Animal Life and Intelligence

It is difficult to realize the wealth, the variety, the diversity, of "animal life." Even if we endeavour to pass in review all that we have seen in woodland and meadow, in pond or pool, in the air, on the earth, in the waters, in temperate or tropical regions; even when we try to remember the results of all anatomical and microscopic investigation displaying new wonders and new diversities hidden from ordinary and unaided vision; even when we call to mind the multifarious contents, recent and fossil, of all the natural history museums we have ever visited, and throw in such mental pictures as we have formed of all the diverse adaptations we have read about or heard described;—even so we cannot but be conscious that not one-tenth, not one-hundredth, part of the diversity and variety of animal life has passed before our mental vision even in sample. It is said that our greatest living poet once, when a young man, left his companions to gaze into the waters of a clear, still pool. "What an imagination God has!" he said, as he rejoined his friends. Fit observation for the poet, whose sensitive nature must be keenly alive to the varied endowments which Nature has lavishly showered upon her animate children.

Certain it is that words, mere words, can never present, though they may aid in recalling, an adequate picture of either the wealth or the beauty of animal life. Fortunately for those who visit London (and who nowadays does not?), we have, in our national collection in South Kensington, the means of getting some insight into the wealth of life. And much is being done there to aid the imagination and to facilitate study for those who are not professed students. Many of the birds are now to be seen set in their natural surroundings, with their life-history illustrated. Our frontispiece is taken from one of these cases. And this admirable system will, no doubt, so far as space permits, be extended; and, perhaps, dramatic incidents may be introduced, like those (notably in the life of heron and hawk) which form so marked a feature in the little museum at Exeter. Anything which leads us to understand the life of animals, and to go forth and study it for ourselves, has an educational value.

In our National Museum, again, much is being wisely done to illustrate the diversity and variety of structure and the principles that underlie them. Observe, as you enter the central hall, the case containing stuffed specimens of ruffs (Machetes pugnax). Among the young autumn birds there is not much difference between males and females, the male being distinguished chiefly by its somewhat larger size. Nor do the old birds, male and female, differ much during the winter months. But in pairing-time, May and June, the females are somewhat richer in colour; while the males not only don the ruff to which the bird owes its popular name, but develop striking colour-tints. Among different individuals it will be seen that the colour-variation is tolerably wide; but the same individual keeps strictly, we are told, in successive seasons, to the same summer dress. Note, next, in a bay to the right, the great variety of form, ornamentation, and colouring among the molluscan shells there exhibited. Observe that the rich colours are often hidden during life by the dull epidermis. Half an hour's attentive study of these varied molluscan forms will give a better idea of the beauty and diversity of these life-products than pages of mere description.

Pass on, too, to note, in a further bay to the right, the extraordinary modifications of the antenna, or feeler, in insects. There is the long, whip-like form in the locust; the clubbed whip in the ant-lion and the butterfly; the feathered form in certain moths and flies; the hooked form characteristic of the sphinx-moths; the many-leaf form in the lamellicorn beetles, like the cockchafer; and the feathered plate of other beetles. Equally wonderful are the diverse developments of the mouth-organs of insects, the spiral tube of the butterfly or moth, the strong jaws of the great beetles, the lancets of the gnat, the sucking-disc of the fly,—all of them special modifications of the same set of structures. Then, in the same bay, note some of the striking differences between the males and females of certain insects. In some there is an extraordinary difference in size (e.g. the locust Xiphocera, and the moth Attacus); in others, like the stag-beetle, it is the size of the jaws that distinguishes the males; in others, again, the most notable differences are in the length, development, or complexity of the antennæ, or feelers; in some beetles the males have great horns on the head or thorax; while in many butterflies it is in richness of colour that the difference chiefly lies—the brilliant green of the Ornithoptera there exhibited contrasting strongly with the sober brown of his larger mate.

The fact that the special characteristics of the male, which we have seen to be variable in the ruff, are also variable among insects, is well exemplified in the case of the stag-beetle, in some males of which the mandibles are far larger than in others. This is shown in Fig. 22, which is copied from the series displayed in the British Museum, by the kind permission of Professor Flower.

Crossing the hall to where the vertebrate structures are displayed, the development of hair, of feathers, of teeth, the modifications of the skull and of legs, wings, and fins are being exemplified. Note here and elsewhere the special adaptations of structure, of which we may select two examples. The first is that seen in the Balistes, or trigger-fish. The anterior dorsal fin is reduced to three spines, of which that which lies in front is a specially modified weapon of defence, while that which follows it is the so-called trigger. These two are so hinged to the underlying interspinous bones and so related to each other that, when once the defensive spine in front is erected, it cannot be forced down until the trigger is lowered. The second example of special adaptation is well displayed in specimens of the mud-tortoise Trionyx. Between the last vertebra of the neck and the first fixed vertebra of the dorsal series is a beautiful hinge-joint, enabling the neck to be bent back, S-fashion, when the creature withdraws its head within the carapace. These are only one or two particular instances of what any one who will visit the National Museum may see for himself admirably displayed and illustrated.

