■^ :--.«^lc:r5lR5!St~ — ggMMMOMMaiaitnnaMnsaMMnMw^ .riimvxKmwsiivyt^y. Jffrnm tl|? ilibrarg of iJpqu^atl|p& by l|tm to tl|p SItbrary of JPnnrrton Ohmlogtral S>ntttnarg 3:^54 * J": 11 1922 *! THE DESCENT OF MAN AND SELECTION IN RELATION TO SEX. BT CHARLES DAR\\^N, M. A., F. R. S., Etc. Wirn ILLUSTRATJONS. IN TWO VOLUMES.— Vol. II. NEW YORK: D. APPLETON ANT) COMPANY, 549 & 551 BROADWAY. 1871. •1 CONTENTS. PART IL SEXUAL SELECTION— contimwd. CHAPTER XII. SECONDARY SEX0AL CHARACTERS OF FISHES, AMminiAXS, AND REPTILES. Fishes : Courtship and Rattles of the Males. — Larf^or Size of tlie Females.^ Males, Briglit Colors and Ornamental Appendaf^es ; other Stran;,'e Cliaracters. — Colors and Appenda;;es acquired by the Males during tlie Breeding-season alone. — Fishes with both Sexes brilliantly colored. — Protective Colors. — Tiie less Conspicuous Colors 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 : Dift'er- ences in Structure and Color between the Se.xes. — Vocal Oigans. Rep- tiles : Chelonians. — Crocodiles. — Snakes, Colors in some Cases pro- tective.— Lizards, Battles of. — Ornamental Appendages. — Strange Dif- ferences ill Structure between the Se-xes. — Colors. — Se.\ual Differences almost as great as with Birds . . . . . p. 1 CHAPTER XIII. SECONDARY SEXUAL CHARACTERS OF BIRDS. Sexual Differences. — Law of Battle. — Special Weapons. — Vocal Organs. — Instrumental Music. — Love-Antics and Dances.— Decorations, Perma- nent anlack. The female, or sordid drag- '0 As quoted in ' The Farmer,' 1808, p. 369. 8 SEXUAL SELECTIOX. [Part II. onct, was considered by Linnaeus and by many subse- quent naturalists as a distinct species; it is of a dingy Fig. 28.— Callionymns Ijra. Upper fifrurc, male ; lower figure, female. reddish-brown, with the dorsal fin brown and the other fins Avhite, The sexes difier also in the proportional size of the head and month, and in the position of the eyes ; " but the most striking diftcrcnce is the extraordinary elon- gation in the male (Fig. 28) of the dorsal fin. The young males resemble, in stiucture and color, the adult females. Throughout the genus Callionymus," the male is gcner- " I have drawn up this description from Yarrcll's 'British Fishes,' vol. i. 1836, pp. 261,266. " ' Catalogue of Acanth. Fishes in the British Museum,' by Dr. Gunther, 1861, pp. 138-151. Chap. XII.] FISHES. 9 ally much more brightly spotted than tho fcnialc, and in several species, not only the dorsal, but the anal 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 dilference in color between them. It is difficult, as ]\Ir. 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 colors with which it, in other respects so ill-favored, is at that time adorned." Both sexes of the Lahrus tnixtus, al- though very ditlerent in color, 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 Cyprinodontida^ — inhabitants of the fresh waters of foreign lands — the sexes sometimes diffi^r much in various characters. In the male of the Mollienesia jjetenefisis,^* the dorsal fin is greatly developed, and is marked with a row of large, round, ocel- lated, bright-colored spots; while the same fin in the fe- male is smaller, of a difierent 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- colored. In the male of an allied form, the ^iphoj^/iorus Ilellerii (Fig. 29), the inferior margin of the anal fin is developed into a long filament, which is striped, as I hear from Dr. GUnther, with bright colors. This filament docs not contain any muscles, and appai'ently cannot be of any direct use to the fish. As in the case of the Callionymus, the males while young resemble in color and structure the adult females. Sexual difierences such as these may be " ' Came Birds of Sweden,' etc., 186T, p. 4G6. " With respect to this and the following species I am indebted to Dr. Giinther for information; see also his paper on the Fishes of Central America, in 'Transact. Zoolog. Soc.' vol. vi. 1868, p. 485. 10 SEXUAL SELECTION [Part IL strictly oomparccl with tliosc which aro so frequent with galhnaceous birds."" > /y Fig. 29. — Xiphophoras Hellerii. Upper figure, male; lower flgufe. female. In a sihiroid fish, inhabiting the fresh waters of South America, namely the Plecostomus harhatus " (Fig. 30), the male has its mouth and interopcrculum 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 stiflf 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 " Dr. Giinthcr makes this remark: 'Catalogue of Fishes in the Brit- ish Museum,' vol. iii. 1861, p. 141. " See Dr. Giinthcr on this genus, in ' Proc. Zoolog. Soc.' 1 868, p. 232. Chap. XII.] FISUES. 11 Fio. 20.— Plccoetomus barMtns. Upptr lii^ui x, head of male ; lower flgnrc, female. 