Correspondence on Biology, etc. 



rigid anterior margin, the slender and flexible hind margin ; 

 the greater resistance to upward than to downward pressure, 

 and the slight concavity of the under surface, are all char- 

 acters common to the wings of birds and most insects, and, 

 considering the totally different structure and homologies of 

 the two, I think there is at least an a priori case for the func- 

 tion they both subserve being dependent upon these peculiari- 

 ties. If I remember rightly, it is on these principles that the 

 Duke of Argyll has explained the flight of birds, in which, 

 however, there are of course some specialities depending 

 on the more perfect organisation of the wing, its greater 

 mobility and flexibility, its capacity for enlargement and 

 contraction, and the peculiar construction and arrangement 

 of the feathers. These, however, are matters of detail ; and 

 there are no doubt many and important differences of detail 

 in the mode of flight of the different types of insects which 

 would require a special study of each. It appeared to me 

 that the Duke of Argyll had given that special study to the 

 flight of birds, and deserved praise for having done so suc- 

 cessfully, although he may not have quite solved the whole 

 problem, or have stated quite accurately the comparative 

 importance of the various causes that combine to effect flight. 

 —Believe me yours very sincerely, Alfred R. Wallace. 



Herbert Spencer to A. R. Wallace 

 57 Queen's Gardens, Bayswater, W. December 5, 1867. 

 My dear Mr. Wallace, — I did not answer your last letter, 

 being busy in getting out my second edition of '^ First 

 Principles." 



I was quite aware of the alleged additional cause of flight 

 which you name, and do not doubt that it is an aid. But I 

 regard it simply as an aid. If you will move an outstretched 

 wing backwards and forwards with equal velocity, I think 

 you will find that the difference of resistance is nothing like 



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