No one can, one would suppose, pass through the galleries in Cromwell Road and remain quite insensible to the beauties of animal life. Beauty of form and beauty of colour are conspicuously combined in many species of birds and insects. And much of this colour-beauty and splendid iridescence is known to be due to minute scales, to thin films of air or fluid, and to microscopically fine lines developed upon scales or feathers. But there is one phase of beauty which cannot be exhibited in the museum—the beauty that comes of life as opposed to death. For this we must go out into the free air of nature, where the animals not only have lived, but are still instinct with the glow of life, and where the silence of the museum galleries is replaced by the song of birds and the hum of insect-wings.

How have this wealth, this diversity, this beauty, this manifold activity, which we summarize under the term "animal life," been produced?

If we answer this question in a word—the word "evolution"[CI]—we must remember that this word merely expresses our belief in a general fact; and we must not forget that many questions remain behind, all centering round that little question, to which an adequate answer is so difficult to give, the question—How? Reduced to its simplest expression, the doctrine of evolution merely states that the animal world as it exists to-day is naturally developed out of the animal world as it existed yesterday, and will in turn develop into the animal world as it shall exist to-morrow. This is the central belief of the evolutionist. No matter what moment in the past history of life you select, the life at that moment was in the act of insensibly passing from the previous towards a future condition. Then at once arises the question—Does life remain the same yesterday, to-day, and to-morrow? A thousand indubitable facts at once make answer—No! Underlying the law of continuity there is a law of change. Life to-day is not what it was yesterday, nor will it be to-morrow the same as to-day. What, then, is the nature of this change? If it be replied that the change must be either for the better or the worse, we shall have to answer the further question—Better or worse in what respects?

Let us narrow our view from the contemplation of life as a whole to the more particular consideration of an organism as one of its constituent units. The individual life of that organism depends on (some would say consists in) its ceaseless adaptation to surrounding circumstances. The circumstances remaining the same, or only varying within constant limits, the adaptation may be more or less perfect. A change in the direction of more perfect adaptation will be a change for the better, a tendency to less perfect adaptation will be a change for the worse.

But the relation of an organism to its circumstances or environment is itself subject to change. The environment itself may alter, or the organism may be brought into relation with a new environment. We have to consider not only the changes in an organism in the direction of more or less perfect adaptation to its environment, but also changes in the environment. These changes are in the direction of increased simplicity or of increased complexity. So that we may say that the modification of life is in the direction of more or of less complete adaptation to simpler or to more complex conditions. Where the adaptation advances to more complex conditions, we speak of elaboration; where it retrogrades to less complex conditions, we speak of degeneration; but both fall under the head of evolution in its more general sense. Viewed as a whole, there can be little doubt that the general tendency of evolution is towards more complete adaptation to more diverse and complex environment. And this tendency is accompanied by a general increase of differentiation and of integration; of differentiation whereby the constituent elements of life, whether cells, tissues, organs, organisms, or groups of organisms, become progressively more specialized and more different from one another; of integration whereby these elements become progressively more interdependent one on the other. We may conveniently sum up the tendency towards more perfect adaptation to more complex circumstances in the word progress; the tendency to differentiation in the word individuality; and the tendency to integration in the word association.

Nobody now doubts the propositions thus briefly summarized, and it is therefore unnecessary to bring forward evidence in their favour.

We may pass, then, to the question—How? Evolution being continuity, associated with change, tending in certain directions, and accompanied by certain processes, how has it been effected? What are its methods?

Natural Selection.

Natural selection claims a foremost place. We have already devoted a chapter to its consideration. Animals vary; more are born than can survive to procreate their kind; hence a struggle for existence, in which the weaker and less adapted are eliminated, the stronger and better adapted surviving to continue the race.

It is scarcely possible to over-estimate what Darwin's labour and genius have done for the study of animal life. Through Darwin's informing spirit, biology has become a science. But now we must be on our guard. So long as natural selection was winning its way to acceptance, every application of the theory had to be made with caution, and was subjected to keen, if sometimes ignorant, criticism. Now there is, perhaps, some danger lest it should suffer the Nemesis of triumphant creeds, and be used blindly as a magic formula.

First, we should be careful not to use the phrase, "of advantage to the species," vaguely and indefinitely, but should in all cases endeavour clearly to indicate wherein lies the particular advantage, and how its possession enables the organism to escape elimination; next, we must remember that the advantage must be immediate and present, prospective advantage being, of course, inoperative; then we must endeavour to show that the advantage is really sufficient to decide the question of elimination or non-elimination; lastly, we must distinguish between indiscriminate and differential destruction, between mere numerical reduction by death or otherwise and selective elimination.

