« 이전계속 »
not only to other men but to the humblest living creature, with his god-like intellect which has penetrated into the movements and constitution of the solar system," must we acknowledge that man "with all these exalted powers" is descended from an Ascidian? Is this a scientific truth resting on scientific evidence, or is it to be classed with the speculations of a bygone age?
With regard to the Origin of Man, Mr. Darwin considers that both "natural selection" and "sexual selection" have acted. We need not on the present occasion discuss the action of natural selection ; but it will be necessary to consider that of " sexual selection" at some length. It plays a very important part in the " descent of man," according to Mr. Darwin's views. He maintains that we owe to it our power of song and our hairlessness of body, and that also to it is due the formation and conservation of the various races and varieties of the human species. In this matter then we fear we shall have to make some demand upon our readers' patience. "Sexual selection" is the cornerstone of Mr. Darwin's theory. It occupies threefourths of his two volumes ; and unless he has clearly established this point, the whole fabric falls to the ground. It is impossible, therefore, to review the book without entering fully into the subject, even at the risk of touching upon some points which, for obvious reasons, we should have preferred to pass over in silence.
Under the head of "sexual selection" Mr. Darwin includes two very distinct processes. One of these consists in the action of superior strength or activity, by which one male succeeds in obtaining possession of mates and in keeping away rivals. This is undoubtedly a vera causa; but may be more conveniently reckoned as one kind of" natural selection" than as a branch of " sexual selection." The second process consists in alleged preference or choice, exercised freely by the female in favor of particular males on account of some attractiveness or beauty of form, color, odor, or voice, which such males may possess. It is this second kind of "sexual selection" (and which alone deserves the name) that is important for the establishment of Mr. Darwin's views, but its valid action has to be proved.
Now, to prove the existence of such a
power of choice Mr. Darwin brings forward a multitude of details respecting the sexual phenomena of animals of various classes; but it is the class of birds which is mainly relied on to afford evidence in support of the exercise of this power of choice by female animals. We contend, however, that not only is the evidence defective even here, but that much of his pwn evidence is in direct opposition to his views. While the unquestionable fact, that male sexual characters (horns, mane, wattles, &c, &c.) have been developed in many cases where sexual selection has certainly not acted, renders it probable, d, priori, that the unknown cause which has operated in these numerous cases has operated in those instances also which seem to favor the hyppthesis supported by Mr. Darwin. Still he contends that the greater part of the beauty and melody of the organic world is due exclusively to this selective process, by which, through countless generations, the tail of the peacock, the throat of the humming-bird, the song of the nightingale, and the chirp of the grasshopper have been developed by females, age after age, selecting for their mates males possessing in a more perfect degree characters which must thus have been continually and constantly preferred.
Yet, after all, Mr. Darwin concedes in principle the very point in dispute, and yields all for which his opponents need argue, when he allows that beautiful and harmonious variations may occur spontaneously and at once, as in the dark or spangled bars on the feathers of Hamburgh fowls. (Descent of Man, vol. i. p. 281.) For what difference is there, other than mere difference of degree, between the spontaneous appearance of a few beautiful new feathers with harmonious markings and the spontaneous appearance of a whole beautiful clothing like that of the Tragopans?
Again, on Mr. Darwin's own showing, it is manifest that male sexual characters, such as he would fain attribute to sexual selection, may arise without any such action whatever. Thus he tells us, "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 "with tortoise-shell cats, the females alone, as a general rule, are thus colored, the males being rusty-red." (vol. i. p. 283.) Now if these cats were only known in a wild' state, Mr. Darwin would certainly bring them forward amongst his other instances of alleged sexual selection, though we now know the phenomenon is not due to any such cause. A more striking instance, however, is the following :—"With the pigeon, the sexes of the parent species do not differ in any external character; nevertheless, in certain domesticated breeds the male is differently colored from the female. 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 "this has arisen, not from, but rather in opposition to, the wishes of the breeder;" which amounts to a positive demonstration that sexual characters may arise spontaneously, and, be it noted, in the class of birds.
The uncertainty which besets these speculations of Mr. Darwin is evident at every turn. What at first could be thought a better instance of sexual selection than the light of the glowworm, exhibited to attract her mate? Yet the discovery of luminous larvae, which of course have no sexual action, leads Mr. Darwin to observe : "It is very doubtful whether the primary use of the light is to guide the male to the female." (vol. i. p. 345.) Again, as to certain British field-bugs, he says: "If in any species the males had differed from the females in an analogous manner, we might have been justified in attributing such conspicuous colors to sexual selection with transference to both sexes." (vol. i. p. 350.) As to the stridulating noises of insects (which is assumed to be the result of sexual selection), Mr. Darwin remarks of certain Neuroptera :—" It is rather surprising that both sexes should have the power of stridulating, as the male is winged and the female wingless" (vol. i. p. 366); and he is again surprised to find that this power is not a sexual character in many Coleoptera.—vol. i. p. 382.
