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$Pretitle{}
$Title{Descent Of Man, The
Chapter 14.3}
$Subtitle{}
$Author{Darwin, Charles}
$Affiliation{}
$Subject{ocelli
peacock
ocellus
fig
species
feathers
part
spots
tail-feathers
upper}
$Date{1874}
$Log{}
Title:       Descent Of Man, The
Book:        Part II: Sexual Selection
Author:      Darwin, Charles
Date:        1874

Chapter 14.3

     Gradation of Secondary Sexual Characters. - Cases of gradation are
important as showing us that highly complex ornaments may be acquired by small
successive steps.  In order to discover the actual steps by which the male of
any existing bird has acquired his magnificent colors or other ornaments we
ought to behold the long line of his extinct progenitors; but this is
obviously impossible.  We may, however, generally gain a clew by comparing all
the species of the same group if it be a large one; for some of them will
probably retain, at least partially, traces of their former characters.
Instead of entering on tedious details respecting various groups, in which
striking instances of gradation could be given, it seems the best plan to take
one or two strongly marked cases, for instance that of the peacock, in order
to see if light can be thrown on the steps by which this bird has become so
splendidly decorated.  The peacock is chiefly remarkable from the
extraordinary length of his tail-coverts; the tail itself not being much
elongated.  The barbs along nearly the whole length of these feathers stand
separate or are decomposed; but this is the case with the feathers of many
species and with some varieties of the domestic fowl and pigeon. The barbs
coalesce toward the extremity of the shaft forming the oval disk or ocellus,
which is certainly one of the most beautiful objects in the world.  It
consists of an iridescent, intensely blue, indented center, surrounded by a
rich green zone, this by a broad coppery-brown zone, and this by five other
narrow zones of slightly different iridescent shades.  A trifling character in
the disk deserves notice; the barbs for a space along one of the concentric
zones are more or less destitute of their barbules, so that a part of the disk
is surrounded by an almost transparent zone, which gives it a highly finished
aspect.  But I have elsewhere described ^887 an exactly analogous variation in
the hackles of a sub-variety of the gamecock in which the tips, having a
metallic luster, "are separated from the lower part of the feather by a
symmetrically shaped transparent zone composed of the naked portions of the
barbs." The lower margin or base of the dark blue center of the ocellus is
deeply indented on the line of the shaft.  The surrounding zones likewise show
traces, as may be seen in the drawing (fig. 54), of indentations, or rather
breaks.  These indentations are common to the Indian and Javan peacocks (Pavo
cristatus and P. muticus); and they seem to deserve particular attention as
probably connected with the development of the ocellus; but for a long time I
could not conjecture their meaning.

[Footnote 887: "Variation of Animals and Plants under Domestication," vol. i,
p. 254.]

     If we admit the principle of gradual evolution there must formerly have
existed many species which presented every successive step between the
wonderfully elongated tail-coverts of the peacock and the short tail-coverts
of all ordinary birds; and again between the magnificent ocelli of the former
and the simpler ocelli or mere colored spots on other birds; and so with all
the other characters of the peacock.  Let us look to the allied Gallinaceae
for any still-existing gradations.  The species and sub-species of
Polyplectron inhabit countries adjacent to the native land of the peacock; and
they so far resemble this bird that they are sometimes called
peacock-pheasants.  I am also informed by Mr. Bartlett that they resemble the
peacock in their voice and in some of their habits.  During the spring the
males, as previously described, strut about before the comparatively
plain-colored females, expanding and erecting their tail and wing feathers,
which are ornamented with numerous ocelli.  I request the reader to turn back
to the drawing (fig. 51) of a Polyplectron.  In P. napoleonis the ocelli are
confined to the tail, and the back is of a rich metallic blue; in which
respects this species approaches the Java peacock.  P. hardwickii possesses a
peculiar top-knot, which is also somewhat like that of the Java peacock.  In
all the species the ocelli on the wings and tail are either circular or oval,
and consist of a beautiful, iridescent, greenish-blue or greenish-purple disk
with a black border.  This border in P. chinquis shades into brown, edged with
cream color, so that the ocellus is here surrounded with variously shaded,
though not bright, concentric zones.  The unusual length of the tail-coverts
is another remarkable character in Polyplectron; for in some of the species
they are half and in others two-thirds as long as the true tail feathers.  The
tail-coverts are ocellated as in the peacock.  Thus the several species of
Polyplectron manifestly make a graduated approach to the peacock in the length
of their tail-coverts, in the zoning of the ocelli, and in some other
characters.

