The lion in South Africa, as I hear from Sir Andrew Smith, sometimes lives with a single female, but generally with more than one, and, in one case, was found with as many as five females, so that he is polygamous. He is, as far as I can discover, the sole polygamist in the whole group of the terrestrial Carnivora, and he alone presents well-marked s.e.xual characters. If, however, we turn to the marine Carnivora, the case is widely different; for many species of seals offer, as we shall hereafter see, extraordinary s.e.xual differences, and they are eminently polygamous. Thus the male sea-elephant of the Southern Ocean, always possesses, according to Peron, several females, and the sea-lion of Forster is said to be surrounded by from twenty to thirty females. In the North, the male sea-bear of Steller is accompanied by even a greater number of females.
With respect to birds, many species, the s.e.xes of which differ greatly from each other, are certainly monogamous. In Great Britain we see well-marked s.e.xual differences in, for instance, the wild-duck which pairs with a single female, with the common blackbird, and with the bullfinch which is said to pair for life. So it is, as I am informed by Mr. Wallace, with the Chatterers or Cotingidae of South America, and numerous other birds. In several groups I have not been able to discover whether the species are polygamous or monogamous. Lesson says that birds of paradise, so remarkable for their s.e.xual differences, are polygamous, but Mr. Wallace doubts whether he had sufficient evidence. Mr. Salvin informs me that he has been led to believe that humming-birds are polygamous. The male widow-bird; remarkable for his caudal plumes, certainly seems to be a polygamist.[343] I have been a.s.sured by Mr.
Jenner Weir and by others, that three starlings not rarely frequent the same nest; but whether this is a case of polygamy or polyandry has not been ascertained.
The Gallinaceae present almost as strongly marked s.e.xual differences as birds of paradise or humming-birds, and many of the species are, as is well known, polygamous; others being strictly monogamous. What a contrast is presented between the s.e.xes by the polygamous peac.o.c.k or pheasant, and the monogamous guinea-fowl or partridge! Many similar cases could be given, as in the grouse tribe, in which the males of the polygamous capercailzie and black-c.o.c.k differ greatly from the females; whilst the s.e.xes of the monogamous red grouse and ptarmigan differ very little. Amongst the Cursores, no great number of species offer strongly-marked s.e.xual differences, except the bustards, and the great bustard (_Otis tarda_), is said to be polygamous. With the Grallatores, extremely few species differ s.e.xually, but the ruff (_Machetes pugnax_) affords a strong exception, and this species is believed by Montagu to be a polygamist. Hence it appears that with birds there often exists a close relation between polygamy and the development of strongly-marked s.e.xual differences. On asking Mr. Bartlett, at the Zoological Gardens, who has had such large experience with birds, whether the male tragopan (one of the Gallinaceae) was polygamous, I was struck by his answering, "I do not know, but should think so from his splendid colours."
It deserves notice that the instinct of pairing with a single female is easily lost under domestication. The wild-duck is strictly monogamous, the domestic-duck highly polygamous. The Rev. W. D. Fox informs me that with some half-tamed wild-ducks, kept on a large pond in his neighbourhood, so many mallards were shot by the gamekeeper that only one was left for every seven or eight females; yet unusually large broods were reared. The guinea-fowl is strictly monogamous; but Mr. Fox finds that his birds succeed best when he keeps one c.o.c.k to two or three hens.[344] Canary-birds pair in a state of nature, but the breeders in England successfully put one male to four or five females; nevertheless the first female, as Mr. Fox has been a.s.sured, is alone treated as the wife, she and her young ones being fed by him; the others are treated as concubines. I have noticed these cases, as it renders it in some degree probable that monogamous species, in a state of nature, might readily become either temporarily or permanently polygamous.
With respect to reptiles and fishes, too little is known of their habits to enable us to speak of their marriage arrangements. The stickle-back Gasterosteus, however, is said to be a polygamist;[345] and the male during the breeding-season differs conspicuously from the female.
To sum up on the means through which, as far as we can judge, s.e.xual selection has led to the development of secondary s.e.xual characters. It has been shewn that the largest number of vigorous offspring will be reared from the pairing of the strongest and best-armed males, which have conquered other males, with the most vigorous and best-nourished females, which are the first to breed in the spring. Such females, if they select the more attractive, and at the same time vigorous, males, will rear a larger number of offspring than the r.e.t.a.r.ded females, which must pair with the less vigorous and less attractive males. So it will be if the more vigorous males select the more attractive and at the same time healthy and vigorous females; and this will especially hold good if the male defends the female, and aids in providing food for the young.
The advantage thus gained by the more vigorous pairs in rearing a larger number of offspring has apparently sufficed to render s.e.xual selection efficient. But a large preponderance in number of the males over the females would be still more efficient; whether the preponderance was only occasional and local, or permanent; whether it occurred at birth, or subsequently from the greater destruction of the females; or whether it indirectly followed from the practice of polygamy.
