. "Descent of Man"
(1901), page 623.) Are there everywhere many unpaired birds? What can
the explanation be?

Mr. Gould assures me that all the nightingales which first come over are
males, and he believes this is so with other migratory birds. But this
does not agree with what the bird-catchers say about the common linnet,
which I suppose migrates within the limits of England.

Many thanks for very curious case of Pavo nigripennis. (438/2. See
"Animals and Plants," Edition II., Volume I., page 306.) I am very glad
to get additional evidence. I have sent your fact to be inserted, if
not too late, in four foreign editions which are now printing. I am
delighted to hear that you approve of my book; I thought every mortal
man would find the details very tedious, and have often repented of
giving so many. You will find pangenesis stiff reading, and I fear will
shake your head in disapproval. Wallace sticks up for the great god Pan
like a man.

The fertility of hybrid canaries would be a fine subject for careful
investigation.

LETTER 439. TO J. JENNER WEIR. Down, April 4th [1868].

I read over your last ten (!) letters this morning, and made an index
of their contents for easy reference; and what a mine of wealth you
have bestowed on me. I am glad you will publish yourself on gay-coloured
caterpillars and birds (439/1. See "Descent of Man," Edition I., Volume
I., page 417, where Mr. Weir's experiments are given; they were made to
test Mr. Wallace's theory that caterpillars, which are protected against
birds by an unpleasant taste, have been rendered conspicuous, so that
they are easily recognised. They thus escape being pecked or tasted,
which to soft-skinned animals would be as fatal as being devoured. See
Mr. Jenner Weir's papers, "Transact. Entomolog. Soc." 1869, page 2;
1870, page 337. In regard to one of these papers Mr. Darwin wrote (May
13th, 1869): "Your verification of Wallace's suggestion seems to me
to amount to quite a discovery."); it seems to me much the best plan;
therefore, I will not forward your letter to Mr. Wallace. I was much
in the Zoological Gardens during my month in London, and picked up
what scraps of knowledge I could. Without my having mentioned your most
interesting observations on the display of the Fringillidae (439/2.
"Descent of Man" (1901), page 738.), Mr. Bartlett told me how the Gold
Pheasant erects his collar and turns from side to side, displaying it
to the hen. He has offered to