$Unique_ID{how01092}
$Pretitle{}
$Title{Descent Of Man, The
Chapter 14.2}
$Subtitle{}
$Author{Darwin, Charles}
$Affiliation{}
$Subject{birds
male
female
species
footnote
white
males
black
certain
feathers}
$Date{1874}
$Log{}
Title:       Descent Of Man, The
Book:        Part II: Sexual Selection
Author:      Darwin, Charles
Date:        1874

Chapter 14.2

     Lichtenstein, who was a good observer and had excellent opportunities of
observation at the Cape of Good Hope, assured Rudolphi that the female
widow-bird (Chera progne) disowns the male when robbed of the long tail
feathers with which he is ornamented during the breeding-season.  I presume
that this observation must have been made on birds under confinement. ^864
Here is an analogous case: Dr. Jaeger, ^865 director of the Zoological Gardens
of Vienna, states that a male silver-pheasant who had been triumphant over all
other males and was the accepted lover of the females had his ornamental
plumage spoiled.  He was then immediately superseded by a rival who got the
upper hand and afterward led the flock.

[Footnote 864: Rudolphi, "Beytrage zur Anthropologie," 1812, s. 184.]

[Footnote 865: "Die Darwin'sche Theorie, und ihre Stellung zu Moral und
Religion," 1869, s. 59.]

     It is a remarkable fact, as showing how important color is in the
courtship of birds, that Mr. Boardman, a well-known collector and observer of
birds for many years in the Northern United States, has never in his large
experience seen an albino paired with another bird; yet he has had
opportunities of observing many albinos belonging to several species. ^866 It
can hardly be maintained that albinos in a state of nature are incapable of
breeding, as they can be raised with the greatest facility under confinement.
It appears, therefore, that we must attribute the fact that they do not pair
to their rejection by their normally colored comrades.

[Footnote 866: This statement is given by Mr. A. Leith Adams, in his "Field
and Forest Rambles," 1873," p. 76, and accords with his own experience.]

     Female birds not only exert a choice, but in some few cases they court
the male, or even fight together for his possession.  Sir R. Heron states that
with peafowl the first advances are always made by the female; something of
the same kind takes place, according to Audubon, with the older females of the
wild turkey.  With the capercailzie, the females flit round the male while he
is parading at one of the places of assemblage and solicit his attention. ^867
We have seen that a tame wild duck seduced an unwilling pintail drake after a
long courtship.  Mr. Bartlett believes that the Lophophorus, like many other
gallinaceous birds, is naturally polygamous, but two females cannot be placed
in the same cage with a male as they fight so much together.  The following
instance of rivalry is more surprising as it relates to bullfinches, which
usually pair for life.  Mr. Jenner Weir introduced a dull-colored and ugly
female into his aviary and she immediately attacked another mated female so
unmercifully that the latter had to be separated. The new female did all the
courtship, and was at last successful, for she paired with the male; but after
a time she met with a just retribution, for, ceasing to be pugnacious, she was
replaced by the old female, and the male then deserted his new and returned to
his old love.

[Footnote 867: In regard to peafowl, see Sir R. Heron, "Proc. Zoolog. Soc.,"
1835, p. 54, and the Rev. E. S. Dixon, "Ornamental Poultry," 1848, p. 8.  For
the turkey, Audubon, ibid, p. 4.  For the capercailzie, Lloyd, "Game Birds of
Sweden," 1867, p. 23.]

     In all ordinary cases the male is so eager that he will accept any
female, and does not, as far as we can judge, prefer one to the other; but, as
we shall hereafter see, exceptions to this rule apparently occur in some few
groups.  With domesticated birds I have heard of only one case of males
showing any preference for certain females, namely, that of the domestic cock,
who, according to the high authority of Mr. Hewitt, prefers the younger to the
older hens.  On the other hand, in effecting hybrid unions between the male
pheasant and common hens, Mr. Hewitt is convinced that the pheasant invariably
prefers the older birds.  He does not appear to be in the least influenced by
their color; but "is most capricious in his attachments;" ^868 from some
inexplicable cause he shows the most determined aversion to certain hens,
which no care on the part of the breeder can overcome.  Mr. Hewitt informs me
that some hens are quite unattractive even to the males of their own species,
so that they may be kept with several cocks during a whole season, and not one
egg out of forty or fifty will prove fertile.  On the other hand, with the
long-tailed duck (Harelda glacialis), "it has been remarked," says M. Ekstrom,
"that certain females are much more courted than the rest. Frequently, indeed,
one sees an individual surrounded by six or eight amorous males." Whether this
statement is credible, I know not; but the native sportsmen shoot these
females in order to stuff them as decoys. ^869

[Footnote 868: Mr. Hewitt, quoted in "Tegetmeier's Poultry Book," 1866, p.
165.]

