e eye be less sensitive
to SPECIFICALLY MALE colours and other VISIBLE signs ENTICING TO THE
FEMALE, than the olfactory sense to specifically male odours, or the
sense of hearing to specifically male sounds? Moreover, the decorative
feathers of birds are almost always spread out and displayed before
the female during courtship. I have elsewhere ("The Evolution Theory",
London, 1904, I. page 219.) pointed out that decorative colouring and
sweet-scentedness may replace one another in Lepidoptera as well as
in flowers, for just as some modestly coloured flowers (mignonette and
violet) have often a strong perfume, while strikingly coloured ones are
sometimes quite devoid of fragrance, so we find that the most beautiful
and gaily-coloured of our native Lepidoptera, the species of Vanessa,
have no scent-scales, while these are often markedly developed in grey
nocturnal Lepidoptera. Both attractions may, however, be combined in
butterflies, just as in flowers. Of course, we cannot explain why both
means of attraction should exist in one genus, and only one of them in
another, since we do not know the minutest details of the conditions
of life of the genera concerned. But from the sporadic distribution of
scent-scales in Lepidoptera, and from their occurrence or absence in
nearly related species, we may conclude that fragrance is a relatively
MODERN acquirement, more recent than brilliant colouring.

One thing in particular that stamps decorative colouring as a product of
selection is ITS GRADUAL INTENSIFICATION by the addition of new spots,
which we can quite well observe, because in many cases the colours have
been first acquired by the males, and later transmitted to the females
by inheritance. The scent-scales are never thus transmitted, probably
for the same reason that the decorative colours of many birds are often
not transmitted to the females: because with these they would be exposed
to too great elimination by enemies. Wallace was the first to point out
that in species with concealed nests the beautiful feathers of the male
occurred in the female also, as in the parrots, for instance, but
this is not the case in species which brood on an exposed nest. In the
parrots one can often observe that the general brilliant colouring of
the male is found in the female, but that certain spots of colour are
absent, and these have probably been acquired comparatively recently by
the male and have not yet been transmitted to the fem