The Descent of Man and Selection in Relation to Sex Volume I Part 12

CHAPTER IX.

SECONDARY s.e.xUAL CHARACTERS IN THE LOWER CLa.s.sES OF THE ANIMAL KINGDOM.

These characters absent in the lowest cla.s.ses-Brilliant colours-Mollusca-Annelids-Crustacea, secondary s.e.xual characters strongly developed; dimorphism; colour; characters not acquired before maturity-Spiders, s.e.xual colours of; stridulation by the males-Myriapoda.

In the lowest cla.s.ses the two s.e.xes are not rarely united in the same individual, and therefore secondary s.e.xual characters cannot be developed. In many cases in which the two s.e.xes are separate, both are permanently attached to some support, and the one cannot search or struggle for the other. Moreover it is almost certain that these animals have too imperfect senses and much too low mental powers to feel mutual rivalry, or to appreciate each other's beauty or other attractions.

Hence in these cla.s.ses, such as the Protozoa, Clenterata, Echinodermata, Scolecida, true secondary s.e.xual characters do not occur; and this fact agrees with the belief that such characters in the higher cla.s.ses have been acquired through s.e.xual selection, which depends on the will, desires, and choice of either s.e.x. Nevertheless some few apparent exceptions occur; thus, as I hear from Dr. Baird, the males of certain Entozoa, or internal parasitic worms, differ slightly in colour from the females; but we have no reason to suppose that such differences have been augmented through s.e.xual selection.

Many of the lower animals, whether hermaphrodites or with the s.e.xes separate, are ornamented with the most brilliant tints, or are shaded and striped in an elegant manner. This is the case with many corals and sea-anemonies (Actiniae), with some jelly-fish (Medusae, Porpita, &c.), with some Planariae, Ascidians, numerous Star-fishes, Echini, &c.; but we may conclude from the reasons already indicated, namely the union of the two s.e.xes in some of these animals, the permanently affixed condition of others, and the low mental powers of all, that such colours do not serve as a s.e.xual attraction, and have not been acquired through s.e.xual selection. With the higher animals the case is very different; for with them when one s.e.x is much more brilliantly or conspicuously coloured than the other, and there is no difference in the habits of the two s.e.xes which will account for this difference, we have reason to believe in the influence of s.e.xual selection; and this belief is strongly confirmed when the more ornamented individuals, which are almost always the males, display their attractions before the other s.e.x. We may also extend this conclusion to both s.e.xes, when coloured alike, if their colours are plainly a.n.a.logous to those of one s.e.x alone in certain other species of the same group.

How, then, are we to account for the beautiful or even gorgeous colours of many animals in the lowest cla.s.ses? It appears very doubtful whether such colours usually serve as a protection; but we are extremely liable to err in regard to characters of all kinds in relation to protection, as will be admitted by every one who has read Mr. Wallace's excellent essay on this subject. It would not, for instance, at first occur to any one that the perfect transparency of the Medusae, or jelly-fishes, was of the highest service to them as a protection; but when we are reminded by Hackel that not only the medusae but many floating mollusca, crustaceans, and even small oceanic fishes partake of this same gla.s.s-like structure, we can hardly doubt that they thus escape the notice of pelagic birds and other enemies.

Notwithstanding our ignorance how far colour in many cases serves as a protection, the most probable view in regard to the splendid tints of many of the lowest animals seems to be that their colours are the direct result either of the chemical nature or the minute structure of their tissues, independently of any benefit thus derived. Hardly any colour is finer than that of arterial blood; but there is no reason to suppose that the colour of the blood is in itself any advantage; and though it adds to the beauty of the maiden's cheek, no one will pretend that it has been acquired for this purpose. So again with many animals, especially the lower ones, the bile is richly coloured; thus the extreme beauty of the Eolidae (naked sea-slugs) is chiefly due, as I am informed by Mr. Hanc.o.c.k, to the biliary glands seen through the translucent integuments; this beauty being probably of no service to these animals.

The tints of the decaying leaves in an American forest are described by every one as gorgeous; yet no one supposes that these tints are of the least advantage to the trees. Bearing in mind how many substances closely a.n.a.logous to natural organic compounds have been recently formed by chemists, and which exhibit the most splendid colours, it would have been a strange fact if substances similarly coloured had not often originated, independently of any useful end being thus gained, in the complex laboratory of living organisms.

