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Descent of Man [ 1871]

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Bullseye
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« Reply #120 on: February 09, 2009, 03:08:22 pm »

From the several foregoing facts it is impossible to admit that
the brilliant colours of butterflies, and of some few moths, have
commonly been acquired for the sake of protection. We have seen that
their colours and elegant patterns are arranged and exhibited as if
for display. Hence I am led to believe that the females prefer or
are most excited by the more brilliant males; for on any other
supposition the males would, as far as we can see, be ornamented to no
purpose. We know that ants and certain lamellicorn beetles are capable
of feeling an attachment for each other, and that ants recognise their
fellows after an interval of several months. Hence there is no
abstract improbability in the Lepidoptera, which probably stand nearly
or quite as high in the scale as these insects, having sufficient
mental capacity to admire bright colours. They certainly discover
flowers by colour. The humming-bird sphinx may often be seen to
swoop down from a distance on a bunch of flowers in the midst of green
foliage; and I have been assured by two persons abroad, that these
moths repeatedly visit flowers painted on the walls of a room, and
vainly endeavour to insert their proboscis into them. Fritz Muller
informs me that several kinds of butterflies in S. Brazil shew an
unmistakable preference for certain colours over others: he observed
that they very often visited the brilliant red flowers of five or
six genera of plants, but never the white or yellow flowering
species of the same and other genera, growing in the same garden;
and I have received other accounts to the same effect. As I hear
from Mr. Doubleday, the common white butterfly often flies down to a
bit of paper on the ground, no doubt mistaking it for one of its own
species. Mr. Collingwood* in speaking of the difficulty in
collecting certain butterflies in the Malay Archipelago, states that
"a dead specimen pinned upon a conspicuous twig will often arrest an
insect of the same species in its headlong flight, and bring it down
within easy reach of the net, especially if it be of the opposite
sex."

  * Rambles of a Naturalist in the Chinese Seas, 1868, p. 182.

  The courtship of butterflies is, as before remarked, a prolonged
affair. The males sometimes fight together in rivalry; and many may be
seen pursuing or crowding round the same female. Unless, then, the
females prefer one male to another, the pairing must be left to mere
chance, and this does not appear probable. If, on the other band,
the females habitually, or even occasionally, prefer the more
beautiful males, the colours of the latter will have been rendered
brighter by degrees, and will have been transmitted to both sexes or
to one sex, according to the law of inheritance which has prevailed.
The process of sexual selection will have been much facilitated, if
the conclusion can be trusted, arrived at from various kinds of
evidence in the supplement to the ninth chapter; namely, that the
males of many Lepidoptera, at least in the imago state, greatly exceed
the females in number.
  Some facts, however, are opposed to the belief that female
butterflies prefer the more beautiful males; thus, as I have been
assured by several collectors, fresh females may frequently be seen
paired with battered, faded, or dingy males; but this is a
circumstance which could hardly fail often to follow from the males
emerging from their cocoons earlier than the females. With moths of
the family of the Bombycidae, the sexes pair immediately after
assuming the imago state; for they cannot feed, owing to the
rudimentary condition of their mouths. The females, as several
entomologists have remarked to me, lie in an almost torpid state,
and appear not to evince the least choice in regard to their partners.
This is the case with the common silk-moth (B. mori), as I have been
told by some continental and English breeders. Dr. Wallace, who has
had great experience in breeding Bombyx cynthia, is convinced that the
females evince no choice or preference. He has kept above 300 of these
moths together, and has often found the most vigorous females mated
with stunted males. The reverse appears to occur seldom; for, as he
believes, the more vigorous males pass over the weakly females, and
are attracted by those endowed with most vitality. Nevertheless, the
Bombycidae, though obscurely-coloured, are often beautiful to our eyes
from their elegant and mottled shades.
  I have as yet only referred to the species in which the males are
brighter coloured than the females, and I have attributed their beauty
to the females for many generations having chosen and paired with
the more attractive males. But converse cases occur, though rarely, in
which the females are more brilliant than the males; and here, as I
believe, the males have selected the more beautiful females, and
have thus slowly added to their beauty. We do not know why in
various classes of animals the males of some few species have selected
the more beautiful females instead of having gladly accepted any
female, as seems to be the general rule in the animal kingdom: but if,
contrary to what generally occurs with the Lepidoptera, the females
were much more numerous than the males, the latter would be likely
to pick out the more beautiful females. Mr. Butler shewed me several
species of Callidryas in the British Museum, in some of which the
females equalled, and in others greatly surpassed the males in beauty;
for the females alone have the borders of their wings suffused with
crimson and orange, and spotted with black. The plainer males of these
species closely resemble each other, shewing that here the females
have been modified; whereas in those cases, where the males are the
more ornate, it is these which have been modified, the females
remaining closely alike.
  In England we have some analogous cases, though not so marked. The
females alone of two species of Thecla have a bright-purple or
orange patch on their fore-wings. In Hipparchia the sexes do not
differ much; but it is the female of H. janira which has a conspicuous
light-brown patch on her wings; and the females of some of the other
species are brighter coloured than their males. Again, the females
of Colias edusa and hyale have "orange or yellow spots on the black
marginal border, represented in the males only by thin streaks"; and
in Pieris it is the females which "are ornamented with black spots
on the fore-wings, and these are only partially present in the males."
Now the males of many butterflies are known to support the females
during their marriage flight; but in the species just named it is
the females which support the males; so that the part which the two
sexes play is reversed, as is their relative beauty. Throughout the
animal kingdom the males commonly take the more active share in
wooing, and their beauty seems to have been increased by the females
having accepted the more attractive individuals; but with these
butterflies, the females take the more active part in the final
marriage ceremony, so that we may suppose that they likewise do so
in the wooing; and in this case we can understand how it is that
they have been rendered the more beautiful. Mr. Meldola, from whom the
foregoing statements have been taken, says in conclusion: "Though I am
not convinced of the action of sexual selection in producing the
colours of insects, it cannot be denied that these facts are
strikingly corroborative of Mr. Darwin's views."*