_The Male generally more modified than the Female._-Throughout the animal kingdom, when the s.e.xes differ from each other in external appearance, it is the male which, with rare exceptions, has been chiefly modified; for the female still remains more like the young of her own species, and more like the other members of the same group. The cause of this seems to lie in the males of almost all animals having stronger pa.s.sions than the females. Hence it is the males that fight together and sedulously display their charms before the females; and those which are victorious transmit their superiority to their male offspring. Why the males do not transmit their characters to both s.e.xes will hereafter be considered. That the males of all mammals eagerly pursue the females is notorious to every one. So it is with birds; but many male birds do not so much pursue the female, as display their plumage, perform strange antics, and pour forth their song, in her presence. With the few fish which have been observed, the male seems much more eager than the female; and so it is with alligators, and apparently with Batrachians.
Throughout the enormous cla.s.s of insects, as Kirby remarks,[346] "the law is, that the male shall seek the female." With spiders and crustaceans, as I hear from two great authorities, Mr. Blackwall and Mr.
C. Spence Bate, the males are more active and more erratic in their habits than the females. With insects and crustaceans, when the organs of sense or locomotion are present in the one s.e.x and absent in the other, or when, as is frequently the case, they are more highly developed in the one than the other, it is almost invariably the male, as far as I can discover, which retains such organs, or has them most developed; and this shews that the male is the more active member in the courtship of the s.e.xes.[347]
The female, on the other hand, with the rarest exception, is less eager than the male. As the ill.u.s.trious Hunter[348] long ago observed, she generally "requires to be courted;" she is coy, and may often be seen endeavouring for a long time to escape from the male. Every one who has attended to the habits of animals will be able to call to mind instances of this kind. Judging from various facts, hereafter to be given, and from the results which may fairly be attributed to s.e.xual selection, the female, though comparatively pa.s.sive, generally exerts some choice and accepts one male in preference to others. Or she may accept, as appearances would sometimes lead us to believe, not the male which is the most attractive to her, but the one which is the least distasteful.
The exertion of some choice on the part of the female seems almost as general a law as the eagerness of the male.
We are naturally led to enquire why the male in so many and such widely distinct cla.s.ses has been rendered more eager than the female, so that he searches for her and plays the more active part in courtship. It would be no advantage and some loss of power if both s.e.xes were mutually to search for each other; but why should the male almost always be the seeker? With plants, the ovules after fertilisation have to be nourished for a time; hence the pollen is necessarily brought to the female organs-being placed on the stigma, through the agency of insects or of the wind, or by the spontaneous movements of the stamens; and with the Algae, &c., by the locomotive power of the antherozooids. With lowly-organised animals permanently affixed to the same spot and having their s.e.xes separate, the male element is invariably brought to the female; and we can see the reason; for the ova, even if detached before being fertilised and not requiring subsequent nourishment or protection, would be, from their larger relative size, less easily transported than the male element. Hence plants[349] and many of the lower animals are, in this respect, a.n.a.logous. In the case of animals not affixed to the same spot, but enclosed within a sh.e.l.l with no power of protruding any part of their bodies, and in the case of animals having little power of locomotion, the males must trust the fertilising element to the risk of at least a short transit through the waters of the sea. It would, therefore, be a great advantage to such animals, as their organisation became perfected, if the males when ready to emit the fertilising element, were to acquire the habit of approaching the female as closely as possible. The males of various lowly-organised animals having thus aboriginally acquired the habit of approaching and seeking the females, the same habit would naturally be transmitted to their more highly developed male descendants; and in order that they should become efficient seekers, they would have to be endowed with strong pa.s.sions.
The acquirement of such pa.s.sions would naturally follow from the more eager males leaving a larger number of offspring than the less eager.
The great eagerness of the male has thus indirectly led to the much more frequent development of secondary s.e.xual characters in the male than in the female. But the development of such characters will have been much aided, if the conclusion at which I arrived after studying domesticated animals, can be trusted, namely, that the male is more liable to vary than the female. I am aware how difficult it is to verify a conclusion of this kind. Some slight evidence, however, can be gained by comparing the two s.e.xes in mankind, as man has been more carefully observed than any other animal. During the Novara Expedition[350] a vast number of measurements of various parts of the body in different races were made, and the men were found in almost every case to present a greater range of variation than the women; but I shall have to recur to this subject in a future chapter. Mr. J. Wood,[351] who has carefully attended to the variation of the muscles in man, puts in italics the conclusion that "the greatest number of abnormalities in each subject is found in the males." He had previously remarked that "altogether in 102 subjects the varieties of redundancy were found to be half as many again as in females, contrasting widely with the greater frequency of deficiency in females before described." Professor Macalister like wise remarks[352] that variations in the muscles "are probably more common in males than females." Certain muscles which are not normally present in mankind are also more frequently developed in the male than in the female s.e.x, although exceptions to this rule are said to occur. Dr.
Burt Wilder[353] has tabulated the cases of 152 individuals with supernumerary digits, of which 86 were males, and 39, or less than half, females; the remaining 27 being of unknown s.e.x. It should not, however, be overlooked that women would more frequently endeavour to conceal a deformity of this kind than men. Whether the large proportional number of deaths of the male offspring of man and apparently of sheep, compared with the female offspring, before, during, and shortly after birth (see supplement), has any relation to a stronger tendency in the organs of the male to vary and thus to become abnormal in structure or function, I will not pretend to conjecture.