[Footnote 869: Quoted in Lloyd's "Game Birds of Sweden," p. 345.]

     With respect to female birds feeling a preference for particular males we
must bear in mind that we can judge of choice being exerted only by analogy.
If an inhabitant of another planet were to behold a number of young rustics at
a fair courting a pretty girl and quarreling about her, like birds at one of
their places of assemblage, he would, by the eagerness of the wooers to please
her and to display their finery, infer that she had the power of choice.  Now
with birds the evidence stands thus; they have acute powers of observation,
and they seem to have some taste for the beautiful both in color and sound.
It is certain that the females occasionally exhibit, from unknown causes, the
strongest antipathies and preferences for particular males.  When the sexes
differ in color or in other ornaments the males with rare exceptions are the
more decorated, either permanently or temporarily during the breeding-season.
They sedulously display their various ornaments, exert their voices, and
perform strange antics in the presence of the females. Even well-armed males,
who, it might be thought, would altogether depend for success on the law of
battle, are in most cases highly ornamented; and their ornaments have been
acquired at the expense of some loss of power.  In other cases ornaments have
been acquired at the cost of increased risk from birds and beasts of prey.
With various species many individuals of both sexes congregate at the same
spot, and their courtship is a prolonged affair.  There is even reason to
suspect that the males and females within the same district do not always
succeed in pleasing each other and pairing.

     What then are we to conclude from these facts and considerations? Does
the male parade his charms with so much pomp and rivalry for no purpose?  Are
we not justified in believing that the female exerts a choice, and that she
receives the addresses of the male who pleases her most?  It is not probable
that she consciously deliberates; but she is most excited or attracted by the
most beautiful, or melodious, or gallant males.  Nor need it be supposed that
the female studies each stripe or spot of color; that the peahen, for
instance, admires each detail in the gorgeous train of the peacock - she is
probably struck only by the general effect.  Nevertheless, after hearing how
carefully the male Argus pheasant displays his elegant primary wing-feathers
and erects his ocellated plumes in the right position for their full effect;
or again, how the male goldfinch alternately displays his gold-bespangled
wings, we ought not to feel too sure that the female does not attend to each
detail of beauty.  We can judge, as already remarked, of choice being exerted,
only from analogy; and the mental powers of birds do not differ fundamentally
from ours.  From these various considerations we may conclude that the pairing
of birds is not left to chance; but that those males, which are best able by
their various charms to please or excite the female, are under ordinary
circumstances accepted.  If this be admitted, there is not much difficulty in
understanding how male birds have gradually acquired their ornamental
characters.  All animals present individual differences, and as man can modify
his domesticated birds by selecting the individuals which appear to him the
most beautiful, so the habitual or even occasional preference by the female of
the more attractive males would almost certainly lead to their modification;
and such modifications might in the course of time be augmented to almost any
extent, compatible with the existence of the species.

     Variability of Birds, and Especially of Their Secondary Sexual
Characters. - Variability and inheritance are the foundations for the work of
selection.  That domesticated birds have varied greatly, their variations
being inherited, is certain.  That birds in a state of nature have been
modified into distinct races is now universally admitted. ^870 Variations may
be divided into two classes; those which appear to our ignorance to arise
spontaneously, and those which are directly related to the surrounding
conditions, so that all or nearly all the individuals of the same species are
similarly modified.  Cases of the latter kind have recently been observed with
care by Mr. J. A. Allen, ^871 who shows that in the United States many species
of birds gradually become more strongly colored in proceeding southward, and
more lightly colored in proceeding westward to the arid plains of the
interior.  Both sexes seem generally to be affected in a like manner, but
sometimes one sex more than the other.  This result is not incompatible with
the belief that the colors of birds are mainly due to the accumulation of
successive variations through sexual selection; for even after the sexes have
been greatly differentiated, climate might produce an equal effect on both
sexes, or a greater effect on one sex than on the other, owing to some
constitutional difference.