_The sub-kingdom of the Mollusca._-Throughout this great division (taken in its largest acceptation) of the animal kingdom, secondary s.e.xual characters, such as we are here considering, never, as far as I can discover, occur. Nor could they be expected in the three lowest cla.s.ses, namely in the Ascidians, Polyzoa, and Brachiopods (const.i.tuting the Molluscoida of Huxley), for most of these animals are permanently affixed to a support or have their s.e.xes united in the same individual.

In the Lamellibranchiata, or bivalve sh.e.l.ls, hermaphroditism is not rare. In the next higher cla.s.s of the Gasteropoda, or marine univalve sh.e.l.ls, the s.e.xes are either united or separate. But in this latter case the males never possess special organs for finding, securing, or charming the females, or for fighting with other males. The sole external difference between the s.e.xes consists, as I am informed by Mr.

Gwyn Jeffreys, in the sh.e.l.l sometimes differing a little in form; for instance, the sh.e.l.l of the male periwinkle (_Littorina littorea_) is narrower and has a more elongated spire than that of the female. But differences of this nature, it may be presumed, are directly connected with the act of reproduction or with the development of the ova.

The Gasteropoda, though capable of locomotion and furnished with imperfect eyes, do not appear to be endowed with sufficient mental powers for the members of the same s.e.x to struggle together in rivalry, and thus to acquire secondary s.e.xual characters. Nevertheless with the pulmoniferous gasteropods, or land-snails, the pairing is preceded by courtship; for these animals, though hermaphrodites, are compelled by their structure to pair together. Aga.s.siz remarks,[410] "Quiconque a eu l'occasion d'observer les amours des limacons, ne saurait mettre en doute la seduction deployee dans les mouvements et les allures qui preparent et accomplissent le double embra.s.s.e.m.e.nt de ces hermaphrodites." These animals appear also susceptible of some degree of permanent attachment: an accurate observer, Mr. Lonsdale, informs me that he placed a pair of land-sh.e.l.ls (_Helix pomatia_), one of which was weakly, into a small and ill-provided garden. After a short time the strong and healthy individual disappeared, and was traced by its track of slime over a wall into an adjoining well-stocked garden. Mr. Lonsdale concluded that it had deserted its sickly mate; but after an absence of twenty-four hours it returned, and apparently communicated the result of its successful exploration, for both then started along the same track and disappeared over the wall.

Even in the highest cla.s.s of the Mollusca, namely the Cephalopoda or cuttle-fishes, in which the s.e.xes are separate, secondary s.e.xual characters of the kind which we are here considering, do not, as far as I can discover, occur. This is a surprising circ.u.mstance, as these animals possess highly-developed sense-organs and have considerable mental powers, as will be admitted by every one who has watched their artful endeavours to escape from an enemy.[411] Certain Cephalopoda, however, are characterised by one extraordinary s.e.xual character, namely, that the male element collects within one of the arms or tentacles, which is then cast off, and, clinging by its sucking-discs to the female, lives for a time an independent life. So completely does the cast-off arm resemble a separate animal, that it was described by Cuvier as a parasitic worm under the name of Hectocotyle. But this marvellous structure may be cla.s.sed as a primary rather than as a secondary s.e.xual character.

Although with the Mollusca s.e.xual selection does not seem to have come into play; yet many univalve and bivalve sh.e.l.ls, such as volutes, cones, scallops, &c., are beautifully coloured and shaped. The colours do not appear in most cases to be of any use as a protection; they are probably the direct result, as in the lowest cla.s.ses, of the nature of the tissues; the patterns and the sculpture of the sh.e.l.l depending on its manner of growth. The amount of light seems to a certain extent to be influential; for although, as repeatedly stated by Mr. Gwyn Jeffreys, the sh.e.l.ls of some species living at a profound depth are brightly coloured, yet we generally see the lower surfaces and the parts covered by the mantle less highly coloured than the upper and exposed surfaces.[412] In some cases, as with sh.e.l.ls living amongst corals or brightly-tinted sea-weeds, the bright colours may serve as a protection.