  * Nature, April 27, 1871, p. 508. Mr. Meldola quotes Donzel, in Soc.
Ent. de France, 1837, p. 77, on the flight of butterflies whilst
pairing. See also Mr. G. Fraser, in Nature, April 20, 1871, p. 489, on
the sexual differences of several British butterflies.

  As sexual selection primarily depends on variability, a few words
must be added on this subject. In respect to colour there is no
difficulty, for any number of highly variable Lepidoptera could be
named. One good instance will suffice. Mr. Bates shewed me a whole
series of specimens of Papilio sesostris and P. childrenae; in the
latter the males varied much in the extent of the beautifully
enamelled green patch on the fore-wings, and in the size of the
white mark, and of the splendid crimson stripe on the hind-wings; so
that there was a great contrast amongst the males between the most and
the least gaudy. The male of Papilio sesostris is much less
beautiful than of P. childrenae; and it likewise varies a little in
the size of the green patch on the fore-wings, and in the occasional
appearance of the small crimson stripe on the hind-wings, borrowed, as
it would seem, from its own female; for the females of this and of
many other species in the Aeneas group possess this crimson stripe.
Hence between the brightest specimens of P. sesostris and the
dullest of P. childrenae, there was but a small interval; and it was
evident that as far as mere variability is concerned, there would be
no difficulty in permanently increasing the beauty of either species
by means of selection. The variability is here almost confined to
the male sex; but Mr. Wallace and Mr. Bates have shewn* that the
females of some species are extremely variable, the males being nearly
constant. In a future chapter I shall have occasion to shew that the
beautiful eye-like spots, or ocelli, found on the wings of many
Lepidoptera, are eminently variable. I may here add that these
ocelli offer a difficulty on the theory of sexual selection; for
though appearing to us so ornamental, they are never present in one
sex and absent in the other, nor do they ever differ much in the two
sexes.*(2) This fact is at present inexplicable; but if it should
hereafter be found that the formation of an ocellus is due to some
change in the tissues of the wings, for instance, occurring at a
very early period of development, we might expect, from what we know
of the laws of inheritance, that it would be transmitted to both
sexes, though arising and perfected in one sex alone.
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