12 SEXUAL SELECTION. [Part IL that stiff hairs and flexible filaments can be useful in any- ordinary way to the males alone. The Monacanthus scopas, which was shown to me in the British Museum by Dr. Gttnther, presents a nearly anah^gous case. The male 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 an inch and a half in length ; the female lias on the same place a cluster 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, w^hile 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 rough- ened in the male and perfectly smooth in the female ; and lastly, in otliers, both sexes have smooth sides. In that strange monster, the Chhncera moy^strosa, the male has a hook-shaped bone on the top of the head, directed for- ward, with its rounded end covered with sharp spines ; in tlie female " this crown is altogether absent," but what its use may be is utterly unknown." 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 their bodies at the same time become more brightly- colored. There can be little doubt that this crest serves as a temporary sexual ornament, for the fi-male 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 this case and in that of the INIonacanthus, we have good instances to how great an extent the sexual characters of closely-allied forms may " F. Buckland, in ' Land and Water,' July, 18f.8, p. Z11, with a figure. >8 Dr. Gunther, 'Catalogue of Fishes,' vol. iii. pp. 221, 240. Chap. XII.] FISHES. 13 differ. In many <>1" tlic Chromida?, for instance, in Geo- phagus and esjK'cially in Cielila, the males, as I liear from Prof. Agassiz,'" have a eonspieuons protuberance on the forehead, which is wholly wanting in the females and in the young males. Prof Agassiz adds : " I have often observed these tislies at the time of spawning when the protuberance is largest, and at other seasons when it is totally wanting and the two sexes show 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 Amazons know nothing about its use." These protuberances in their periodical appearance resem- ble the fleshy caruncles on the heads of certain birds ; but whether they serve as ornaments must reniain at present doubtful. The males of those fishes, which differ permanently in color from the females, often become more brilliant, as I hear from Prof. Agassiz and Dr. Gilnther, during the breeding-season. This is likewise the case with a multi- tude of fishes, the sexes of which at all other seasons of the year are identical in color. The tench, roach, and perch, may be given as instances. The male salmon at this season is " marked on the cheeks with orange-colored stripes, which gave it the a})i)earance of a Labrus, and the body jiartakes of a golden-orange tinge. The females arc (lark in color, and are commoidy called blackfish."" An analogous and even greater change takes place with the ^Sdlmo t'n'ojr^ or bull-trout ; the males of the char {S. umblti) are likewise at this season rather brighter in color than the females.^' The colors of the pike {Esox re- '» See also 'A .Tourney in Urazil,' by Prof. :inunctatus) is " brownish gray above, passing into yellow beneath, which 33 Bell, "History of British Reptiles," 2d edit. 1849, pp. 156-159. 24 SEXUAL SELECTION. [Part IL in tlic spring Lecomcs a I'icli bright orange, marked evcry- "wliere with round dark spots." The edge of the crest also is then tipped with V)riglit red or violet. The female is usually of a yellowish-brown color with scattered brown dots ; and the lower surface is often quite plain." The young are obscurely tinted. The ova are fertilized during the act of deposition and are not subsequently tended by either parent. We may therefore conclude that the males acquired their strongly-marked colors and ornamental ap- pendages through sexual selection ; these being trans- mitted either to the male oft'spring alone or to both sexes. A?mra or JBatrachia. — With many frogs and toads the colors evidently serve as a protection, such as the bright-green tints of tree-frogs and the obscure mot- tled shades of many terrestrial species. The most con- spicuously-colored toad which I ever saw, iiamely, the l^hryniscKS nigricans,*" had the whole upjior 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 colors 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 hy- drophobia. I was the more struck with the conspicuous colors of this toad, as close by I found a lizard [Proctotre- tus multimaculatvs) which, Avhen frightened, flattened its body, closed its eyes, and then from its mottled tints could hardly be distinguishable fioiu the sun-ounding sand. 3» Bell, ibid. pp. 14G, 151. *" ' Zoology of the Voyage of the " Beagle," ' 1843. " Reptiles," by Mr. Bell, p. 49. Chap. XII.] AMPHIBIANS. 