Moths and butterflies, however, are the insects which Mr. Darwin treats of at the greatest length in support of sexual selection. Yet even here he supplies us with positive evidence that in certain cases beauty does not charm the female. He tells us :—
"Some facts, however, are opposed to the belief that female butterflies prefer the more beautiful males; thus, as I have been assured by several observers, fresh females may frequently be seen paired with bat
tered, faded, or dingy males."—vol. i. p. 400.
As to the Bombycidas he adds :—
"The females lie in an almost torpid state, and appear not to evince the least choice in regard to their partners. This is the case with the common silk-moth (£. mori). Dr. Wallace, who has had such immense experience in breeding Bombyx cynthia, is convinced that the females evince no choice or preference. He has kept above 300 of these moths living together, and has often found the most vigorous females mated with stunted males."
Nevertheless, we do not find, for all this,' any defect of color or markings, for, as Mr. Alfred Wallace observes (jVature, March 15th, 1871, p. 182), "the Bombyces are amongst the most elegantly colored of all moths."
Mr. Darwin gives a number of instances of sexual characters, such as horns, spines, &c, in beetles and other insects; but there is no fragment of evidence that such structures are in any way due to feminine caprice. Other structures are described and figured which doubtless do aid the sexual act, as the claws of certain Crustacea; but these are often of such size and strength (e. g., in Callianassa and Orchestin) as to render any power of choice on the part of the female in the highest degree incredible.
Similarly with the higher classes, ;'. e., Fishes, Reptiles, and Beasts, we have descriptions and representations of a number of sexual peculiarities, but no evidence whatever that such characters are due to female selection. Often we have statements which conflict strongly with a belief in any such action. Thus, e. g., Mr. Darwin quotes Mr. R. Buist, Superintendent of Fisheries, as saying that male salmon
"Are constantly fighting and tearing each other on the spawning-beds, and many so injure each other as to cause the death of numbers, many being seen swimming near the banks of the river in a state of exhaustion, and apparently in a dying state." . . . "The keeper of Stormontfield found in the northern Tyne about 300 dead salmon, all of which with one exception were males; and he was convinced that they had lost their lives by fighting."—vol. ii. p. 3.
The female's choice must here be much limited, and the only kind of sexual selection which can operate is that first kind, determined by combat, which, we before observed, must rather be ranked as a kind of " natural selection." Even with regard to this, however, we may well hesitate, when Mr. Darwin tells us, as he does, that seeing the habitual contests of the males, "it is surprising that they have not generally become, through the effects of sexual selection, larger and stronger than the females ;" and this the more as "the males suffer from their small size," being "liable to be devoured by the females of their own species." (vol. ii. p. 7.) The cases cited by our author with regard to fishes do not even tend to prove the existence of sexual selection, and the same may be said as to the numerous details given by him about Reptiles and Amphibians. Nay, rather the facts are hostile to his views. Thus, he says himself, "It is surprising that frogs and toads should not have acquired more strongly-marked sexual differences; for though coldblooded, their passions are strong." (vol. ii. p. 26.) But he cites a fact, than which it would be difficult to find one less favorable to his cause. He adds: "Dr. Glinther informs me that he has several times found an unfortunate female toad dead and smothered from having been so closely embraced by three or four males." If female selection was difficult in the case of the female salmon, it must be admitted to have been singularly infelicitous, to the female toad.
We will now notice some facts brought forward by Mr. Darwin with regard to beasts. And first, as to the existence of choice on the part of the females, it may be noted that " Mr. Blenkiron, the greatest breeder of race-horses in the world, says that stallions are so frequently capricious in their choice, rejecting one mare and without any apparent cause taking to another, that various artifices have to be habitually used." "He has never known a mare to reject a horse;" though this has occurred in Mr. Wright's stable.
Some of the most marked sexual characters found amongst mammals are those which exist in apes. These are abundantly noticed by Mr. Darwin, but his treatment of them seems to show his inability to bring them within the scope of his theory.
