing may be accounted for 

 by the fact that on the narrow part of the wing near the tip 

 the feathers are crowded so that the under coverts become 

 sandwiched in between the remiges, and must necessarily 

 have the conforming overlap. Thus the overlap of the 



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