Darwin, and After Darwin Volume I Part 16

Nevertheless, that it does discharge under suitable stimulation has been proved by Professor Burdon Sanderson by means of a telephone; for he found that every time he stimulated the animal its electrical discharge was rendered audible by the telephone. Here, then, the difficulty arises. For of what conceivable use is such an organ to its possessor?

We can scarcely suppose that any aquatic animal is more sensitive to electric shocks than is the human hand; and even if such were the case, a discharge of so feeble a kind taking place in water would be short-circuited in the immediate vicinity of the skate itself. So there can be no doubt that such weak discharges as the skate is able to deliver must be wholly imperceptible alike to prey and to enemies. Yet for the delivery of such discharges there is provided an organ of such high peculiarity and huge complexity, that, regarded as a piece of living mechanism, it deserves to rank as at once the most extremely specialized and the most highly elaborated structure in the whole animal kingdom. Thousands of separately formed elements are ranged in row after row, all electrically insulated one from another, and packed away into the smallest possible s.p.a.ce, with the obvious end, or purpose, of conspiring together for the simultaneous delivery of an electric shock.

Nevertheless, the shock when delivered is, as we have just seen, too slight to be of any conceivable use to the skate. Therefore it appears impossible to suggest how this astonis.h.i.+ng structure--much more astonis.h.i.+ng, in my opinion, than the human eye or the human hand--can ever have been begun, or afterwards developed, by means of natural selection. For if it be not even yet of any conceivable use to its possessor, clearly thus far survival of the fittest can have had nothing to do with its formation. On the other hand, seeing that electric organs when of larger size, as in the _Gymnotus_ and _Torpedo_, are of obvious use to their possessors, the facts of the case, so far as the skate is concerned, a.s.suredly do appear to sanction the doctrine of "prophetic germs." The organ in the skate seems to be on its way towards becoming such an organ as we meet with in these other animals; and, therefore, unless we can show that it is now, and in all previous stages of its evolution has throughout been, of use to the skate, the facts do present a serious difficulty to the theory of natural selection, while they readily lend themselves to the interpretation of a disposing or fore-ordaining mind, which knows how to construct an electric battery by thus transforming muscular tissue into electric tissue, and is now actually in process of constructing such an apparatus for the prospective benefit of future creatures.

Should it be suggested that possibly the electric organ of the skate may be in process of degeneration, and therefore that it is now the practically functionless remnant of an organ which in the ancestors of the skate was of larger size and functional use--against so obvious a suggestion there lie the whole results of Professor Ewart's investigations, which go to indicate that the organ is here not in a stage of degeneration, but of evolution. For instance, in _Raia radiata_, it does not begin to be formed out of the muscular tissue until some time after the animal has left the egg-capsule, and a.s.sumed all the normal proportions (though not yet the size) of the adult creature. The organ, therefore, is one of the very latest to appear in the ontogeny of _R. radiata_; and, moreover, it does not attain its full _development_ (i. e. not merely _growth_, but transforming of muscular fibres into electrical elements) till the fish attains maturity. Read in the light of embryology, these facts prove, (1) that the electric organ of _R. radiata_ must be one of the very latest products of the animal's phylogeny; and, (2) that as yet, at all events, it has not begun to degenerate. But, if not, it must either be at a stand-still, or it must be in course of further evolution; and, whichever of these alternatives we adopt, the difficulty of accounting for its present condition remains. In this connexion also it is worth while to remark that the electric organ, even after it has attained its full _development_, continues its _growth_ with the growth of the fish, and this in a much higher ratio, either than the tail alone, or the whole animal. Lastly, Prof. Burdon Sanderson finds that _section for section_ the organ in the skate is as efficient as it is in _Torpedo_. It is evident that these facts also point to the skate's organ being in course of phylogenetic evolution.

[Ill.u.s.tration: Fig. 118.--_Raia radiata_, representing the life size of the youngest individual in which muscle fibres have been found developing into electric cells].

[Ill.u.s.tration: Fig. 119.--Electric organ of the skate. The left-hand drawing (i) represents the entire organ (natural size) of a full-grown _r. radiata_. This is a small skate, which rarely exceeds 50 centms. in length; but in the large _r. batis_, the organ may exceed two feet in length. The other drawings represent single muscle-fibres in successive stages of transition. In the first of the series (ii) the motor plate, and the nerves connected with it, have already been considerably enlarged. In the other three specimens, the fibre becomes more and more club-like, and eventually cup-like. These changes of shape are expressive of great changes of structure, as may be seen in the last of the series (v), where the shallow cup is seen in partial section. The electric plate lines the concavity of the cup, and is richly supplied with nerves (only a few of which are represented in the last drawing); the thick walls of the cup are composed of muscular fibres, the striation of which is distinctly visible.]