But many of the nudibranch mollusca, or sea-slugs, are as beautifully coloured as any sh.e.l.ls, as may be seen in Messrs. Alder and Hanc.o.c.k's magnificent work; and from information kindly given me by Mr. Hanc.o.c.k, it is extremely doubtful whether these colours usually serve as a protection. With some species this may be the case, as with one which lives on the green leaves of algae, and is itself bright-green. But many brightly-coloured, white or otherwise conspicuous species, do not seek concealment; whilst again some equally conspicuous species, as well as other dull-coloured kinds, live under stones and in dark recesses. So that with these nudibranch molluscs, colour apparently does not stand in any close relation to the nature of the places which they inhabit.

These naked sea-slugs are hermaphrodites, yet they pair together, as do land-snails, many of which have extremely pretty sh.e.l.ls. It is conceivable that two hermaphrodites, attracted by each others' greater beauty, might unite and leave offspring which would inherit their parents' greater beauty. But with such lowly-organised creatures this is extremely improbable. Nor is it at all obvious how the offspring from the more beautiful pairs of hermaphrodites would have any advantage, so as to increase in numbers, over the offspring of the less beautiful, unless indeed vigour and beauty generally coincided. We have not here a number of males becoming mature before the females, and the more beautiful ones selected by the more vigorous females. If, indeed, brilliant colours were beneficial to an hermaphrodite animal in relation to its general habits of life, the more brightly-tinted individuals would succeed best and would increase in number; but this would be a case of natural and not of s.e.xual selection.

_Sub-kingdom of the Vermes or Annulosa_: Cla.s.s, _Annelida (or Sea-worms)_.-In this cla.s.s, although the s.e.xes (when separate) sometimes differ from each other in characters of such importance that they have been placed under distinct genera or even families, yet the differences do not seem of the kind which can be safely attributed to s.e.xual selection. These animals, like those in the preceding cla.s.ses, apparently stand too low in the scale, for the individuals of either s.e.x to exert any choice in selecting a partner, or for the individuals of the same s.e.x to struggle together in rivalry.

_Sub-kingdom of the Arthropoda_: Cla.s.s, _Crustacea_.-In this great cla.s.s we first meet with undoubted secondary s.e.xual characters, often developed in a remarkable manner. Unfortunately the habits of crustaceans are very imperfectly known, and we cannot explain the uses of many structures peculiar to one s.e.x. With the lower parasitic species the males are of small size, and they alone are furnished with perfect swimming-legs, antennae and sense-organs; the females being dest.i.tute of these organs, with their bodies often consisting of a mere distorted ma.s.s. But these extraordinary differences between the two s.e.xes are no doubt related to their widely different habits of life, and consequently do not concern us. In various crustaceans, belonging to distinct families, the anterior antennae are furnished with peculiar thread-like bodies, which are believed to act as smelling-organs, and these are much more numerous in the males than in the females. As the males, without any unusual development of their olfactory organs, would almost certainly be able sooner or later to find the females, the increased number of the smelling-threads has probably been acquired through s.e.xual selection, by the better provided males having been the most successful in finding partners and in leaving offspring. Fritz Muller has described a remarkable dimorphic species of Tanais, in which the male is represented by two distinct forms, never graduating into each other. In the one form the male is furnished with more numerous smelling-threads, and in the other form with more powerful and more elongated chelae or pincers which serve to hold the female. Fritz Muller suggests that these differences between the two male forms of the same species must have originated in certain individuals having varied in the number of the smelling-threads, whilst other individuals varied in the shape and size of their chelae; so that of the former, those which were best able to find the female, and of the latter, those which were best able to hold her when found, have left the greater number of progeny to inherit their respective advantages.[413]

[Ill.u.s.tration: Fig. 3. Labidocera Darwinii, (from Lubbock).

_a._ Part of right-hand anterior antenna of male, forming a prehensile organ.

_b._ Posterior pair of thoracic legs of male.

_c._ Ditto of female.]

In some of the lower crustaceans, the right-hand anterior antenna of the male differs greatly in structure from the left-hand one, the latter resembling in its simple tapering joints the antennae of the female. In the male the modified antenna is either swollen in the middle or angularly bent, or converted (fig. 3) into an elegant, and sometimes wonderfully complex, prehensile organ.[414] It serves, as I hear from Sir J. Lubbock, to hold the female, and for this same purpose one of the two posterior legs (_b_) on the same side of the body is converted into a forceps. In another family the inferior or posterior antennae are "curiously zigzagged" in the males alone.