(1) In illustration of the first point, we may select a passage from the writings of even so great a biologist as Professor Weismann. As is well known, Professor Weismann believes that senility and death are no part of the natural heritage of animal life, but have been introduced among the metazoa on utilitarian grounds. In his earlier papers, he attributed the introduction of death, and the tissue-degeneration that precedes it, to the direct action of natural selection.[CJ] More lately, he attributes it to the cessation of selection.[CK] Concerning this later view, we shall have somewhat to say presently; we may now consider the former as an example of too indefinite a use of such phrases as "of advantage to the species." "Worn-out individuals," says Professor Weismann, "are not only valueless to the species, but they are even harmful, for they take the places of those which are sound. Hence, by the operation of natural selection, the life of our hypothetically immortal individual would be shortened by the amount which was useless to the species. It would be reduced to a length which would afford the most favourable conditions of existence of as large a number as possible of vigorous individuals at the same time." This may be so, but, as it stands, the modus operandi is not given, and is not obvious. We start with a hypothetically immortal metazoon. Barring accidents, it will go on existing indefinitely. But you cannot bar accidents for an indefinite time; hence, the longer the individual lives, the more defective and crippled it becomes. There is neither natural decay nor natural death here. The organism is gradually crippled through accident and injury. But the crippled individuals are harmful to the species, because they take the places of those which are sound. Therefore, says Professor Weismann, natural decay and death step in to take them off before they have time to become cripples. Now, the point I wish to notice is that there is no definite statement how or why natural decrepitude should thus be introduced. We must remember that it is not until a late stage in evolution that, through the association of its members, groups of organisms compete with other groups. In the earlier stages, when we must suppose decrepitude and death to arise on Professor Weismann's hypothesis, the law of the struggle for existence is—each for himself against all. The question, therefore, is—What advantage to the individual is there in natural decay and death to enable it, through the possession of these attributes, to escape elimination? Surely none as such. At the same time, it is quite conceivable that natural decay and death may be the penalty the individual has to pay for increased strength and vitality in the early stages of life. This, probably, was Professor Weismann's meaning. But, if so, it would surely have been better to state the matter in such a way as to lay the chief stress on the really important feature, and to say that, through natural selection, those individuals have survived which exhibited predominant strength and vitality for a shortened period, even at the expense of natural decay and death. The increased life-power, not the seeds of decay and death, was that which natural selection picked out for survival, or rather that which elimination allowed to survive.

In such ways—a short life with heightened activity being of advantage to some forms, a more prolonged existence at a lower level of vitality being essential to others—natural selection may have determined in some degree the relative longevity of different organisms. That it caused the introduction of senility as a preparation for death is a less tenable hypothesis.

And here we may note, in passing, that in using the phrase, "of advantage to the race or species," we must steadily bear in mind the fact that it is with individuals that the process of elimination deals. In the individual it is that every modification must make good its claim to existence and transmission. Where the principle of association for mutual benefit obtains, as in the case of social insects, it is still the individual that must resist elimination. Self-sacrifice, whether conscious or unconscious, must not be carried so far as to lead to the elimination of the self-sacrificing individual, for in this event it cannot but defeat its own ends. Within these limits, self-sacrifice is of advantage, as in the case of parental self-sacrifice, in that it enables certain other individuals to escape elimination. We should endeavour, then, not to use the phrase, "of advantage to the species," vaguely and indefinitely, but to indicate in what particular ways certain individuals are to be so advantaged as to escape the Nemesis of elimination.

(2) The second point that I mentioned above scarcely needs exemplification. That the advantage which enables an organism to escape elimination must be present and existent, not merely prospective, is obvious. Still, the mistake is sometimes made. I have heard it stated that feathers were evolved for the sake of flight. But clearly, unless the wing sprang into existence already sufficiently developed for flight, this would be impossible. The same is true of the first stages of many structures which could not be of service for the purpose and use to which they were subsequently turned. Not impossibly, the earliest "wings" were for diving, and flight was, so to speak, an after-thought. Undoubtedly, structures which have been fostered under the wing of one form of advantage have been subsequently applied to new purposes, and fostered through new modes of adaptation. Teeth, for example, are probably modified scales, such as are found in the thorn-back skate. But the early development of these scales could have had no reference to their future application to purposes subservient to alimentation.

Again, such and such a structure is sometimes spoken of as a "prevision against emergencies." In his interesting and valuable work on "The Colours of Animals," for example, Mr. E. B. Poulton says, "Dimorphism [in the larvæ of butterflies and moths] is also valuable in another way: the widening range of a species may carry it into countries in which one of its forms may be especially well concealed, while in other countries the other form may be more protected. Thus a dimorphic form is more fully provided against emergencies than one with only a single form." And after giving, as an example, the fact that the convolvulus hawk-moth has a browner and a greener form of caterpillar, of which the browner is more prevalent under European conditions, and the greener under those which obtain in the Canary Islands, Mr. Poulton adds, "This result appears to have been brought about by the ordinary operation of natural selection, leading to the extermination of the less-protected variety." Now, I do not mean for one moment to imply that so careful and able a naturalist as Mr. Poulton believes that any character has been evolved through natural selection in prevision for future emergencies. But I do think that his statement is open to this criticism.

