The Descent of Man and Selection in Relation to Sex Volume II Part 1

The Descent of Man and Selection in Relation to s.e.x.

by Charles Darwin.

s.e.xUAL SELECTION.

CHAPTER XII.

SECONDARY s.e.xUAL CHARACTERS OF FISHES, AMPHIBIANS, AND REPTILES.

FISHES: Courtship and battles of the males-Larger size of the females-Males, bright colours and ornamental appendages; other strange characters-Colours and appendages acquired by the males during the breeding-season alone-Fishes with both s.e.xes brilliantly coloured-Protective colours-The less conspicuous colours of the female cannot be accounted for on the principle of protection-Male fishes building nests, and taking charge of the ova and young. AMPHIBIANS: Differences in structure and colour between the s.e.xes-Vocal organs. REPTILES: Chelonians-Crocodiles-Snakes, colours in some cases protective-Lizards, battles of-Ornamental appendages-Strange differences in structure between the s.e.xes-Colours-s.e.xual differences almost as great as with birds.

We have now arrived at the great sub-kingdom of the Vertebrata, and will commence with the lowest cla.s.s, namely 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 so many of the lower animals. Besides the claspers, the males of many rays have cl.u.s.ters 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 the 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 spines.[1]

Owing to the element which fishes inhabit, little is known about their courtship, and not much about their battles. 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 acc.u.mulated 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."[2] The males are said to be polygamists;[3] 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,[4] "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."

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 salmons 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 sp.a.w.ning. The males "are constantly fighting and tearing each other on the sp.a.w.ning-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."[5]

The keeper of the Stormontfield breeding-ponds visited, as Mr. Buist informs me, in June, 1868, 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.

[Ill.u.s.tration: Fig. 26. Head of male of common salmon (_Salmo salar_) during the breeding-season.

(This drawing, as well as all the others in the present chapter, have been executed by the well-known artist, Mr. G. Ford, under the kind superintendence of Dr. Gunther, from specimens in the British Museum.)]

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."[6] (Figs. 26 and 27.) 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[7] believes, is permanent and best marked in the older males which have previously ascended the rivers. In these old males the jaws become developed into immense hook-like projections, and the teeth grow into regular fangs, often more than half an inch in length. With the European salmon, according to Mr. Lloyd,[8] 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.

[Ill.u.s.tration: Fig. 27. Head of female salmon.]

The salmon is not the only fish in which the teeth differ in the two s.e.xes. 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, forming a pavement; so that these teeth differ in the two s.e.xes 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 s.e.xual characters, both s.e.xes of some species of rays, for instance _R. batis_, possess, when adult, 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 s.e.xes. The teeth are likewise pointed in both s.e.xes of _R. maculata_, but only when completely adult; the males acquiring them at an earlier age than the females. We shall hereafter meet with a.n.a.logous cases with certain birds, in which the male acquires the plumage common to both adult s.e.xes, at a somewhat earlier age than the female. With other species of rays the males even when old never possess sharp teeth, and consequently both s.e.xes when adult are provided with broad, flat teeth like those of the young, and of the mature females of the above-mentioned species.[9] As the rays are bold, strong and voracious fishes, 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.

In regard to size, M. Carbonnier[10] maintains that with almost all fishes the female 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 the female. As with many kinds of fishes the males habitually fight together; it is surprising that they have not generally become through the effects of s.e.xual selection larger and stronger than the females.

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.

[Ill.u.s.tration: Fig. 28. Callionymus lyra. Upper figure, male; lower figure, female.]

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 peac.o.c.k. I am indebted for most of the following facts to the great kindness of Dr. Gunther. There is reason to suspect that many tropical fishes differ s.e.xually 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 freshly taken 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 a.n.a.l 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 s.e.xes differ also in the proportional size of the head and mouth, and in the position of the eyes;[11] but the most striking difference is the extraordinary elongation in the male (fig.

28) of the dorsal fin. The young males resemble in structure and colour the adult females. Throughout the genus Callionymus,[12] the male is generally much more brightly spotted than the female, and in several species, not only the dorsal, but the a.n.a.l fin of the male is much elongated.