[Footnote 870: According to Dr. Blasius ("Ibis," vol. ii, 1860, p. 297), there
are 425 indubitable species of birds which breed in Europe, besides sixty
forms, which are frequently regarded as distinct species.  Of the latter,
Blasius thinks that only ten are really doubtful and that the other fifty
ought to be united with their nearest allies; but this shows that there must
be a considerable amount of variation with some of our European birds.  It is
also an unsettled point with naturalists, whether several North American birds
ought to be ranked as specifically distinct from the corresponding European
species.  So again many North American forms which until lately were named as
distinct species, are now considered to be local races.]

[Footnote 871: "Mammals and Birds of East Florida," also an "Ornithological
Reconnaissance of Kansas, etc." Notwithstanding the influence of climate on
the colors of birds, it is difficult to account for the dull or dark tints of
almost all the species inhabiting certain countries, for instance, the
Galapagos Islands under the equator, the wide, temperate plains of Patagonia,
and, as it appears, Egypt (see Mr. Hartshorne in the "American Naturalist,"
1873, p. 747).  These countries are open and afford little shelter to birds;
but it seems doubtful whether the absence of brightly colored species can be
explained on the principle of protection, for on the Pampas, which are equally
open, though covered by green grass, and where the birds would be equally
exposed to danger, many brilliant and conspicuously colored species are
common.  I have sometimes speculated whether the prevailing dull tints of the
scenery in the above-named countries may not have affected the appreciation of
bright colors by the birds inhabiting them.]

     Individual differences between the members of the same species are
admitted by every one to occur under a state of nature.  Sudden and strongly
marked variations are rare; it is also doubtful whether if beneficial they
would often be preserved through selection and transmitted to succeeding
generations. ^872 Nevertheless, it may be worth while to give the few cases
which I have been able to collect, relating chiefly to color - simple albinism
and melanism being excluded.  Mr. Gould is well known to admit the existence
of few varieties, for he esteems very slight differences as specific; yet he
states ^873 that near Bogota certain humming-birds belonging to the genus
Cynanthus are divided into two or three races or varieties, which differ from
each other in the coloring of the tail - "some having the whole of the
feathers blue, while others have the eight central ones tipped with beautiful
green." It does not appear that intermediate gradations have been observed in
this or the following cases.  In the males alone of one of the Australian
paroquets "the thighs in some are scarlet, in others grass-green." In another
paroquet of the same country "some individuals have the band across the
wing-coverts bright yellow, while in others the same part is tinged with red."
^874 In the United States some few of the males of the Scarlet Tanager
(Tanagra rubra) have "a beautiful transverse band of glowing red on the
smaller wing-coverts;" ^875 but this variation seems to be somewhat rare, so
that its preservation through sexual selection would follow only under
unusually favorable circumstances.  In Bengal the honey buzzard (Pernis
cristata) has either a small rudimental crest on its head, or none at all; so
slight a difference, however, would not have been worth notice, had not this
same species possessed in Southern India a well-marked occipital crest formed
of several graduated feathers." ^876

[Footnote 872: "Origin of Species," 5th edit., 1869, p. 104.  I had always
perceived that rare and strongly marked deviations of structure, deserving to
be called monstrosities, could seldom be preserved through natural selection,
and that the preservation of even highly beneficial variations would depend to
a certain extent on chance.  I had also fully appreciated the importance of
mere individual differences, and this led me to insist so strongly on the
importance of that unconscious form of selection by man which follows from the
preservation of the most valued individuals of each breed, without any
intention on his part to modify the characters of the breed.  But until I read
an able article in the "North British Review" (March, 1867, p. 289, et seq.),
which has been of more use to me than any other review, I did not see how
great the chances were against the preservation of variations, whether slight
or strongly pronounced, occurring only in single individuals.]

[Footnote 873: "Introduct. to the Trochilidae," p, 102.]

[Footnote 874: Gould, "Hand-book to Birds of Australia," vol. ii, pp. 32, 68.]