(3) It is sometimes said, in bold metaphor, that natural selection is constantly on the watch to select any modification, however slight, which is of advantage to the species. And it is true that elimination is ceaselessly operative. But it is equally certain that the advantage must be of sufficient value to decide the question whether its possessor should be eliminated or should escape elimination. If it does not reach this value, Natural Selection, watch she never so carefully, can make no use of it. Elimination need not, however, be to the death; exclusion from any share in continuing the species is sufficient. To breed or not to breed, that is the question. Any advantage affecting this essential life-function will at once catch the eye of a vigilant natural selection. But it must be of sufficient magnitude for the machinery of natural selection to deal with. That machinery is the elimination of a certain proportion of the individuals which are born. Which shall be eliminated, and which shall survive, depends entirely on the way in which the individuals themselves come out in life's competitive examination. The manner in which that examination is conducted is often rude and coarse, too rough-and-ready to weigh minute and infinitesimal advantages.

What must be the value of a favourable or advantageous modification to decide the question of elimination, to make it an available advantage, must remain a matter of conjecture. It will vary with the nature and the pressure of the eliminative process. And perhaps it is scarcely too much to say that, at present, we have not observational grounds on which to base a reliable estimate in a single instance. We must not let our conviction of its truth and justice blind us to the fact that natural selection is a logical inference rather than a matter of direct observation. A hundred are born, and two survive; the ninety-eight are eliminated in the struggle for existence; we may therefore infer that the two escaped elimination in virtue of their possession of certain advantageous characters. There is no flaw in the logic that has thus convinced the world that natural selection is a factor in evolution. But by what percentage of elimination-marks the second of the two successful candidates beats the senior on the list of failures we do not know. We can only see that, on the hypothesis of natural selection, it must have been sufficiently appreciable to determine success or failure.

(4) And then, to come to our fourth point, we must remember that, apart from the differentiating process of elimination, there is much fortuitous destruction. A hundred are born, and but two survive. But of the ninety-eight which die, and fail to procreate, how many are eliminated, how many are fortuitously destroyed, we do not find it easy to say. And indiscriminate destruction gets rid of good, bad, and indifferent alike. It is a mistake to say that of the hundred born the two survivors are necessarily the very best of the lot. It is quite possible that indiscriminate destruction got rid of ninety of all sorts, and left only ten subject to the action of a true elimination. "In the majority of birds," says Professor Weismann, "the egg, as soon as it is laid, becomes exposed to the attacks of enemies; martens and weasels, cats and owls, buzzards and crows, are all on the look out for it. At a later period, the same enemies destroy numbers of the helpless young, and in winter many succumb in the struggle against cold and hunger, or to the numerous dangers which attend migration over land and sea—dangers which decimate the young birds." There is here, first, a certain amount of fortuitous destruction; secondly, some selection applied to the eggs; thirdly, a selection among the very young nestlings; and, fourthly, a selection among the young migratory birds. What may be the proportion of elimination to destruction at each stage it is difficult to say. Among the eggs and fry of fishes fortuitous destruction probably very far outbalances the truly differentiating process.

Panmixia and Disuse.

We may now pass on to consider shortly some of the phenomena of degeneration, and the dwindling or disappearance of structures which are no longer of use.

Many zoologists believe, or until lately have believed, that disuse is itself a factor in the process. Just as the well-exercised muscle is strengthened, so is the neglected muscle rendered weak and flabby. Until recently it was generally held that the effects of such use or disuse are inherited. But now Professor Weismann has taught us, if not to doubt ourselves, at least to admit that doubt is permissible. On the older view, the gradual dwindling of unused parts was readily comprehensible. But now, if Professor Weismann is right, we must seek another explanation of the facts; and, in any case, we may be led to recognize other factors (than that of disuse alone) in the process.