35 With respect to sexual differences of color, Dr. Giin- ther 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. Gtinther know of any striking difference in ex- ternal structure between tlie sexes, excepting the promi- nences which become developed during the breeding-sea- son on the front-legs of the male, by which he is enabled to hold the female. The Megalophrys montana'^^ (Fig. 32) offers the best case of a certain amount of structural difference between the sexes ; for in the male the tip of the nose and the eyelids aro 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 sexual differ- ences ; for, though cold blooded, their passions are strong. Dr. Gtinther informs me that he has several times found an unfortunate female toad dead and smothered from hav- ing been so closely embraced ly three or four males. These animals, however, offer one interesting sexual difference, namely, in the musical powers possessed by the males ; but to speak of music, when applied to the discord- ant and overwhelming sounds emitted by male bull-frogs and some other species, seems, according to our taste, a singularly inapprojjriate 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 Hylse, which, perched on blades of grass close to the water, sent forth sweet chirping notes in har- mony. The various sounds are emitted chiefly by the males durmg the breeding-season, as in the case of the croaking of our common frog." In accordance with this 41 'The Reptiles of India,' by Dr. A. Giinther, Ray Soc. 1864, p. 413. 42 Bell, 'History of Britis.^ Reptile?,' 1849, p. 93. 21 26 SRXUAL SELECTION. [Pakt II. fact the vocal organs of the males are more highly de- veloped than those of the females. In some genera the males alotoe are provided with sacs wliich open into the larynx." For instance, in the edible frog {Rana esculen- ta) " the sacs are peculiar to the male.^, and become, when filled with air in tlio act of croaking, large globular blad- Fio. .32.— Mcgalophrys TOontana. The two left-band flares, the male; the two right-hand figures, the female. ders, 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 ; while that of the female is only a slight groaning noise." The vocal organs differ considerably in structure in the several genera of the family ; and their development in all cases may be attrib- uted to sexual selection. Reptiles. Chelonia. — Tortoises and turtles do not ' offer well- marked sexual differences. In some species, the tail of *■> .1. Hishop, in ' Toild's Cyclop, of An;it. and Thys.' vol. iv. p. 1503. « Bell, ibid. pp. 112-114. Chap. XII.J REPTILES. 37 tlie male is longer than that of the female. In some, the plastron or lower surface of the shell of the male is slight- ly 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.^° Crocodilia. — The sexes apparently do not differ in color; nor do I know that the males fight together, though this is probable, for some kinds make a prodi gious 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 ex- tent ready to burst, with his head and tail lifted up, he spins or twirls round on the surface of the water, like an Indian chief rehearsing his feats of war." During the sea- son of love, a musky odor is emitted by the submaxillary glands of the crocodile, and pervades their haunts." Ophidia. — I have little to say about Snakes. Dr. Gtinther 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 color, Di-. Gilnther can almost al- ways distinguish the male from the female \)j his more *^ Mr. C. J. Maynard, ' The American Naturalist,' Dec. 1869, p. 555. *^ See my ' Journal of Researches during the Yoyage of the " Bea- gle," ' 1845, p. 384. « 'Travels through Carolina,' etc., 1Y91, p. 128. *^ Owen, ' Anatomy of Vertebrates,' vol. i. 1866, p. 615. 28 SEXUAL SELECTION. [Part II. strongly-pronounced tints ; thus tlie black zigzag band on the back of the male English viper is more distinctly de- fined than in the female. The difference is much plainer in the Rattlesnakes of North America, the male of Avhich, as the kee2)er in the Zoological Gardens showed me, can instantly be distinguished from the female by having more lurid yellow about its whole body. In South Africa the Ut(cephalus capensis presents an analogous difference, for the female " is never so fully A-ariegated with yellow on the sides, as the male." " The male of the Indian Dtpsas cynodon^ on the other hand, is blackish-brown, with the belly partly black, while 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-colored." No doubt the colors of some snakes serve as a protection, as the green tints of tree-snakes and the various mottled shades of the species which live in sandy places ; biit it is doubtful whether the colors of many kinds, for instance of the common English snake or viper, serve to conceal them ; and this is still more doubtful with the many for- eign species which are colored with extreme elegance. During the breeding-season their anal scent-glands are in active function ; " and so it is with the same glands in lizards, and as we have seen with the submaxillary 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 « Sir Andrew Smith, ' Zoolog. of South Africa: Ileptilia,' 1849, pi. x. '0 Dr. A. Giiiither, 'Reptiles of British India,' Ray Soc. 18C4, pp. 304, 308. " Owen, 'Anatomy of Vertebrates,' vol. i. 1806, p. 615. "Till" celebrated botanist Schleiden incidentally lemarks ('Uebei den Darwinismus : Insere Zeit,' I860, s. 269), that Rattlesnakes use Chap. XII.] REPTILES. 