[Ill.u.s.tration: Fig. 120.--Electric cells of _raia radiata_. The drawing on the left represents one of the clubs magnified, as in the preceding wood-cut. The drawing on the right represents a number of these clubs, less highly magnified, _in situ_.]

Again, it cannot be answered that the principle of correlation may be drawn upon in mitigation of the difficulty. The structure of the electric organ is far too elaborate, far too specialized, and far too obviously directed to a particular end, to admit of our conceivably supposing it due to any accidental correlation with structural changes going on elsewhere. Even as regards the initial changes of muscle-elements into electrical-elements, I do not think the principle of correlation can be reasonably adduced by way of explanation; for, as shown in the ill.u.s.trations, even this initial change is most extraordinarily peculiar, elaborate, and specialized. But, be this as it may, I am perfectly certain that the principle of correlation cannot possibly be adduced to explain the subsequent _a.s.sociation of these electrical elements into an electric battery_, actuated by a special nervous mechanism of enormous size and elaboration--unless of course, the progress of such a structure were a.s.sumed to have been throughout of some utility. Under this supposition, however, the principle of correlation would be forsaken in favour of that of natural selection; and we should again be in the presence of the same difficulty as that with which we started.

But now, and further, if we do thus abandon correlation in favour of natural selection, and therefore if for the sake of saving an hypothesis we a.s.sume that the organ as it now stands _must_ be of some use to the existing skate, we should still have to face the question--Of what conceivable use can those initial stages of its formation have been, when first the muscle-elements began to be changed into the very different electrical-elements, and when therefore they became useless as muscles while not yet capable of performing even so much of the electrical function as they now perform?

Lastly, we must remember that not only have we here the most highly specialized, the most complex, and altogether the most elaboratively adaptive organ in the animal kingdom; but also that in the formation of this structure there has been needed an altogether unparalleled expenditure of the most physiologically expensive of all materials--namely, nervous tissue. Whether estimated by volume or by weight, the quant.i.ty of nervous tissue which is consumed in the electric organ of the skate is in excess of all the rest of the nervous system put together. It is needless to say that nowhere else in the animal kingdom--except, of course, in other electric fishes--is there any approach to so enormous a development of nervous tissue for the discharge of a special function. Therefore, as nervous tissue is, physiologically speaking, the most valuable of all materials, we are forced to conclude that natural selection ought strongly to have _opposed_ the evolution of such organs, unless from the first moment of their inception, and throughout the whole course of their development, they were of some such paramount importance as biologically to justify so unexampled an expenditure. Yet this paramount importance does not admit of being so much as surmised, even where the organ has already attained the size and degree of elaboration which it presents in the skate.

In view of all these considerations taken together, I freely confess that the difficulty presented by this case appears to me of a magnitude and importance altogether unequalled by that of any other single case--or any series of cases--which has. .h.i.therto been encountered by the theory of natural selection. So that, if there were many other cases of the like kind to be met with in nature, I should myself at once allow that the theory of natural selection would have to be discarded. But inasmuch as this particular case stands so far entirely by itself, and therefore out of a.n.a.logy with thousands, or even millions, of other cases throughout the whole range of organic nature, I am constrained to feel it more probable that the electric organ of the skate will some day admit of being marshalled under the general law of natural selection--in just the same way as proved to be the case with the conspicuous colouring of those caterpillars, which, as explained in the last chapter, at one time seemed to const.i.tute a serious difficulty to the theory, and yet, through a better knowledge of all the relations involved, has now come to const.i.tute one of the strongest witnesses in its favour.

I have now stated all the objections of any importance which have hitherto been brought against the theory of natural selection, excepting three, which I left to be dealt with together because they form a logically connected group. With a brief consideration of these, therefore, I will bring this chapter to a close.

The three objections to which I allude are, (1) that a large proportional number of specific, as well as of higher taxonomic characters, are seemingly useless characters, and therefore do not lend themselves to explanation by the Darwinian theory; (2) that the most general of all specific characters--viz. cross-infertility between allied species--cannot possibly be due to natural selection, as is demonstrated by Darwin himself; (3) that the swamping effects of free intercrossing must always render impossible by natural selection alone any evolution of species in divergent (as distinguished from serial) lines of change.