In various cla.s.ses of animals a few exceptional cases occur, in which the female instead of the male has acquired well p.r.o.nounced secondary s.e.xual characters, such as brighter colours, greater size, strength, or pugnacity. With birds, as we shall hereafter see, there has sometimes been a complete transposition of the ordinary characters proper to each s.e.x; the females having become the more eager in courtship, the males remaining comparatively pa.s.sive, but apparently selecting, as we may infer from the results, the more attractive females. Certain female birds have thus been rendered more highly coloured or otherwise ornamented, as well as more powerful and pugnacious than the males, these characters being transmitted to the female offspring alone.
It may be suggested that in some cases a double process of selection has been carried on; the males having selected the more attractive females, and the latter the more attractive males. This process however, though it might lead to the modification of both s.e.xes, would not make the one s.e.x different from the other, unless indeed their taste for the beautiful differed; but this is a supposition too improbable in the case of any animal, excepting man, to be worth considering. There are, however, many animals, in which the s.e.xes resemble each other, both being furnished with the same ornaments, which a.n.a.logy would lead us to attribute to the agency of s.e.xual selection. In such cases it may be suggested with more plausibility, that there has been a double or mutual process of s.e.xual selection; the more vigorous and precocious females having selected the more attractive and vigorous males, the latter having rejected all except the more attractive females. But from what we know of the habits of animals, this view is hardly probable, the male being generally eager to pair with any female. It is more probable that the ornaments common to both s.e.xes were acquired by one s.e.x, generally the male, and then transmitted to the offspring of both s.e.xes. If, indeed, during a lengthened period the males of any species were greatly to exceed the females in number, and then during another lengthened period under different conditions the reverse were to occur, a double, but not simultaneous, process of s.e.xual selection might easily be carried on, by which the two s.e.xes might be rendered widely different.
We shall hereafter see that many animals exist, of which neither s.e.x is brilliantly coloured or provided with special ornaments, and yet the members of both s.e.xes or of one alone have probably been modified through s.e.xual selection. The absence of bright tints or other ornaments may be the result of variations of the right kind never having occurred, or of the animals themselves preferring simple colours, such as plain black or white. Obscure colours have often been acquired through natural selection for the sake of protection, and the acquirement through s.e.xual selection of conspicuous colours, may have been checked from the danger thus incurred. But in other cases the males have probably struggled together during long ages, through brute force, or by the display of their charms, or by both means combined, and yet no effect will have been produced unless a larger number of offspring were left by the more successful males to inherit their superiority, than by the less successful males; and this, as previously shewn, depends on various complex contingencies.
s.e.xual selection acts in a less rigorous manner than natural selection.
The latter produces its effects by the life or death at all ages of the more or less successful individuals. Death, indeed, not rarely ensues from the conflicts of rival males. But generally the less successful male merely fails to obtain a female, or obtains later in the season a r.e.t.a.r.ded and less vigorous female, or, if polygamous, obtains fewer females; so that they leave fewer, or less vigorous, or no offspring. In regard to structures acquired through ordinary or natural selection, there is in most cases, as long as the conditions of life remain the same, a limit to the amount of advantageous modification in relation to certain special ends; but in regard to structures adapted to make one male victorious over another, either in fighting or in charming the female, there is no definite limit to the amount of advantageous modification; so that as long as the proper variations arise the work of s.e.xual selection will go on. This circ.u.mstance may partly account for the frequent and extraordinary amount of variability presented by secondary s.e.xual characters. Nevertheless, natural selection will determine that characters of this kind shall not be acquired by the victorious males, which would be injurious to them in any high degree, either by expending too much of their vital powers, or by exposing them to any great danger. The development, however, of certain structures-of the horns, for instance, in certain stags-has been carried to a wonderful extreme; and in some instances to an extreme which, as far as the general conditions of life are concerned, must be slightly injurious to the male. From this fact we learn that the advantages which favoured males have derived from conquering other males in battle or courtship, and thus leaving a numerous progeny, have been in the long run greater than those derived from rather more perfect adaptation to the external conditions of life. We shall further see, and this could never have been antic.i.p.ated, that the power to charm the female has been in some few instances more important than the power to conquer other males in battle.
LAWS OF INHERITANCE.