29 appearing so sluggish, are amorous ; for many have been observed crowding round the same female, and even round the dead body of a female. They are not known to fight together from rivalry. Their intellectual powers are higli- er than might have been anticipated, ^n excellent ob- server in Ceylon, Mr, E. Layard,^^ saw a Cobra thrust its head through a narrow hole and swallow a toad. " With this encumbrance he could not witlidraw himself; finding this, he reluctantly disgorged the precious morsel, which began to move olF; this was too much for snake philoso- phy 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 learned, and the toad was seized by one leg, withdrawn, and then swallowed in triumph." It does not, however, follow because snakes have some reasoning power and strong passions, that they should likewise be endowed Avitli sufficient taste to admire bril- liant colors in their partners, so as to lead to the adorn- ment of the species through sexual selection. Neverthe- less, it is difficult to account in any other manner for the extreme beauty of certain species ; for instance, of the coral-snakes of South 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 colored in this peculiar manner, as Mr. Wallace states on the authority of Dr. Giinther," are found nowhere else in their rattles as a sexual call, by which the two sexes find each other. I do not know whether this suggestion rests on any direct observations. These snakes pair in the Zoological Gardens, but the keepers have never observed that they use their rattles at this season more than at any other. *^ " Rambles in Ceylon," ' Annals and Mag. of Nat. Hist.' 2d series vol. ix. 1852, p. 333. " ' Westminster Review,' July 1, 1867, p. 32. 30 SEXUAL SELECTION. [Paut IL the world except in South America, and liere 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 be- longing to these distinct genera inhabit tlie 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 Ijelieves, the innocuous kinds have probably acquired their colors as a protection, on the principle of imitation; for they would naturally be thought dangerous by their enemies. The cause, how- ever, of the bright colors of the venomous Elaps remains to be explained, and this may perhaps be sexual selection. Lacertilia. — The males of some, pi-obably of many kinds of lizards, fight together from rivalry. Thus the arboreal Anolis cristatellus of South 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, 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. Gilnther has been able to ascertain, is the general rule with lizards of all kinds. The sexes often difter greatly in various external char- acters. The male of the above-mentioned Anolis is fur- " Mr. N. L. Austen kept these aniraals alive for a cousiderabic thue ; sec ' Liuid and Water,' July, 18G7, p. 9. Chap. XII.] REPTILES. 31 nishecl with a crest, which runs along tlie back and tail, and can be erected at pleasui'e ; but of this crest the female does not exhibit a trace. In the Indian Cophotis ceylanica, the female possesses a dorsal crest, though much less de- veloj)ed than in the male ; and so it is, as Dr. Giinther in- forms 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 tubercu- lata. In tlie genus Sitana, the males alone ai'e furnished with a large throat-pouch (Fig. 33), which can be folded up like a fan, and is colored blue, black, and red; but these splendid colors are exhibited only during the pair- ing-season. The female does not possess even a rudiment of this appendage. In the Anolls cristatellus, according to Mr. Austen, the throat-pouch, which is bright red mar- bled with yellow, is present, though in a rudimental con- dition, in the female. Again, in certain other lizards, both sexes are equally well provided with throat-pouches. Here, as in so many previous cases, we see, with species belonging to the same group, the same character confined to the males, or more largely developed in the males than in the fe- males, or 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 colors baffle description, are furnished with skinny appendages to the throat, '' like the wattles of gallinaceous birds." These be- come erected when the animal is excited. They occur in both sexes, but are best developed in the male when ar- FiG. 33. — Sitana minor. Male, with tlie gular pouch expanded (from Gun- ther's 'Keptiles of India'). 