These three objections have been urged from time to time by not a few of the most eminent botanists and zoologists of our century; and from one point of view I cannot myself have the smallest doubt that the objections thus advanced are not only valid in themselves, but also by far the most formidable objections which the theory of natural selection has encountered. From another point of view, however, I am equally convinced that they all admit of absolute annihilation. This strong ant.i.thesis arises, as I have said, from differences of standpoint, or from differences in the view which we take of the theory of natural selection itself. If we understand this theory to set forth natural selection as the sole cause of organic evolution, then all the above objections to the theory are not merely, as already stated, valid and formidable, but as I will now add, logically insurmountable. On the other hand, if we take theory to consist merely in setting forth natural selection as a factor of organic evolution, even although we believe it to have been the chief factor or princ.i.p.al cause, all the three objections in question necessarily vanish. For in this case, even if it be satisfactorily proved that the theory of natural selection is unable to explain the three cla.s.ses of facts above mentioned, the theory is not thereby affected: facts of each and all of these cla.s.ses may be consistently left by the theory to be explained by causes other than natural selection--whether these be so far capable or incapable of hypothetical formulation. Thus it is evident that whether the three objections above named are to be regarded as logically insurmountable by the theory, or as logically non-existent in respect to it, depends simply upon the manner in which the theory itself is stated.

In the next volume a great deal more will have to be said upon these matters--especially with regard to the causes other than natural selection which in my opinion are capable of explaining these so-called "difficulties." In the present connexion, however, all I have attempted to show is, that, whatever may be thought touching the supplementary theories whereby I shall endeavour to explain the facts of inutility, cross-sterility, and non-occurrence of free intercrossing, no one of these facts is ent.i.tled to rank as an objection against the theory of natural selection, unless we understand this theory to claim an exclusive prerogative in the field of organic evolution. This, as we have previously seen, is what Mr. Wallace does claim for it; while on the other hand, Mr. Darwin expressly--and even vehemently--repudiates the claim: from which it follows that all the three main objections against the theory of natural selection are objections which vitally affect the theory only as it has been stated and upheld by Wallace. As the theory has been stated and upheld by Darwin, all these objections are irrelevant. This is a fact which I had not myself perceived at the time when I mentioned these objections in a paper ent.i.tled _Physiological Selection_, which was published in 1886. The discussions to which that paper gave rise, however, led me to consider these matters more closely; and further study of Darwin's writings, with these matters specially in view, has led me to see that none of the objections in question are relevant to his theory, as distinguished from that of Mr.

Wallace. This, I acknowledge, I ought to have perceived before I published the paper just alluded to; but in those days I had had no occasion to follow out the differences between Darwin and Wallace to all their consequences, and therefore adopted the prevalent view that their theories of evolution were virtually identical. Now, however, I have endeavoured to make it clear that the points wherein they differ involve the important consequences above set forth. All these the most formidable objections against the theory of natural selection arise simply and solely from what I conceive to be the erroneous manner in which the theory has been presented by Darwin's distinguished colleague.

I have now considered, as impartially as I can, all the main criticisms and objections which have been brought against the theory of natural selection; and the result is to show that, neither singly nor collectively, are they ent.i.tled to much weight. On the other hand, as we have seen in the preceding chapter, there is a vast acc.u.mulation of evidence in favour of the theory. Hence, it is no wonder that the theory has now been accepted by all naturalists, with scarcely any one notable exception, as at any rate the best working hypothesis which has ever been propounded whereby to explain the facts of organic evolution.

Moreover, in the opinion of those most competent to judge, the theory is ent.i.tled to be regarded as something very much more than a working hypothesis: it is held to be virtually a completed induction, or, in other words, the proved exhibition of a general law, whereby the causation of organic evolution admits of being in large part--if not altogether--explained.

Now, whether or not we subscribe to this latter conclusion ought, I think, to depend upon what we mean by an explanation in the case which is before us. If we mean only that, given the large cla.s.s of known facts and unknown causes which are conveniently summarized under the terms Heredity and Variability, then the further facts of Struggle and Survival serve, in some considerable degree or another, to account for the phenomena of adaptive evolution, I cannot see any room to question that the evidence is sufficient to prove the statement. But it is clear that by taking for granted these great facts of Heredity and Variability, we have a.s.sumed the larger part of the problem as a whole.