[Ill.u.s.tration: Fig. 4. Anterior part of body of Calliana.s.sa (from Milne-Edwards), showing the unequal and differently-constructed right and left-hand chelae of the male.

N.B.-The artist by mistake has reversed the drawing, and made the left-hand chela the largest.]

[Ill.u.s.tration: Fig. 5. Second leg of male Orchestia Tucuratinga (from Fritz Muller).]

[Ill.u.s.tration: Fig. 6. Ditto of female.]

In the higher crustaceans the anterior legs form a pair of chelae or pincers, and these are generally larger in the male than in the female.

In many species the chelae on the opposite sides of the body are of unequal size, the right-hand one being, as I am informed by Mr. C.

Spence Bate, generally, though not invariably, the largest. This inequality is often much greater in the male than in the female. The two chelae also often differ in structure (figs. 4, 5, and 6), the smaller one resembling those of the female. What advantage is gained by their inequality in size on the opposite sides of the body, and by the inequality being much greater in the male than in the female; and why, when they are of equal size, both are often much larger in the male than in the female, is not known. The chelae are sometimes of such length and size that they cannot possibly be used, as I hear from Mr. Spence Bate, for carrying food to the mouth. In the males of certain freshwater prawns (Palaemon) the right leg is actually longer than the whole body.[415] It is probable that the great size of one leg with its chelae may aid the male in fighting with his rivals; but this use will not account for their inequality in the female on the opposite sides of the body. In Gelasimus, according to a statement quoted by Milne-Edwards,[416] the male and female live in the same burrow, which is worth notice, as shewing that they pair, and the male closes the mouth of the burrow with one of its chelae, which is enormously developed; so that here it indirectly serves as a means of defence.

Their main use, however, probably is to seize and to secure the female, and this in some instances, as with Gammarus, is known to be the case.

The s.e.xes, however, of the common sh.o.r.e-crab (_Carcinus maenas_), as Mr.

Spence Bate informs me, unite directly after the female has moulted her hard sh.e.l.l, and when she is so soft that she would be injured if seized by the strong pincers of the male; but as she is caught and carried about by the male previously to the act of moulting, she could then be seized with impunity.

Fritz Muller states that certain species of Melita are distinguished from all other amphipods by the females having "the c.o.xal lamellae of the penultimate pair of feet produced into hook-like processes, of which the males lay hold with the hands of the first pair." The development of these hook-like processes probably resulted from those females which were the most securely held during the act of reproduction, having left the largest number of offspring. Another Brazilian amphipod (_Orchestia Darwinii_, fig. 7) is described by Fritz Muller, as presenting a case of dimorphism, like that of Tanais; for there are two male forms, which differ in the structure of their chelae.[417] As chelae of either shape would certainly have sufficed to hold the female, for both are now used for this purpose, the two male forms probably originated, by some having varied in one manner and some in another; both forms having derived certain special, but nearly equal advantages, from their differently shaped organs.

It is not known that male crustaceans fight together for the possession of the females, but this is probable; for with most animals when the male is larger than the female, he seems to have acquired his greater size by having conquered during many generations other males. Now Mr.

Spence Bate informs me that in most of the crustacean orders, especially in the highest or the Brachyura, the male is larger than the female; the parasitic genera, however, in which the s.e.xes follow different habits of life, and most of the Entomostraca must be excepted. The chelae of many crustaceans are weapons well adapted for fighting. Thus a Devil-crab (_Portunus p.u.b.er_) was seen by a son of Mr. Bate fighting with a _Carcinus maenas_, and the latter was soon thrown on its back, and had every limb torn from its body. When several males of a Brazilian Gelasimus, a species furnished with immense pincers, were placed together by Fritz Muller in a gla.s.s vessel, they mutilated and killed each other. Mr. Bate put a large male _Carcinus maenas_ into a pan of water, inhabited by a female paired with a smaller male; the latter was soon dispossessed, but, as Mr. Bate adds, "if they fought, the victory was a bloodless one, for I saw no wounds." This same naturalist separated a male sand-skipper (so common on our sea-sh.o.r.es), _Gammarus marinus_, from its female, both of which were imprisoned in the same vessel with many individuals of the same species. The female being thus divorced joined her comrades. After an interval the male was again put into the same vessel and he then, after swimming about for a time, dashed into the crowd, and without any fighting at once took away his wife. This fact shews that in the Amphipoda, an order low in the scale, the males and females recognise each other, and are mutually attached.