     Notwithstanding this approach, the first species of Polyplectron which I
examined almost made me give up the search; for I found not only that the true
tail-feathers which in the peacock are quite plain, were ornamented with
ocelli, but that the ocelli on all the feathers differed fundamentally from
those of the peacock, in there being two on the same feather (fig. 55), one on
each side of the shaft.  Hence I concluded that the early progenitors of the
peacock could not have resembled a Polyplectron.  But on continuing my search
I observed that in some of the species the two ocelli stood very near each
other; that in the tail-feathers of P. hardwickii they touched each other; and
finally that on the tail-coverts of this same species as well as of P.
malaccense (fig. 56) they were actually confluent.  As the central part alone
is confluent, an indentation is left at both the upper and lower ends; and the
surrounding colored zones are likewise indented.  A single ocellus is thus
formed on each tail-covert, though still plainly betraying its double origin.
These confluent ocelli differ from the single ocelli of the peacock in having
an indentation at both ends instead of only at the lower or basal end.  The
explanation, however, of this difference is not difficult; in some species of
Polyplectron the two oval ocelli on the same feather stand parallel to each
other; in other species (as in P. chinquis) they converge toward one end; now
the partial confluence of two convergent ocelli would manifestly leave a much
deeper indentation at the divergent than at the convergent end.  It is also
manifest that if the convergence were strongly pronounced and the confluence
complete, the indentation at the convergent end would tend to disappear.

     The tail-feathers in both species of the peacock are entirely destitute
of ocelli, and this apparently is related to their being covered up and
concealed by the long tail-coverts.  In this respect they differ remarkably
from the tail-feathers of Polyplectron, which in most of the species are
ornamented with larger ocelli than those on the tail-coverts.  Hence I was led
carefully to examine the tail-feathers of the several species, in order to
discover whether their ocelli showed any tendency to disappear; and to my
great satisfaction this appeared to be so.  The central tail-feathers of P.
napoleonis have the two ocelli on each side of the shaft perfectly developed;
but the inner ocellus becomes less and less conspicuous on the more exterior
tail-feathers, until a mere shadow or rudiment is left on the inner side of
the outermost feather.  Again, in P. malaccense, the ocelli on the
tail-coverts are, as we have seen, confluent; and these feathers are of
unusual length, being two-thirds of the length of the tail-feathers, so that
in both these respects they approach the tail-coverts of the peacock.  Now in
P. malaccense the two central tail-feathers alone are ornamented, each with
two brightly colored ocelli, the inner occellus having completely disappeared
from all the other tail-feathers.  Consequently the tail-coverts and
tail-feathers of this species of Polyplectron make a near approach in
structure and ornamentation to the corresponding feathers of the peacock.

     As far, then, as gradation throws light on the steps by which the
magnificent train of the peacock has been acquired, hardly anything more is
needed.  If we picture to ourselves a progenitor of the peacock in an almost
exactly intermediate condition between the existing peacock with his
enormously elongated tail-coverts ornamented with single ocelli, and an
ordinary gallinaceous bird with short tail-coverts merely spotted with some
color, we shall see a bird allied to Polyplectron - that is, with tail-coverts
capable of erection and expansion, ornamented with two partially confluent
ocelli, and long enough almost to conceal the tail-feathers, the latter having
already partially lost their ocelli. The indentation of the central disk and
of the surrounding zones of the ocellus in both species of peacock speaks
plainly in favor of this view and is otherwise inexplicable.  The males of the
Polyplectron are no doubt beautiful birds, but their beauty, when viewed from
a little distance, cannot be compared with that of the peacock.  Many female
progenitors of the peacock must, during a long line of descent, have
appreciated this superiority; for they have unconsciously, by the continued
preference of the most beautiful males, rendered the peacock the most splendid
of living birds.