In order to understand how s.e.xual selection has acted, and in the course of ages has produced conspicuous results with many animals of many cla.s.ses, it is necessary to bear in mind the laws of inheritance, as far as they are known. Two distinct elements are included under the term "inheritance," namely the transmission and the development of characters; but as these generally go together, the distinction is often overlooked. We see this distinction in those characters which are transmitted through the early years of life, but are developed only at maturity or during old age. We see the same distinction more clearly with secondary s.e.xual characters, for these are transmitted through both s.e.xes, though developed in one alone. That they are present in both s.e.xes, is manifest when two species, having strongly-marked s.e.xual characters, are crossed, for each transmits the characters proper to its own male and female s.e.x to the hybrid offspring of both s.e.xes. The same fact is likewise manifest, when characters proper to the male are occasionally developed in the female when she grows old or becomes diseased; and so conversely with the male. Again, characters occasionally appear, as if transferred from the male to the female, as when, in certain breeds of the fowl, spurs regularly appear in the young and healthy females; but in truth they are simply developed in the female; for in every breed each detail in the structure of the spur is transmitted through the female to her male offspring. In all cases of reversion, characters are transmitted through two, three, or many generations, and are then under certain unknown favourable conditions developed. This important distinction between transmission and development will be easiest kept in mind by the aid of the hypothesis of pangenesis, whether or not it be accepted as true. According to this hypothesis, every unit or cell of the body throws off gemmules or undeveloped atoms, which are transmitted to the offspring of both s.e.xes, and are multiplied by self-division. They may remain undeveloped during the early years of life or during successive generations; their development into units or cells, like those from which they were derived, depending on their affinity for, and union with, other units or cells previously developed in the due order of growth.
_Inheritance at Corresponding Periods of Life._-This tendency is well established. If a new character appears in an animal whilst young, whether it endures throughout life or lasts only for a time, it will reappear, as a general rule, at the same age and in the same manner in the offspring. If, on the other hand, a new character appears at maturity, or even during old age, it tends to reappear in the offspring at the same advanced age. When deviations from this rule occur, the transmitted characters much oftener appear before than after the corresponding age. As I have discussed this subject at sufficient length in another work,[354] I will here merely give two or three instances, for the sake of recalling the subject to the reader"s mind. In several breeds of the Fowl, the chickens whilst covered with down, in their first true plumage, and in their adult plumage, differ greatly from each other, as well as from their common parent-form, the _Gallus bankiva_; and these characters are faithfully transmitted by each breed to their offspring at the corresponding period of life. For instance, the chickens of spangled Hamburghs, whilst covered with down, have a few dark spots on the head and rump, but are not longitudinally striped, as in many other breeds; in their first true plumage, "they are beautifully pencilled," that is each feather is transversely marked by numerous dark bars; but in their second plumage the feathers all become spangled or tipped with a dark round spot.[355] Hence in this breed variations have occurred and have been transmitted at three distinct periods of life.
The Pigeon offers a more remarkable case, because the aboriginal parent-species does not undergo with advancing age any change of plumage, excepting that at maturity the breast becomes more iridescent; yet there are breeds which do not acquire their characteristic colours until they have moulted two, three, or four times; and these modifications of plumage are regularly transmitted.
_Inheritance at Corresponding Seasons of the Year._-With animals in a state of nature innumerable instances occur of characters periodically appearing at different seasons. We see this with the horns of the stag, and with the fur of arctic animals which becomes thick and white during the winter. Numerous birds acquire bright colours and other decorations during the breeding-season alone. I can throw but little light on this form of inheritance from facts observed under domestication. Pallas states,[356] that in Siberia domestic cattle and horses periodically become lighter-coloured during the winter; and I have observed a similar marked change of colour in certain ponies in England. Although I do not know that this tendency to a.s.sume a differently coloured coat during different seasons of the year is transmitted, yet it probably is so, as all shades of colour are strongly inherited by the horse. Nor is this form of inheritance, as limited by season, more remarkable than inheritance as limited by age or s.e.x.
_Inheritance as Limited by s.e.x._-The equal transmission of characters to both, s.e.xes is the commonest form of inheritance, at least with those animals which do not present strongly-marked s.e.xual differences, and indeed with many of these. But characters are not rarely transferred exclusively to that s.e.x, in which they first appeared. Ample evidence on this head has been advanced in my work on Variation under Domestication; but a few instances may here be given. There are breeds of the sheep and goat, in which the horns of the male differ greatly in shape from those of the female; and these differences, acquired under domestication, are regularly transmitted to the same s.e.x. With tortoise-sh.e.l.l cats the females alone, as a general rule, are thus coloured, the males being rusty-red. With most breeds of the fowl, the characters proper to each s.e.x are transmitted to the same s.e.x alone. So general is this form of transmission that it is an anomaly when we see in certain breeds variations transmitted equally to both s.e.xes. There are also certain sub-breeds of the fowl in which the males can hardly be distinguished from each other, whilst the females differ considerably in colour. With the pigeon the s.e.xes of the parent-species do not differ in any external character; nevertheless in certain domesticated breeds the male is differently coloured from the female.[357] The wattle in the English Carrier pigeon and the crop in the Pouter are more highly developed in the male than in the female; and although these characters have been gained through long-continued selection by man, the difference between the two s.e.xes is wholly due to the form of inheritance which has prevailed; for it has arisen, not from, but rather in opposition to, the wishes of the breeder.