33 SEXUAL SELECTION. [Part II. rived at maturity, at whicli age the middle appendage is Bometimes twice as long as the liead. ISIost of the species likewise have a low crest running along the neck ; and this is much more developed in the fuli-grown males than in the females or young male?." There are other and much more remarkable differences between the sexes of certain lizai'd^;. The male of Cerato- j)l(ora 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 ( C. Stoddartif, Fig. 34), the whole api)cnd- age is converted into a horn, which is usually of a white color, but assumes apur])lish tint when the animal is excited. In the adult male of this latter species the horn is half an inch in length, but is of quite minute size in the female and in the young. These appendages, as Dr. Gunther has remarked to me, may be com- pared with the combs of galli- naceous birds, and apparently serve as ornaments. In the genus Chama^leon we come to the climax of dil- ference between the sexes. The v.]i])vt ]iart of the skull of the male C. hifurcus (Fig. 35), an inhabitant of ]Mada- '« All these statements and quotations, in regard to Cophotis, Fitana, and Draco, as well as the followinfi facts in regard to Ceratophora, arc taken from Dr. Gunther's magnificent work on the 'Rc^itiles of British India,' Ray Soc. 1864, pp. 122, 130, 135. Fig. 34.— Ccrntophora Stoddartii. Upper figure, male ; lower figure, female. Chap. XII.] REPTILES. 33 gascar, is produced into two great, solid, bony projec- tions, covered with scales like the rest of the head ; and of this wonderful modification of structure the female ex- hibits only a rudiment. Again, in Chamceleon Owenii (Fig. 36), from the West Coast of Africa, the male bears Fig. 35. — Chamseleon bifurcus. Upper figuie, male ; lower figure, female. on his snout and forehead three curious horns, of which the female has not a trace. These horns consist of an ex- crescence 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. Althouo;h the three horns 34 SEXUAL SELECTION. [Part IL differ so mucli in appearance from the two great prolon- gations 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; but Dr. GUnther, to whom I am in- debted for the foregoing details, does not believe that such peaceable creatures would ever become pugnacious. Hence we are driven to infer tliat these almost monstrous devia- tions of structure serve as mascu- line ornaments. With many kinds of lizards, the sexes differ slightly in color, the tints and stripes of the males being Fia. 36.— Chamaelcon Owenii. Upper figure, male ; brighter and lower figure, female. '^ -,• . more distinctly defined than in the females. This, for instance, is the case with the previously-mentioned Cophotis and "with the Acanthodactylus cajyensis of South Africa. In a Cordylus of the latter coimtry, the male is either much redder or greener than the female. In the Indian Calotes nigrilahris there is a greater difference in color between the sexes ; the lips also of the male are black, while those of the female are green. In our common little vivij>arous lizard [Zootoca tu'vipara), " the umlcr side of the body and base of the tail in the male are bright orange, spotted with black ; in the female these parts are Chap. XII.] REPTILES. 35 pale grayish-green without spots." " We have seen that the males alone of Sitana possess a throat-pouch ; and this is splendidly tinted witli blue, black, and red. In the Proctotretus tenuis of Chili the male alone is mai'ked with spots of blue, green, and coppery-red.^^ I collected in South America fourteen species of this genus, and though I neglected to record the sexes, I observed that certain individuals alone were marked with emerald-like green spots, while others had orange-colored gorges ; and these in both cases no doubt were the males. In the foregoing species, the males are more brightly colored than the females, but with many lizards both sexes are colored in the same elegant or even magnificent manner; and there is no reason to suj^pose that such con- spicuous colors are protective. With some lizards, how- ever, the green tints no doubt serve for concealment ; and an instance has already been incidently given of one species of Proctotretus which closely resembles the sand on which it lives. On the whole we may conclude with tolerable safety that the beautiful colors of many lizards, as well as various appendages and other strange modi- fications of structure, have been gained by the males through sexual selection for the sake of ornament, and have been transmitted either to their male oft'spring alone or to both sexes. Sexual selection, indeed, seems to have played almost as important a part with reptiles as with birds. But the less conspicuous colors of the females in comparison with those of the males cannot be accounted for, as Mr. Wallace believes to be the case with birds, by the exposure of the females to danger during incubation. s'' Bell, 'History of British Reptiles,' 2d edit. 1849, p. 40. 