     Argus Pheasant. - Another excellent case for investigation is offered by
the ocelli on the wing-feathers of the Argus pheasant, which are shaded in so
wonderful a manner as to resemble balls lying loose within sockets and
consequently differ from ordinary ocelli.  No one, I presume, will attribute
the shading, which has excited the admiration of many experienced artists, to
chance - to the fortuitous concourse of atoms of coloring matter.  That these
ornaments should have been formed through the selection of many successive
variations, not one of which was originally intended to produce the
ball-and-socket effect, seems as incredible as that one of Rapheal's Madonnas
should have been formed by the selection of chance daubs of paint made by a
long succession of young artists, not one of whom intended at first to draw
the human figure.  In order to discover how the ocelli have been developed we
cannot look to a long line of progenitors nor to many closely allied forms,
for such do not now exist.  But fortunately the several feathers on the wing
suffice to give us a clew to the problem, and they prove to demonstration that
a gradation is at least possible from a mere spot to a finished
ball-and-socket ocellus.

     The wing-feathers, bearing the ocelli, are covered with dark stripes
(fig. 57) or with rows of dark spots (fig. 59), each stripe or row of spots
running obliquely down the outer side of the shaft to one of the ocelli.  The
spots are generally elongated in a line transverse to the row in which they
stand.  They often become confluent either in the line of the row - and then
they form a longitudinal stripe - or transversely, that is, with the spots in
the adjoining rows, and then they form transverse stripes.  A spot sometimes
breaks up into smaller spots, which still stand in their proper places.

     It will be convenient first to describe a perfect ball-and-socket
ocellus.  This consists of an intensely black circular ring, surrounding a
space shaded so as exactly to resemble a ball.  The figure here given has been
admirably drawn by Mr. Ford and well engraved, but a wood-cut cannot exhibit
the exquisite shading of the original.  The ring is almost always slightly
broken or interrupted (see fig. 57) at a point in the upper half a little to
the right of and above the white shade on the inclosed ball; it is also
sometimes broken toward the base on the right hand.  These little breaks have
an important meaning.  The ring is always much thickened, with the edges
ill-defined toward the lefthand upper corner, the feather being held erect in
the position in which it is here drawn.  Beneath this thickened part there is
on the surface of the ball an oblique, almost pure white mark which shades off
downward into a pale-leaden hue, and this into yellowish and brown tints,
which insensibly become darker and darker toward the lower part of the ball.
It is this shading which gives so admirably the effect of light shining on a
convex surface.  If one of the balls be examined it will be seen that the
lower part is of a brown tint and is indistinctly separated by a curved
oblique line from the upper part, which is yellower and more leaden; this
curved oblique line runs at right angles to the longer axis of the white patch
of light, and indeed of all the shading; but this difference in color, which
cannot of course be shown in the wood-cut, does not in the least interfere
with the perfect shading of the ball.  It should be particularly observed that
each ocellus stands in obvious connection either with a dark stripe or with a
longitudinal row of dark spots, for both occur indifferently on the same
feather.  Thus in fig. 57 stripe A runs to ocellus a; B runs to ocellus b;
stripe C is broken in the upper part and runs down to the next succeeding
ocellus, not represented in the wood-cut; D to the next lower one, and so with
the stripes E and F.  Lastly the several ocelli are separated from each other
by a pale surface bearing irregular black marks.

     I will next describe the other extreme of the series, namely, the first
trace of an ocellus.  The short secondary wing-feather (fig. 58), nearest to
the body, is marked like the other feathers, with oblique, longitudinal,
rather irregular rows of very dark spots.  The basal spot, or that nearest the
shaft in the five lower rows (excluding the lowest one), is a little larger
than the other spots of the same row, and a little more elongated in a
transverse direction.  It differs also from the other spots by being bordered
on its upper side with some dull fulvous shading.  But this spot is not in any
way more remarkable than those on the plumage of many birds, and might easily
be overlooked.  The next higher spot does not differ at all from the upper
ones in the same row.  The larger basal spots occupy exactly the same relative
position on these feathers as do the perfect ocelli on the longer
wing-feathers.