Most of our domestic races have been formed by the acc.u.mulation of many slight variations; and as some of the successive steps have been transmitted to one s.e.x alone, and some to both s.e.xes, we find in the different breeds of the same species all gradations between great s.e.xual dissimilarity and complete similarity. Instances have already been given with the breeds of the fowl and pigeon; and under nature a.n.a.logous cases are of frequent occurrence. With animals under domestication, but whether under nature I will not venture to say, one s.e.x may lose characters proper to it, and may thus come to resemble to a certain extent the opposite s.e.x; for instance, the males of some breeds of the fowl have lost their masculine plumes and hackles. On the other hand the differences between the s.e.xes may be increased under domestication, as with merino sheep, in which the ewes have lost their horns. Again, characters proper to one s.e.x may suddenly appear in the other s.e.x; as with those sub-breeds of the fowl in which the hens whilst young acquire spurs; or, as in certain Polish sub-breeds, in which the females, as there is reason to believe, originally acquired a crest, and subsequently transferred it to the males. All these cases are intelligible on the hypothesis of pangenesis; for they depend on the gemmules of certain units of the body, although present in both s.e.xes, becoming through the influence of domestication dormant in the one s.e.x; or if naturally dormant, becoming developed.
There is one difficult question which it will be convenient to defer to a future chapter; namely, whether a character at first developed in both s.e.xes, can be rendered through selection limited in its development to one s.e.x alone. If, for instance, a breeder observed that some of his pigeons (in which species characters are usually transferred in an equal degree to both s.e.xes) varied into pale blue; could he by long-continued selection make a breed, in which the males alone should be of this tint, whilst the females remained unchanged? I will here only say, that this, though perhaps not impossible, would be extremely difficult; for the natural result of breeding from the pale-blue males would be to change his whole stock, including both s.e.xes, into this tint. If, however, variations of the desired tint appeared, which were from the first limited in their development to the male s.e.x, there would not be the least difficulty in making a breed characterised by the two s.e.xes being of a different colour, as indeed has been effected with a Belgian breed, in which the males alone are streaked with black. In a similar manner, if any variation appeared in a female pigeon, which was from the first s.e.xually limited in its development, it would be easy to make a breed with the females alone thus characterised; but if the variation was not thus originally limited, the process would be extremely difficult, perhaps impossible.
_On the Relation between the period of Development of a Character and its transmission to one s.e.x or to both s.e.xes._-Why certain characters should be inherited by both s.e.xes, and other characters by one s.e.x alone, namely by that s.e.x in which the character first appeared, is in most cases quite unknown. We cannot even conjecture why with certain sub-breeds of the pigeon, black striae, though transmitted through the female, should be developed in the male alone, whilst every other character is equally transferred to both s.e.xes. Why, again, with cats, the tortoise-sh.e.l.l colour should, with rare exceptions, be developed in the female alone. The very same characters, such as deficient or supernumerary digits, colour-blindness, &c., may with mankind be inherited by the males alone of one family, and in another family by the females alone, though in both cases transmitted through the opposite as well as the same s.e.x.[358] Although we are thus ignorant, two rules often hold good, namely that variations which, first appear in either s.e.x at a late period of life, tend to be developed in the same s.e.x alone; whilst variations which first appear early in life in either s.e.x tend to be developed in both s.e.xes. I am, however, far from supposing that this is the sole determining cause. As I have not elsewhere discussed this subject, and as it has an important bearing on s.e.xual selection, I must here enter into lengthy and somewhat intricate details.
It is in itself probable that any character appearing at an early age would tend to be inherited equally by both s.e.xes, for the s.e.xes do not differ much in const.i.tution, before the power of reproduction is gained.
On the other hand, after this power has been gained and the s.e.xes have come to differ in const.i.tution, the gemmules (if I may again use the language of pangenesis) which are cast off from each varying part in the one s.e.x would be much more likely to possess the proper affinities for uniting with the tissues of the same s.e.x, and thus becoming developed, than with those of the opposite s.e.x.
I was first led to infer that a relation of this kind exists, from the fact that whenever and in whatever manner the adult male has come to differ from the adult female, he differs in the same manner from the young of both s.e.xes. The generality of this fact is quite remarkable: it holds good with almost all mammals, birds, amphibians, and fishes; also with many crustaceans, spiders and some few insects, namely certain orthoptera and libellulae. In all these cases the variations, through the acc.u.mulation of which the male acquired his proper masculine characters, must have occurred at a somewhat late period of life; otherwise the young males would have been similarly characterised; and conformably with our rule, they are transmitted to and developed in the adult males alone. When, on the other hand, the adult male closely resembles the young of both s.e.xes (these, with rare exceptions, being alike), he generally resembles the adult female; and in most of these cases the variations through which the young and old acquired their present characters, probably occurred in conformity with our rule during youth.
But there is here room for doubt, as characters are sometimes transferred to the offspring at an earlier age than that at which they first appeared in the parents, so that the parents may have varied when adult, and have transferred their characters to their offspring whilst young. There are, moreover, many animals, in which the two s.e.xes closely resemble each other, and yet both differ from their young; and here the characters of the adults must have been acquired late in life; nevertheless, these characters in apparent contradiction to our rule, are transferred to both s.e.xes. We must not, however, overlook the possibility or even probability of successive variations of the same nature sometimes occurring, under exposure to similar conditions, simultaneously in both s.e.xes at a rather late period of life; and in this case the variations would be transferred to the offspring of both s.e.xes at a corresponding late age; and there would be no real contradiction to our rule of the variations which occur late in life being transferred exclusively to the s.e.x in which they first appeared.
