History of the World/History of the World (Bureau Development, Inc.)(1992).BIN/dp/0109/01098.txt

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$Unique_ID{how01098}
$Pretitle{}
$Title{Descent Of Man, The
Chapter 16.3}
$Subtitle{}
$Author{Darwin, Charles}
$Affiliation{}
$Subject{birds
males
young
species
females
sexes
selection
colors
white
colored}
$Date{1874}
$Log{}
Title:       Descent Of Man, The
Book:        Part II: Sexual Selection
Author:      Darwin, Charles
Date:        1874

Chapter 16.3

     Class VI.  The young in their first plumage differ from each other
according to sex; the young males resembling more or less closely the adult
males, and the young females more or less closely the adult females. - The
cases in the present class, though occurring in various groups, are not
numerous; yet it seems the most natural thing that the young should at first
somewhat resemble the adults of the same sex, and gradually become more and
more like them.  The adult male blackcap (Sylvia atricapilla) has a black
head, that of the female being reddish-brown; and I am informed by Mr. Blyth,
that the young of both sexes can be distinguished by this character even as
nestlings.  In the family of thrushes an unusual number of similar cases have
been noticed; thus, the male blackbird (Turdus merula) can be distinguished in
the nest from the female.  The two sexes of the mocking-bird (Turdus
polyglottus, Linn.) differ very little from each other, yet the males can
easily be distinguished at a very early age from the females by showing more
pure white. ^971 The males of a forest-thrush and of a rock-thrush (Orocetes
erythrogastra and Petrocincla cyanea) have much of their plumage of a fine
blue, while the females are brown; and the nestling males of both species have
their main wing and tail feathers edged with blue, while those of the female
are edged with brown. ^972 In the young blackbird the wing-feathers assume
their mature character and become black after the others; on the other hand,
in the two species just named the wing-feathers become blue before the others.
The most probable view with reference to the cases in the present class is
that the males, differently from what occurs in Class I, have transmitted
their colors to their male offspring at an earlier age than that at which they
were first acquired; for, if the males had varied while quite young, their
characters would probably have been transmitted to both sexes. ^973

[Footnote 971: Audubon "Ornith. Biography," vol. i, p. 113.]

[Footnote 972: Mr. C. A. Wright, in "Ibis," vol. vi, 1864, p. 65. Jerdon,
"Birds of India," vol. i, p. 515.  See also on the blackbird, Blyth, in
Charlesworth's "Mag. of Nat. History," vol. i, 1837, p. 113.]

[Footnote 973: The following additional cases may be mentioned: the young
males of Tanagra rubra can be distinguished from the young females (Audubon,
"Ornith. Biography," vol. iv, p. 392), and so it is within the nestlings of a
blue nuthatch, Dendrophila frontalis, of India (Jerdon, "Birds of India," vol.
i, p. 389).  Mr. Blyth also informs me that the sexes of the stonechat,
Saxicola rubicola, are distinguishable at a very early age.  Mr. Salvin gives
("Proc. Zoolog. Soc.," 1870, p. 206), the case of a humming-bird, like the
following one of Eustephanus.]

     In Aithurus polytmus, a humming-bird, the male is splendidly colored
black and green, and two of the tail-feathers are immensely lengthened; the
female has an ordinary tail and inconspicuous colors; now the young males,
instead of resembling the adult female, in accordance with the common rule,
begin from the first to assume the colors proper to their sex, and their
tail-feathers soon become elongated.  I owe this information to Mr. Gould, who
has given me the following more striking and as yet unpublished case.  Two
humming-birds belonging to the genus Eustephanus, both beautifully colored,
inhabit the small island of Juan Fernandez, and have always been ranked as
specifically distinct.  But it has lately been ascertained that the one which
is of a rich chestnut-brown color with a golden-red head, is the male, while
the other, which is elegantly variegated with green and white, with a metallic
green head, is the female.  Now the young from the first somewhat resemble the
adults of the corresponding sex, the resemblance gradually becoming more and
more complete.

