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Descent of Man [ 1871]

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Author Topic: Descent of Man [ 1871]  (Read 5938 times)
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« Reply #120 on: February 09, 2009, 03:09:07 pm »

The colouring of insects is a complex and obscure subject. When
the male differs slightly from the female, and neither are
brilliantly-coloured, it is probable that the sexes have varied in a
slightly different manner, and that the variations have been
transmitted by each sex to the same without any benefit or evil thus
accruing. When the male is brilliantly-coloured and differs
conspicuously from the female, as with some dragonflies and many
butterflies, it is probable that he owes his colours to sexual
selection; whilst the female has retained a primordial or very ancient
type of colouring, slightly modified by the agencies before explained.
But in some cases the female has apparently been made obscure by
variations transmitted to her alone, as a means of direct
protection; and it is almost certain that she has sometimes been
made brilliant, so as to imitate other protected species inhabiting
the same district. When the sexes resemble each other and both are
obscurely coloured, there is no doubt that they have been in a
multitude of cases so coloured for the sake of protection. So it is in
some instances when both are brightly-coloured, for they thus
imitate protected species, or resemble surrounding objects such as
flowers; or they give notice to their enemies that they are
unpalatable. In other cases in which the sexes resemble each other and
are both brilliant, especially when the colours are arranged for
display, we may conclude that they have been gained by the male sex as
an attraction, and have been transferred to the female. We are more
especially led to this conclusion whenever the same type of coloration
prevails throughout a whole group, and we find that the males of
some species differ widely in colour from the females, whilst others
differ slightly or not at all with intermediate gradations
connecting these extreme states.
  In the same manner as bright colours have often been partially
transferred from the males to the females, so it has been with the
extraordinary horns of many lamellicorn and some other beetles. So
again, the sound-producing organs proper to the males of the Homoptera
and Orthoptera have generally been transferred in a rudimentary, or
even in a nearly perfect condition, to the females; yet not
sufficiently perfect to be of any use. It is also an interesting fact,
as bearing on sexual selection, that the stridulating organs of
certain male Orthoptera are not fully developed until the last
moult; and that the colours of certain male dragon-flies are not fully
developed until some little time after their emergence from the
pupal state, and when they are ready to breed.
  Sexual selection implies that the more attractive individuals are
preferred by the opposite sex; and as with insects, when the sexes
differ, it is the male which, with some rare exceptions, is the more
ornamented, and departs more from the type to which the species
belongs;- and as it is the male which searches eagerly for the female,
we must suppose that the females habitually or occasionally prefer the
more beautiful males, and that these have thus acquired their
beauty. That the females in most or all the Orders would have the
power of rejecting any particular male, is probable from the many
singular contrivances possessed by the males, such as great jaws,
adhesive cushions, spines, elongated legs, &c., for seizing the
female; for these contrivances shew that there is some difficulty in
the act, so that her concurrence would seem necessary. Judging from
what we know of the perceptive powers and affections of various
insects, there is no antecedent improbability in sexual selection
having come largely into play; but we have as yet no direct evidence
on this head, and some facts are opposed to the belief.
Nevertheless, when we see many males pursuing the same female, we
can hardly believe that the pairing is left to blind chance- that
the female exerts no choice, and is not influenced by the gorgeous
colours or other ornaments with which the male is decorated.
  If we admit that the females of the Homoptera and Orthoptera
appreciate the musical tones of their male partners, and that the
various instruments have been perfected through sexual selection,
there is little improbability in the females of other insects
appreciating beauty in form or colour, and consequently in such
characters having been thus gained by the males. But from the
circumstance of colour being so variable, and from its having been
so often modified for the sake of protection, it is difficult to
decide in how large a proportion of cases sexual selection has
played a part. This is more especially difficult in those Orders, such
as Orthoptera, Hymenoptera, and Coleoptera, in which the two sexes
rarely differ much in colour; for we are then left to mere analogy.
With the Coleoptera, however, as before remarked, it is in the great
lamellicorn group, placed by some authors at the head of the Order,
and in which we sometimes see a mutual attachment between the sexes,
that we find the males of some species possessing weapons for sexual
strife, others furnished with wonderful horns, many with
stridulating organs, and others ornamented with splendid metallic
tints. Hence it seems probable that all these characters have been
gained through the same means, namely sexual selection. With
butterflies we have the best evidence, as the males sometimes take
pains to display their beautiful colours; and we cannot believe that
they would act thus, unless the display was of use to them in their
  When we treat of birds, we shall see that they present in their
secondary sexual characters the closest analogy with insects. Thus,
many male birds are highly pugnacious, and some are furnished with
special weapons for fighting with their rivals. They possess organs
which are used during the breeding-season for producing vocal and
instrumental music. They are frequently ornamented with combs,
horns, wattles and plumes of the most diversified kinds, and are
decorated with beautiful colours, all evidently for the sake of
display. We shall find that, as with insects, both sexes in certain
groups are equally beautiful, and are equally provided with
ornaments which are usually confined to the male sex. In other
groups both sexes are equally plain-coloured and unornamented. Lastly,
in some few anomalous cases, the females are more beautiful than the
males. We shall often find, in the same group of birds, every
gradation from no difference between the sexes, to an extreme
difference. We shall see that female birds, like female insects, often
possess more or less plain traces or rudiments of characters which
properly belong to the males and are of use only to them. The analogy,
indeed, in all these respects between birds and insects is curiously
close. Whatever explanation applies to the one class probably
applies to the other; and this explanation, as we shall hereafter
attempt to shew in further detail, is sexual selection.

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