[Ill.u.s.tration: Fig. 7. Orchestia Darwinii (from Fritz Muller), showing the differently-constructed chelae of the two male forms.]

The mental powers of the Crustacea are probably higher than might have been expected. Any one who has tried to catch one of the sh.o.r.e-crabs, so numerous on many tropical coasts, will have perceived how wary and alert they are. There is a large crab (_Birgus latro_), found on coral islands, which makes at the bottom of a deep burrow a thick bed of the picked fibres of the cocoa-nut. It feeds on the fallen fruit of this tree by tearing off the husk, fibre by fibre; and it always begins at that end where the three eye-like depressions are situated. It then breaks through one of these eyes by hammering with its heavy front pincers, and turning round, extracts the alb.u.minous core with its narrow posterior pincers. But these actions are probably instinctive, so that they would be performed as well by a young as by an old animal. The following case, however, can hardly be so considered: a trustworthy naturalist, Mr. Gardner,[418] whilst watching a sh.o.r.e-crab (Gelasimus) making its burrow, threw some sh.e.l.ls towards the hole. One rolled in, and three other sh.e.l.ls remained within a few inches of the mouth. In about five minutes the crab brought out the sh.e.l.l which had fallen in, and carried it away to the distance of a foot; it then saw the three other sh.e.l.ls lying near, and evidently thinking that they might likewise roll in, carried them to the spot where it had laid the first. It would, I think, be difficult to distinguish this act from one performed by man by the aid of reason.

With respect to colour which so often differs in the two s.e.xes of animals belonging to the higher cla.s.ses, Mr. Spence Bate does not know of any well-marked instances with our British crustaceans. In some cases, however, the male and female differ slightly in tint, but Mr.

Bate thinks not more than may be accounted for by their different habits of life, such as by the male wandering more about and being thus more exposed to the light. In a curious Bornean crab, which inhabits sponges, Mr. Bate could always distinguish the s.e.xes by the male not having the epidermis so much rubbed off. Dr. Power tried to distinguish by colour the s.e.xes of the species which inhabit the Mauritius, but always failed, except with one species of Squilla, probably the _S. stylifera_, the male of which is described as being "of a beautiful blueish-green," with some of the appendages cherry-red, whilst the female is clouded with brown and grey, "with the red about her much less vivid than in the male."[419] In this case, we may suspect the agency of s.e.xual selection.

With Saphirina (an oceanic genus of Entomostraca, and therefore low in the scale) the males are furnished with minute shields or cell-like bodies, which exhibit beautiful changing colours; these being absent in the females, and in the case of one species in both s.e.xes.[420] It would, however, be extremely rash to conclude that these curious organs serve merely to attract the females. In the female of a Brazilian species of Gelasimus, the whole body, as I am informed by Fritz Muller, is of a nearly uniform greyish-brown. In the male the posterior part of the cephalo-thorax is pure white, with the anterior part of a rich green, shading into dark brown; and it is remarkable that these colours are liable to change in the course of a few minutes-the white becoming dirty grey or even black, the green "losing much of its brilliancy." The males apparently are much more numerous than the females. It deserves especial notice that they do not acquire their bright colours until they become mature. They differ also from the females in the larger size of their chelae. In some species of the genus, probably in all, the s.e.xes pair and inhabit the same burrow. They are also, as we have seen, highly intelligent animals. From these various considerations it seems highly probable that the male in this species has become gaily ornamented in order to attract or excite the female.

It has just been stated that the male Gelasimus does not acquire his conspicuous colours until mature and nearly ready to breed. This seems the general rule in the whole cla.s.s with the many remarkable differences in structure between the two s.e.xes. We shall hereafter find the same law prevailing throughout the great sub-kingdom of the Vertebrata, and in all cases it is eminently distinctive of characters which have been acquired through s.e.xual selection. Fritz Muller[421] gives some striking instances of this law; thus the male sand-hopper (Orchestia) does not acquire his large claspers, which are very differently constructed from those of the female, until nearly full-grown; whilst young his claspers resemble those of the female. Thus, again, the male Brachyscelus possesses, like all other amphipods, a pair of posterior antennae; the female, and this is a most extraordinary circ.u.mstance, is dest.i.tute of them, and so is the male as long as he remains immature.