     In considering this last case, if as before we take the plumage of the
young as our guide, it would appear that both sexes have been rendered
beautiful independently; and not that one sex has partially transferred its
beauty to the other.  The male apparently has acquired his bright colors
through sexual selection in the same manner as, for instance, the peacock or
pheasant in our first class of cases; and the female in the same manner as the
female Rhynchaea or Turnix in our second class of cases.  But there is much
difficulty in understanding how this could have been effected at the same time
with the two sexes of the same species.  Mr. Salvin states, as we have seen in
the eighth chapter, that with certain humming-birds the males greatly exceed
the females in number, while with other species inhabiting the same country
the females greatly exceed the males.  If, then, we might assume that during
some former lengthened period the males of the Juan Fernandez species had
greatly exceeded the females in number, but that during another lengthened
period the females had far exceeded the males, we could understand how the
males at one time, and the females at another, might have been rendered
beautiful by the selection of the brighter-colored individuals of either sex;
both sexes transmitting their characters to their young at a rather earlier
age than usual.  Whether this is the true explanation I will not pretend to
say; but the case is too remarkable to be passed over without notice.

     We have now seen in all six classes that an intimate relation exists
between the plumage of the young and the adults, either of one sex or both.
These relations are fairly well explained on the principle that one sex - this
being in the great majority of cases the male - first acquired through
variation and sexual selection bright colors or other ornaments and
transmitted them in various ways in accordance with the recognized laws of
inheritance.  Why variations have occurred at different periods of life, even
sometimes with species of the same group, we do not know, but with respect to
the form of transmission one important determining cause seems to be the age
at which the variations first appear.

     From the principle of inheritance at corresponding ages and from any
variations in color which occurred in the males at an early age not being then
selected - on the contrary being often eliminated as dangerous - while similar
variations occurring at or near the period of reproduction have been
preserved, it follows that the plumage of the young will often have been left
unmodified, or but little modified.  We thus get some insight into the
coloring of the progenitors of our existing species.  In a vast number of
species in five out of our six classes of cases the adults of one sex or of
both are bright colored, at least during the breeding-season, while the young
are invariably less brightly colored than the adults, or are quite dull
colored; for no instance is known, as far as I can discover, of the young of
dull-colored species displaying bright colors, or of the young of
bright-colored species being more brilliant than their parents.  In the fourth
class, however, in which the young and the old resemble each other, there are
many species (though by no means all), of which the young are bright colored,
and, as these form old groups, we may infer that their early progenitors were
likewise bright.  With this exception, if we look to the birds of the world,
it appears that their beauty has been much increased since that period, of
which their immature plumage gives us a partial record.

     On the Color of the Plumage in Relation to Protection. - It will have
been seen that I cannot follow Mr. Wallace in the belief that dull colors,
when confined to the females, have been in most cases specially gained for the
sake of protection.  There can, however, be no doubt, as formerly remarked,
that both sexes of many birds have had their colors modified so as to escape
the notice of their enemies; or in some instances, so as to approach their
prey unobserved, just as owls have had their plumage rendered soft, that their
flight may not be overheard.  Mr. Wallace remarks ^974 that "it is only in the
tropics, among forests which never lose their foliage, that we find whole
groups of birds whose chief color is green." It will be admitted by every one
who has ever tried how difficult it is to distinguish parrots in a
leaf-covered tree. Nevertheless, we must remember that many parrots are
ornamented with crimson, blue and orange tints, which can hardly be
protective. Woodpeckers are eminently arboreal, but besides green species
there are many black and black-and-white kinds - all the species being
apparently exposed to nearly the same dangers.  It is therefore probable that
with tree-haunting birds strongly pronounced colors have been acquired through
sexual selection, but that a green tint has been acquired oftener than any
other from the additional advantage of protection.

[Footnote 974: "Westminster Review," July, 1867, p. 5.]