This latter rule seems to hold true more generally than the second rule, namely, that variations which occur in either s.e.x early in life tend to be transferred to both s.e.xes. As it was obviously impossible even to estimate in how large a number of cases throughout the animal kingdom these two propositions hold good, it occurred to me to investigate some striking or crucial instances, and to rely on the result.
An excellent case for investigation is afforded by the Deer Family. In all the species, excepting one, the horns are developed in the male alone, though certainly transmitted through the female, and capable of occasional abnormal development in her. In the reindeer, on the other hand, the female is provided with horns; so that in this species, the horns ought, according to our rule, to appear early in life, long before the two s.e.xes had arrived at maturity and had come to differ much in const.i.tution. In all the other species of deer the horns ought to appear later in life, leading to their development in that s.e.x alone, in which they first appeared in the progenitor of the whole Family. Now in seven species, belonging to distinct sections of the family and inhabiting different regions, in which the stags alone bear horns, I find that the horns first appear at periods varying from nine months after birth in the roebuck to ten or twelve or even more months in the stags of the six other larger species.[359] But with the reindeer the case is widely different, for as I hear from Prof. Nilsson, who kindly made special enquiries for me in Lapland, the horns appear in the young animals within four or five weeks after birth, and at the same time in both s.e.xes. So that here we have a structure, developed at a most unusually early age in one species of the family, and common to both s.e.xes in this one species.
In several kinds of antelopes the males alone are provided with horns, whilst in the greater number both s.e.xes have horns. With respect to the period of development, Mr. Blyth informs me that there lived at one time in the Zoological Gardens a young koodoo (_Ant. strepsiceros_), in which species the males alone are horned, and the young of a closely-allied species, viz. the eland (_Ant. oreas_), in which both s.e.xes are horned.
Now in strict conformity with our rule, in the young male koodoo, although arrived at the age of ten months, the horns were remarkably small considering the size ultimately attained by them: whilst in the young male eland, although only three months old, the horns were already very much larger than in the koodoo. It is also worth notice that in the p.r.o.ng-horned antelope,[360] in which species the horns, though present in both s.e.xes, are almost rudimentary in the female, they do not appear until about five or six months after birth. With sheep, goats, and cattle, in which the horns are well developed in both s.e.xes, though not quite equal in size, they can be felt, or even seen, at birth or soon afterwards.[361] Our rule, however, fails in regard to some breeds of sheep, for instance merinos, in which the rams alone are horned; for I cannot find on enquiry,[362] that the horns are developed later in life in this breed than in ordinary sheep in which both s.e.xes are horned. But with domesticated sheep the presence or absence of horns is not a firmly fixed character; a certain proportion of the merino ewes bearing small horns, and some of the rams being hornless; whilst with ordinary sheep hornless ewes are occasionally produced.
In most of the species of the splendid family of the Pheasants, the males differ conspicuously from the females, and they acquire their ornaments at a rather late period of life. The eared pheasant (_Crossoptilon auritum_), however, offers a remarkable exception, for both s.e.xes possess the fine caudal plumes, the large ear-tufts and the crimson velvet about the head; and I find on enquiry in the Zoological Gardens that all these characters, in accordance with our rule, appear very early in life. The adult male can, however, be distinguished from the adult female by one character, namely by the presence of spurs; and conformably with our rule, these do not begin to be developed, as I am a.s.sured by Mr. Bartlett, before the age of six months, and even at this age, can hardly be distinguished in the two s.e.xes.[363] The male and female Peac.o.c.k differ conspicuously from each other in almost every part of their plumage, except in the elegant head-crest, which is common to both s.e.xes; and this is developed very early in life, long before the other ornaments which are confined to the male. The wild-duck offers an a.n.a.logous case, for the beautiful green speculum on the wings is common to both s.e.xes, though duller and somewhat smaller in the female, and it is developed early in life, whilst the curled tail-feathers and other ornaments peculiar to the male are developed later.[364] Between such extreme cases of close s.e.xual resemblance and wide dissimilarity, as those of the Crossoptilon and peac.o.c.k, many intermediate ones could be given, in which the characters follow in their order of development our two rules.
As most insects emerge from their pupal state in a mature condition, it is doubtful whether the period of development determines the transference of their characters to one or both s.e.xes. But we do not know that the coloured scales, for instance, in two species of b.u.t.terflies, in one of which the s.e.xes differ in colour, whilst in the other they are alike, are developed at the same relative age in the coc.o.o.n. Nor do we know whether all the scales are simultaneously developed on the wings of the same species of b.u.t.terfly, in which certain coloured marks are confined to one s.e.x, whilst other marks are common to both s.e.xes. A difference of this kind in the period of development is not so improbable as it may at first appear; for with the Orthoptera, which a.s.sume their adult state, not by a single metamorphosis, but by a succession of moults, the young males of some species at first resemble the females, and acquire their distinctive masculine characters only during a later moult. Strictly a.n.a.logous cases occur during the successive moults of certain male crustaceans.
