Chapter XII Secondary Sexual Characteristics of Fishes, Amphibians, and Reptiles
WE have now arrived at the great sub-kingdom of the Vertebrata,
and will commence with the lowest class, that of fishes. The males
of plagiostomous fishes (sharks, rays) and of chimaeroid fishes are
provided with claspers which serve to retain the female, like the
various structures possessed by many of the lower animals. Besides the
claspers, the males of many rays have clusters of strong sharp
spines on their heads, and several rows along "the upper outer surface
of their pectoral fins." These are present in the males of some
species, which have other parts of their bodies smooth. They are
only temporarily developed during the breeding-season; and Dr. Gunther
suspects that they are brought into action as prehensile organs by the
doubling inwards and downwards of the two sides of the body. It is a
remarkable fact that the females and not the males of some species, as
of Raia clavata, have their backs studded with large hook-formed
* Yarrell's Hist. of British Fishes, vol. ii., 1836, pp 417, 425,
436. Dr. Gunther informs me that the spines in R. clavata are peculiar
to the female.
The males alone of the capelin (Mallotus villosus, one of
Salmonidae), are provided with a ridge of closely-set, brush-like
scales, by the aid of which two males, one on each side, hold the
female, whilst she runs with great swiftness on the sandy beach, and
there deposits her spawn.* The widely distinct Monacanthus scopas
presents a somewhat analogous structure. The male, as Dr. Gunther
informs me, has a cluster of stiff, straight spines, like those of a
comb, on the sides of the tail; and these in a specimen six inches
long were nearly one and a half inches in length; the female has in
the same place a cluster of bristles, which may be compared with those
of a tooth-brush. In another species, M. peronii, the male has a brush
like that possessed by the female of the last species, whilst the
sides of the tail in the female are smooth. In some other species of
the same genus the tail can be perceived to be a little roughened in
the male and perfectly smooth in the female; and lastly in others,
both sexes have smooth sides.
* The American Naturalist, April, 1871, p. 119.
The males of many fish fight for the possession of the females. Thus
the male stickleback (Gasterosteus leiurus) has been described as "mad
with delight," when the female comes out of her hiding-place and
surveys the nest which he has made for her. "He darts round her in
every direction, then to his accumulated materials for the nest,
then back again in an instant; and as she does not advance he
endeavours to push her with his snout, and then tries to pull her by
the tail and side-spine to the nest."* The males are said to be
polygamists;*(2) they are extraordinarily bold and pugnacious,
whilst "the females are quite pacific." Their battles are at times
desperate; "for these puny combatants fasten tight on each other for
several seconds, tumbling over and over again until their strength
appears completely exhausted." With the rough-tailed stickleback (G.
trachurus) the males whilst fighting swim round and round each
other, biting and endeavouring to pierce each other with their
raised lateral spines. The same writer adds,*(3) "the bite of these
little furies is very severe. They also use their lateral spines
with such fatal effect, that I have seen one during a battle
absolutely rip his opponent quite open, so that he sank to the
bottom and died." When a fish is conquered, "his gallant bearing
forsakes him; his gay colours fade away; and he hides his disgrace
among his peaceable companions, but is for some time the constant
object of his conqueror's persecution."
* See Mr, R. Warington's interesting articles in Annals and Magazine
of Natural History, October, 1852, and November, 1855.
*(2) Noel Humphreys. River Gardens, 1857.
*(3) Loudon's Magazine of Natural History, vol. iii., 1830, p. 331.
The male salmon is as pugnacious as the little stickleback; and so
is the male trout, as I hear from Dr. Gunther. Mr. Shaw saw a
violent contest between two male salmon which lasted the whole day;
and Mr. R. Buist, Superintendent of Fisheries, informs me that he
has often watched from the bridge at Perth the males driving away
their rivals, whilst the females were spawning The males "are
constantly fighting and tearing each other on the spawning-beds, and
many so injure each other as to cause the death of numbers, many being
seen swimming near the banks of the river in a state of exhaustion,
and apparently in a dying state."* Mr. Buist informs me, that in
June 1868, the keeper of the Stormontfield breeding-ponds visited
the northern Tyne and found about 300 dead salmon, all of which with
one exception were males; and he was convinced that they had lost
their lives by fighting.
* The Field, June 29, 1867. For Mr. Shaw's statements, see Edinburgh
Review, 1843. Another experienced observer (Scrope's Days of Salmon
Fishing, p. 60) remarks that like the stag, the male would, if he
could, keep all other males away.
The most curious point about the male salmon is that during the
breeding-season, besides a slight change in colour, "the lower jaw
elongates, and a cartilaginous projection turns upwards from the
point, which, when the jaws are closed, occupies a deep cavity between
the intermaxillary bones of the upper jaw."* (See figs. 27 and 28.) In
our salmon this change of structure lasts only during the
breeding-season; but in the Salmo lycaodon of N. W. America the
change, as Mr. J. K. Lord*(2) believes, is permanent, and best
marked in the older males which have previously ascended the rivers.
In these old males the jaw becomes developed into an immense hook-like
projection, and the teeth grow into regular fangs, often more than
half an inch in length. With the European salmon, according to Mr.
Lloyd,*(3) the temporary hook-like structure serves to strengthen
and protect the jaws, when one male charges another with wonderful
violence; but the greatly developed teeth of the male American
salmon may be compared with the tusks of many male mammals, and they
indicate an offensive rather than a protective purpose.
* Yarrell, History of British Fishes, vol. ii., 1836, p. 10.
*(2) The Naturalist in Vancouver's Island, vol. i., 1866, p. 54.
*(3) Scandinavian Adventures, vol. i., 1854, pp. 100, 104.
The salmon is not the only fish in which the teeth differ in the two
sexes; as this is the case with many rays. In the thornback (Raia
clavata) the adult male has sharp, pointed teeth, directed
backwards, whilst those of the female are broad and flat, and form a
pavement; so that these teeth differ in the two sexes of the same
species more than is usual in distinct genera of the same family.
The teeth of the male become sharp only when he is adult: whilst young
they are broad and flat like those of the female. As so frequently
occurs with secondary sexual characters, both sexes of some species of
rays (for instance R. batis), when adult, possess sharp pointed teeth;
and here a character, proper to and primarily gained by the male,
appears to have been transmitted to the offspring of both sexes. The
teeth are likewise pointed in both sexes of R. maculata, but only when
quite adult; the males acquiring them at an earlier age than the
females. We shall hereafter meet with analogous cases in certain
birds, in which the male acquires the plumage common to both sexes
when adult, at a somewhat earlier age than does the female. With other
species of rays the males even when old never possess sharp teeth, and
consequently the adults of both sexes are provided with broad, flat
teeth like those of the young, and like those of the mature females of
the above-mentioned species.* As the rays are bold, strong and
voracious fish, we may suspect that the males require their sharp
teeth for fighting with their rivals; but as they possess many parts
modified and adapted for the prehension of the female, it is
possible that their teeth may be used for this purpose.