     In regard to birds which live on the ground, every one admits that they
are colored so as to imitate the surrounding surface.  How difficult it is to
see a partridge, snipe, woodcock, certain plovers, larks and night-jars when
crouched on ground.  Animals inhabiting deserts offer the most striking cases,
for the bare surface affords no concealment, and nearly all the smaller
quadrupeds, reptiles and birds depend for safety on their colors.  Mr.
Tristram has remarked in regard to the inhabitants of the Sahara, that all are
protected by their "isabelline or sand color." ^975 Calling to my recollection
the desert-birds of South America, as well as most of the ground-birds of
Great Britain, it appeared to me that both sexes in such cases are generally
colored nearly alike. Accordingly, I applied to Mr. Tristram with respect to
the birds of the Sahara, and he has kindly given me the following information:
There are twenty-six species belonging to fifteen genera, which manifestly
have their plumage colored in a protective manner; and this coloring is all
the more striking, as with most of these birds it differs from that of their
congeners.  Both sexes of thirteen out of the twenty-six species are colored
in the same manner; but these belong to genera in which this rule commonly
prevails, so that they tell us nothing about the protective colors being the
same in both sexes of desert-birds.  Of the other thirteen species three
belong to genera in which the sexes usually differ from each other, yet here
they have the sexes alike.  In the remaining ten species the male differs from
the female; but the difference is confined chiefly to the under surface of the
plumage, which is concealed when the bird crouches on the ground; the head and
back being of the same sand-colored hue in the two sexes.  So that in these
ten species the upper surfaces of both sexes have been acted on and rendered
alike through natural selection for the sake of protection; while the lower
surfaces of the males alone have been diversified through sexual selection for
the sake of ornament.  Here, as both sexes are equally well protected, we
clearly see that the females have not been prevented by natural selection from
inheriting the colors of their male parents; so that we must look to the law
of sexually limited transmission.

[Footnote 975: "Ibis," 1859, vol. i, p. 429, et seq.  Dr. Rohlfs, however,
remarks to me in a letter that, according to his experience of the Sahara,
this statement is too strong.]

     In all parts of the world both sexes of many soft-billed birds,
especially those which frequent reeds or hedges, are obscurely colored. No
doubt if their colors had been brilliant, they would have been much more
conspicuous to their enemies; but whether their dull tints have been specially
gained for the sake of protection seems, as far as I can judge, rather
doubtful.  It is still more doubtful whether such dull tints can have been
gained for the sake of ornament.  We must, however, bear in mind that male
birds, though dull-colored, often differ much from their females (as with the
common sparrow), and this leads to the belief that such colors have been
gained through sexual selection from being attractive.  Many of the
soft-billed birds are songsters; and a discussion in a former chapter should
not be forgotten, in which it was shown that the best songsters are rarely
ornamented with bright tints. It would appear that female birds, as a general
rule, have selected their mates either for their sweet voices or gay colors,
but not for both charms combined.  Some species which are manifestly colored
for the sake of protection, such as the jack-snipe, woodcock and night-jar,
are likewise marked and shaded, according to our standard of taste, with
extreme elegance.  In such cases we may conclude that both natural and sexual
selection have acted conjointly for protection and ornament. Whether any bird
exists which does not possess some special attraction by which to charm the
opposite sex may be doubted.  When both sexes are so obscurely colored that it
would be rash to assume the agency of sexual selection, and when no direct
evidence can be advanced showing that such colors serve as a protection, it is
best to own complete ignorance of the cause, or, which comes to nearly the
same thing, to attribute the result to the direct action of the conditions of
life.

     Both sexes of many birds are conspicuously, though not brilliantly,
colored, such as the numerous black, white, or piebald species; and these
colors are probably the result of sexual selection.  With the common
blackbird, capercailzie, blackcock, black scoter-duck (Oidemia), and even with
one of the birds of paradise (Lophorina atra) the males alone are black, while
the females are brown or mottled; and there can hardly be a doubt that
blackness in these cases has been a sexually selected character.  Therefore,
it is in some degree probable that the complete or partial blackness of both
sexes in such birds as crows, certain cockatoos, storks and swans, and many
marine birds, is likewise the result of sexual selection, accompanied by equal
transmission to both sexes; for blackness can hardly serve in any case as a
protection.  With several birds, in which the male alone is black, and in
others in which both sexes are black, the beak or skin about the head is
brightly colored, and the contrast thus afforded adds much to their beauty; we
see this in the bright yellow beak of the male blackbird, in the crimson skin
over the eyes of the blackcock and capercailzie, in the brightly and variously
colored beak of the scoter-drake (Oidemia), in the red beak of the chough
(Corvus graculus, Linn.), of the black swan and the black stork.  This leads
me to remark that it is not incredible that toucans may owe the enormous size
of their beaks to sexual selection, for the sake of displaying the diversified
and vivid stripes of color with which these organs are ornamented. ^976 The
naked skin, also, at the base of the beak and round the eyes is likewise often
brilliantly colored; and Mr. Gould, in speaking of one species, ^977 says that
the colors of the beak "are doubtless in the finest and most brilliant state
during the time of pairing." There is no greater improbability that toucans
should be encumbered with immense beaks, though rendered as light as possible
by their cancellated structure, for the display of fine colors (an object
falsely appearing to us unimportant), than that the male Argus pheasant and
some other birds should be encumbered with plumes so long as to impede their
flight.