We have as yet only considered the transference of characters, relatively to their period of development, with species in a natural state; we will now turn to domesticated animals; first touching on monstrosities and diseases. The presence of supernumerary digits, and the absence of certain phalanges, must be determined at an early embryonic period-the tendency to profuse bleeding is at least congenital, as is probably colour-blindness-yet these peculiarities, and other similar ones, are often limited in their transmission to one s.e.x; so that the rule that characters which are developed at an early period tend to be transmitted to both s.e.xes, here wholly fails. But this rule, as before remarked, does not appear to be nearly so generally true as the converse proposition, namely, that characters which appear late in life in one s.e.x are transmitted exclusively to the same s.e.x. From the fact of the above abnormal peculiarities becoming attached to one s.e.x, long before the s.e.xual functions are active, we may infer that there must be a difference of some kind between the s.e.xes at an extremely early age. With respect to s.e.xually-limited diseases, we know too little of the period at which they originate, to draw any fair conclusion.
Gout, however, seems to fall under our rule; for it is generally caused by intemperance after early youth, and is transmitted from the father to his sons in a much more marked manner than to his daughters.
In the various domestic breeds of sheep, goats, and cattle, the males differ from their respective females in the shape or development of their horns, forehead, mane, dewlap, tail, and hump on the shoulders; and these peculiarities, in accordance with our rule, are not fully developed until rather late in life. With dogs, the s.e.xes do not differ, except that in certain breeds, especially in the Scotch deer-hound, the male is much larger and heavier than the female; and as we shall see in a future chapter, the male goes on increasing in size to an unusually late period of life, which will account, according to our rule, for his increased size being transmitted to his male offspring alone. On the other hand, the tortoise-sh.e.l.l colour of the hair, which is confined to female cats, is quite distinct at birth, and this case violates our rule. There is a breed of pigeons in which the males alone are streaked with black, and the streaks can be detected even in the nestlings; but they become more conspicuous at each successive moult, so that this case partly opposes and partly supports the rule. With the English Carrier and Pouter pigeon the full development of the wattle and the crop occurs rather late in life, and these characters, conformably with our rule, are transmitted in full perfection to the males alone. The following cases perhaps come within the cla.s.s previously alluded to, in which the two s.e.xes have varied in the same manner at a rather late period of life, and have consequently transferred their new characters to both s.e.xes at a corresponding late period; and if so, such cases are not opposed to our rule. Thus there are sub-breeds of the pigeon, described by Neumeister,[365] both s.e.xes of which change colour after moulting twice or thrice, as does likewise the Almond Tumbler; nevertheless these changes, though occurring rather late in life, are common to both s.e.xes.
One variety of the Canary-bird, namely the London Prize, offers a nearly a.n.a.logous case.
With the breeds of the Fowl the inheritance of various characters by one s.e.x or by both s.e.xes, seems generally determined by the period at which such characters are developed. Thus in all the many breeds in which the adult male differs greatly in colour from the female and from the adult male parent-species, he differs from the young male, so that the newly acquired characters must have appeared at a rather late period of life.
On the other hand with most of the breeds in which the two s.e.xes resemble each other, the young are coloured in nearly the same manner as their parents, and this renders it probable that their colours first appeared early in life. We have instances of this fact in all black and white breeds, in which the young and old of both s.e.xes are alike; nor can it be maintained that there is something peculiar in a black or white plumage, leading to its transference to both s.e.xes; for the males alone of many natural species are either black or white, the females being very differently coloured. With the so-called Cuckoo sub-breeds of the fowl, in which the feathers are transversely pencilled with dark stripes, both s.e.xes and the chickens are coloured in nearly the same manner. The laced plumage of the Sebright bantam is the same in both s.e.xes, and in the chickens the feathers are tipped with black, which makes a near approach to lacing. Spangled Hamburghs, however, offer a partial exception, for the two s.e.xes, though not quite alike, resemble each other more closely than do the s.e.xes of the aboriginal parent-species, yet they acquire their characteristic plumage late in life, for the chickens are distinctly pencilled. Turning to other characters besides colour: the males alone of the wild parent-species and of most domestic breeds possess a fairly well developed comb, but in the young of the Spanish fowl it is largely developed at a very early age, and apparently in consequence of this it is of unusual size in the adult females. In the Game breeds pugnacity is developed at a wonderfully early age, of which curious proofs could be given; and this character is transmitted to both s.e.xes, so that the hens, from their extreme pugnacity, are now generally exhibited in separate pens. With the Polish breeds the bony protuberance of the skull which supports the crest is partially developed even before the chickens are hatched, and the crest itself soon begins to grow, though at first feebly;[366] and in this breed a great bony protuberance and an immense crest characterise the adults of both s.e.xes.
Finally, from what we have now seen of the relation which exists in many natural species and domesticated races, between the period of the development of their characters and the manner of their transmission-for example the striking fact of the early growth of the horns in the reindeer, in which both s.e.xes have horns, in comparison with their much later growth in the other species in which the male alone bears horns-we may conclude that one cause, though not the sole cause, of characters being exclusively inherited by one s.e.x, is their development at a late age. And secondly, that one, though apparently a less efficient, cause of characters being inherited by both s.e.xes is their development at an early age, whilst the s.e.xes differ but little in const.i.tution. It appears, however, that some difference must exist between the s.e.xes even during an early embryonic period, for characters developed at this age not rarely become attached to one s.e.x.