* See Yarrell's account of the rays in his History of British
Fishes, vol. ii., 1836, p. 416, with an excellent figure, and pp. 422,
In regard to size, M. Carbonnier* maintains that the female of
almost all fishes is larger than the male; and Dr. Gunther does not
know of a single instance in which the male is actually larger than
the female. With some cyprinodonts the male is not even half as large.
As in many kinds of fishes the males habitually fight together, it
is surprising that they have not generally become larger and
stronger than the females through the effects of sexual selection. The
males suffer from their small size, for according to M. Carbonnier,
they are liable to be devoured by the females of their own species
when carnivorous, and no doubt by other species. Increased size must
be in some manner of more importance to the females, than strength and
size are to the males for fighting with other males; and this
perhaps is to allow of the production of a vast number of ova.
* As quoted in the Farmer, 1868, p. 369.
In many species the male alone is ornamented with bright colours; or
these are much brighter in the male than the female. The male, also,
is sometimes provided with appendages which appear to be of no more
use to him for the ordinary purposes of life, than are the tail
feathers to the peacock. I am indebted for most of the following facts
to the kindness of Dr. Gunther. There is reason to suspect that many
tropical fishes differ sexually in colour and structure; and there are
some striking cases with our British fishes. The male Callionymus lyra
has been called the gemmeous dragonet "from its brilliant gem-like
colours." When fresh caught from the sea the body is yellow of various
shades, striped and spotted with vivid blue on the head; the dorsal
fins are pale brown with dark longitudinal bands; the ventral, caudal,
and anal fins being bluish-black. The female, or sordid dragonet,
was considered by Linnaeus, and by many subsequent naturalists, as a
distinct species; it is of a dingy reddish-brown, with the dorsal
fin brown and the other fins white. The sexes differ also in the
proportional size of the head and mouth, and in the position of the
eyes;* but the most striking difference is the extraordinary
elongation in the male (see fig. 29) of the dorsal fin. Mr. W. Saville
Kent remarks that this "singular appendage appears from my
observations of the species in confinement, to be subservient to the
same end as the wattles, crests, and other abnormal adjuncts of the
male in gallinaceous birds, for the purpose of fascinating their
mates."*(2) The young males resemble the adult females in structure
and colour. Throughout the genus Callionymus,*(3) the male is
generally much more brightly spotted than the female, and in several
species, not only the dorsal, but the anal fin is much elongated in
* I have drawn up this description from Yarrell's British Fishes,
vol. i., 1836, pp. 261 and 266.
*(2) Nature, July, 1873, p. 264.
*(3) Catalogue of Acanth. Fishes in the British Museum, by Dr.
Gunther, 1861, pp. 138-151.
The male of the Cottus scorpius, or sea-serpent, is slenderer and
smaller than the female. There is also a great difference in colour
between them. It is difficult, as Mr. Lloyd* remarks, "for any one,
who has not seen this fish during the spawning-season, when its hues
are brightest, to conceive the admixture of brilliant colours with
which it, in other respects so ill-favoured, is at that time
adorned. Both sexes of the Labrus mixtus, although very different in
colour, are beautiful; the male being orange with bright blue stripes,
and the female bright red with some black spots on the back.
* Game Birds of Sweden, &c., 1867, p. 466.
In the very distinct family of the Cyprinodontidae- inhabitants of
the fresh waters of foreign lands- the sexes sometimes differ much
in various characters. In the male of the Mollienesia petenensis,* the
dorsal fin is greatly developed and is marked with a row of large,
round, ocellated, bright-coloured spots; whilst the same fin in the
female is smaller, of a different shape, and marked only with
irregularly curved brown spots. In the male the basal margin of the
anal fin is also a little produced and dark coloured. In the male of
an allied form, the Xiphophorus Hellerii (see fig. 30), the inferior
margin of the caudal fin is developed into a long filament, which,
as I hear from Dr. Gunther, is striped with bright colours. This
filament does not contain any muscles, and apparently cannot be of any
direct use to the fish. As in the case of the Callionymus, the males
whilst young resemble the adult females in colour and structure.
Sexual differences such as these may be strictly compared with those
which are so frequent with gallinaceous birds.*(2)
* With respect to this and the following species I am indebted to
Dr. Gunther for information: see also his paper on the "Fishes of
Central America," in Transact. Zoological Soc., vol. vi., 1868, p.
*(2) Dr. Gunther makes this remark, Catalogue of Fishes in the
British Museum, vol. iii., 1861, p. 141.
In a siluroid fish, inhabiting the fresh waters of South America,
the Plecostomus barbatus* (see fig. 31), the male has its mouth and
interoperculum fringed with a beard of stiff hairs, of which the
female shows hardly a trace. These hairs are of the nature of
scales. In another species of the same genus, soft flexible
tentacles project from the front part of the head of the male, which
are absent in the female. These tentacles are prolongations of the
true skin, and therefore are not homologous with the stiff hairs of
the former species; but it can hardly be doubted that both serve the
same purpose. What this purpose may be, is difficult to conjecture;
ornament does not here seem probable, but we can hardly suppose that
stiff hairs and flexible filaments can be useful in any ordinary way
to the males alone. In that strange monster, the Chimaera monstrosa,
the male has a hook-shaped bone on the top of the head, directed
forwards, with its end rounded and covered with sharp spines; in the
female "this crown is altogether absent," but what its use may be to
the male is utterly unknown.*(2)
* See Dr. Gunther on this genus, in Proceedings of the Zoological
Society, 1868, p. 232.
*(2) F. Buckland, in Land and Water, July, 1868, p. 377, with a
figure. Many other cases could be added of structures peculiar to
the male, of which the uses are not known.