[Footnote 976: No satisfactory explanation has ever been offered of the
immense size, and still less of the bright colors, of the toucan's beak. Mr.
Bates ("The Naturalist on the Amazons," vol. ii, 1863, p. 341) states that
they use their beaks for reaching fruit at the extreme tips of the branches;
and likewise, as stated by other authors, for extracting eggs and young birds
from the nests of other birds.  But, as Mr. Bates admits, the beak "can
scarcely be considered a very perfectly formed instrument for the end to which
it is applied." The great bulk of the beak, as shown by its breadth, depth, as
well as length, is not intelligible on the view, that it serves merely as an
organ of prehension.  Mr. Belt believes ("The Naturalist in Nicaragua," p.
197) that the principal use of the beak is as a defense against enemies,
especially to the female while nesting in a hole in a tree.]

[Footnote 977: Rhamphastos carinatus, Gould's "Monograph of Rhamphastidae."]

     In the same manner, as the males alone of various species are black, the
females being dull-colored; so in a few cases the males alone are either
wholly or partially white, as with the several bell-birds of South America
(Chasmorhynchus), the Antarctic goose (Bernicla antarctica), the silver
pheasant, etc., while the females are brown or obscurely mottled. Therefore,
on the same principle as before, it is probable that both sexes of many birds,
such as white cockatoos, several egrets with their beautiful plumes, certain
ibises, gulls, terns, etc., have acquired their more or less completely white
plumage through sexual selection.  In some of these cases the plumage becomes
white only at maturity.  This is the case with certain gannets, tropic-birds,
etc., and with the snow-goose (Anser hyperboreus).  As the latter breeds on
the "barren grounds," when not covered with snow, and as it migrates southward
during the winter there is no reason to suppose that its snow-white adult
plumage serves as a protection.  In the Anastomus oscitans we have still
better evidence that the white plumage is a nuptial character, for it is
developed only during the summer; the young in their immature state and the
adults in their winter dress being gray and black.  With many kinds of gulls
(Larus) the head and neck become pure white during the summer, being gray or
mottled during the winter and in the young state.  On the other hand, with the
smaller gulls, or sea-mews (Gavia) and with some terns (Sterna) exactly the
reverse occurs; for the heads of the young birds during the first year, and of
the adults during the winter, are either pure white or much paler colored than
during the breeding-season.  These latter cases offer another instance of the
capricious manner in which sexual selection appears often to have acted. ^978

[Footnote 978: On Larus, Gavia and Sterna, see Macgillivray, "Hist. Brit.
Birds," vol. v, pp. 515, 584, 626.  On the Anser hyperboreus, Audubon,
"Ornith. Biography," vol. iv, p. 562.  On the Anastomus, Mr. Blyth, in "Ibis,"
1867, p. 173.]

     That aquatic birds have acquired a white plumage so much oftener than
terrestrial birds probably depends on their large size and strong powers of
flight, so that they can easily defend themselves or escape from birds of
prey, to which, moreover, they are not much exposed. Consequently sexual
selection has not here been interfered with or guided for the sake of
protection.  No doubt with birds which roam over the open ocean, the males and
females could find each other much more easily when made conspicuous either by
being perfectly white or intensely black; so that these colors may possibly
serve the same end as the call-notes of many land-birds. ^979 A white or black
bird when it discovers and flies down to a carcass floating on the sea or cast
upon the beach, will be seen from a great distance, and will guide other birds
of the same and other species to the prey; but as this would be a disadvantage
to the first finders, the individuals which were the whitest or blackest would
not thus procure more food than the less strongly colored individuals. Hence
conspicuous colors cannot have been gradually acquired for this purpose
through natural selection.

[Footnote 979: It may be noticed that with vultures, which roam far and wide
high in the air, like marine birds over the ocean, three or four species are
almost wholly or largely white, and that many others are black.  So that here
again conspicuous colors may possibly aid the sexes in finding each other
during the breeding season.]