It is well known that certain apes are distinguished by the lively colors or peculiarities as to hair possessed by the males, while it is also notorious that their vastly superior strength of body and length of fang would render resistance on the
part of the female difficult and perilous, even were we to adopt the utterly gratuitous supposition, that at seasons of sexual excitement the female shows any disposition to coyness. Mr. Darwin has no facts to bring forward to prove the exercise of any choice on the part ot female apes, but gives in support of his views the following remarkable passage :—
"Must we attribute to mere purposeless variability in the male all these appendages of hair and skin? It cannot be denied that this is possible; for, with many domesticated quadrupeds, certain characters, apparently not derived through reversion from any wild parent-form, have appeared in, and are confined to, the males, or are more largely developed in them than in the females,—for instance, the hump in the male zebu-cattle of India, the tail in fat-tailed rams, the arched outline of the forehead in the males ot several breeds of sheep, the mane in the ram of an African breed, and, lastly, the mane, long hairs on the hinder legs, and the dewlap in the male alone of the Berbura goat."—vol. ii. p. 284.
If these are due, as is probable, to simple variability, then, he adds,—
"It would appear reasonable to extend the same view to the many analogous characters occurring in animals under a state of nature. Nevertheless I cannot persuade myself that this view is applicable in many cases, as in that of the extraordinary development of hair on the throat and fore-legs of the male Ammotragus, or of the immense beard of the Pithecia (monkey)."—vol. ii. p. 285.
But one naturally asks, Why not? Mr. Darwin gives no reason (if such it may be called) beyond that implied in the gratuitous use of the epithet " purposeless" in the passage cited, and to which we shall return.
In the Rhesus monkey the female appears to be more vividly colored than the male; therefore Mr. Darwin infers (grounding his inference on alleged phenomena in birds) that sexual selection is reversed, and that in this case the male selects. This hypothetical reversion of a hypothetical process to meet an exceptional case will appear to many rash indeed, when they reflect that as to teeth, whiskers, general size, and superciliary ridges this monkey "follows the common rule of the male excelling the female."—vol. ii. p. 294.
To turn now to the class on which Mr. Darwin especially relies, we shall find that even birds supply us with numerous instances which conflict with his hypothesis. Thus, speaking of the battling of male waders, our author tells us :—" Two were seen to be thus engaged for half an hour until 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." (vol. ii. p. 41.) As these battles must take place generally in the absence of spectators, their doubtless frequently fatal termination must limit greatly the power of selection Mr. Darwin attributes to the females. The same limit is certainly imposed in the majority of gallinaceous birds, the cocks of which fight violently; and there can be little doubt but that, as an almost invariable rule, the victorious birds mate with the comparatively passive hens.
Again, how can we explain, on Mr. Darwin's hypothesis, the existence of distinguishing male sexual marks, where it is the male and not the female bird which selects? Yet the wild turkey-cock, a distinguished bird enough, is said by Mr. Darwin (vol. ii. p. 207) to be courted by the females ; and he quotes (vol. ii. p. 120) Sir R. Heron as saying, " that with peafowl, the first advances are always made by the female." And of the capercailzie he says, "the females flit round the male while he is parading, and solicit his attention."
But though, of course, the sexual instinct always seeks its gratification, does the female ever select a particular plumage? The strongest instance given by Mr. Darwin is as follows :—
"Sir R. Heron during many years kept an account of the habits of the peafowl, which he bred in large numbers. He states that the hens have frequently great preference for a particular peacock. They were all so fond of an old pied cock, that one vear, when he was confined though still in view, they were constantly assembled close to the trellice-walls of his prison, and would not suffer a japanned peacock to touch them. On his being let out in the autumn, the oldest of the hens instantly courted him, and was successful in her courtship. The next year he was shut up in a stable, and then the hens all courted his rival. This rival was a japanned or black-winged peacock, which to our eyes is a more beautiful bird than the common kind."—vol. ii. p. 119.
Now no one disputes as to birds showing
preferences one for another, but it is quite a gratuitous suggestion that the pied plumage of the venerable paterfamilias was the charm which attracted the opposite sex; and even if such were the case, it would seem (from Mr. Darwin's concluding remark) to prove either that the peahen's taste is so different from ours, that the peacock's plumage could never have been developed by it, or (if the taste of these peahens was different from that of most peahens) that such is the instability of a vicious feminine caprice, that no constancy of coloration could be produced by its selective action.
Mr. Darwin bases his theory of sexual selection greatly on the fact that the male birds display the beauty of their plumage with elaborate parade and many curious and uncouth gestures. But this display is not exclusively used in attracting and stimulating the hens. Thus he admits that "the males will sometimes display their ornaments when not in the presence of the females, as occasionally occurs with the grouse at their balz-places, and as may be noticed with the peacock; this latter bird, however, evidently wishes for a spectator of some kind, and will show off his finery, as I have often seen, before poultry or even pigs." (vol. ii. p. 86.) Again, as to the brilliant Rupieola crocea, Sir R. Schomburgk says: "A male was capering to the apparent delight of several others."—vol. ii. p. 87.