Or, more correctly, by thus generalizing, in a merely verbal form, all the unknown causes which are concerned in these two great factors of the process in question, we are not so much as attempting to explain the precedent causation which serves as a condition to the process. Much more than half the battle would already have been won, had Darwin's predecessors been able to explain the causes of Heredity and Variation; hence it is but a very partial victory which we have hitherto gained in our recent discovery of the effects of Struggle and Survival.

Yet partial though it be in relation to the whole battle, in itself, or considered absolutely, there can be no reasonable doubt that it const.i.tutes the greatest single victory which has ever been gained by the science of Biology. For this very reason, however, it behoves us to consider all the more carefully the extent to which it goes. But my discussion of this matter must be relegated to the next volume, where I hope to give abundant proof of the soundness of Darwin's judgment as conveyed in the words:--"I am convinced that natural selection has been the main, but not the exclusive, means of modification."

CHAPTER X.

THE THEORY OF s.e.xUAL SELECTION, AND CONCLUDING REMARKS.

Although the explanatory value of the Darwinian theory of natural selection is, as we have now seen, incalculably great, it nevertheless does not meet those phenomena of organic nature which perhaps more than any other attract the general attention, as well as the general admiration, of mankind: I mean all that cla.s.s of phenomena which go to const.i.tute the Beautiful. Whatever value beauty as such may have, it clearly has not a life-preserving value. The gorgeous plumage of a peac.o.c.k, for instance, is of no advantage to the peac.o.c.k in his struggle for life, and therefore cannot be attributed to the agency of natural selection. Now this fact of beauty in organic structures is a fact of wide generality--almost as wide, indeed, as is the fact of their utility. Mr. Darwin, therefore, suggested another hypothesis whereby to render a scientific explanation of this fact. Just as by his theory of natural selection he sought to explain the major fact of utility, so did he endeavour to explain the minor fact of beauty by a theory of what he termed s.e.xual Selection.

It is a matter of observation that the higher animals do not pair indiscriminately; but that the members of either s.e.x prefer those individuals of the opposite s.e.x which are to them most attractive. It is important to understand _in limine_ that n.o.body has ever attempted to challenge this statement. In other words, it is an unquestionable fact that among many of the higher animals there literally and habitually occurs a _s.e.xual selection_; and this fact is not a matter of inference, but, as I have said, a matter of observation. The inference only begins where, from this observable fact, it is argued,--1st, that the s.e.xual selection has reference to an aesthetic taste on the part of the animals themselves; and 2nd, that, supposing the selection to be determined by such a taste, the cause thus given is adequate to explain the phenomena of beauty which are presented by these animals. I will consider these two points separately.

From the evidence which Darwin has collected, it appears to me impossible to doubt that an aesthetic sense is displayed by many birds, and not a few mammals. This of course does not necessarily imply that the _standards_ of such a sense are the same as our own; nor does it necessarily imply that there is any constant relation between such a sense and high levels of intelligence in other respects. In point of fact, such is certainly not the case, because the best evidence that we have of an aesthetic sense in animals is derived from birds, and not from mammals. The most cogent cases to quote in this connexion are those of the numerous species of birds which habitually adorn their nests with gaily coloured feathers, wool, cotton, or any other gaudy materials which they may find lying about the woods and fields. In many cases a marked preference is shown for particular objects--as, for instance, in the case of the Syrian nut-hatch, which chooses the iridescent wings of insects, or that of the great crested fly-catcher, which similarly chooses the cast-off skins of snakes. But no doubt the most remarkable of these cases is that of the baya-bird of Asia, which after having completed its bottle-shaped and chambered nest[47], studs it over with small lumps of clay, both inside and out, upon which the c.o.c.k-bird sticks fire-flies, apparently for the sole purpose of securing a brilliantly decorative effect. Other birds, such as the hammer-head of Africa, adorn the surroundings of their nests (which are built upon the ground) with sh.e.l.ls, bones, pieces of broken gla.s.s and earthenware, or any objects of a bright and conspicuous character which they may happen to find. The most consummate artists in this respect are, however, the bower-birds; for the species of this family construct elaborate play-houses in the form of arched tunnels, built of twigs upon the ground. Through and around such a tunnel they chase one another; and it is always observable that not only is the floor paved with a great collection of sh.e.l.ls, bones, coloured stones, and any other brilliant objects which they are able to carry in their beaks, but also that the walls are decorated with the most gaudy articles which the birds can find. There is one genus, in Papua, which even goes so far as to provide the theatre with a surrounding garden. A level piece of ground is selected as a site for the building. The latter is about two feet high, and constructed round the growing stalk of a shrub, which therefore serves as a central pillar to which the frame-work of the roof is attached. Twigs are woven into this frame-work until the whole is rendered rain-proof. The tent thus erected is about nine feet in circ.u.mference at its base, and presents a large arch as an entrance. The central pillar is banked up with moss at its base, and a gallery is built round the interior of the edifice. This gallery is decorated with flowers, fruits, fungi, &c. These are also spread over the garden, which covers about the same area as the play-house. The flowers are said to be removed when they fade, while fresh ones are gathered to supply their places. Thus the garden is always kept bright with flowers, as well as with the brilliant green of mosses, which are collected and distributed in patches, resembling tiny lawns.