     As sexual selection depends on so fluctuating an element as taste, we can
understand how it is that, within the same group of birds having nearly the
same habits, there should exist white or nearly white, as well as black or
nearly black, species - for instance, both white and black cockatoos, storks,
ibises, swans, terns and petrels.  Piebald birds likewise sometimes occur in
the same groups together with black and white species; for instance, the
black-necked swan, certain terns and the common magpie.  That a strong
contrast in color is agreeable to birds we may conclude by looking through any
large collection, for the sexes often differ from each other in the male
having the pale parts of a purer white, and the variously colored dark parts
of still darker tints than the female.

     It would even appear that mere novelty, or slight changes for the sake of
change, have sometimes acted on female birds as a charm, like changes of
fashion with us.  Thus the males of some parrots can hardly be said to be more
beautiful than the females, at least according to our taste, but they differ
in such points, as in having a rose-colored collar instead of "a bright,
emeraldine, narrow green collar;" or in the male having a black collar instead
of "a yellow demi-collar in front," with a pale roseate instead of a plum-blue
head. ^980 As so many male birds have elongated tail-feathers or elongated
crests for their chief ornament, the shortened tail, formerly described in the
male of a humming-bird, and the shortened crest of the male goosander, seem
like one of the many changes of fashion which we admire in our own dresses.

[Footnote 980: See Jerdon on the genus Palaeornis, "Birds of India," vol. 1,
pp. 258-260.]

     Some members of the heron family offer a still more curious case of
novelty in coloring, having, as it appears, been appreciated for the sake of
novelty.  The young of the Ardea asha are white, the adults being dark
slate-colored; and not only the young, but the adults in their winter plumage,
of the allied Buphus coromandus are white, this color changing into a rich
golden-buff during the breeding-season.  It is incredible that the young of
these two species, as well as of some other members of the same family, ^981
should for any special purpose have been rendered pure white and thus made
conspicuous to their enemies; or that the adults of one of these two species
should have been specially rendered white during the winter in a country which
is never covered with snow.  On the other hand, we have good reason to believe
that whiteness has been gained by many birds as a sexual ornament.  We may
therefore conclude that some early progenitor of the Ardea asha and the Buphus
acquired a white plumage for nuptial purposes, and transmitted this color to
their young; so that the young and the old became white like certain existing
egrets; and that the whiteness was afterward retained by the young, while it
was exchanged by the adults for more strongly pronounced tints.  But if we
could look still further back to the still earlier progenitors of these two
species we should probably see the adults dark-colored.  I infer that this
would be the case from the analogy of many other birds which are dark while
young and when adult are white; and more especially from the case of the Ardea
gularis, the colors of which are the reverse of those of A. asha, for the
young are dark-colored and the adults white, the young having retained a
former state of plumage.  It appears therefore that during a long line of
descent, the adult progenitors of the Ardea asha, the Buphus, and of some
allies, have undergone the following changes of color: firstly, a dark shade;
secondly, pure white, and thirdly, owing to another change of fashion (if I
may so express myself), their present slaty, reddish, or golden-buff tints.
These successive changes are intelligible only on the principle of novelty
having been admired by birds for its own sake.

[Footnote 981: The young of Ardea rufescens and A. coerulea of the United
States are likewise white, the adults being colored in accordance with their
specific names.  Audubon ("Ornith. Biography," vol. iii, p. 416; vol. iv, p.
58) seems rather pleased at the thought that this remarkable change of plumage
will greatly "disconcert the systematists."]

     Several writers have objected to the whole theory of sexual selection by
assuming that with animals and savages the taste of the female for certain
colors or other ornaments would not remain constant for many generations; that
first one color and then another would be admired, and consequently that no
permanent effect could be produced.  We may admit that taste is fluctuating,
but it is not quite arbitrary.  It depends much on habit, as we see in
mankind; and we may infer that this would hold good with birds and other
animals.  Even in our own dress the general character lasts long, and the
changes are to a certain extent graduated.  Abundant evidence will be given in
two places in a future chapter that savages of many races have admired for
many generations the same cicatrices on the skin, the same hideously
perforated lips, nostrils, or ears, distorted heads, etc.; and these
deformities present some analogy to the natural ornaments of various animals.
Nevertheless, with savages such fashions do not endure forever, as we may
infer from the differences in this respect between allied tribes on the same
continent.  So, again, the raisers of fancy animals certainly have admired for
many generations and still admire the same breeds; they earnestly desire
slight changes, which are considered as improvements, but any great or sudden
change is looked at as the greatest blemish. With birds in a state of nature
we have no reason to suppose that they would admire an entirely new style of
coloration, even if great and sudden variations often occurred, which is far
from being the case.  We know that dove-cote pigeons do not willingly
associate with the variously colored fancy breeds; that albino birds do not
commonly get partners in marriage; and that the black ravens of the Feroe
Islands chase away their piebald brethren.  But this dislike of a sudden
change would not preclude their appreciating slight changes any more than it
does in the case of man.  Hence, with respect to taste, which depends on many
elements, but partly on habit and partly on a love of novelty, there seems no
improbability in animals admiring for a very long period the same general
style of ornamentation or other attractions, and yet appreciating slight
changes in colors, form, or sound.