The male of the _Cottus scorpius_, or sea-scorpion, is more slender and smaller than the female. There is also a great difference in colour between them. It is difficult, as Mr. Lloyd[13] remarks, "for any one, who has not seen this fish during the sp.a.w.ning-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 s.e.xes 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.

In the very distinct family of the Cyprinodontidae-inhabitants of the fresh waters of foreign lands-the s.e.xes sometimes differ much in various characters. In the male of the _Mollienesia petenensis_,[14] 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 a.n.a.l fin is also a little produced and dark-coloured. In the male of an allied form, the _Xiphophorus h.e.l.lerii_ (fig. 29), the inferior margin of the a.n.a.l fin is developed into a long filament, which is striped, as I hear from Dr. Gunther, 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 in colour and structure the adult females. s.e.xual differences such as these may be strictly compared with those which are so frequent with gallinaceous birds.[15]

[Ill.u.s.tration: Fig. 29. Xiphophorus h.e.l.lerii. Upper figure, male; lower figure, female.]

In a siluroid fish, inhabiting the fresh waters of South America, namely the _Plecostomus barbatus_[16] (fig. 30), the male has its mouth and interoperculum fringed with a beard of stiff hairs, of which the female shews 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 h.o.m.ologous 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 it 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. The _Monacanthus scopas_, which was shewn to me in the British Museum by Dr.

Gunther, presents a nearly a.n.a.logous case. The male has a cl.u.s.ter 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 an inch and a half in length; the female has on the same place a cl.u.s.ter of bristles, which may be compared with those of a tooth-brush. In another species, the _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 the same part of the tail can be perceived to be a little roughened in the male and perfectly smooth in the female; and lastly in others, both s.e.xes have smooth sides. In that strange monster, the _Chimaera monstrosa_, the male has a hook-shaped bone on the top of the head, directed forwards, with its rounded end covered with sharp spines; in the female "this crown is altogether absent," but what its use may be is utterly unknown.[17]

[Ill.u.s.tration: Fig. 30. Plecostomus barbatus. Upper figure, head of male; lower figure, female.]

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[18] a crest is developed on the head of the male only during the breeding-season, and their bodies at the same time become more brightly-coloured. There can be little doubt that this crest serves as a temporary s.e.xual ornament, for the female does not exhibit a trace of it. In other species of the same genus both s.e.xes possess a crest, and in at least one species neither s.e.x is thus provided. In this case and in that of the Monacanthus, we have good instances to how great an extent the s.e.xual characters of closely-allied forms may differ. In many of the Chromidae, for instance in Geophagus and especially in Cichla, the males, as I hear from Professor Aga.s.siz,[19] have a conspicuous protuberance on the forehead, which is wholly wanting in the females and in the young males. Professor Aga.s.siz adds, "I have often observed these fishes at the time of sp.a.w.ning when the protuberance is largest, and at other seasons when it is totally wanting and the two s.e.xes 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 in their periodical appearance resemble the fleshy caruncles on the heads of certain birds; but whether they serve as ornaments must remain at present doubtful.

The males of those fishes, which differ permanently in colour from the females, often become more brilliant, as I hear from Professor Aga.s.siz and Dr. Gunther, during the breeding-season. This is likewise the case with a mult.i.tude of fishes, the s.e.xes of which at all other seasons of the year are identical in colour. 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."[20] An a.n.a.logous 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.[21] 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.[22] Another striking instance out of many is afforded by the male stickleback (_Gasterosteus leiurus_), which is described by Mr. Warington,[23] 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 l.u.s.tre 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 subside.