Professor Weismann regards panmixia, or free intercrossing, when the preserving influence of natural selection is suspended, as the efficient cause of a reduction or deterioration in the organ concerned. And Mr. Romanes had, in England, drawn attention to the fact that the "cessation of natural selection" would lead to some dwindling of the organ concerned, since it was no longer kept up to standard. In illustration of his panmixia, Professor Weismann says, "A goose or duck must possess strong powers of flight in the natural state, but such powers are no longer necessary for obtaining food when it is brought into the poultry-yard, so that a rigid selection of individuals with well-developed wings at once ceases among its descendants. Hence, in the course of generations, a deterioration of the organs of flight must necessarily ensue, and the other members and organs of the bird will be sensibly affected."[CL] And, again, "As at each stage of retrogressive transformation individual fluctuations always occur, a continued decline from the original degree of development will inevitably, although very slowly, take place, until the last remnant finally disappears."[CM] Now, I think it can be shown that panmixia, or the cessation of selection, alone cannot affect much reduction. It can only affect a reduction from the "survival-mean" to the "birth-mean." This was referred to in the chapter on "Heredity and the Origin of Variations," but may be again indicated. Suppose the number of births among wild ducks be represented by the number nine, of which six are eliminated through imperfections in the organs of flight. Let us place the nine in order of merit in this respect, as is done in the table on p. 172. The average wing-power of the nine will be found in No. 5, there being four ducks with superior wing-power (1-4), and four with inferior wing-power (6-9). The birth-mean will therefore be at the level of No. 5, as indicated to the left of the table. But if six ducks with the poorest wings be eliminated, only three survive. The average wing-power will now be found in No. 2, one duck being superior and one inferior to it in this respect. It is clear that this survival-mean is at a level of higher excellence than the birth-mean. Now, when the ducks are placed in a poultry-yard, selection in the matter of flight ceases, and, since all nine ducks survive, the survival-mean drops to the birth-mean. We may variously estimate this retrogression; but it cannot be a large percentage—I should suppose, in the case under consideration, one or two per cent. at most. But Professor Weismann says, "A continued decline from the original degree of development must inevitably take place." It is not evident why such decline should continue. If variations continue in the same proportion as before, the birth-mean will be preserved, since there are as many positive or favourable variations above the mean as there are negative or unfavourable variations below the mean. A continuous decline must result from a preponderance of negative over positive variations, and for this some other principle, such as atavism, or reversion to ancestral characters, must be called in. But in the case of so long-established and stable an organ as that of flight, fixed and rendered constant through so many generations, it is hardly probable that reversion would be an important factor. Mr. Galton has calculated that among human-folk the offspring inherits one-fourth from each parent, one-sixteenth from each grandparent, leaving one-fourth to be contributed by more remote ancestors. There is no doubt, however, that among domesticated animals reversion occurs to characters which have been lost for many generations. But we should probably have to go a very long way back in the ancestry of wild ducks for any marked diminution in wing-power. It must be remembered that, in the case of the artificial selection of domesticated animals, man has been working against and not with the stream of ancestral tendency. Reversion in their case is towards a standard which was long maintained and had become normal before man's interference. Reversion in domesticated ducks should therefore be towards the greater wing-power of their normal ancestry before domestication, not in the direction of lessened wing-power and diminished wing-structure. The whole question of reversion is full of interest, and needs further investigation.

In the dwindling of disused structures, Mr. Romanes has suggested "failure of heredity" as an efficient cause. I find it difficult, however, to distinguish this failure of heredity from the effects of disuse. To what other cause is the failure of heredity due? If natural selection has intervened to hasten this failure, this can only be because the failure is advantageous, since it permits the growth-force to be applied more advantageously elsewhere. And this involves a different principle. Even so it is difficult to exclude the possibility (to put it no stronger) that the diversion of growth-force from a less useful to a more useful organ is in part due to the use of the one and the disuse of the other. But of disuse Mr. Romanes says, "There is the gravest possible doubt lying against the supposition that any really inherited decrease is due to the inherited effects of disuse." We may fairly ask Mr. Romanes, therefore, to explain to what cause the failure of heredity is due. In any case, Professor Weismann and his school are not likely to accept this failure of heredity as an efficient factor in the process. Nor is Professor Weismann likely to fall back upon any innate tendency to degeneration. Unless, therefore, some cause be shown why the negative variations should be prepotent over the positive variations, we must, I think, allow that unaided panmixia cannot affect any great amount of reduction.

In this connection we may notice Professor Weismann's newer view of the introduction of bodily mortality. He says, "The problem is very easily solved if we seek assistance from the principle of panmixia. As soon as natural selection ceases to operate upon any character, structural or functional, it begins to disappear. As soon, therefore, as the immortality of somatic [body-] cells became useless, they would begin to lose this attribute."[CN] Even granting that panmixia could continuously reduce the size of ducks' wings, it is not easy to see how it could get rid of immortality. The essence of the idea of panmixia is that, when the natural selection which has raised an organ to a high functional level, and sustains it there, ceases or is suspended, the organ drops back from its high level. But on Professor Weismann's hypothesis, immortality has neither been produced nor is it sustained by natural selection. How, therefore, the cessation of selection can cause the disappearance of immortality—a character with which natural selection has had nothing whatever to do—Professor Weismann does not explain. He seems to be using "panmixia" in the same vague way that, in his previous explanation, he used "natural selection."

If panmixia alone cannot, to any very large extent, reduce an organ no longer sustained by natural selection, to what efficient cause are we to look? Mr. Romanes has drawn attention to the reversal of selection as distinguished from its mere cessation. When an organ is being improved or sustained by selection, elimination weeds out all those which have the organ in an ill-developed form. Under a reversal of selection, elimination will weed out all those which possess the organ well developed. In burrowing animals, the eyes may have been reduced in size, or even buried beneath the skin, through a reversal of selection. The tuco-tuco (Ctenomys), a burrowing rodent of South America, is frequently blind. One which Darwin kept alive was in this condition, the immediate cause being inflammation of the nictitating membrane. "As frequent inflammation of the eyes," says Darwin, "must be injurious to any animal, and as eyes are certainly not necessary to animals having subterranean habits, a reduction in their size, with the adhesion of the eyelids and growth of fur over them, might in such cases be an advantage; and, if so, natural selection would aid the effect of disuse."[CO] Granting that the inflammation of the eyes is a sufficient disadvantage to lead to elimination, such cases may be assigned to the effects of a reversal of selection.