_Summary and concluding remarks._-From the foregoing discussion on the various laws of inheritance, we learn that characters often or even generally tend to become developed in the same s.e.x, at the same age, and periodically at the same season of the year, in which they first appeared in the parents. But these laws, from unknown causes, are very liable to change. Hence the successive steps in the modification of a species might readily be transmitted in different ways; some of the steps being transmitted to one s.e.x, and some to both; some to the offspring at one age, and some at all ages. Not only are the laws of inheritance extremely complex, but so are the causes which induce and govern variability. The variations thus caused are preserved and acc.u.mulated by s.e.xual selection, which is in itself an extremely complex affair, depending, as it does, on ardour in love, courage, and the rivalry of the males, and on the powers of perception, taste, and will of the female. s.e.xual selection will also be dominated by natural selection for the general welfare of the species. Hence the manner in which the individuals of either s.e.x or of both s.e.xes are affected through s.e.xual selection cannot fail to be complex in the highest degree.
When variations occur late in life in one s.e.x, and are transmitted to the same s.e.x at the same age, the other s.e.x and the young are necessarily left unmodified. When they occur late in life, but are transmitted to both s.e.xes at the same age, the young alone are left unmodified. Variations, however, may occur at any period of life in one s.e.x or in both, and be transmitted to both s.e.xes at all ages, and then all the individuals of the species will be similarly modified. In the following chapters it will be seen that all these cases frequently occur under nature.
s.e.xual selection can never act on any animal whilst young, before the age for reproduction has arrived. From the great eagerness of the male it has generally acted on this s.e.x and not on the females. The males have thus become provided with weapons for fighting with their rivals, or with organs for discovering and securely holding the female, or for exciting and charming her. When the s.e.xes differ in these respects, it is also, as we have seen, an extremely general law that the adult male differs more or less from the young male; and we may conclude from this fact that the successive variations, by which the adult male became modified, cannot have occurred much before the age for reproduction. How then are we to account for this general and remarkable coincidence between the period of variability and that of s.e.xual selection,- principles which are quite independent of each other? I think we can see the cause: it is not that the males have never varied at an early age, but that such variations have commonly been lost, whilst those occurring at a later age have been preserved.
All animals produce more offspring than can survive to maturity; and we have every reason to believe that death falls heavily on the weak and inexperienced young. If then a certain proportion of the offspring were to vary at birth or soon afterwards, in some manner which at this age was of no service to them, the chance of the preservation of such variations would be small. We have good evidence under domestication how soon variations of all kinds are lost, if not selected. But variations which occurred at or near maturity, and which were of immediate service to either s.e.x, would probably be preserved; as would similar variations occurring at an earlier period in any individuals which happened to survive. As this principle has an important bearing on s.e.xual selection, it may be advisable to give an imaginary ill.u.s.tration. We will take a pair of animals, neither very fertile nor the reverse, and a.s.sume that after arriving at maturity they live on an average for five years, producing each year five young. They would thus produce 25 offspring; and it would not, I think, be an unfair estimate to a.s.sume that 18 or 20 out of the 25 would perish before maturity, whilst still young and inexperienced; the remaining seven or five sufficing to keep up the stock of mature individuals. If so, we can see that variations which occurred during youth, for instance in brightness, and which were not of the least service to the young, would run a good chance of being utterly lost. Whilst similar variations, which occurring at or near maturity in the comparatively few individuals surviving to this age, and which immediately gave an advantage to certain males, by rendering them more attractive to the females, would be likely to be preserved. No doubt some of the variations in brightness which occurred at an earlier age would by chance be preserved, and eventually give to the male the same advantage as those which appeared later; and this will account for the young males commonly partaking to a certain extent (as may be observed with many birds) of the bright colours of their adult male parents. If only a few of the successive variations in brightness were to occur at a late age, the adult male would be only a little brighter than the young male; and such cases are common.
In this ill.u.s.tration I have a.s.sumed that the young varied in a manner which was of no service to them; but many characters proper to the adult male would be actually injurious to the young,-as bright colours from making them conspicuous, or horns of large size from expending much vital force. Such variations in the young would promptly be eliminated through natural selection. With the adult and experienced males, on the other hand, the advantage thus derived in their rivalry with other males would often more than counterbalance exposure to some degree of danger.
Thus we can understand how it is that variations which must originally have appeared rather late in life have alone or in chief part been preserved for the development of secondary s.e.xual characters; and the remarkable coincidence between the periods of variability and of s.e.xual selection is intelligible.
As variations which give to the male an advantage in lighting with other males, or in finding, securing, or charming the female, would be of no use to the female, they will not have been preserved in this s.e.x either during youth or maturity. Consequently such variations would be extremely liable to be lost; and the female, as far as these characters are concerned, would be left unmodified, excepting in so far as she may have received them by transference from the male. No doubt if the female varied and transferred serviceable characters to her male offspring, these would be favoured through s.e.xual selection; and then both s.e.xes would thus far be modified in the same manner. But I shall hereafter have to recur to these more intricate contingencies.