[Footnote 875: Audubon, "Ornitholog. Biography," 1838, vol. iv, p. 389.]

[Footnote 876: Jerdon, "Birds of India," vol. i, p. 108; and Mr. Blyth, in
"Land and Water," 1868, p. 381.]

     The following case is in some respects more interesting.  A pied variety
of the raven, with the head, breast, abdomen, and parts of the wings and
tail-feathers white is confined to the Feroe Islands.  It is not very rare
there, for Graba saw during his visit from eight to ten living specimens.
Although the characters of this variety are not quite constant, yet it has
been named by several distinguished ornithologists as a distinct species.  The
fact of the pied birds being pursued and persecuted with much clamor by the
other ravens of the island was the chief cause which led Brunnich to conclude
that they were specifically distinct; but this is now known to be an error.
^877 This case seems analogous to that lately given of albino birds not
pairing from being rejected by their comrades.

[Footnote 877: Graba, "Tagebuch Reise nach Faro," 1830, ss. 51-54.
Macgillivray, "Hist. British Birds," vol. iii, p. 745.  "Ibis," vol. v, 1863,
p. 469.]

     In various parts of the northern seas a remarkable variety of the common
Guillemot (Uria troile) is found; and in Feroe one out of every five birds,
according to Graba's estimation, presents this variation.  It is characterized
^878 by a pure white ring round the eye, with a curved narrow white line an
inch and a half in length extending back from the ring.  This conspicuous
character has caused the bird to be ranked by several ornithologists as a
distinct species under the name of U. lacrymans, but it is now known to be
merely a variety.  It often pairs with the common kind, yet intermediate
gradations have never been seen; nor is this surprising, for variations which
appear suddenly are often, as I have elsewhere shown, ^879 transmitted either
unaltered or not at all.  We thus see that two distinct forms of the same
species may co-exist in the same district, and we cannot doubt that if the one
had possessed any advantage over the other it would soon have been multiplied
to the exclusion of the latter.  If, for instance, the male pied ravens,
instead of being persecuted by their comrades, had been highly attractive
(like the above pied peacock) to the black female ravens their numbers would
have rapidly increased.  And this would have been a case of sexual selection.

[Footnote 878: Graba, ibid, s. 54.  Macgillivray, ibid, vol. v, p. 327.]

[Footnote 879: "Variation of Animals and Plants under Domestication," vol. ii,
p. 92.]

     With respect to the slight individual differences which are common, in a
greater or less degree, to all the members of the same species, we have every
reason to believe that they are by far the most important for the work of
selection.  Secondary sexual characters are eminently liable to vary, both
with animals in a state of nature and under domestication. ^880 There is also
reason to believe, as we have seen in our eighth chapter, that variations are
more apt to occur in the male than in the female sex.  All these contingencies
are highly favorable for sexual selection.  Whether characters thus acquired
are transmitted to one sex or to both sexes depends, as we shall see in the
following chapter, on the form of inheritance which prevails.

[Footnote 880: On these points see also "Variation of Animals and Plants under
Domestication," vol. i, p. 253; vol. ii, pp. 73, 75.]