Cla.s.s, _Arachnida_ (Spiders).-The males are often darker, but sometimes lighter than the females, as may be seen in Mr. Blackwall's magnificent work.[422] In some species the s.e.xes differ conspicuously from each other in colour; thus the female of _Spara.s.sus smaragdulus_ is dullish-green; whilst the adult male has the abdomen of a fine yellow, with three longitudinal stripes of rich red. In some species of Thomisus the two s.e.xes closely resemble each other; in others they differ much; thus in _T. citreus_ the legs and body of the female are pale-yellow or green, whilst the front legs of the male are reddish-brown: in _T.

floricolens_, the legs of the female are pale-green, those of the male being ringed in a conspicuous manner with various tints. Numerous a.n.a.logous cases could be given in the genera Epeira, Nephila, Philodromus, Theridion, Linyphia, &c. It is often difficult to say which of the two s.e.xes departs most from the ordinary coloration of the genus to which the species belong; but Mr. Blackwall thinks that, as a general rule, it is the male. Both s.e.xes whilst young, as I am informed by the same author, usually resemble each other; and both often undergo great changes in colour during their successive moults before arriving at maturity. In other cases the male alone appears to change colour.

Thus the male of the above-mentioned brightly-coloured Spara.s.sus at first resembles the female and acquires his peculiar tints only when nearly adult. Spiders are possessed of acute senses, and exhibit much intelligence. The females often shew, as is well known, the strongest affection for their eggs, which they carry about enveloped in a silken web. On the whole it appears probable that well-marked differences in colour between the s.e.xes have generally resulted from s.e.xual selection, either on the male or female side. But doubts may be entertained on this head from the extreme variability in colour of some species, for instance of _Theridion lineatum_, the s.e.xes of which differ when adult; this great variability indicates that their colours have not been subjected to any form of selection.

Mr. Blackwall does not remember to have seen the males of any species fighting together for the possession of the female. Nor, judging from a.n.a.logy, is this probable; for the males are generally much smaller than the females, sometimes to an extraordinary degree.[423] Had the males been in the habit of fighting together, they would, it is probable, have gradually acquired greater size and strength. Mr. Blackwall has sometimes seen two or more males on the same web with a single female; but their courtship is too tedious and prolonged an affair to be easily observed. The male is extremely cautious in making his advances, as the female carries her coyness to a dangerous pitch. De Geer saw a male that "in the midst of his preparatory caresses was seized by the object of his attentions, enveloped by her in a web and then devoured, a sight which, as he adds, filled him with horror and indignation."[424]

Westring has made the interesting discovery that the males of several species of Theridion[425] have the power of making a stridulating sound (like that made by many beetles and other insects, but feebler), whilst the females are quite mute. The apparatus consists of a serrated ridge at the base of the abdomen, against which the hard hinder part of the thorax is rubbed; and of this structure not a trace could be detected in the females. From the a.n.a.logy of the Orthoptera and h.o.m.optera, to be described in the next chapter, we may feel almost sure that the stridulation serves, as Westring remarks, either to call or to excite the female; and this is the first case in the ascending scale of the animal kingdom, known to me, of sounds emitted for this purpose.

Cla.s.s, _Myriapoda_.-In neither of the two orders in this cla.s.s, including the millipedes and centipedes, can I find any well-marked instances of s.e.xual differences such as more particularly concern us. In _Glomeris limbata_, however, and perhaps in some few other species, the males differ slightly in colour from the females; but this Glomeris is a highly variable species. In the males of the Diplopoda, the legs belonging to one of the anterior segments of the body, or to the posterior segment, are modified into prehensile hooks which serve to secure the female. In some species of Iulus the tarsi of the male are furnished with membranous suckers for the same purpose. It is a much more unusual circ.u.mstance, as we shall see when we treat of Insects, that it is the female in Lithobius which is furnished with prehensile appendages at the extremity of the body for holding the male.[426]

CHAPTER X.

SECONDARY s.e.xUAL CHARACTERS OF INSECTS.