5S por Proctotretus see 'Zoology of the Voyage of the "Beagle:" Reptiles,' by Mr. Bell, p. 8. For the Lizards of South Africa, see ' Zool- ogy of South Africa : Reptiles,' by Sir Andrew Smith, pis. 25, 39. For the Indian Calotes, see ' Reptiles of British India,' by Dr. Giinther, p. 143. 36 SEXUAL SELECTION. [Pakt U. CHAPTER XIII. Second AKY Sexual Characters of Birds. Sexual Differences. — Law of Battle. — Special Weapons. — Vocal Organs. — Instrumental Music. — Love-Antics and Dances.— Decorations, Penna- nent and Seasonal. — Double and Single Annual Moults. — Display of Ornaments by the Males. Secondary sexual characters are more divei-sified and conspicuous in birds, though not perhaps entailing more important changes of structure, than in any other class of animals. I shall, therefore, treat the subject at consider- able length. Male birds sometimes, though rarely, pos- sess special weapons for fighting with each other. They charm the females by vocal or instrumental music of the most varied kinds. They are ornamented by all sorts of combs, wattles, protuberances, horns, air-distended sacs, topknots, naked shafts, plumes and lengthened feathers gracefully springing from all parts of the body. The beak and naked skin about the head and the feathers are often gorgeously colored. The males sometimes pay their court by dancing, or by fantastic antics performed either on the ground or in the air. In one instance, at least, the male emits a musky odor which we may suppose serves to charm or excite the female ; for that excellent observ- er, Mr. Kanisay,' says of the Australian musk-duck [Jiizi- ura lohata) that " the smell which the mule emits during the summer months is confined to that sex, and in some individuals is retained througliotit the year; I have never, even in the breeding-season, shot a female Avhich had any ' ' Ibis,' vol. ill. (new series) 18G7, p. 414. Chap. XIIL] BIRDS. 37 smell of musk." So powerful Is this odor during the pair- ing-season, that it can be detected long before the bird can be seen." On the whole, birds appear to be the most aesthetic of all animals, excepting of course man, and they have nearly the same taste for the beautiful as we have. This is shown by our enjoyment of the singing of birds, and by our women, both civilized and savage, decking their heads with borrowed plumes, and using gems which are hardly more brilliantly colored than the naked skin and wattles of certain birds. Before treating of the characters with which we are here more particulafly concerned, I may just allude to certain diiFei*ences between the sexes which apparently depend on differences in their habits of life ; for such cases, though conamon in the lower, are rare in the higher classes. Two humming-birds belonging to the genus Eustephanus, which inhabit the island of Juan Fernandez, were long thought to be specifically distinct, but are now known, as Mr. Gould informs me, to be the sexes of the same species, and they differ slightly in the form of the beak. In another genus of humming-birds [Grt/piis), the beak of the male is serrated along the margin and hooked at the extremity, thus differing much from that of the female. In the curious Neomorpha of New Zealand, there is a still wider difference in the form of the beak ; and Mr. Gould has been informed that the male with his " straight and stout beak " tears off the bark of trees, in order that the female may feed on the uncovered larvfe with her weaker and more curved beak. Something of the same kind may be observed with our goldfinch ( Car- duelis elegmis), for I am assured by Mr. J. Jenner Weir that the bird-catchers can distinguish the males by their slightly longer beaks. The flocks of males, as an old and trustworthy bird-catcher asserted, are commonly found 2 Gould, ' Hand-book to the Birds of Australia,' 18C5, vol. ii. p. 383. 38 SEXUAL SELECTION: BIRDS. [Part I L feeding on tl)e seeds of the teazle (Dipsaciis) which they can reach with their elongated beaks, while the females more commonly feed on the seeds of the betony or Scrophularia. With a slight diiference of this nature as a foundation, we can see how the beaks of the two sexes might be made to difter greatly through natural selection. In all these cases, however, especially in that of the quar- relsome humming-birds, it is possible that the differences in the beaks may have been first acquired by the males in relation to their battles, and afterward led to slightly changed habits of life. Law of J^attle. — Almost all male birds are extremely pugnacious, using their beaks, wings, and legs, for fighting together. We see this every spring with our robins and sparrows. The smallest of all birds, namely, the hum- ming-bird, is one of the most quarrelsome. Mr. Gosse ^ describes a battle, in which a pair of humming-birds seized hold of each other's beaks, and whirled round and round, till they almost fell to the ground ; and M. Montes de Oca, in speaking of another genus, says that two males rarely meet Avithout a fierce aerial encounter : when kept in cages " their fighting has mostly ended in the splitting of the tongue of one of the two, which then surely dies from being unable to feed." * With Waders, the males of the common water-hen [Gallinida chloropus) "when pairing, fight violently for the females : they stand nearly upright in the water and strike with their feet." Two were seen to be thus engaged for half an hour, imtil one got hold of the head of the other, which would have been killed had not the observer interfered ; the female all the time looking on as a quiet spectator.