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Alfred Russel Wallace: Letters and Reminiscences, Vol. 2 by James Marchant
page 33 of 414 (07%)
of Argyll had given that special study to the flight of birds, and
deserved praise for having done so successfully, 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 commensurate with the difference in size between the
muscles that raise the wings and the muscles that depress them. It seems
to me quite out of the question that the principles of flight are
fundamentally different in a bat and a bird, which they must be if the
Duke of Argyll's interpretation is correct. I write, however, not so
much to reply to your argument as to correct a misapprehension which my
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