     It is sometimes difficult to form an opinion whether certain slight
differences between the sexes of birds are simply the result of variability
with sexually limited inheritance without the aid of sexual selection or
whether they have been augmented through this latter process.  I do not here
refer to the many instances where the male displays splendid colors or other
ornaments of which the female partakes to a slight degree; for these are
almost certainly due to characters primarily acquired by the male having been
more or less transferred to the female.  But what are we to conclude with
respect to certain birds in which, for instance, the eyes differ slightly in
color in the two sexes? ^881 In some cases the eyes differ conspicuously; thus
with the storks of the genus Xenorhynchus, those of the male are
blackish-hazel, while those of the female are gamboge-yellow; with many
hornbills (Buceros), as I hear from Mr. Blyth, ^882 the males have intense
crimson eyes, and those of the females are white.  In the Buceros bicornis,
the hind margin of the casque and a stripe on the crest of the beak are black
in the male, but not so in the female.  Are we to suppose that these black
marks and the crimson color of the eyes have been preserved or augmented
through sexual selection in the males?  This is very doubtful; for Mr.
Bartlett showed me in the Zoological Gardens that the inside of the mouth of
this buceros is black in the male and flesh-colored in the female; and their
external appearance or beauty would not be thus affected.  I observed in Chili
^883 that the iris in the condor, when about a year old, is dark-brown, but
changes at maturity into a yellowish-brown in the male, and into bright red in
the female.  The male has also a small, longitudinal, leaden-colored, fleshy
crest or comb. The comb of many gallinaceous birds is highly ornamental, and
assumes vivid colors during the act of courtship; but what are we to think of
the dull-colored comb of the condor which does not appear to us in the least
ornamental?  The same question may be asked in regard to various other
characters, such as the knob on the base of the beak of the Chinese goose
(Anser cygnoides), which is much larger in the male than in the female. No
certain answer can be given to these questions; but we ought to be cautious in
assuming that knobs and various fleshy appendages cannot be attractive to the
female, when we remember that with savage races of man various hideous
deformities - deep scars on the face with the flesh raised into protuberances,
the septum of the nose pierced by sticks or bones, holes in the ears and lips
stretched widely open - are all admired as ornamental.

[Footnote 881: See, for instance, on the irides of a Podica and Gallicrex in
"Ibis," vol. ii, 1860, p. 206; and vol. v, 1863, p. 426.]

[Footnote 882: See also Jerdon, "Birds of India," vol. i, pp. 243-245.]

[Footnote 883: "Zoology of the Voyage of H. M. S. 'Beagle,'" 1841, p. 6.]

     Whether or not unimportant differences between the sexes, such as those
just specified, have been preserved through sexual selection, these
differences, as well as all others, must primarily depend on the laws of
variation.  On the principle of correlated development, the plumage often
varies on different parts of the body, or over the whole body, in the same
manner.  We see this well illustrated in certain breeds of the fowl. In all
the breeds the feathers on the neck and loins of the males are elongated and
are called hackles; now when both sexes acquire a top-knot, which is a new
character in the genus, the feathers on the head of the male become
hackle-shaped, evidently on the principle of correlation; while those on the
head of the female are of the ordinary shape.  The color also of the hackles
forming the top-knot of the male is often correlated with that of the hackles
on the neck and loins, as may be seen by comparing these feathers in the
golden and silver-spangled Polish, the Houdans and Creve-coeur breeds.  In
some natural species we may observe exactly the same correlation in the colors
of these same feathers, as in the males of the splendid gold and Amherst
pheasants.

     The structure of each individual feather generally causes any change in
its coloring to be symmetrical; we see this in the various laced, spangled,
and penciled breeds of the fowl; and on the principle of correlation the
feathers over the whole body are often colored in the same manner.  We are
thus enabled without much trouble to rear breeds with their plumage marked
almost as symmetrically as in natural species. In laced and spangled fowls the
colored margins of the feathers are abruptly defined; but in a mongrel raised
by me from a black Spanish cock glossed with green, and a white game-hen, all
the feathers were greenish-black, excepting toward their extremities, which
were yellowish-white; but between the white extremities and the black bases
there was on each feather a symmetrical, curved zone of dark-brown.  In some
instances the shaft of the feather determines the distribution of the tints;
thus with the body-feathers of a mongrel from the same black Spanish cock and
a silver-spangled Polish hen, the shaft, together with a narrow space on each
side, was greenish-black, and this was surrounded by a regular zone of
dark-brown, edged with brownish-white.  In these cases we have feathers
symmetrically shaded, like those which give so much elegance to the plumage of
many natural species.  I have also noticed a variety of the common pigeon with
the wingbars symmetrically zoned with three bright shades, instead of being
simply black on a slaty-blue ground, as in the parent-species.

     In many groups of birds the plumage is differently colored in the several
species, yet certain spots, marks, or stripes are retained by all.  Analogous
cases occur with the breeds of the pigeon, which usually retain the two
wingbars, though they may be colored red, yellow, white, black, or blue, the
rest of the plumage being of some wholly different tint.  Here is a more
curious case, in which certain marks are retained, though colored in a manner
almost exactly the opposite of what is natural; the aboriginal pigeon has a
blue tail, with the terminal halves of the outer webs of the two outer
tail-feathers white; now there is a sub-variety having a white instead of a
blue tail, with precisely that part black which is white in the
parent-species." ^884

[Footnote 884: Bechstein, "Naturgeschichte Deutschlands," B. iv, 1795, s. 31,
on a sub-variety of the Monck pigeon.]