The structures as yet referred to are permanent in the male after he
has arrived at maturity; but with some blennies, and in another allied
genus,* a crest is developed on the head of the male only during the
breeding-season, and the body at the same time becomes more
brightly-coloured. There can be little doubt that this crest serves as
a temporary sexual ornament, for the female does not exhibit a trace
of it. In other species of the same genus both sexes possess a
crest, and in at least one species neither sex is thus provided. In
many of the Chromidae, for instance in Geophagus and especially in
Cichla, the males, as I hear from Professor Agassiz,*(2) have a
conspicuous protuberance on the forehead, which is wholly wanting in
the females and in the young males. Professor Agassiz adds, "I have
often observed these fishes at the time of spawning when the
protuberance is largest, and at other seasons when it is totally
wanting, and the two sexes shew no difference whatever in the
outline of the profile of the head. I never could ascertain that it
subserves any special function, and the Indians on the Amazon know
nothing about its use." These protuberances resemble, in their
periodical appearance, the fleshy carbuncles on the heads of certain
birds; but whether they serve as ornaments must remain at present
* Dr. Gunther, Catalogue of Fishes, vol. iii., pp. 221 and 240.
*(2) See also A Journey in Brazil, by Prof. and Mrs. Agassiz,
1868, p. 220.
I hear from Professor Agassiz and Dr. Gunther, that the males of
those fishes, which differ permanently in colour from the females,
often become more brilliant during the breeding-season. This is
likewise the case with a multitude of fishes, the sexes of which are
identical in colour at all other seasons of the year. The tench,
roach, and perch may be given as instances. The male salmon at this
season is marked on the cheeks with orange-coloured stripes, which
give it the appearance of a Labrus, and the body partakes of a
golden orange tinge. The females are dark in colour, and are
commonly called black-fish."* An analogous and even greater change
takes place with the Salmo eriox or bull trout; the males of the
char (S. umbla) are likewise at this season rather brighter in
colour than the females.*(2) The colours of the pike (Esox
reticulatus) of the United States, especially of the male, become,
during the breeding-season, exceedingly intense, brilliant, and
iridescent.*(3) Another striking instance out of many is afforded by
the male stickleback (Gasterosteus leiurus), which is described by Mr.
Warington,*(4) as being then "beautiful beyond description." The
back and eyes of the female are simply brown and the belly white.
The eyes of the male, on the other hand, are "of the most splendid
green, having a metallic lustre like the green feathers of some
humming-birds. The throat and belly are of a bright crimson, the
back of an ashy-green, and the whole fish appears as though it were
somewhat translucent and glowed with an internal incandescence." After
the breeding-season these colours all change, the throat and belly
become of a paler red, the back more green, and the glowing tints
* Yarrell, History of British Fishes, vol. ii., 1836, pp. 10, 12,
*(2) W. Thompson, in Annals and Magazine of Natural History, vol.
vi., 1841, p. 440.
*(3) The American Agriculturalist, 1868, p. 100.
*(4) Annals and Mag. of Nat. Hist., Oct., 1852.
With respect to the courtship of fishes, other cases have been
observed since the first edition of this book appeared, besides that
already given of the stickleback. Mr. W. S. Kent says that the male of
the Labrus mixtus, which, as we have seen, differs in colour from
the female, makes "a deep hollow in the sand of the tank, and then
endeavours in the most persuasive manner to induce a female of the
same species to share it with him, swimming backwards and forwards
between her and the completed nest, and plainly exhibiting the
greatest anxiety for her to follow." The males of Cantharus lineatus
become, during the breeding-season, of deep leaden-black; they then
retire from the shoal, and excavate a hollow as a nest. "Each male now
mounts vigilant guard over his respective hollow, and vigorously
attacks and drives away any other fish of the same sex. Towards his
companions of the opposite sex his conduct is far different; many of
the latter are now distended with spawn, and these he endeavours by
all the means in his power to lure singly to his prepared hollow,
and there to deposit the myriad ova with which they are laden, which
he then protects and guards with the greatest care."*
* Nature, May, 1873, p. 25.
A more striking case of courtship, as well as of display, by the
males of a Chinese Macropus has been given by M. Carbonnier, who
carefully observed these fishes under confinement.* The males are most
beautifully coloured, more so than the females. During the
breeding-season they contend for the possession of the females; and,
in the act of courtship, expand their fins, which are spotted and
ornamented with brightly coloured rays, in the same manner,
according to M. Carbonnier, as the peacock. They then also bound about
the females with much vivacity, and appear by "l'etalage de leurs
vives couleurs chercher a attirer l'attention des femelles, lesquelles
ne paraissaient indifferentes a ce manege, elles nageaient avec une
molle lenteur vers les males et semblaient se complaire dans leur
voisinage." After the male has won his bride, he makes a little disc
of froth by blowing air and mucus out of his mouth. He then collects
the fertilised ova, dropped by the female, in his mouth; and this
caused M. Carbonnier much alarm, as he thought that they were going to
be devoured. But the male soon deposits them in the disc of froth,
afterwards guarding them, repairing the froth, and taking care of
the young when hatched. I mention these particulars because, as we
shall presently see, there are fishes, the males of which hatch
their eggs in their mouths; and those who do not believe in the
principle of gradual evolution might ask how could such a habit have
originated; but the difficulty is much diminished when we know that
there are fishes which thus collect and carry the eggs; for if delayed
by any cause in depositing them, the habit of hatching them in their
mouths might have been acquired.
* Bulletin de la Societe d'Acclimation, Paris, July, 1869, and Jan.,
To return to our more immediate subject. The case stands thus:
female fishes, as far as I can learn, never willingly spawn except
in the presence of the males; and the males never fertilise the ova
except in the presence of the females. The males fight for the
possession of the females. In many species, the males whilst young
resemble the females in colour; but when adult become much more
brilliant, and retain their colours throughout life. In other
species the males become brighter than the females and otherwise
more highly ornamented, only during the season of love. The males
sedulously court the females, and in one case, as we have seen, take
pains in displaying their beauty before them. Can it be believed
that they would thus act to no purpose during their courtship? And
this would be the case, unless the females exert some choice and
select those males which please or excite them most. If the female
exerts such choice, all the above facts on the ornamentation of the
males become at once intelligible by the aid of sexual selection.