Perhaps the best instances of the reversal of selection are to be found in the insects of wind-swept islands, in which, as we have already seen (p. 81), the power of flight has been gradually reduced or even done away with. Such instances are, however, exceptional. And one can hardly suppose that such reversal of selection can be very far-reaching in its effects, at least, through any direct disadvantage from the presence of the organ. One can hardly suppose that the presence of an eye in a cave-dwelling fish[CP] could be of such direct disadvantage as to lead to the elimination of those members which still possess this structure.

But may it not be of indirect disadvantage? May not this structure be absorbing nutriment which would be more advantageously utilized elsewhere? This is Darwin's principle of economy. Granting its occurrence, is it effective? We may put the matter in this way: The crustacea which have been swept into a dark cave may be divided into three classes so far as fortuitous variations of eyes and antennæ are concerned. First, those which preserve eyes and antennæ in the original absolute and relative proportion and value; secondly, those in which, while the eyes remain the same, the antennæ are longer and more sensitive; thirdly, those in which, while the antennæ are longer and more sensitive, the eyes are reduced in size and elaboration. According to the principle of economy, the third class have sufficient advantage over the first and second to enable them to survive and escape the elimination which removes those with fully developed eyes. It may be so. We cannot estimate the available advantage with sufficient accuracy to deny it. But we may fairly suppose that, in general, it is only where the useless organ in question is of relatively large size, and where nutriment is deficient, that economy of growth is an important factor.

We may here note the case of the hermit crab as one which exemplifies degeneration through the reversal of natural selection. This animal, as is well known, adopts an empty whelk-shell or other gasteropod shell as its own. The hinder part of the body which is thus thrust into the shell loses its protective armour, and is quite soft. Professor Weismann seems to regard this loss of the hardened cuticle as due entirely to panmixia. If what has been urged above has weight, this explanation cannot be correct. No amount of promiscuous interbreeding of crabs could reduce the cuticle to a level indefinitely below that of any of the interbreeding individuals. But it is clear that an armour-sheathed "tail" would be exceedingly ill adapted to thrusting into a whelk-shell. Hence there would, by natural selection, be an adaptation to new needs, involving not the higher development of cuticle, but the reverse. So far as the cuticle is concerned, it is a case of reversed selection. Whether this reversal alone will adequately account for the facts is another matter.

Mr. Herbert Spencer has made a number of observations and measurements of the jaws of pet dogs, which lead him to conclude that there has been a reduction in size and muscular power due to disuse. The creatures being fed on sops, have no need to use to any large extent the jaw-muscles. In this case, he argues, the principle of economy is not likely to be operative, since the pampered pet habitually overeats, and has therefore abundant nutriment and to spare to keep up the jaws. It is possible, however, that artificial selection has here been a factor. There may have been a competition among the old ladies who keep such pets to secure the dear little dog that never bites, while the nasty little wretch that does occasionally use his jaws for illegitimate purposes may have been speedily eliminated. Pet dogs are, moreover, a pampered, degenerate, and for the most part unhealthy race, often deteriorated by continued in-breeding, so that we must not build too much on Mr. Spencer's observations, interesting as they undoubtedly are.

There is one feature about the reduction of organs which must not be lost sight of. They are very apt to persist for a long time as remnants or vestiges. The pineal gland is the vestigial remnant of a structure connected with the primitive, median, or pineal eye. The whalebone whales and the duck-bill platypus have teeth which never cut the gum and are of no functional value. With regard to these, it may be asked—If disuse leads to the reduction of unused structures, how comes it that it has not altogether swept away these quite valueless structures? In considering this point, we must notice the unfortunate and misleading way in which disuse is spoken of as if it were a positive determinant, instead of the mere absence of free and full and healthy exercise. Few will question the fact that in the individual, if an organ is to be kept up to its full standard of perfection, it must be healthily and moderately exercised; and that, if not so exercised, it will not only cease to increase in size, but will tend to degenerate. The healthy, functionally valuable tissue passes into the condition of degenerate, comparatively useless tissue. Now, those who hold that the inheritance of functional modifications is still a tenable hypothesis, carry on into the history of the race that which they find to hold good in the history of the individual. They believe that, in the race, the continued functional activity of an organ is necessary for the maintenance of the integrity and perfection of its structure, and that, if not so exercised, the organ will inevitably tend to dwindle to embryonic proportions and to degenerate. The healthy, functionally valuable tissue passes at last into the condition of degenerate, comparatively useless tissue. The force of heredity will long lead to the production in the embryo of the structure which, in the ancestral days of healthy exercise, was to be of service to the organism. At this stage of life the conditions have not changed. The degeneration sets in at that period when the ancestral use is persistently denied. There is no reason why "disuse" should in all cases remove all remnants of a structure; but if the presence of the degenerate tissue is a source of danger to the organism which possesses it, that organism will be eliminated, and those (1) which possess it in an inert, harmless form, or (2) in which it is absent, will survive. Thus natural selection (which will fall under Mr. Romanes's reversed selection) will step in—will in some cases reduce the organ to a harmless and degenerate rudiment, and in others remove the last vestiges of the organ.

