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Descent of Man [ 1871]

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Author Topic: Descent of Man [ 1871]  (Read 5970 times)
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« Reply #120 on: February 09, 2009, 03:08:37 pm »

* Wallace on the "Papilionidae of the Malayan Region," in
Transact. Linn. Soc., vol. xxv., 1865, pp. 8, 36. A striking case of a
rare variety, strictly intermediate between two other well-marked
female varieties, is given by Mr. Wallace. See also Mr. Bates, in
Proc. Entomolog. Soc., Nov. 19, 1866, p. xl.
  *(2) Mr. Bates was so kind as to lay this subject before the
Entomological Society, and I have received answers to this effect from
several entomologists.

  On the whole, although many serious objections may be urged, it
seems probable that most of the brilliantly-coloured species of
Lepidoptera owe their colours to sexual selection, excepting in
certain cases, presently to be mentioned, in which conspicuous colours
have been gained through mimicry as a protection. From the ardour of
the male throughout the animal kingdom, he is generally willing to
accept any female; and it is the female which usually exerts a choice.
Hence, if sexual selection has been efficient with the Lepidoptera,
the male, when the sexes differ, ought to be the more brilliantly
coloured, and this undoubtedly is the case. When both sexes are
brilliantly coloured and resemble each other, the characters
acquired by the males appear to have been transmitted to both. We
are led to this conclusion by cases, even within the same genus, of
gradation from an extraordinary amount of difference to identity in
colour between the two sexes.
  But it may be asked whether the difference in colour between the
sexes may not be accounted for by other means besides sexual
selection. Thus the males and females of the same species of butterfly
are in several cases known* to inhabit different stations, the
former commonly basking in the sunshine, the latter haunting gloomy
forests. It is therefore possible that different conditions of life
may have acted directly on the two sexes; but this is not probable*(2)
as in the adult state they are exposed to different conditions
during a very short period; and the larvae of both are exposed to
the same conditions. Mr. Wallace believes that the difference
between the sexes is due not so much to the males having been
modified, as to the females having in all or almost all cases acquired
dull colours for the sake of protection. It seems to me, on the
contrary, far more probable that it is the males which have been
chiefly modified through sexual selection, the females having been
comparatively little changed. We can thus understand how it is that
the females of allied species generally resemble one another so much
more closely than do the males. They thus shew us approximately the
primordial colouring of the parent-species of the group to which
they belong. They have, however, almost always been somewhat
modified by the transfer to them of some of the successive variations,
through the accumulation of which the males were rendered beautiful.
But I do not wish to deny that the females alone of some species may
have been specially modified for protection. In most cases the males
and females of distinct species will have been exposed during their
prolonged larval state to different conditions, and may have been thus
affected; though with the males any slight change of colour thus
caused will generally have been masked by the brilliant tints gained
through sexual selection. When we treat of birds, I shall have to
discuss the whole question, as to how far the differences in colour
between the sexes are due to the males having been modified through
sexual selection for ornamental purposes, or to the females having
been modified through natural selection for the sake of protection, so
that I will here say but little on the subject.

  * H. W. Bates, The Naturalist on the Amazons, vol. ii., 1863, p.
228. A. R. Wallace, in Transactions, Linnean Society, vol. xxv., 1865,
p. 10.
  *(2) On this whole subject see The Variation of Animals and Plants
under Domestication, 1868, vol. ii., chap. xxiii.

  In all the cases in which the more common form of equal
inheritance by both sexes has prevailed, the selection of
bright-coloured males would tend to make the females
bright-coloured; and the selection of dull-coloured females would tend
to make the males dull. If both processes were carried on
simultaneously, they would tend to counteract each other; and the
final result would depend on whether a greater number of females
from being well protected by obscure colours, or a greater number of
males by being brightly-coloured and thus finding partners,
succeeded in leaving more numerous offspring.
  In order to account for the frequent transmission of characters to
one sex alone, Mr. Wallace expresses his belief that the more common
form of equal inheritance by both sexes can be changed through natural
selection into inheritance by one sex alone, but in favour of this
view I can discover no evidence. We know from what occurs under
domestication that new characters often appear, which from the first
are transmitted to one sex alone; and by the selection of such
variations there would not be the slightest difficulty in giving
bright colours to the males alone, and at the same time or
subsequently, dull colours to the females alone. In this manner the
females of some butterflies and moths have, it is probable, been
rendered inconspicuous for the sake of protection, and widely
different from their males.
  I am, however, unwilling without distinct evidence to admit that two
complex processes of selection, each requiring the transference of new
characters to one sex alone, have been carried on with a multitude
of species,- that the males have been rendered more brilliant by
beating their rivals, and the females more dull-coloured by having
escaped from their enemies. The male, for instance, of the common
brimstone butterfly (Gonepteryx), is of a far more intense yellow than
the female, though she is equally conspicuous; and it does not seem
probable that she specially acquired her pale tints as a protection,
though it is probable that the male acquired his bright colours as a
sexual attraction. The female of Anthocharis cardamines does not
possess the beautiful orange wing-tips of the male; consequently she
closely resembles the white butterflies (Pieris) so common in our
gardens; but we have no evidence that this resemblance is beneficial
to her. As, on the other hand, she resembles both sexes of several
other species of the genus inhabiting various quarters of the world,
it is probable that she has simply retained to a large extent her
primordial colours.
  Finally, as we have seen, various considerations lead to the
conclusion that with the greater number of brilliantly-coloured
Lepidoptera it is the male which has been chiefly modified through
sexual selection; the amount of difference between the sexes mostly
depending on the form of inheritance which has prevailed.
Inheritance is govemed by so many unknown laws or conditions, that
it seems to us to act in a capricious manner;* and we can thus, to a
certain extent, understand how it is that with closely allied
species the sexes either differ to an astonishing degree, or are
identical in colour. As all the successive steps in the process of
variation are necessarily transmitted through the female, a greater or
less number of such steps might readily become developed in her; and
thus we can understand the frequent gradations from an extreme
difference to none at all between the sexes of allied species. These
cases of gradation, it may be added, are much too common to favour the
supposition that we here see females actually undergoing the process
of transition and losing their brightness for the sake of
protection; for we have every reason to conclude that at any one
time the greater number of species are in a fixed condition.

  * The Variation of Animals and Plants under Domestication, vol. ii.,
chap. xii., p. 17.
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