ers 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 function they both subserve being dependent upon
these peculiarities. 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 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
expressions seem to have given you. The objections are not made by
Tyndall or Huxley; but they are objections made by me, which I stated to
them, and in which they agreed--Tyndall expressing the opinion that I
ought to make them public. I name this because you may otherwise some
day startle Tynda