; but the adult male is a
beautiful white, excepting the extraordinary structure with which we are
at present concerned. This is a tube about three inches long, which
rises from the base of the beak. It is jet black, and dotted over with
small downy feathers. The tube is closed at the top, but its cavity
communicates with the palate, and thus the whole admits of being
inflated from within, when, of course, it stands erect as represented in
one of the two drawings. When not thus inflated, it hangs down, as
shown in the second figure, which represents the plumage of a young
male. (Fig. 124.)

[Illustration: FIG. 124.--The Bell-bird (_Chasmorhynchus niveus_,
1/4 natural size). Drawn from nature (_R. Coll. Surg. Mus._). In the
drawing of the adult male the ornamental appendage is represented in
its inflated condition, during courtship; in the drawing of the
young male it is shown in its flaccid condition.]

In another species of the genus there are three of these appendages--the
two additional ones being mounted on the corners of the mouth. (Fig.
125.) In all species of the genus (four in number) the tubes are
inflated during courtship, and therefore perform the function of sexual
embellishments. Now the point to which I wish to draw attention is, that
so specialized and morphologically elaborate a structure cannot be
regarded as merely adventitious. It must have been developed by some
definite cause, acting through a long series of generations. And as no
other function can be assigned to it than that of charming the female
when it is erected in courtship, the peculiarity of form and mechanism
which it presents--like the elaboration of patterns in cases where
colour only is concerned--virtually compels us to recognise in sexual
selection the only conceivable cause of its production.

[Illustration: FIG. 125.--_C. tricarunculatus_, 1/4 natural size.
Copied from the _Ibis_. The ornamental appendages of the male are
represented in a partly inflated condition.]

For these reasons I think that Mr. Wallace's main objection falls to the
ground. Passing on to his subsidiary objections, I do not see much
weight in his merely negative difficulty as to there being an absence of
evidence upon hen birds being charmed by the plumage, or the voice, of
their consorts. For, on the one hand, it is not very safe to infer what
sentiments may be in the mind of a hen; and, on the other hand, it is
impossible t