pair later in the season? With respect to the mysterious cases of which
you have given me so many, in addition to those previously collected, of
when one bird of a pair is shot another immediately supplying its place, I
was drawing to the conclusion that there must be in each district several
unpaired birds; yet this seems very improbable. You allude, also, to the
unknown causes which keep down the numbers of birds; and often and often
have I marvelled over this subject with respect to many animals.


LETTER 440. TO A.R. WALLACE.

(440/1. The following refers to Mr. Wallace's article "A Theory of Birds'
Nests," in Andrew Murray's "Journal of Travel," Volume I., page 73. He
here treats in fuller detail the view already published in the "Westminster
Review," July 1867, page 38. The rule which Mr. Wallace believes, with
very few exceptions, to hold good is, "that when both sexes are of
strikingly gay and conspicuous colours, the nest is...such as to conceal
the sitting bird; while, whenever there is a striking contrast of colours,
the male being gay and conspicuous, the female dull and obscure, the nest
is open, and the sitting bird exposed to view." At this time Mr. Wallace
allowed considerably more influence to sexual selection (in combination
with the need of protection) than in his later writings. The following
extract from a letter from Mr. Wallace to Darwin (July 23rd, 1877) fixes
the period at which the change in his views occurred: "I am almost afraid
to tell you that in going over the subject of the colours of animals, etc.,
etc., for a small volume of essays, etc., I am preparing, I have come to
conclusions directly opposed to voluntary sexual selection, and believe
that I can explain (in a general way) all the phenomena of sexual ornaments
and colours by laws of development aided by simple 'Natural Selection.'"
He finally rejected Mr. Darwin's theory that colours "have been developed