We have next to inquire whether this view of the bright colours of
certain male fishes having been acquired through sexual selection can,
through the law of the equal transmission of characters to both sexes,
be extended to those groups in which the males and females are
brilliant in the same, or nearly the same degree and manner. In such a
genus as Labrus, which includes some of the most splendid fishes in
the world- for instance, the peacock Labrus (L. pavo), described,*
with pardonable exaggeration, as formed of polished scales of gold,
encrusting lapis-lazuli, rubies, sapphires, emeralds, and amethysts-
we may, with much probability, accept this belief; for we have seen
that the sexes in at least one species of the genus differ greatly
in colour. With some fishes, as with many of the lowest animals,
splendid colours may be the direct result of the nature of their
tissues and of the surrounding conditions, without the aid of
selection of any kind. The gold-fish (Cyprinus auratus), judging
from the analogy of the golden variety of the common carp, is
perhaps a case in point, as it may owe its splendid colours to a
single abrupt variation, due to the conditions to which this fish
has been subjected under confinement. It is, however, more probable
that these colours have been intensified through artificial selection,
as this species has been carefully bred in China from a remote
period.*(2) Under natural conditions it does not seem probable that
beings so highly organised as fishes, and which live under such
complex relations, should become brilliantly coloured without
suffering some evil or receiving some benefit from so great a
change, and consequently without the intervention of natural
* Bory de Saint Vincent, in Dict. Class. d'Hist. Nat., tom. ix.,
1826, p. 151.
*(2) Owing to some remarks on this subject, made in my work On the
Variation of Animals under Domestication, Mr. W. F. Mayers (Chinese
Notes and Queries, Aug., 1868, p. 123) has searched the ancient
Chinese encyclopedias. He finds that gold-fish were first reared in
confinement during the Sung Dynasty, which commenced A.D. 960. In
the year 1129 these fishes abounded. In another place it is said
that since the year 1548 there has been "produced at Hangchow a
variety called the fire-fish, from its intensely red colour. It is
universally admired, and there is not a household where it is not
cultivated, in rivalry as to its colour, and as a source of profit."
What, then, are we to conclude in regard to the many fishes, both
sexes of which are splendidly coloured? Mr. Wallace* believes that the
species which frequent reefs, where corals and other brightly-coloured
organisms abound, are brightly coloured in order to escape detection
by their enemies; but according to my recollection they were thus
rendered highly conspicuous. In the fresh-waters of the tropics
there are no brilliantly-coloured corals or other organisms for the
fishes to resemble; yet many species in the Amazons are beautifully
coloured, and many of the carnivorous Cyprinidae in India are
ornamented with "bright longitudinal lines of various tints."*(2)
Mr. M'Clelland, in describing these fishes, goes so far as to
suppose that "the peculiar brilliancy of their colours" serves as "a
better mark for king-fishers, terns, and other birds which are
destined to keep the number of these fishes in check"; but at the
present day few naturalists will admit that any animal has been made
conspicuous as an aid to its own destruction. It is possible that
certain fishes may have been rendered conspicuous in order to warn
birds and beasts of prey that they were unpalatable, as explained when
treating of caterpillars; but it is not, I believe, known that any
fish, at least any fresh-water fish, is rejected from being
distasteful to fish-devouring animals. On the whole, the most probable
view in regard to the fishes, of which both sexes are brilliantly
coloured, is that their colours were acquired by the males as a sexual
ornament, and were transferred equally, or nearly so, to the other
* Westminster Review, July, 1867, p. 7.
*(2) "Indian Cyprinidae," by Mr. M'Clelland, Asiatic Researches,
vol. xix., part ii., 1839, p. 230.
We have now to consider whether, when the male differs in a marked
manner from the female in colour or in other ornaments, he alone has
been modified, the variations being inherited by his male offspring
alone; or whether the female has been specially modified and
rendered inconspicuous for the sake of protection, such
modifications being inherited only by the females. It is impossible to
doubt that colour has been gained by many fishes as a protection: no
one can examine the speckled upper surface of a flounder, and overlook
its resemblance to the sandy bed of the sea on which it lives. Certain
fishes, moreover, can through the action of the nervous system
change their colours in adaptation to surrounding objects, and that
within a short time.* One of the most striking instances ever recorded
of an animal being protected by its colour (as far as it can be judged
of in preserved specimens), as well as by its form, is that given by
Dr. Gunther*(2) of a pipe-fish, which, with its reddish streaming
filaments, is hardly distinguishable from the sea-weed to which it
clings with its prehensile tail. But the question now under
consideration is whether the females alone have been modified for this
object. We can see that one sex will not be modified through natural
selection for the sake of protection more than the other, supposing
both to vary, unless one sex is exposed for a longer period to danger,
or has less power of escaping from such danger than the other; and
it does not appear that with fishes the sexes differ in these
respects. As far as there is any difference, the males, from being
generally smaller and from wandering more about, are exposed to
greater danger than the females; and yet, when the sexes differ, the
males are almost always the more conspicuously coloured. The ova are
fertilised immediately after being deposited; and when this process
lasts for several days, as in the case of the salmon,*(3) the
female, during the whole time, is attended by the male. After the
ova are fertilised they are, in most cases, left unprotected by both
parents, so that the males and females, as far as oviposition is
concerned, are equally exposed to danger, and both are equally
important for the production of fertile ova; consequently the more
or less brightly-coloured individuals of either sex would be equally
liable to be destroyed or preserved, and both would have an equal
influence on the colours of their offspring.
* G. Pouchet, L'Institut., Nov. 1, 1871, p. 134.
*(2) Proc. Zoolog. Soc., 1865, p. 327, pls. xiv. and xv.
*(3) Yarrell, British Fishes, vol. ii., p. 11.
Certain fishes belonging to several families, make nests, and some
of them take care of their young when hatched. Both sexes of the
bright-coloured Crenilabrus massa and melops work together in building
their nests with seaweed, shells, &c.* But the males of certain fishes
do all the work, and afterward take exclusive charge of the young.
This is the case with the dull-coloured gobies,*(2) in which the sexes
are not known to differ in colour, and likewise with the
sticklebacks (Gasterosteus), in which the males become brilliantly
coloured during the spawning season. The male of the smooth-tailed
stickleback (G. leiurus) performs the duties of a nurse with exemplary
care and vigilance during a long time, and is continually employed
in gently leading back the young to the nest, when they stray too far.
He courageously drives away all enemies including the females of his
own species. It would indeed be no small relief to the male, if the
female, after depositing her eggs, were immediately devoured by some
enemy, for he is forced incessantly to drive her from the nest.*(3)
* According to the observations of M. Gerbe; see Gunther's Record of
Zoolog. Literature, 1865, p. 194.
*(2) Cuvier, Regne Animal, vol. ii., 1829, p. 242.
*(3) See Mr. Warington's most interesting description of the
habits of the Gasterosteus leiurus in Annals and Magazine of Nat.
History, November, 1855.