     Summary of the Four Chapters on Birds. - Most male birds are highly
pugnacious during the breeding-season, and some possess weapons adapted for
fighting with their rivals.  But the most pugnacious and the best armed males
rarely or never depend for success solely on their power to drive away or kill
their rivals, but have special means for charming the female.  With some it is
the power of song, or of giving forth strange cries, or instrumental music,
and the males in consequence differ from the females in their vocal organs, or
in the structure of certain feathers.  From the curiously diversified means
for producing various sounds we gain a high idea of the importance of this
means of courtship. Many birds endeavor to charm the females by love dances or
antics performed on the ground or in the air, and sometimes at prepared
places. But ornaments of many kinds, the most brilliant tints, combs and
wattles, beautiful plumes, elongated feathers, top-knots, and so forth, are by
far the commonest means.  In some cases mere novelty appears to have acted as
a charm.  The ornaments of the males must be highly important to them, for
they have been acquired in not a few cases at the cost of increased danger
from enemies, and even at some loss of power in fighting with their rivals.
The males of very many species do not assume their ornamental dress until they
arrive at maturity, or they assume it only during the breeding-season, or the
tints then become more vivid.  Certain ornamental appendages become enlarged,
turgid, and brightly colored during the act of courtship.  The males display
their charms with elaborate care and to the best effect; and this is done in
the presence of the females.  The courtship is sometimes a prolonged affair,
and many males and females congregate at an appointed place.  To suppose that
the females do not appreciate the beauty of the males is to admit that their
splendid decorations, all their pomp and display, are useless; and this is
incredible.  Birds have fine powers of discrimination, and in some few
instances it can be shown that they have a taste for the beautiful.  The
females, moreover, are known occasionally to exhibit a marked preference or
antipathy for certain individual males.

     If it be admitted that the females prefer, or are unconsciously excited
by the more beautiful males, then the males would slowly but surely be
rendered more and more attractive through sexual selection. That it is this
sex which has been chiefly modified, we may infer from the fact that, in
almost every genus where the sexes differ, the males differ much more from one
another than do the females; this is well shown in certain closely allied
representative species, in which the females can hardly be distinguished,
while the males are quite distinct.  Birds in a state of nature offer
individual differences which would amply suffice for the work of sexual
selection; but we have seen that they occasionally present more strongly
marked variations which recur so frequently that they would immediately be
fixed, if they served to allure the female.  The laws of variation must
determine the nature of the initial changes, and will have largely influenced
the final result.  The gradations, which may be observed between the males of
allied species, indicate the nature of the steps through which they have
passed.  They explain also in the most interesting manner how certain
characters have originated, such as the indented ocelli on the tail-feathers
of the peacock, and the ball-and-socket ocelli on the wing-feathers of the
Argus pheasant.  It is evident that the brilliant colors, top-knots, fine
plumes, etc., of many male birds cannot have been acquired as a protection;
indeed, they sometimes lead to danger.  That they are not due to the direct
and definite action of the conditions of life, we may feel assured, because
the females have been exposed to the same conditions, and yet often differ
from the males to an extreme degree.  Although it is probable that changed
conditions acting during a lengthened period have in some cases produced a
definite effect on both sexes, or sometimes on one sex alone, the more
important result will have been an increased tendency to vary or to present
more strongly marked individual differences; and such differences will have
afforded an excellent groundwork for the action of sexual selection.