^ The males of an allied 3 Quoted l>y Mr. Gould, ' Introduction to the Trochilidae,' 1861, p. 29. * Gould, ibid. p. 52. ' W. Thompson, ' Nat. Hist, of Ireland : Birds,' vol. ii. 1850, p. 327. Chap. XIII.J LAW OF BATTLE. 39 bird {Gallicrex cristatus), as Mr. Blyth informs me, are one-third larger than the females, and are so pugnacious during the breeding-season, that they are kept by the natives of Eastern Bengal for the sake of fighting. Various other birds are kept in India for the same pur- pose, for instance, the Bulbuls [Pycnonotus hcBmorrhous) which " fight with great spirit." * The polygamous Ruif {^Machetes pugnax. Fig. 37) is notorious for his extreme pugnacity ; and in the spring, the males, which ai'e considerably larger than the females, congregate day after day at a particular spot, where the females propose to lay their eggs. The fowlers discover these spots by the turf being trampled somewhat bare. Here they fight very much like game-cocks, seizing each other with their beaks and striking with their wings. The great rufl^" of feathers round the neck is then erected, and according to Colonel Montagu " sweeps the ground as a shield to defend the more tender parts ; " and this is the only instance known to me in the case of birds, of any structure serving as a shield. The rufi" of feathers, how- ever, from its varied and rich colors pi-obably serves in chief part as an ornament. Like most pugnacious birds, they seem always ready to fight, and when closely confined often kill each other; but Montagu observed that their pugnacity becomes greater during the sprmg, when the long feathers on their necks are fully developed ; and at this'period the least movement by any one bird provokes a general battle.' Of the pugnacity of web-footed birds, two instances will suffice : in Guiana " bloody fights occur during the breeding-season between the males of the wild musk-duck ( Gairina moschata) ; and where these fights have occurred the river is covered for some distance with * Jerdon, 'Birds of India,' 1863, vol. ii. p. 96 ' Macgillivray, ' Hist. Brit. Birds,' vol. iv. 1852, pp. I'Z'T-lSl. } 40 SEXUAL SELECTION: BIRDS. [Pakt II feathers." * Birds which seem ill-adapted for fighting en- gage in fierce conflicts ; thus with the pelican the stronger * Sir R. Schomburgk, in ' Journal of R. Geograph. Soc' vol. xiii. 1843, p. 31. Chap. XIII.] LAW OF BATTLE. 41 males drive away the weaker ones, snapping with their huge beaks and giving heavy blows with their wings. Male snipes fight together, "tugging and pushing each other with their bills in the most curious manner miagi- nable." Some few species are believed never to fight ; this is the case, according to Audubon, with one of the wood- peckers of the United States {Picus auratus), although " the hens are followed by even half a dozen of their gay suitors." ' The males of many birds are larger than the females, and this no doubt is an advantage to them in their bat- tles with their rivals, and has been gained through sexiial selection. The diiference in size between the two sexes is carried to an extreme point in several Australian species ; thus the male musk-duck (Biziura) and the male Cinclo- ramphus cruralis (allied to our pipits) are by measure- ment actually twice as large as their respective females.'" With many other bii'ds the females are larger than the males ; and as formerly remarked, the explanation often given, namely, that the females have most of the work in feeding their young, will not suffice. In some few cases, as we shall hereafter see, the females apparently have ac- quired their greater size and strength for the sake of con- quering other females and obtaining possession of the males. The males of many gallinaceous birds, especially of the polygamous kinds, are furnished with special weapons for fighting with their rivals, namely spurs, which can be used with fearful eifect. It has been recorded by a trust- worthy writer" that in Derbyshire a kite struck at a * ' Ornithological Biography,' vol. i. p. 191. For pelicans and snipes, see vol. iii. pp. 381, 4*77. "* Gould, ' Hand-book of Birds of Australia,' vol. i. p. 395 ; vol. ii. p. 383. " Mr. Hewitt m the ' Poultry Book by Tegetmeier,' 1866, p. 137. 42 SEXUAL SELECTION: BIHDS. [Part IL gamc-hcn accompanie