but no such phenomena
as in the Papilionidae.--With best wishes, believe me, my dear Darwin,
yours very faithfully,

ALFRED R. WALLACE.

* * * * *

_6 Queen Anne Street, W. Monday, January, 1867._

My dear Wallace,--I return by this post the _Journal_.[56] Your resume of
glacier action seems to me very good, and has interested my brother
much, and as the subject is new to him he is a better judge. That is
quite a new and perplexing point which you specify about the freshwater
fishes during the glacial period.

I have also been very glad to see the article on Lyell, which seems to
me to be done by some good man.

I forgot to say when with you--but I then indeed did not know so much as
I do now--that the sexual, i.e. _ornamental_, differences in fishes,
which differences are sometimes very great, offer a difficulty in the
wide extension of the view that the female is not brightly coloured on
account of the danger which she would incur in the propagation of the
species.

I very much enjoyed my long conversation with you; and to-day we return
home, and I to my horrid dull work of correcting proof-sheets.--Believe
me, my dear Wallace, yours very sincerely,

CHARLES DARWIN.

P.S.--I had arranged to go and see your collection on Saturday evening,
but my head suddenly failed after luncheon, and I was forced to lie down
all the rest of the day.

* * * * *

_Down, Bromley, Kent, S.E. February 23, 1867._

Dear Wallace,--I much regretted that I was unable to call on you, but
after Monday I was unable even to leave the house. On Monday evening I
called on Bates and put a difficulty before him, which he could not
answer, and, as on some former similar occasion, his first suggestion
was, "You had better ask Wallace." My difficulty is, why are
caterpillars sometimes so beautifully and artistically coloured? Seeing
that many are coloured to escape danger, I can hardly attribute their
bright colour in other cases to mere physical conditions. Bates says the
most gaudy caterpillar he ever saw in Amazonia (of a Sphinx) was
conspicuous at the distance of yards from its black and red colouring
whilst feeding on large green leaves. If anyone objected to male
butterflies having been made beautiful by sexual selection, and asked
why should they not have been made beautiful as well as their
caterpillars, what would you answer? I could not answer, but should
maintain m