On the whole, even taking into consideration the effects of panmixia, of reversed selection, and of the principle of economy, the reduction of organs is difficult to explain, unless we call into play "disuse" as a co-operating factor.

Sexual Selection, or Preferential Mating.

It is well known that, in addition to and apart from the primary sexual differences in animals, there are certain secondary characters by which the males, or occasionally the females, are conspicuous. The antlers of stags, the tail of the peacock, the splendid plumes of the male bird of paradise, the horns or pouches of lizards, the brilliant frilled crest of the newt, the gay colours of male sticklebacks, the metallic hues of male butterflies, and the large horns or antennæ of other insects,—these and many other examples which will at once occur to the reader are illustrations of the fact.

As a contribution towards the explanation of this order of phenomena, Darwin brought forward his hypothesis of sexual selection, of which there are two modes. In the first place, the males struggle together for their mates; in this struggle the weakest are eliminated; those possessed of the most efficient weapons of offence and defence escape elimination. In the second place, the females are represented as exercising individual choice, and selecting (in the true sense of the word) those mates whose bright colours, clear voices, or general strength and vigour render them most pleasing and attractive. For this mode I shall employ the term "preferential mating." Combining these two in his summary, Darwin says, "It has been shown that the largest number of vigorous offspring will be reared from the pairing of the strongest and best-formed males, victorious in contests over other males, with the most vigorous and best-nourished females, which are the first to breed in the spring. If such females select the more attractive and, at the same time, vigorous males, they will rear a larger number of offspring than the retarded 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 sexual selection efficient."[CQ]

With regard to the first of the two modes, little need be said. There can be no question that there are both elimination by battle and elimination by competition in the struggle for mates. It is well known that the emperor moth discovers his mate by his keen sense of smell residing probably in the large, branching antennæ. There can be little doubt that, if an individual is deficient in this sense, or misinterprets the direction in which the virgin female lies, he will be unsuccessful in the competition for mates; he will be eliminated from procreation. And it is a familiar observation of the poultry-yard that the law of battle soon determines which among the cock birds shall procreate their kind. The law of battle for mates is, indeed, an established fact among many animals, especially those which are polygamous, and the elimination of the unfit in this respect is a logical necessity.

It is when we come to the second of the two modes, that which involves selection proper, that we find differences of opinion among naturalists.

Darwin, as we have seen, suggested that those secondary sexual characters which can be of no value in aiding their possessor to escape elimination by combat result from the preferential choice of the female, the female herself remaining comparatively unaffected. But Mr. Wallace made an exceedingly valuable suggestion with regard to these comparatively dull colours of the female. He pointed out that conspicuousness (unless, as we have seen, accompanied by some protective character, such as a sting or a bitter taste) increased the risk of elimination by enemies. Now, the males, since they are generally the stronger, more active, and more pugnacious, could better afford to run this risk than their mates. They could to some extent take care of themselves. Moreover, when impregnation was once effected, the male's business in procreation was over. Not so the female; she had to bear the young or to lay the eggs, often to foster or nourish her offspring. Not only were her risks greater, but they extended over a far longer period of time. Hence, according to Mr. Wallace, the dull tints of the females, as compared with those of the males, are due to natural selection eliminating the conspicuous females in far greater proportion than the gaudy males.

There is clearly no reason why this view should not be combined with Darwin's; preferential mating being one factor, natural elimination being another factor; both being operative at the same time, and each contributing to that marked differentiation of male and female which we find to prevail in certain classes of the animal kingdom.

But Mr. Wallace will not accept this compromise. He rejects preferential mating altogether, or, in any case, denies that through its agency secondary sexual characters have been developed. He admits, of course, the striking and beautiful nature of some of these characters; he admits that the male in courtship takes elaborate pains to display all his finery before his would-be mate; he admits that the "female birds may be charmed or excited by the fine display of plumage by the males;" but he concludes that "there is no proof whatever that slight differences in that display have any effect in determining their choice of a partner."[CR]

How, then, does Mr. Wallace himself suppose that these secondary sexual characters have arisen? His answer is that "ornament is the natural outcome and direct product of superabundant health and vigour," and is "due to the general laws of growth and development."[CS] At which one rubs one's eyes and looks to the title-page to see that Mr. Wallace's name is really there, and not that of Professor Mivart or the Duke of Argyll. For, if the plumage of the argus pheasant and the bird of paradise is due to the general laws of growth and development, why not the whole animal? If Darwin's sexual selection is to be thus superseded, why not Messrs. Darwin and Wallace's natural selection?