     Formation and Variability of the Ocelli or Eye-Like Spots on the Plumage
of Birds. - As no ornaments are more beautiful than the ocelli on the feathers
of various birds, on the hairy coats of some mammals, on the scales of
reptiles and fishes, on the skin of amphibians, on the wings of many
Lepidoptera and other insects, they deserve to be especially noticed.  An
ocellus consists of a spot within a ring of another color, like the pupil
within the iris, but the central spot is often surrounded by additional
concentric zones.  The ocelli on the tail-coverts of the peacock offer a
familiar example, as well as those on the wings of the peacock-butterfly
(Vanessa).  Mr. Trimen has given me a description of a South African moth
(Gynanisa isis), allied to our emperor moth, in which a magnificent ocellus
occupies nearly the whole surface of each hinder wing; it consists of a black
center, including a semi-transparent crescent-shaped mark, surrounded by
successive ocher-yellow, black, ocher-yellow, pink, white, pink, brown and
whitish zones.  Although we do not know the steps by which these wonderfully
beautiful and complex ornaments have been developed the process has probably
been a simple one, at least with insects; for, as Mr. Trimen writes to me, "no
characters of mere marking or coloration are so unstable in the Lepidoptera as
the ocelli, both in number and size." Mr. Wallace, who first called my
attention to this subject, showed me a series of specimens of our common
meadow-brown butterfly (Hipparchia janira) exhibiting numerous gradations from
a simple minute black spot to an elegantly shaded ocellus.  In a South African
butterfly (Cyllo leda, Linn.), belonging to the same family, the ocelli are
even still more variable.  In some specimens (a, fig. 53) large spaces on the
upper surface of the wings are colored black, and include irregular white
marks; and from this state a complete gradation can be traced into a tolerably
perfect ocellus (a1), and this results from the contraction of the irregular
blotches of color.  In another series of specimens a gradation can be followed
from excessively minute white dots, surrounded by a scarcely visible black
line (b), into perfectly symmetrical and large ocelli (b1). ^885 In cases like
these, the development of a perfect ocellus does not require a long course of
variation and selection.

[Footnote 885: This wood-cut has been engraved from a beautiful drawing, most
kindly made for me by Mr. Trimen; see also his description of the wonderful
amount of variation in the coloration and shape of the wings of this
butterfly, in his "Rhopalocera Africae Australis," p. 186.]

     With birds and many other animals it seems to follow from the comparison
of allied species that circular spots are often generated by the breaking up
and contraction of stripes.  In the Tragopan pheasant faint white lines in the
female represent the beautiful white spots in the male; ^886 and something of
the same kind may be observed in the two sexes of the Argus pheasant.  However
this may be, appearances strongly favor the belief that, on the one hand, a
dark spot is often formed by the coloring matter being drawn toward a central
point from a surrounding zone, which latter is thus rendered lighter; and, on
the other hand, that a white spot is often formed by the color being driven
away from a central point, so that it accumulates in a surrounding darker
zone.  In either case an ocellus is the result.  The coloring matter seems to
be a nearly constant quantity, but is redistributed, either centripetally or
centrifugally.  The feathers of the common guinea-fowl offer a good instance
of white spots surrounded by darker zones; and wherever the white spots are
large and stand near each other the surrounding dark zones become confluent.
In the same wing-feather of the Argus pheasant dark spots may be seen
surrounded by a pale zone and white spots by a dark zone.  Thus the formation
of an ocellus in its most elementary state appears to be a simple affair.  By
what further steps the more complex ocelli, which are surrounded by many
successive zones of color, have been generated, I will not pretend to say.
But the zoned feathers of the mongrels from differently colored fowls, and the
extraordinary variability of the ocelli on many Lepidoptera, lead us to
conclude that their formation is not a complex process, but depends on some
slight and graduated change in the nature of the adjoining tissues.

[Footnote 886: Jerdon, "Birds of India," vol. iii, p. 517.]