     By looking to the next two or three succeeding wing-feathers, an
absolutely insensible gradation can be traced from one of the last described
basal spots, together with the next higher one in the same row, to a curious
ornament, which cannot be called an ocellus, and which I will name, from the
want of a better term, an "elliptic ornament." These are shown in the
accompanying figure (fig. 59).  We here see several oblique rows, A, B, C, D,
etc. (see the lettered diagram on the right hand), of dark spots of the usual
character.  Each row of spots runs down to and is connected with one of the
elliptic ornaments, in exactly the same manner as each stripe in fig. 57 runs
down to, and is connected with, one of the ball-and-socket ocelli.  Looking to
any one row, for instance, B, in fig. 59, the lowest mark (b) is thicker and
considerably longer than the upper spots, and has its left extremity pointed
and curved upward.  This black mark is abruptly bordered on itsupper side by a
rather broad space of richly shaded tints, beginning with a narrow brown zone,
which passes into orange, and this into a pale leaden tint, with the end
toward the shaft much paler.  These shaded tints together fill up the whole
inner space of the elliptic ornament.  The mark (b) corresponds in every
respect with the basal shaded spot of the simple feather described in the last
paragraph (fig. 58), but is more highly developed and more brightly colored.
Above and to the right of this spot (b fig. 59), with its bright shading,
there is a long, narrow, black mark (c), belonging to the same row, and which
is arched a little downward so as to face (b).  This mark is sometimes broken
into two portions.  It is also narrowly edged on the lower side with a fulvous
tint.  To the left of and above (c), in the same oblique direction, but always
more or less distinct from it, there is another black mark (d).  This mark is
generally sub-triangular and irregular in shape, but in the one lettered in
the diagram it is unusually narrow, elongated and regular.  It apparently
consists of a lateral and broken prolongation of the mark (c), together with
its confluence with a broken and prolonged part of the next spot above; but I
do not feel sure of this.  These three marks, (b), (c) and (d), with the
intervening bright shades, form together the so-called elliptic ornament.
These ornaments placed parallel to the shaft, manifestly correspond in
position with the ball-and-socket ocelli.  Their extremely elegant appearance
cannot be appreciated in the drawing, as the orange and leaden tints,
contrasting so well with the black marks, cannot be shown.

     Between one of the elliptic ornaments and a perfect ball-and-socket
ocellus the gradation is so perfect that it is scarcely possible to decide
when the latter term ought to be used.  The passage from the one into the
other is effected by the elongation and greater curvature in the opposite
directions of the lower black mark (b fig. 59), and more especially of the
upper one (c), together with the contraction of the elongated sub-triangular
or narrow mark (d), so that at last these three marks become confluent,
forming an irregular elliptic ring.  This ring is gradually rendered more and
more circular and regular, increasing at the same time in diameter.  I have
here given a drawing (fig. 60) of the natural size of an ocellus not as yet
quite perfect.  The lower part of the black ring is much more curved than is
the lower mark in the elliptic ornament (b fig. 59).  The upper part of the
ring consists of two or three separate portions; and there is only a trace of
the thickening of the portion which forms the black mark above the white
shade.  This white shade itself is not as yet much concentrated; and beneath
it the surface is brighter colored than in a perfect ball-and-socket ocellus.
Even in the most perfect ocelli traces of the junction of three or four
elongated black marks, by which the ring has been formed, may often be
detected. The irregular sub-triangular or narrow mark (d fig. 59), manifestly
forms, by its contraction and equalization, the thickened portion of the ring
above the white shade on a perfect ball-and-socket ocellus.  The lower part of
the ring is invariably a little thicker than the other parts (see fig. 57),
and this follows from the lower black mark of the elliptic ornament (b fig.
59) having originally been thicker than the upper mark (c).  Every step can be
followed in the process of confluence and modification; and the black ring
which surrounds the ball of the ocellus is unquestionably formed by the union
and modification of the three black marks, b, c, d, of the elliptic ornament.
The irregular zigzag black marks between the successive ocelli (see again fig.
57) are plainly due to the breaking up of the somewhat more regular but
similar marks between the elliptic ornaments.

     The successive steps in the shading of the ball-and-socket ocelli can be
followed out with equal clearness.  The brown, orange and pale-leadened narrow
zones which border the lower black mark of the elliptic ornament can be seen
gradually to become more and more softened and shaded into each other, with
the upper lighter part toward the left-hand corner rendered still lighter, so
as to become almost white and at the same time more contracted.  But even in
the most perfect ball-and-socket ocelli a slight difference in the tints,
though not in the shading, between the upper and lower parts of the ball can
be perceived, as before noticed; and the line of separation is oblique in the
same direction as the bright-colored shades of the elliptic ornaments.  Thus
almost every minute detail in the shape and coloring of the ball-and-socket
ocelli can be shown to follow from gradual changes in the elliptic ornaments;
and the development of the latter can be traced by equally small steps from
the union of two almost simple spots, the lower one (fig. 58) having some dull
fulvous shading on its upper side.