From the fact of "display" Mr. Darwin concludes that "it is obviously probable that the females appreciate the beauty of their suitors." (vol. ii. p. in.) Our author, however, only ventures to call it "probable," and he significantly adds: "It is, however, difficult to obtain direct evidence of their capacity to appreciate beauty." And again he says of the hen bird: "It is not probable that she consciously deliberates; but she is most excited or attracted by the most beautiful, or melodious, or gallant males." (vol. ii. p. 123.) No doubt the plumage, song, &c, all play their parts in aiding the various processes of life; but to stimulate the sexual instinct, even supposing this to be the object, is one thing—to supply the occasion for the exercise of a power of choice is quite another. Certainly we cannot admit what Mr. Darwin affirms (vol. ii. p. 124), that an "even occasional preference by the female of the more attractive males would almost certainly lead to their modification." •
A singular instance is given by Mr. Darwin (vol. ii. p. m) in support of his view, on the authority of Mr. J. Weir. It is that of a bullfinch which constantly attacked a reed-bunting, newly put into the aviary; and this attack is attributed to a sort of jealousy on the part of the blackheaded bullfinch of the black head of the bunting. But the bullfinch could hardly be aware of the color of the top of its own head!
Mr. Wallace accounts for the brilliant colors of caterpillars and many birds in another way. The caterpillars which are distasteful must have gained if "some outward sign indicated to their would-be destroyer that its prey was a disgusting morsel." As to birds, he believes that brilliance of plumage is developed where not hurtful, and that the generally more sober plumage of the hens has been produced by natural selection, killing off the more brilliant ones exposed during incubation to trying conditions.
Now as Mr. Wallace disposes of Mr. Darwin's views by his objections, so Mr. Darwin's remarks tend to refute Mr. Wallace's positions, and the result seems to point to the existence of some unknown innate and internal law which determines at the same time both coloration and its transmission to either or to both sexes. At the same time these authors, indeed, show the harmony of natural laws and processes one with another, and their mutual interaction and aid.
It cannot be pretended that there is any evidence for sexual selection except in the class of birds. Certain of the phenomena which Mr. Darwin generally attributes to Vich selection must be due, in some other classes, to other causes, and there is no proof that sexual selection acts, even amongst birds.
But in other classes, as we have seen, sexual characters are as marked as ihey are in the feathered group. Mr. Darwin, indeed, argues that birds select, and assumes that their sexual characters have been produced by such sexual selection, and that, therefore, the sexual characters of beasts have been similarly evolved. But we may turn the argument round and say that sexual characters not less strongly marked exist in many beasts, reptiles, and insects, which characters cannot be due
to sexual selection; that it is, therefore, probable the sexual characters of birds are not due to sexual selection either, but that some unknown internal cause has equally operated in each case. The matter, indeed, stands thus: Of animals possessing sexual characters there are some in which sexual selection cannot have acted; others in which it may possibly have acted; others again in which, according to Mr. Darwin, it has certainly acted. It is a somewhat singular conclusion to deduce from this that sexual selection is the one universal cause of sexual characters, when similar effects to those which it is supposed to cause take place in its absence.
But, indeed, what are the data on which Mr. Darwin relies as regards birds? As before said, they are "display" by the males, the "greater brilliancy and ornamentation of these," and the "occasional preference" by females in confinement for particular males. Is there here any sufficient foundation for such a superstructure? In the first place, in insects, e. g., butterflies, we have often many brilliant males crowding in pursuit of a single female. Yet, as Mr. Wallace justly observes, "Surely the male who finally obtains the female will be either the most vigorous, or the strongest-winged, or the most patient—the one who tires out or beats off the rest." Similarly in birds, strength and perseverance will, no doubt, generally reward the suitor possessing those qualities. Doubtless, also, this will • generally be the most beautiful or most melodious; but this will simply be because extra beauty of plumage, or of song, will accompany supereminent vigor of constitution and fulness of vitality. What has been before said as to the fierce combats of cockbirds must be borne in mind.
But that internal spontaneous powers are sufficient to produce all the most varied or bizarre sexual characters which any birds exhibit, is actually demonstrated by the class of insects, especially caterpillars, which from their sexless undeveloped state can have nothing to do with the kind of selection Mr. Darwin advocates. Yet amongst caterpillars we not only find some ornamented with spots, bands, stripes, and curious patterns, "perfectly definite in character and of the most brilliantly contrasted hues. We have also many ornamental appendages; beautiful