[47] The chambers are three in number. The two upper ones are occupied respectively by the male and the sitting female. The lower one serves as a general living room when the young are hatched.

[Ill.u.s.tration: FIG. 121.--The Garden Bower-bird (_Amblyornis inornata_). Reduced from _Gould's Birds of New Guinea_ to 1/4 nat.

size.]

Now these sundry cases alone seem to prove a high degree of the aesthetic sense as occurring among birds; for, it is needless to say, none of the facts just mentioned can be due to natural selection, seeing that they have no reference to utility, or the preservation of life. But if an aesthetic sense occurs in birds, we should expect, on _a priori_ grounds, that it would probably be exercised with reference to the personal appearance of the s.e.xes. And this expectation is fully realized. For it is an observable fact that in most species of birds where the males are remarkable for the brilliancy of their plumage, not only is this brilliancy most remarkable during the pairing season, but at this season also the male birds take elaborate pains to display their charms before the females. Then it is that the peac.o.c.k erects his tail to strut round and round the hens, taking care always to present to them a front view, where the coloration is most gorgeous. And the same is true of all other gaily coloured male birds. During the pairing season they actively compete with one another in exhibiting their attractiveness to the females; and in many cases there are added all sorts of extraordinary antics in the way of dancings and crowings. Again, in the case of all song-birds, the object of the singing is to please the females; and for this purpose the males rival one another to the best of their musical ability.

Thus there can be no question that the courts.h.i.+p of birds is a highly elaborate business, in which the males do their best to surpa.s.s one another in charming the females. Obviously the inference is that the males do not take all this trouble for nothing; but that the females give their consent to pair with the males whose personal appearance, or whose voice, proves to be the most attractive. But, if so, the young of the male bird who is thus _selected_ will inherit his superior beauty; and thus, in successive generations, a continuous advance will be made in the beauty of plumage or of song, as the case may be,--both the origin and development of beauty in the animal world being thus supposed due to the aesthetic taste of animals themselves.

Such is the theory of s.e.xual selection in its main outlines; and with regard to it we must begin by noting two things which are of most importance. In the first place, it is a theory wholly and completely distinct from the theory of natural selection; so that any truth or error in the one does not in the least affect the other. The second point is, that there is not so great a wealth of evidence in favour of s.e.xual selection as there is in favour of natural selection; and, therefore, that while all naturalists nowadays accept natural selection as _a_ (whether or not _the_) cause of adaptive, useful, or life-preserving structures, there is no such universal--but only a very general--agreement with reference to s.e.xual selection as a cause of decorative, beautiful, or life-embellis.h.i.+ng structures. Nevertheless, the evidence in favour of s.e.xual selection is both large in amount and ma.s.sive in weight.

Our consideration of this evidence will bring us to the second division of our subject, as previously marked out for discussion--namely, granting that an aesthetic sense occurs in certain large divisions of the animal kingdom, what is the proof that such a sense is a cause of the beauty which is presented by the animals in question?