The males of certain other fishes inhabiting South America and
Ceylon, belonging to two distinct Orders, have the extraordinary habit
of hatching within their mouths, or branchial cavities, the eggs
laid by the females.* I am informed by Professor Agassiz that the
males of the Amazonian species which follow this habit, "not only
are generally brighter than the females, but the difference is greater
at the spawning-season than at any other time." The species of
Geophagus act in the same manner; and in this genus, a conspicuous
protuberance becomes developed on the forehead of the males during the
breeding-season. With the various species of chromids, as Professor
Agassiz likewise informs me, sexual differences in colour may be
observed, "whether they lay their eggs in the water among aquatic
plants, or deposit them in holes, leaving them to come out without
further care, or build shallow nests in the river mud, over which they
sit, as our Pomotis does. It ought also to be observed that these
sitters are among the brightest species in their respective
families; for instance, Hygrogonus is bright green, with large black
ocelli, encircled with the most brilliant red." Whether with all the
species of chromids it is the male alone which sits on the eggs is not
known. It is, however, manifest that the fact of the eggs being
protected or unprotected by the parents, has had little or no
influence on the differences in colour between the sexes. It is
further manifest, in all the cases in which the males take exclusive
charge of the nests and young, that the destruction of the
brighter-coloured males would be far more influential on the character
of the race, than the destruction of the brighter-coloured females;
for the death of the male during the period of incubation or nursing
would entail the death of the young, so that they could not inherit
his peculiarities; yet, in many of these very cases the males are more
conspicuously coloured than the females.
* Prof. Wyman, in Proc. Boston Soc. of Nat. Hist., Sept. 15, 1857.
Also Prof. Turner, in Journal of Anatomy and Physiology, Nov. 1, 1866,
p. 78. Dr. Gunther has likewise described other cases.
In most of the Lophobranchii (pipe-fish, Hippocampi, &c.) the
males have either marsupial sacks or hemispherical depressions on
the abdomen, in which the ova laid by the female are hatched. The
males also shew great attachment to their young.* The sexes do not
commonly differ much in colour; but Dr. Gunther believes that the male
Hippocampi are rather brighter than the females. The genus
Solenostoma, however, offers a curious exceptional case,*(2) for the
female is much more vividly-coloured and spotted than the male, and
she alone has a marsupial sack and hatches the eggs; so that the
female of Solenostoma differs from all the other Lophobranchii in this
latter respect, and from almost all other fishes, in being more
brightly-coloured than the male. It is improbable that this remarkable
double inversion of character in the female should be an accidental
coincidence. As the males of several fishes, which take exclusive
charge of the eggs and young, are more brightly coloured than the
females, and as here the female Solenostoma takes the same charge
and is brighter than the male, it might be argued that the conspicuous
colours of that sex which is the more important of the two for the
welfare of the offspring, must be in some manner protective. But
from the large number of fishes, of which the males are either
permanently or periodically brighter than the females, but whose
life is not at all more important for the welfare of the species
than that of the female, this view can hardly be maintained. When we
treat of birds we shall meet with analogous cases, where there has
been a complete inversion of the usual attributes of the two sexes,
and we shall then give what appears to be the probable explanation,
namely, that the males have selected the more attractive females,
instead of the latter having selected, in accordance with the usual
rule throughout the animal kingdom, the more attractive males.
* Yarrell, History of British Fishes, vol. ii., 1836, pp. 329, 338.
*(2) Dr. Gunther, since publishing an account of this species in The
Fishes of Zanzibar, by Col. Playfair, 1866, p. 137, has re-examined
the specimens, and has given me the above information.
On the whole we may conclude, that with most fishes, in which the
sexes differ in colour or in other ornamental characters, the males
originally varied, with their variations transmitted to the same
sex, and accumulated through sexual selection by attracting or
exciting the females. In many cases, however, such characters have
been transferred, either partially or completely, to the females. In
other cases, again, both sexes have been coloured alike for the sake
of protection; but in no instance does it appear that the female alone
has had her colours or other characters specially modified for this
The last point which need be noticed is that fishes are known to
make various noises, some of which are described as being musical. Dr.
Dufosse, who has especially attended to this subject, says that the
sounds are voluntarily produced in several ways by different fishes:
by the friction of the pharyngeal bones- by the vibration of certain
muscles attached to the swim bladder, which serves as a resounding
board- and by the vibration of the intrinsic muscles of the swim
bladder. By this latter means the Trigla produces pure and
long-drawn sounds which range over nearly an octave. But the most
interesting case for us is that of two species of Ophidium, in which
the males alone are provided with a sound-producing apparatus,
consisting of small movable bones, with proper muscles, in
connection with the swim bladder.* The drumming of the Umbrinas in the
European seas is said to be audible from a depth of twenty fathoms;
and the fishermen of Rochelle assert "that the males alone make the
noise during the spawning-time; and that it is possible by imitating
it, to take them without bait."*(2) From this statement, and more
especially from the case of Ophidium, it is almost certain that in
this, the lowest class of the Vertebrata, as with so many insects
and spiders, sound-producing instruments have, at least in some cases,
been developed through sexual selection, as a means for bringing the
* Comptes-Rendus, tom. xlvi., 1858, p. 353; tom. xlvii., 1858, p.
916; tom. liv., 1862, p. 393. The noise made by the Umbrinas
(Sciaena aquila), is said by some authors to be more like that of a
flute or organ, than drumming: Dr. Zouteveen, in the Dutch translation
of this work (vol. ii., p. 36), gives some further particulars on
the sounds made by fishes.
*(2) The Rev. C. Kingsley, in Nature, May, 1870, p. 40.
URODELA.- I will begin with the tailed amphibians. The sexes of
salamanders or newts often differ much both in colour and structure.
In some species prehensile claws are developed on the fore-legs of the
males during the breeding-season: and at this season in the male
Triton palmipes the hind-feet are provided with a swimming-web,
which is almost completely absorbed during the winter; so that their
feet then resemble those of the female.* This structure no doubt
aids the male in his eager search and pursuit of the female. Whilst
courting her he rapidly vibrates the end of his tail. With our
common newts (Triton punctatus and cristatus) a deep, much indented
crest is developed along the back and tail of the male during the
breeding-season, which disappears during the winter. Mr. St. George
Mivart informs me that it is not furnished with muscles, and therefore
cannot be used for locomotion. As during the season of courtship it
becomes edged with bright colours, there can hardly be a doubt that it
is a masculine ornament. In many species the body presents strongly
contrasted, though lurid tints, and these become more vivid during the
breeding-season. The male, for instance, of our common little newt
(Triton punctatus) is "brownish-grey above, passing into yellow
beneath, which in the spring becomes a rich bright orange, marked
everywhere with round dark spots." The edge of the crest also is
then tipped with bright red or violet. The female is usually of a
yellowish-brown colour with scattered brown dots, and the lower
surface is often quite plain.*(2) The young are obscurely tinted.
