anding his conviction
that the hypothesis was sound, Darwin was quite aware that it was
probably the most vulnerable part of the "Origin". Thus he wrote to H.W.
Bates, April 4, 1861: "If I had to cut up myself in a review I would
have (worried?) and quizzed sexual selection; therefore, though I am
fully convinced that it is largely true, you may imagine how pleased I
am at what you say on your belief." ("More Letters", I. page 183.)

The existence of sound-producing organs in the males of insects was,
Darwin considered, the strongest evidence in favour of the operation
of sexual selection in this group. ("Life and Letters", III. pages 94,
138.) Such a conclusion has received strong support in recent years by
the numerous careful observations of Dr F.A. Dixey ("Proc. Ent. Soc.
Lond." 1904, page lvi; 1905, pages xxxvii, liv; 1906, page ii.) and Dr
G.B. Longstaff ("Proc. Ent. Soc. Lond." 1905, page xxxv; "Trans. Ent.
Soc. Lond." 1905, page 136; 1908, page 607.) on the scents of male
butterflies. The experience of these naturalists abundantly confirms and
extends the account given by Fritz Muller ("Jen. Zeit." Vol. XI. 1877,
page 99; "Trans. Ent. Soc. Lond." 1878, page 211.) of the scents
of certain Brazilian butterflies. It is a remarkable fact that the
apparently epigamic scents of male butterflies should be pleasing to
man while the apparently aposematic scents in both sexes of species with
warning colours should be displeasing to him. But the former is far more
surprising than the latter. It is not perhaps astonishing that a scent
which is ex hypothesi unpleasant to an insect-eating Vertebrate should
be displeasing to the human sense; but it is certainly wonderful that an
odour which is ex hypothesi agreeable to a female butterfly should also
be agreeable to man.

Entirely new light upon the seasonal appearance of epigamic characters
is shed by the recent researches of C.W. Beebe ("The American
Naturalist", Vol. XLII. No. 493, Jan. 1908, page 34.), who caused the
scarlet tanager (Piranga erythromelas) and the bobolink (Dolichonyx
oryzivorus) to retain their breeding plumage through the whole year by
means of fattening food, dim illumination, and reduced activity. Gradual
restoration to the light and the addition of meal-worms to the diet
invariably brought back the spring song, even in the middle of winter.
A sudden alteration of temperature, either higher or lower, caused the
birds nearly to stop feeding, and one tana