Before proceeding to state this proof, however, it is desirable to observe that under the theory of s.e.xual selection Darwin has included two essentially different cla.s.ses of facts. For besides the large cla.s.s of facts to which I have thus far been alluding,--i. e. the cases where two s.e.xes of the same species differ from one another in respect of ornamentation,--there is another cla.s.s of facts equally important, namely, the cases where the two s.e.xes of the same species differ from one another in respect of size, strength, and the possession of natural weapons, such as spurs, horns, &c. In most of these cases it is the males which are thus superiorly endowed; and it is a matter of observation that in all cases where they are so endowed they use their superior strength and natural weapons for fighting together, in order to secure possession of the females. Hence results what Mr. Darwin has called the Law of Battle between males of the same species; and this law of battle he includes under his theory of s.e.xual selection. But it is evident that the principle which is operative in the law of battle differs from the principle which is concerned in the form of s.e.xual selection that has to do with embellishment, and consequent charm. The law of battle, in fact, more nearly approaches the law of natural selection; seeing that it expresses the natural advantages of brute force in the struggling of rival animals, and so frequently results in _death of the less fitted_, as distinguished from a mere failure to propagate. Now against this doctrine of the law of battle, and the consequences to which it leads in the superior fighting powers of male animals, no objection has been raised in any quarter. It is only with regard to the other aspect of the theory of s.e.xual selection--or that which is concerned with the superior embellishment of male animals--that any difference of opinion obtains. I will now proceed to give the main arguments on both sides of this question, beginning with a _resume_ of the evidences in favour of s.e.xual selection.

In the first place, the fact that secondary s.e.xual characters of the embellis.h.i.+ng kind are so generally restricted to the male s.e.x in itself seems to const.i.tute very cogent proof that, in some way or another, such characters are connected with the part which is played by the male in the act of propagation. Moreover, secondary s.e.xual characters of this kind are of quite as general occurrence as are those of the other kind which have to do with rivalry in battle; and the former are usually of the more elaborate description. Therefore, as there is no doubt that secondary s.e.xual characters of the one order have an immediate purpose to serve in the act of propagation, we are by this close a.n.a.logy confirmed in our surmise that secondary s.e.xual characters of the other, and still more elaborate, order are likewise so concerned. Moreover, this view of their meaning becomes still further strengthened when we take into consideration the following facts. Namely, (_a_) secondary s.e.xual characters of the embellis.h.i.+ng kind are, as a rule, developed only at maturity; and most frequently during only a part of the year, which is invariably the breeding season: (_b_) they are always more or less seriously affected by emasculation: (_c_) they are always, and only, displayed in perfection during the act of courts.h.i.+p: (_d_) then, however, they are displayed with the most elaborate pains; yet always, and only, before the females: (_e_) they appear, at all events in many cases, to have the effect of charming the females into a performance of the s.e.xual act; while it is certain that in many cases, both among quadrupeds and birds, individuals of the one s.e.x are capable of feeling a strong antipathy against, or a strong preference for, certain individuals of the opposite s.e.x.

Such are the main lines of evidence in favour of the theory of s.e.xual selection. And although it is enough that some of them should be merely stated as above in order that their immense significance should become apparent, in the case of others a bare statement is not sufficient for this purpose. More especially is this the case as regards the enormous profusion, variety, and elaboration of s.e.xually-embellis.h.i.+ng characters which occur in birds and mammals--not to mention several divisions of Arthropoda; together with the extraordinary amount of trouble which, in a no less extraordinary number of different ways, is taken by the male animals to display their embellishments before the females. And even in many cases where to our eyes there is no particular embellishment to display, the process of courts.h.i.+p consists in such an elaborate performance of dancings, struttings, and att.i.tudinizings that it is scarcely possible to doubt their object is to incite the opposite s.e.x.

Here, for instance, is a series of drawings ill.u.s.trating the courts.h.i.+p of spiders. I choose this case as an example, partly because it is the one which has been published most recently, and partly because it is of particular interest as occurring so low down in the zoological scale. I am indebted to the kindness of Mr. and Mrs. Peckham for permission to reproduce these few selected drawings from their very admirable work, which is published by the Natural History Society of Wisconsin, U.S. It is evident at a glance that all these elaborate, and to our eyes ludicrous, performances are more suggestive of incitation than of any other imaginable purpose. And this view of the matter is strongly corroborated by the fact that it is the most brightly coloured parts of the male spiders which are most obtruded upon the notice of the female by these peculiar att.i.tudes--in just the same way as is invariably the case in the a.n.a.logous phenomena of courts.h.i.+p among birds, insects, &c.

[Ill.u.s.tration: FIG. 122.--Courts.h.i.+p of Spiders. A few examples of some of the att.i.tudes adopted by different species of males when approaching their females. (After Peckham.)]

[Ill.u.s.tration: FIG. 123.--Courts.h.i.+p of Spiders. Continued from Fig.