That with fishes there exists some close relation between their colours and their s.e.xual functions we can clearly see;-firstly, from the adult males of certain species being differently coloured from the females, and often much more brilliantly;-secondly, from these same males, whilst immature, resembling the mature females;-and, lastly, from the males, even of those species which at all other times of the year are identical in colour with the females, often acquiring brilliant tints during the sp.a.w.ning-season. We know that the males are ardent in their courtship and sometimes fight desperately together. If we may a.s.sume that the females have the power of exerting a choice and of selecting the more highly-ornamented males, all the above facts become intelligible through the principle of s.e.xual selection. On the other hand, if the females habitually deposited and left their ova to be fertilised by the first male which chanced to approach, this fact would be fatal to the efficiency of s.e.xual selection; for there could be no choice of a partner. But, as far as is known, the female never willingly sp.a.w.ns except in the close presence of a male, and the male never fertilises the ova except in the close presence of a female. It is obviously difficult to obtain direct evidence with respect to female fishes selecting their partners. An excellent observer,[24] who carefully watched the sp.a.w.ning of minnows (_Cyprinus phoxinus_), remarks that owing to the males, which were ten times as numerous as the females, crowding closely round them, he could "speak only doubtfully on their operations. When a female came among a number of males they immediately pursued her; if she was not ready for shedding her sp.a.w.n, she made a precipitate retreat; but if she was ready, she came boldly in among them, and was immediately pressed closely by a male on each side; and when they had been in that situation a short time, were superseded by other two, who wedged themselves in between them and the female, who appeared to treat all her lovers with the same kindness."

Notwithstanding this last statement, I cannot, from the several previous considerations, give up the belief that the males which are the most attractive to the females, from their brighter colours or other ornaments, are commonly preferred by them; and that the males have thus been rendered more beautiful in the course of ages.

We have next to inquire whether this view can be extended, through the law of the equal transmission of characters to both s.e.xes, 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 Peac.o.c.k Labrus (_L. pavo_), described,[25] 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 s.e.xes in at least one species 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 any aid from selection. The goldfish (_Cyprinus auratus_), judging from the a.n.a.logy 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.[26] 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 selection.

What, then, must we conclude in regard to the many fishes, both s.e.xes of which are splendidly coloured? Mr. Wallace[27] 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."[28] 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 kingfishers, 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 (as explained when treating of caterpillars) that they were unpalatable; but it is not, I believe, known that any fish, at least any freshwater 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 s.e.xes are brilliantly coloured, is that their colours have been acquired by the males as a s.e.xual ornament, and have been transferred in an equal or nearly equal degree to the other s.e.x.

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, with the variations inherited only by his male offspring; 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 acquired by many fishes as a protection: no one can behold the speckled upper surface of a flounder, and overlook its resemblance to the sandy bed of the sea on which it lives. One of the most striking instances ever recorded of an animal gaining protection by its colour (as far as can be judged in preserved specimens) and by its form, is that given by Dr. Gunther[29] 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. Fishes offer valuable evidence on this head. We can see that one s.e.x will not be modified through natural selection for the sake of protection more than the other, supposing both to vary, unless one s.e.x is exposed for a longer period to danger, or has less power of escaping from such danger than the other s.e.x; and it does not appear that with fishes the s.e.xes differ in these respects. As far as there is any difference, the males, from being generally of smaller size, and from wandering more about, are exposed to greater danger than the females; and yet, when the s.e.xes differ, the males are almost always the most 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,[30] 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 s.e.x would be equally liable to be destroyed or preserved, and both would have an equal influence on the colours of their offspring or the race.

Certain fishes, belonging to several families, make nests; and some of these fishes take care of their young when hatched. Both s.e.xes of the brightly-coloured _Crenilabrus ma.s.sa_ and _melops_ work together in building their nests with sea-weed, sh.e.l.ls, &c.[31] But the males of certain fishes do all the work, and afterwards take exclusive charge of the young. This is the case with the dull-coloured gobies,[32] in which the s.e.xes are not known to differ in colour, and likewise with the sticklebacks (Gasterosteus), in which the males become brilliantly coloured during the sp.a.w.ning-season. The male of the smooth-tailed stickleback (_G. leiurus_) performs during a long time the duties of a nurse with exemplary care and vigilance, 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.[33]

The males of certain other fishes inhabiting South America and Ceylon, and belonging to two distinct orders, have the extraordinary habit of hatching the eggs laid by the females within their mouths or branchial cavities.[34] With the Amazonian species which follow this habit, the males, as I am informed by the kindness of Professor Aga.s.siz, "not only are generally brighter than the females, but the difference is greater at the sp.a.w.ning-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 Aga.s.siz likewise informs me, s.e.xual 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 Promotis 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, has had little or no influence on the differences in colour between the s.e.xes. 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 these could not inherit his peculiarities; yet, in many of these very cases the males are more conspicuously coloured than the females.