The ova are fertilised during the act of deposition, and are not
subsequently tended by either parent. We may therefore conclude that
the males have acquired their strongly-marked colours and ornamental
appendages through sexual selection; these being transmitted either to
the male offspring alone, or to both sexes.
* Bell, History of British Reptiles, 2nd ed., 1849, pp. 156-159.
*(2) Bell, History of British Reptiles, 2nd ed., 1849, pp. 146, 151.
ANURA or BATRACHIA.- With many frogs and toads the colours evidently
serve as a protection, such as the bright green tints of tree frogs
and the obscure mottled shades of many terrestrial species. The most
conspicuously-coloured toad which I ever saw, the Phryniscus
nigricans,* had the whole upper surface of the body as black as ink,
with the soles of the feet and parts of the abdomen spotted with the
brightest vermilion. It crawled about the bare sandy or open grassy
plains of La Plata under a scorching sun, and could not fail to
catch the eye of every passing creature. These colours are probably
beneficial by making this animal known to all birds of prey as a
* Zoology of the Voyage of the Beagle, 1843. Bell, ibid., p. 49.
In Nicaragua there is a little frog "dressed in a bright livery of
red and blue" which does not conceal itself like most other species,
but hops about during the daytime, and Mr. Belt says* that as soon
as he saw its happy sense of security, he felt sure that it was
uneatable. After several trials he succeeded in tempting a young
duck to snatch up a young one, but it was instantly rejected; and
the duck "went about jerking its head, as if trying to throw off
some unpleasant taste."
* The Naturalist in Nicaragua, 1874, p. 321.
With respect to sexual differences of colour, Dr. Gunther does not
know of any striking instance either with frogs or toads; yet he can
often distinguish the male from the female by the tints of the
former being a little more intense. Nor does he know of any striking
difference in external structure between the sexes, excepting the
prominences which become developed during the breeding-season on the
front legs of the male, by which he is enabled to hold the female.* It
is surprising that these animals have not acquired more
strongly-marked sexual characters; for though cold-blooded their
passions are strong. Dr. Gunther informs me that he has several
times found an unfortunate female toad dead and smothered from
having been so closely embraced by three or four males. Frogs have
been observed by Professor Hoffman in Giessen fighting all day long
during the breeding-season, and with so much violence that one had its
body ripped open.
* The male alone of the Bufo sikimmensis (Dr. Anderson, Proc.
Zoolog. Soc., 1871, p. 204) has two plate-like callosities on the
thorax and certain rugosities on the fingers, which perhaps subserve
the same end as the above-mentioned prominences.
Frogs and toads offer one interesting sexual difference, namely,
in the musical powers possessed by the males; but to speak of music,
when applied to the discordant and overwhelming sounds emitted by male
bullfrogs and some other species, seems, according to our taste, a
singularly inappropriate expression. Nevertheless, certain frogs
sing in a decidedly pleasing manner. Near Rio Janeiro I used often
to sit in the evening to listen to a number of little Hylae, perched
on blades of grass close to the water, which sent forth sweet chirping
notes in harmony. The various sounds are emitted chiefly by the
males during the breeding-season, as in the case of the croaking of
our common frog.* In accordance with this fact the vocal organs of the
males are more highly-developed than those of the females. In some
genera the males alone are provided with sacs which open into the
larynx.*(2) For instance, in the edible frog (Rana esculenta) "the
sacs are peculiar to the males, and become, when filled with air in
the act of croaking, large globular bladders, standing out one on each
side of the head, near the corners of the mouth." The croak of the
male is thus rendered exceedingly powerful; whilst that of the
female is only a slight groaning noise.*(3) In the several genera of
the family the vocal organs differ considerably in structure, and
their development in all cases may be attributed to sexual selection.
* Bell, History British Reptiles, 1849, p. 93.
*(2) J. Bishop, in Todd's Cyclopaedia of Anatomy and Physiology,
vol. iv., p. 1503.
*(3) Bell, ibid., pp. 112-114.
CHELONIA.- Tortoises and turtles do not offer well-marked sexual
differences. In some species, the tail of the male is longer than that
of the female. In some, the plastron or lower surface of the shell
of the male is slightly concave in relation to the back of the female.
The male of the mud-turtle of the United States (Chrysemys picta)
has claws on its front feet twice as long as those of the female;
and these are used when the sexes unite.* With the huge tortoise of
the Galapagos Islands (Testudo nigra) the males are said to grow to
a larger size than the females: during the pairing-season, and at no
other time, the male utters a hoarse bellowing noise, which can be
heard at the distance of more than a hundred yards; the female, on the
other hand, never uses her voice.*(2)
* Mr. C. J. Maynard, the American Naturalist, Dec., 1869, p. 555.
*(2) See my Journal of Researches during the Voyage of the Beagle,
1845, p. 384.
With the Testudo elegans of India, it is said "that the combats of
the males may be heard at some distance, from the noise they produce
in butting against each other."*
* Dr. Gunther, Reptiles of British India, 1864, p. 7.
CROCODILIA.- The sexes apparently do not differ in colour; nor do
I know that the males fight together, though this is probable, for
some kinds make a prodigious display before the females. Bartram*
describes the male alligator as striving to win the female by
splashing and roaring in the midst of a lagoon, "swollen to an
extent ready to burst, with its head and tail lifted up, he springs or
twirls round on the surface of the water, like an Indian chief
rehearsing his feats of war." During the season of love, a musky odour
is emitted by the sub-maxiliary glands of the crocodile, and
pervades their haunts.*(2)
* Travels through Carolina, &c., 1791, p. 128.
*(2) Owen, Anatomy of Vertebrates, vol. i., 1866, p. 615.