     The laws of inheritance, irrespectively of selection, appear to have
determined whether the characters acquired by the males for the sake of
ornament, for producing various sounds and for fighting together, have been
transmitted to the males alone or to both sexes either permanently or
periodically during certain seasons of the year.  Why various characters
should have been transmitted sometimes in one way and sometimes in another is
not in most cases known; but the period of variability seems often to have
been the determining cause.  When the two sexes have inherited all characters
in common they necessarily resemble each other; but as the successive
variations may be differently transmitted every possible gradation may be
found, even within the same genus, from the closest similarity to the widest
dissimilarity between the sexes.  With many closely allied species, following
nearly the same habits of life, the males have come to differ from each other
chiefly through the action of sexual selection; while the females have come to
differ chiefly from partaking more or less of the characters thus acquired by
the males.  The effects, moreover, of the definite action of the conditions of
life will not have been masked in the females as in the males by the
accumulation through sexual selection of strongly pronounced colors and other
ornaments.  The individuals of both sexes, however affected, will have been
kept at each successive period nearly uniform by the free intercrossing of
many individuals.

     With species in which the sexes differ in color it is possible or
probable that some of the successive variations often tended to be transmitted
equally to both sexes; but that when this occurred the females were prevented
from acquiring the bright colors of the males by the destruction which they
suffered during incubation.  There is no evidence that it is possible by
natural selection to convert one form of transmission into another.  But there
would not be the least difficulty in rendering a female dull-colored, the male
being still kept bright-colored, by the selection of successive variations
which were from the first limited in their transmission to the same sex.
Whether the females of many species have actually been thus modified must at
present remain doubtful.  When, through the law of the equal transmission of
characters to both sexes, the females were rendered as conspicuously colored
as the males, their instincts appear often to have been modified so that they
were led to build domed or concealed nests.

     In one small and curious class of cases the characters and ad habits of
the two sexes have been completely transposed, for the females are larger,
stronger, more vociferous and brighter colored than the males. They have also
become so quarrelsome that they often fight together for the possession of the
males like the males of other pugnacious species for the possession of the
females.  If, as seems probable, such females habitually drive aways their
rivals, and by the display of their bright colors or other charms endeavor to
attract the males, we can understand how it is that they have gradually been
rendered by sexual selection and sexually limited transmission more beautiful
than the males - the latter being left unmodified or only slightly modified.

     Whenever the law of inheritance at corresponding ages prevails, but not
that of sexually limited transmission, then if the parents vary late in life -
and we know that this constantly occurs with our poultry and occasionally with
other birds - the young will be left unaffected, while the adults of both
sexes will be modified.  If both these laws of inheritance prevail and either
sex varies late in life, that sex alone will be modified, the other sex and
the young being unaffected.  When variations in brightness or in other
conspicuous characters occur early in life, as no doubt often happens, they
will not be acted on through sexual selection until the period of reproduction
arrives; consequently if dangerous to the young they will be eliminated
through natural selection.  Thus we can understand how it is that variations
arising late in life have so often been preserved for the ornamentation of the
males; the females and the young being left almost unaffected, and therefore
like each other.  With species having a distinct summer and winter plumage,
the males of which either resemble or differ from the females during both
seasons or during the summer alone, the degrees and kinds of resemblance
between the young and the old are exceedingly complex; and this complexity
apparently depends on characters, first acquired by the males, being
transmitted in various ways and degrees, as limited by age, sex and season.

     As the young of so many species have been but little modified in color
and in other ornaments, we are enabled to form some judgment with respect to
the plumage of their early progenitors; and we may infer that the beauty of
our existing species, if we look to the whole class, has been largely
increased since that period, of which the immature plumage gives us an
indirect record.  Many birds, especially those which live much on the ground,
have undoubtedly been obscurely colored for the sake of protection.  In some
instances the upper exposed surface of the plumage has been thus colored in
both sexes, while the lower surface in the males alone has been variously
ornamented through sexual selection. Finally, from the facts given in these
four chapters, we may conclude that weapons for battle, organs for producing
sound, ornaments of many kinds, bright and conspicuous colors, have generally
been acquired by the males through variation and sexual selection and have
been transmitted in various ways according to the several laws of inheritance
- the females and the young being left comparatively but little modified. ^982

[Footnote 982: I am greatly indebted to the kindness of Mr. Sclater for having
looked over these four chapters on birds, and the two following ones on
mammals.  In this way I have been saved from making mistakes about the names
of the species, and from stating anything as a fact which is known to this
distinguished naturalist to be erroneous.  But of course he is not at all
answerable for the accuracy of the statements quoted by me from various
authorities.]