iving room when the young are hatched.

[Illustration: FIG. 121.--The Garden Bower-bird (_Amblyornis
inornata_). Reduced from _Gould's Birds of New Guinea_ to 1/4 nat.
size.]

Now these sundry cases alone seem to prove a high degree of the aesthetic
sense as occurring among birds; for, it is needless to say, none of the
facts just mentioned can be due to natural selection, seeing that they
have no reference to utility, or the preservation of life. But if an
aesthetic sense occurs in birds, we should expect, on _a priori_ grounds,
that it would probably be exercised with reference to the personal
appearance of the sexes. And this expectation is fully realized. For it
is an observable fact that in most species of birds where the males are
remarkable for the brilliancy of their plumage, not only is this
brilliancy most remarkable during the pairing season, but at this season
also the male birds take elaborate pains to display their charms before
the females. Then it is that the peacock erects his tail to strut round
and round the hens, taking care always to present to them a front view,
where the coloration is most gorgeous. And the same is true of all other
gaily coloured male birds. During the pairing season they actively
compete with one another in exhibiting their attractiveness to the
females; and in many cases there are added all sorts of extraordinary
antics in the way of dancings and crowings. Again, in the case of all
song-birds, the object of the singing is to please the females; and for
this purpose the males rival one another to the best of their musical
ability.

Thus there can be no question that the courtship of birds is a highly
elaborate business, in which the males do their best to surpass one
another in charming the females. Obviously the inference is that the
males do not take all this trouble for nothing; but that the females
give their consent to pair with the males whose personal appearance, or
whose voice, proves to be the most attractive. But, if so, the young of
the male bird who is thus _selected_ will inherit his superior beauty;
and thus, in successive generations, a continuous advance will be made
in the beauty of plumage or of song, as the case may be,--both the
origin and development of beauty in the animal world being thus supposed
due to the aesthetic taste of animals themselves.

Such is the theory of sexual selection in its main outlines; and with
regard to it we must beg