OPHIDIA.- Dr. Gunther informs me that the males are always smaller
than the females, and generally have longer and slenderer tails; but
he knows of no other difference in external structure. In regard to
colour, be can almost always distinguish the male from the female,
by his more strongly-pronounced tints; thus the black zigzag band on
the back of the male English viper is more distinctly defined than
in the female. The difference is much plainer in the rattle-snakes
of N. America, the male of which, as the keeper in the Zoological
Gardens shewed me, can at once be distinguished from the female by
having more lurid yellow about its whole body. In S. Africa the
Bucephalus capensis presents an analogous difference, for the female
"is never so fully variegated with yellow on the sides as the
male."* The male of the Indian Dipsas cynodon, on the other hand, is
blackish-brown, with the belly partly black, whilst the female is
reddish or yellowish-olive, with the belly either uniform yellowish or
marbled with black. In the Tragops dispar of the same country the male
is bright green, and the female bronze-coloured.*(2) No doubt the
colours of some snakes are protective, as shewn by the green tints
of tree-snakes, and the various mottled shades of the species which
live in sandy places; but it is doubtful whether the colours of many
kinds, for instance of the common English snake and viper, serve to
conceal them; and this is still more doubtful with the many foreign
species which are coloured with extreme elegance. The colours of
certain species are very different in the adult and young states.*(3)
* Sir Andrew Smith, Zoology of S. Africa: Reptilia, 1849, pl. x.
*(2) Dr. A. Gunther, "Reptiles of British India," Ray Soc., 1864,
pp. 304, 308.
*(3) Dr. Stoliczka, Journal of Asiatic Society of Bengal., vol.
xxxix, 1870, pp. 205, 211.
During the breeding-season the anal scentglands of snakes are in
active function;* and so it is with the same glands in lizards, and as
we have seen with the submaxiliary glands of crocodiles. As the
males of most animals search for the females, these odoriferous glands
probably serve to excite or charm the female, rather than to guide her
to the spot where the male may be found. Male snakes, though appearing
so sluggish, are amorous; for many have been observed crowding round
the same female, and even round her dead body. They are not known to
fight together from rivalry. Their intellectual powers are higher than
might have been anticipated. In the Zoological Gardens they soon learn
not to strike at the iron bar with which their cages are cleaned;
and Dr. Keen of Philadelphia informs me that some snakes which he kept
learned after four or five times to avoid a noose, with which they
were at first easily caught. An excellent observer in Ceylon, Mr. E.
Layard, saw*(2) a cobra thrust its head through a narrow hole and
swallow a toad. "With this encumbrance be could not withdraw
himself; finding this, he reluctantly disgorged the precious morsel,
which began to move off; this was too much for snake philosophy to
bear, and the toad was again seized, and again was the snake, after
violent efforts to escape, compelled to part with its prey. This time,
however, a lesson had been learnt, and the toad was seized by one leg,
withdrawn, and then swallowed in triumph."
* Owen, Anatomy of Vertebrates, vol. i., 1866, p. 615.
*(2) "Rambles in Ceylon," in Annals and Magazine of Natural History,
2nd series, vol. ix., 1852, p. 333.
The keeper in the Zoological Gardens is positive that certain
snakes, for instance Crotalus and Python, distinguish him from all
other persons. Cobras kept together in the same cage apparently feel
some attachment towards each other.*
* Dr. Gunther, Reptiles of British India, 1864, p. 340.
It does not, however, follow because snakes have some reasoning
power, strong passions and mutual affection, that they should likewise
be endowed with sufficient taste to admire brilliant colours in
their partners, so as to lead to the adornment of the species
through sexual selection. Nevertheless, it is difficult to account
in any other manner for the extreme beauty of certain species; for
instance, of the coral-snakes of S. America, which are of a rich red
with black and yellow transverse bands. I well remember how much
surprise I felt at the beauty of the first coral-snake which I saw
gliding across a path in Brazil. Snakes coloured in this peculiar
manner, as Mr. Wallace states on the authority of Dr. Gunther,* are
found nowhere else in the world except in S. America, and here no less
than four genera occur. One of these, Elaps, is venomous; a second and
widely-distinct genus is doubtfully venomous, and the two others are
quite harmless. The species belonging to these distinct genera inhabit
the same districts, and are so like each other that no one "but a
naturalist would distinguish the harmless from the poisonous kinds."
Hence, as Mr. Wallace believes, the innocuous kinds have probably
acquired their colours as a protection, on the principle of imitation;
for they would naturally be thought dangerous by their enemies. The
cause, however, of the bright colours of the venomous Elaps remains to
be explained, and this may perhaps be sexual selection.
* Westminster Review, July 1, 1867, p. 32.
Snakes produce other sounds besides hissing. The deadly Echis
carinata has on its sides some oblique rows of scales of a peculiar
structure with serrated edges; and when this snake is excited these
scales are rubbed against each other, which produces "a curious
prolonged, almost hissing sound."* With respect to the rattling of the
rattle-snake, we have at last some definite information: for Professor
Aughey states,*(2) that on two occasions, being himself unseen, he
watched from a little distance a rattle-snake coiled up with head
erect, which continued to rattle at short intervals for half an
hour: and at last he saw another snake approach, and when they met
they paired. Hence be is satisfied that one of the uses of the
rattle is to bring the sexes together. Unfortunately he did not
ascertain whether it was the male or the female which remained
stationary and called for the other. But it by no means follows from
the above fact that the rattle may not be of use to snakes in other
ways, as a warning to animals which would otherwise attack them. Nor
can I quite disbelieve the several accounts which have appeared of
their thus paralysing their prey with fear. Some other snakes also
make a distinct noise by rapidly vibrating their tails against the
surrounding stalks of plants; and I have myself heard this in the case
of a Trigonocephalus in S. America.
* Dr. Anderson, Proc. Zoolog. Soc., 1871, p. 196.
*(2) The American Naturalist, 1873, p. 85.
LACERTILIA.- The males of some, probably of many kinds of lizards,
fight together from rivalry. Thus the arboreal Anolis cristatellus
of S. America is extremely pugnacious: "During the spring and early
part of the summer, two adult males rarely meet without a contest.
On first seeing one another, they nod their heads up and down three or
four times, and at the same time expanding the frill or pouch
beneath the throat; their eyes glisten with rage, and after waving
their tails from side to side for a few seconds, as if to gather
energy, they dart at each other furiously, rolling over and over,
and holding firmly with their teeth. The conflict generally ends in
one of the combatants losing his tail, which is often devoured by
the victor." The male of this species is considerably larger than
the female;* and this, as far as Dr. Gunther has been able to
ascertain, is the general rule with lizards of all kinds. The male
alone of the Cyrtodactylus rubidus of the Andaman Islands possesses
pre-anal pores; and these pores, judging from analogy, probably
serve to emit an odour.*(2)
* Mr. N. L. Austen kept these animals alive for a considerable time;
see Land and Water, July, 1867, P. 9.
*(2) Stoliczka, Journal of the Asiatic Society of Bengal, vol.
xxxiv., 1870, p. 166.