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.[35] The s.e.xes 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 very curious exceptional case,[36] 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 the s.e.x which is the most important of the two for the welfare of the offspring must serve, in some manner, as a protection. But from the mult.i.tude of fishes, the males of which are either permanently or periodically brighter than the females, but whose life is not at all more important than that of the female for the welfare of the species, this view can hardly be maintained. When we treat of birds we shall meet with a.n.a.logous cases in which there has been a complete inversion of the usual attributes of the two s.e.xes, 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.

On the whole we may conclude, that with most fishes, in which the s.e.xes differ in colour or in other ornamental characters, the males originally varied, with their variations transmitted to the same s.e.x, and acc.u.mulated through s.e.xual 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 s.e.xes 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 purpose.

The last point which need be noticed is that in many parts of the world fishes are known to make peculiar noises, which are described in some cases as being musical. Very little has been ascertained with respect to the means by which such sounds are produced, and even less about their purpose. The drumming of the Umbrinas in the European seas is said to be audible from a depth of twenty fathoms. The fishermen of Roch.e.l.le a.s.sert "that the males alone make the noise during the sp.a.w.ning-time; and that it is possible by imitating it, to take them without bait."[37] If this statement is trustworthy, we have an instance in this, the lowest cla.s.s of the Vertebrata, of what we shall find prevailing throughout the other vertebrate cla.s.ses, and which prevails, as we have already seen, with insects and spiders; namely, that vocal and instrumental sounds so commonly serve as a love-call or as a love-charm, that the power of producing them was probably first developed in connection with the propagation of the species.

AMPHIBIANS.

[Ill.u.s.tration: Fig. 31. Triton cristatus (half natural size, from Bell's 'British Reptiles'). Upper figure, male during the breeding-season; lower figure, female.]

_Urodela._-First for the tailed amphibians. The s.e.xes of salamanders or newts often differ much both in colour and structure. In some species prehensile claws are developed on the forelegs 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.[38] This structure no doubt aids the male in his eager search and pursuit of the female. 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, being absorbed during the winter. It is not furnished, as Mr. St. George Mivart informs me, with muscles, and therefore cannot be used for locomotion. As during the season of courtship it becomes edged with bright colours, it serves, there can hardly be a doubt, as 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, pa.s.sing 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.[39] 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 acquired their strongly-marked colours and ornamental appendages through s.e.xual selection; these being transmitted either to the male offspring alone or to both s.e.xes.

_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, namely the _Phryniscus nigricans_[40] 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 gra.s.sy plains of La Plata under a scorching sun, and could not fail to catch the eye of every pa.s.sing creature. These colours may be beneficial by making this toad known to all birds of prey as a nauseous mouthful; for it is familiar to every one that these animals emit a poisonous secretion, which causes the mouth of a dog to froth, as if attacked by hydrophobia. I was the more struck with the conspicuous colours of this toad, as close by I found a lizard (_Proctotretus multimaculatus_) which, when frightened, flattened its body, closed its eyes, and then from its mottled tints could hardly be distinguishable from the surrounding sand.

With respect to s.e.xual differences of colour, Dr. Gunther knows of no striking instance 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 Dr. Gunther know of any striking difference in external structure between the s.e.xes, 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. The _Megalophrys montana_[41] (fig. 32) offers the best case of a certain amount of structural difference between the s.e.xes; for in the male the tip of the nose and the eyelids are produced into triangular flaps of skin, and there is a little black tubercle on the back-characters which are absent, or only feebly developed, in the females. It is surprising that frogs and toads should not have acquired more strongly-marked s.e.xual differences; for though cold-blooded, their pa.s.sions 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.

[Ill.u.s.tration: Fig. 32. Megalophrys montana. The two left-hand figures, the male; the two right-hand figures, the female.]

These animals, however, offer one interesting s.e.xual 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 bull-frogs and some other species, seems, according to our taste, a singularly inappropriate expression. Nevertheless certain frogs sing in a decidedly pleasing manner. Near Rio de Janeiro I used often to sit in the evening to listen to a number of little Hylae, which, perched on blades of gra.s.s close to the water, 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.[42]

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.[43] 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.[44] The vocal organs differ considerably in structure in the several genera of the family; and their development in all cases may be attributed to s.e.xual selection.