122, similarly showing some of the att.i.tudes of approach adopted by males of yet other different species. (After Peckham.)]

But so great is the ma.s.s of material which Darwin has collected in proof of all the points mentioned in the foregoing paragraph, that to attempt anything in the way of an epitome would really be to damage its evidential force. Therefore I deem it best simply to refer to it as it stands in his _Descent of Man_, concluding, as he concludes,--"This surprising uniformity in the laws regulating the differences between the s.e.xes in so many and such widely separated cla.s.ses is intelligible if we admit the action throughout all the higher divisions of the animal kingdom of one common cause, namely, s.e.xual selection"; while, as he might well have added, it is difficult to imagine that all the large cla.s.ses of facts which an admission of this common cause serves to explain, can ever admit of being rendered intelligible by any other theory.

We may next proceed to consider the objections which have been brought against the theory of s.e.xual selection. And this is virtually the same thing as saying that we may now consider Mr. Wallace's views upon the subject.

Reserving for subsequent consideration the most general of these objections--namely, that at best the theory can only apply to the more intelligent animals, and so must necessarily fail to explain the phenomena of beauty in the less intelligent, or in the non-intelligent, as well as in all species of plants--we may take _seriatim_ the other objections which, in the opinion of Mr. Wallace, are sufficient to dispose of the theory even as regards the higher animals.

In the first place, he argues that the princ.i.p.al cause of the greater brilliancy of male animals in general, and of male birds in particular, is that they do not so much stand in need of protection arising from concealment as is the case with their respective females. Consequently natural selection is not so active in repressing brilliancy of colour in the males, or, which amounts to the same thing, is more active in "repressing in the female those bright colours which are normally produced in both s.e.xes by general laws."

Next, he argues that not only does natural selection thus exercise a negative influence in pa.s.sively permitting more heightened colour to appear in the males, but even exercises a positive influence in actively promoting its development in the males, while, at the same time, actively repressing its appearance in the females. For heightened colour, he says, is correlated with health and vigour; and as there can be no doubt that healthy and vigorous birds best provide for their young, natural selection, by always placing its premium on health and vigour in the males, thus also incidentally promotes, through correlated growth, their superior coloration.

Again, with regard to the display which is practised by male birds, and which const.i.tutes the strongest of all Mr. Darwin's arguments in favour of s.e.xual selection, Mr. Wallace points out that there is no evidence of the females being in any way affected thereby. On the other hand, he argues that this display may be due merely to general excitement; and he lays stress upon the more special fact that moveable feathers are habitually erected under the influence of anger and rivalry, in order to make the bird look more formidable in the eyes of antagonists.

Furthermore, he adduces the consideration that, even if the females are in any way affected by colour and its display on the part of the males, and if, therefore, s.e.xual selection be conceded a true principle in theory, still we must remember that, as a matter of fact, it can only operate in so far as it is allowed to operate by natural selection. Now, according to Mr. Wallace, natural selection must wholly neutralize any such supposed influence of s.e.xual selection. For, unless the survivors in the general struggle for existence happen to be those which are also the most highly ornamented, natural selection must neutralize and destroy any influence that may be exerted by female selection. But obviously the chances against the otherwise best fitted males happening to be likewise the most highly ornamented must be many to one, unless, as Wallace supposes, there is some correlation between embellishment and general perfection, in which case, as he points out, the theory of s.e.xual selection lapses altogether, and becomes but a special case of natural selection.

Once more, Mr. Wallace argues that the evidence collected by Mr. Darwin himself proves that each bird finds a mate under any circ.u.mstances--a general fact which in itself must quite neutralize any effect of s.e.xual selection of colour or ornament, since the less highly coloured birds would be at no disadvantage as regards the leaving of healthy progeny.

Lastly, he urges the high improbability that through thousands of generations all the females of any particular species--possibly spread over an enormous area--should uniformly and always have displayed exactly the same taste with respect to every detail of colour to be presented by the males.

Now, without any question, we have here a most powerful array of objections against the theory of s.e.xual selection. Each of them is ably developed by Mr. Wallace himself in his work on _Tropical Nature_; and although I have here s.p.a.ce only to state them in the most abbreviated of possible forms, I think it will be apparent how formidable these objections appear. Unfortunately the work in which they are mainly presented was published several years after the second edition of the _Descent of Man_, so that Mr. Darwin never had a suitable opportunity of replying. But, if he had had such an opportunity, as far as I can judge it seems that his reply would have been more or less as follows.