The sexes often differ greatly in various external characters. The
male of the above-mentioned Anolis is furnished with a crest which
runs along the back and tail, and can be erected at pleasure; but of
this crest the female does not exhibit a trace. In the Indian Cophotis
ceylanica, the female has a dorsal crest, though much less developed
than in the male; and so it is, as Dr. Gunther informs me, with the
females of many iguanas, chameleons, and other lizards. In some
species, however, the crest is equally developed in both sexes, as
in the Iguana tuberculata. In the genus Sitana, the males alone are
furnished with a large throat pouch (see fig. 33), which can be folded
up like a fan, and is coloured blue, black, and red; but these
splendid colours are exhibited only during the pairing-season. The
female does not possess even a rudiment of this appendage. In the
Anolis cristatellus, according to Mr. Austen, the throat pouch,
which is bright red marbled with yellow, is present in the female,
though in a rudimental condition. Again, in certain other lizards,
both sexes are equally well provided with throat pouches. Here we
see with species belonging to the same group, as in so many previous
cases, the same character either confined to the males, or more
largely developed in them than in the females, or again equally
developed in both sexes. The little lizards of the genus Draco,
which glide through the air on their rib-supported parachutes, and
which in the beauty of their colours baffle description, are furnished
with skinny appendages to the throat "like the wattles of gallinaceous
birds." These become erected when the animal is excited. They occur in
both sexes, but are best developed when the male arrives at
maturity, at which age the middle appendage is sometimes twice as long
as the head. Most of the species likewise have a low crest running
along the neck; and this is much more developed in the full-grown
males than in the females or young males.*
* All the foregoing statements and quotations, in regard to
Cophotis, Sitana and Draco, as well as the following facts in regard
to Ceratophora and Chamaeleon, are from Dr. Gunther himself, or from
his magnificent work on the "Reptiles of British India," Ray Soc.,
1864, pp. 122, 130, 135.
A Chinese species is said to live in pairs during the spring; "and
if one is caught, the other falls from the tree to the ground, and
allows itself to be captured with impunity"- I presume from despair.*
* Mr. Swinhoe, Proc. Zoolog. Soc., 1870, p. 240.
There are other and much more remarkable differences between the
sexes of certain lizards. The male of Ceratophora aspera bears on
the extremity of his snout an appendage half as long as the head. It
is cylindrical, covered with scales, flexible, and apparently
capable of erection: in the female it is quite rudimental. In a second
species of the same genus a terminal scale forms a minute horn on
the summit of the flexible appendage; and in a third species (see C.
stoddartii, fig. 34) the whole appendage is converted into a horn,
which is usually of a white colour, but assumes a purplish tint when
the animal is excited. In the adult male of this latter species the
horn is half an inch in length, but it is of quite minute size in
the female and in the young. These appendages, as Dr. Gunther has
remarked to me, may be compared with the combs of gallinaceous
birds, and apparently serve as ornaments.
In the genus Chamaeleon we come to the acme of difference between
the sexes. The upper part of the skull of the male C. bifurcus (see
fig. 35), an inhabitant of Madagascar, is produced into two great,
solid, bony projections, covered with scales like the rest of the
head; and of this wonderful modification of structure the female
exhibits only a rudiment. Again, in Chamaeleo owenii (see fig. 36),
from the west coast of Africa, the male bears on his snout and
forehead three curious horns, of which the female has not a trace.
These horns consist of an excrescence of bone covered with a smooth
sheath, forming part of the general integuments of the body, so that
they are identical in structure with those of a bull, goat, or other
sheath-horned ruminant. Although the three horns differ so much in
appearance from the two great prolongations of the skull in C.
bifurcus, we can hardly doubt that they serve the same general purpose
in the economy of these two animals. The first conjecture, which
will occur to every one, is that they are used by the males for
fighting together; and as these animals are very quarrelsome,* this is
probably a correct view. Mr. T. W. Wood also informs me that he once
watched two individuals of C. pumilus fighting violently on the branch
of a tree; they flung their heads about and tried to bite each
other; they then rested for a time and afterwards continued their
* Dr. Buchholz, Monatsbericht K. Preuss. Akad., Jan., 1874, p. 78.
With many lizards the sexes differ slightly in colour, the tints and
stripes of the males being brighter and more distinctly defined than
in the females. This, for instance, is the case with the above
Cophotis and with the Acanthodactylus capensis of S. Africa. In a
Cordylus of the latter country, the male is either much redder or
greener than the female. In the Indian Calotes nigrilabris there is
a still greater difference; the lips also of the male are black,
whilst those of the female are green. In our common little
viviparous lizard (Zootoca vivipara) "the under side of the body and
base of the tail in the male are bright orange, spotted with black; in
the female these parts are pale-greyish-green without spots."* We have
seen that the males alone of Sitana possess a throat-pouch; and this
is splendidly tinted with blue, black, and red. In the Proctotretus
tenuis of Chile the male alone is marked with spots of blue, green,
and coppery-red.*(2) In many cases the males retain the same colours
throughout the year, but in others they become much brighter during
the breeding-season; I may give as an additional instance the
Calotes maria, which at this season has a bright red head, the rest of
the body being green.*(3)
* Bell, History of British Reptiles, 2nd ed., 1849, p. 40.
*(2) For Proctotretus, see Zoology of the Voyage of the Beagle;
Reptiles by Mr. Bell, p. 8. For the lizards of S. Africa, see
Zoology of S. Africa: Reptiles, by Sir Andrew Smith, pls. 25 and 39.
For the Indian Calotes, see Reptiles of British India, by Dr. Gunther,
*(3) Gunther in Proceedings, Zoological Society, 1870, p. 778,
with a coloured figure.
Both sexes of many species are beautifully coloured exactly alike;
and there is no reason to suppose that such colours are protective. No
doubt with the bright green kinds which live in the midst of
vegetation, this colour serves to conceal them; and in N. Patagonia
I saw a lizard (Proctotretus multimaculatus) which, when frightened,
flattened its body, closed its eyes, and then from its mottled tints
was hardly distinguishable from the surrounding sand. But the bright
colours with which so many lizards are ornamented, as well as their
various curious appendages, were probably acquired by the males as
an attraction, and then transmitted either to their male offspring, or
to both sexes. Sexual selection, indeed, seems to have played almost
as important a part with reptiles as with birds; and the less
conspicuous colours of the females in comparison with the males cannot
be accounted for, as Mr. Wallace believes to be the case with birds,
by the greater exposure of the females to danger during incubation.