     The extremities of the longer secondary feathers which bear the perfect
ball-and-socket ocelli are peculiarly ornamented (fig. 61).  The oblique
longitudinal stripes suddenly cease upward and become confused; and above this
limit the whole upper end of the feather (a) is covered with white dots
surrounded by little black rings standing on a dark ground.  The oblique
stripe belonging to the uppermost ocellus (b) is barely represented by a very
short irregular black mark with the usual curved, transverse base.  As this
stripe is thus abruptly cut off we can perhaps understand from what has gone
before how it is that the upper thickened part of the ring is here absent;
for, as before stated, this thickened part apparently stands in some relation
with a broken prolongation from the next higher spot.  From the absence of the
upper and thickened part of the ring the uppermost ocellus, though perfect in
all other respects, appears as if its top had been obliquely sliced off. It
would, I think, perplex any one who believes that the plumage of the Argus
pheasant was created as we now see it to account for the imperfect condition
of the uppermost ocellus.  I should add that on the secondary wing-feather
farthest from the body all the ocelli are smaller and less perfect than on the
other feathers and have the upper part of the ring deficient, as in the case
just mentioned.  The imperfection here seems to be connected with the fact
that the spots on this feather show less tendency than usual to become
confluent into stripes; they are, on the contrary, often broken up into
smaller spots, so that two or three rows run down to the same ocellus.

     There still remains another very curious point, first observed by Mr. T.
W. Wood, ^888 which deserves attention.  In a photograph, given me by Mr.
Ward, of a specimen mounted as in the act of display, it may be seen that on
the feathers which are held perpendicularly, the white marks on the ocelli,
representing light reflected from a convex surface, are at the upper or
farther end, that is, are directed upward; and the bird while displaying
himself on the ground would naturally be illuminated from above.  But here
comes the curious point, the outer feathers are held almost horizontally, and
their ocelli ought likewise to appear as if illuminated from above, and
consequently the white marks ought to be placed on the upper sides of the
ocelli; and, wonderful as is the fact, they are thus placed!  Hence the ocelli
on the several feathers, though occupying very different positions with
respect to the light, all appear as if illuminated from above, just as an
artist would have shaded them. Nevertheless they are not illuminated from
strictly the same point as they ought to be; for the white marks on the ocelli
of the feathers, which are held almost horizontally, are placed rather too
much toward the farther end; that is, they are not sufficiently lateral.  We
have, however, no right to expect absolute perfection in a part rendered
ornamental through sexual selection, any more than we have in a part modified
through natural selection for real use; for instance, in that wondrous organ
the human eye.  And we know what Helmholtz, the highest authority in Europe on
the subject, has said about the human eye, that if an optician had sold him an
instrument so carelessly made, he would have thought himself fully justified
in returning it. ^889

[Footnote 888: The "Field," May 28, 1870.]

[Footnote 889: "Popular Lectures on Scientific Subjects," Eng. trans., 1873,
pp. 219, 227, 269, 390.]

     We have now seen that a perfect series can be followed, from simple spots
to the wonderful ball-and-socket ornaments.  Mr. Gould, who kindly gave me
some of these feathers, fully agrees with me in the completeness of the
gradation.  It is obvious that the stages in development exhibited by the
feathers on the same bird do not at all necessarily show us the steps passed
through by the extinct progenitors of the species; but they probably give us
the clue to the actual steps, and they at least prove to demonstration that a
gradation is possible.  Bearing in mind how carefully the male Argus pheasant
displays his plumes before the female, as well as the many facts rendering it
probable that female birds prefer the more attractive males, no one who admits
the agency of sexual selection in any case will deny that a simple dark spot
with some fulvous shading might be converted, through the approximation and
modification of two adjoining spots, together with some slight increase of
color, into one of the so-called elliptic ornaments.  These latter ornaments
have been shown to many persons, and all have admitted that they are
beautiful, some thinking them even more so than the ball-and-socket ocelli.
As the secondary plumes became lengthened through sexual selection, and as the
elliptic ornaments increased in diameter, their colors apparently became less
bright; and then the ornamentation of the plumes had to be gained by an
improvement in the pattern and shading; and this process was carried on until
the wonderful ball-and-socket ocelli were finally developed.  Thus we can
understand - and in no other way as it seems to me - the present condition and
origin of the ornaments on the wing-feathers of the Argus pheasant.

     From the light afforded by the principle of gradation - from what we know
of the laws of variation - from the changes which have taken place in many of
our domesticated birds - and, lastly, from the character (as we shall
hereafter see more clearly) of the immature plumage of young birds - we can
sometimes indicate, with a certain amount of confidence, the probable steps by
which the males have acquired their brilliant plumage and various ornaments;
yet in many cases we are involved in complete darkness.  Mr. Gould several
years ago pointed out to me a humming-bird, the Urosticte benjamini,
remarkable for the curious differences between the sexes.  The male, besides a
splendid gorget, has greenish-black tail-feathers with the four central ones
tipped with white; in the female, as with most of the allied species, the
three outer tail-feathers on each side are tipped with white, so that the male
has the four central, while the female has the six exterior feathers
ornamented with white tips.  What makes the case more curious is that,
although the coloring of the tail differs remarkably in both sexes of many
kinds of humming-birds, Mr. Gould does not know a single species, besides the
Urosticte, in which the male has the four central feathers tipped with white.

     The Duke of Argyll, in commenting on this case, ^890 passes over sexual
selection, and asks: "What explanation does the law of natural selection give
of such specific varieties as these?" He answers "none whatever;" and I quite
agree with him.  But can this be so confidently said of sexual selection?
Seeing in how many ways the tail-feathers of humming-birds differ, why should
not the four central feathers have varied in this one species alone, so as to
have acquired white tips?  The variations may have been gradual or somewhat
abrupt, as in the case recently given of the humming-birds near Bogota, in
which certain individuals alone have the "central tail-feathers tipped with
beautiful green." In the female of the Urosticte I noticed extremely minute or
rudimental white tips to the two outer of the four central black
tail-feathers; so that here we have an indication of change of some kind in
the plumage of this species.  If we grant the possibility of the central
tail-feathers of the male varying in whiteness, there is nothing strange in
such variations having been sexually selected.  The white tips, together with
the small white ear-tuffs, certainly add, as the Duke of Argyll admits, to the
beauty of the male; and whiteness is apparently apprecited by other birds, as
may be inferred from such cases as the snow-white male of the bell-bird.  The
statement made by Sir R. Heron should not be forgotten, namely, that his
peahens, when debarred from access to the pied peacock, would not unite with
any other male, and during that season produced no offspring.  Nor is it
strange that variations in the tail-feathers of the Urosticte should have been
specially selected for the sake of ornament, for the next succeeding genus in
the family takes its name of Metallura from the splendor of these feathers.
We have, moreover, good evidence that humming-birds take especial pains in
displaying their tail-feathers; Mr. Belt, ^891 after describing the beauty of
the Florisuga mellivora, says: "I have seen the female sitting on a branch and
two males displaying their charms in front of her.  One would shoot up like a
rocket, then, suddenly expanding the snow-white tail, like an inverted
parachute, slowly descend in front of her, turning round gradually to show off
back and front. . . . The expanded white tail covered more space than all the
rest of the bird, and was evidently the grand feature in the performance.
While one male was descending the other would shoot up and come slowly down
expanded.  The entertainment would end in a fight between the two performers;
but whether the most beautiful or the most pugnacious was the accepted suitor,
I know not." Mr. Gould, after describing the peculiar plumage of the
Urosticte, adds, "that ornament and variety is the sole object, I have myself
but little doubt." ^892 If this be admitted, we can perceive that the males
which during former times were decked in the most elegant and novel manner
would have gained an advantage, not in the ordinary struggle for life, but in
rivalry with other males, and would have left a larger number of offspring to
inherit their newly acquired beauty.

[Footnote 890: "The Reign of Law," 1867, p. 247.]

[Footnote 891: "The Naturalist in Nicaragua," 1874, p. 112.]

[Footnote 892: "Introduction to the Trochilidae," 1861, p. 110.]