Must we not confess that Mr. Wallace, for whose genius I have the profoundest admiration, has here allowed himself to confound together the question of origin and the question of guidance or direction? Natural selection by elimination and sexual selection through preferential mating are, supposing them to be veræ causæ, guiding or selecting agencies. Given the variations, however caused, these agencies will deal with them, eliminating some, selecting others, with the ultimate result that those specially fitted for their place in nature will survive. Neither the one nor the other deals with the origin of variations. That is a wholly different matter, and constitutes the leading biological problem of our day. Mr. Wallace's suggestion is one which concerns the origin of variations, and as such is worthy of careful consideration. It does not touch the question of their guidance into certain channels or the maintenance of specific standards. Concerning this Mr. Wallace is silent or confesses ignorance. "Why, in allied species," he says, "the development of accessory plumes has taken different forms, we are unable to say, except that it may be due to that individual variability which has served as the starting-point for so much of what seems to us strange in form or fantastic in colour, both in the animal and vegetable world."[CT] It is clear, however, that "individual variability" cannot be regarded as a vera causa of the maintenance of a specific standard—a standard maintained in spite of variability.

The only directive agency (apart from that of natural selection) to which Mr. Wallace can point is that suggested by Mr. Alfred Tylor, in an interesting, if somewhat fanciful, posthumous work on "Coloration in Animals and Plants," "namely, that diversified coloration follows the chief lines of structure, and changes at points, such as the joints, where function changes." But even if we admit that coloration-bands or spots originate at such points or along such lines—and the physiological rationale is not altogether obvious—even if we admit that in butterflies the spots and bands usually have reference to the form of the wing and the arrangement of the nervures, and that in highly coloured birds the crown of the head, the throat, the ear-coverts, and the eyes have usually distinct tints, still it can hardly be maintained that this affords us any adequate explanation of the specific colour-tints of the humming-birds, or the pheasants, or the Papilionidæ among butterflies. If, as Mr. Wallace argues, the immense tufts of golden plumage in the bird of paradise owe their origin to the fact that they are attached just above the point where the arteries and nerves for the supply of the pectoral muscles leave the interior of the body, are there no other birds in which similar arteries and nerves are found in a similar position? Why have these no similar tufts? And why, in the birds of paradise themselves, does it require four years (for it takes so long for the feathers of the male to come to maturity) ere these nervous and arterial influences take effect upon the plumage? Finally, one would inquire how the colour is determined and held constant in each species. The difficulty of the Tylor-Wallace view, even as a matter of origin, is especially great in those numerous cases in which the colour is determined by delicate lines, thin plates, or thin films of air or fluid.[CU]

Under natural selection, as we have seen, the development of colour is fostered under certain conditions. The colour is either protective, rendering the organism inconspicuous amid its normal surroundings, or it is of warning value, advertising the organism as inedible or dangerous, or, in the form of recognition-marks, it is of service in enabling the members of a species to recognize each other. Now, in the case of both warning colour and recognition-marks, their efficacy depends upon the perceptual powers of animals. Unless there be a rapidly acquired and close association of the quality we call nastiness with the quality we call gaudiness (though, for the animal, there is no such isolation of these qualities as is implied in our words [CV]), such that the sight of the gaudy insect suggests that it will be unpleasant to eat, the gaudiness will be of no avail. And if there is any truth in the doctrine of mimicry, the association is particular. It is not merely that bright colours are suggestive of a nasty taste. The insect-eating birds associate nastiness especially with certain markings and coloration—"the tawny Danais, the barred Heliconias, the blue-black Euplæas, and the fibrous Acræas;" and this is proved by the fact that sweet insects mimicking these particular forms are thereby protected.

So, too, with recognition-marks. If the bird or the mammal have not sufficient perceptive powers to distinguish between the often not very different recognition-marks, of what service can they be?

Recognition-marks and mimicry seem, therefore, to show that in the former case many animals, and in the latter the insect-eating birds, mammals, lizards, and other animals concerned, have considerable powers of perception and association.

Among other associations are those which are at the base of what I have termed preferential mating. We must remember how deeply ingrained in the animal nature is the mating instinct. We may find it difficult to distinguish closely allied species. But the individuals of that species are led to mate together by an impelling instinct that is so well known as to elicit no surprise. Instinct though it be, however, the mating individuals must recognize each other in some way. The impulse that draws them together must act through perceptual agency. It is not surprising, therefore, to find, when we come to the higher animals, that, built upon this basis, there are well-marked mating preferences. And this, as we have before pointed out, following Wallace, is an efficient factor in segregation. Let us, however, hear Mr. Wallace himself in the matter.

There is, he says,[CW] "a very powerful cause of isolation in the mental nature—the likes and dislikes—of animals; and to this is probably due the fact of the rarity of hybrids in a state of nature. The differently coloured herds of cattle in the Falkland Islands, each of which keeps separate, have been already mentioned. Similar facts occur, however, among our domestic animals, and are well known to breeders. Professor Low, one of the greatest authorities on our domesticated animals, says, 'The female of the dog, when not under restraint, makes selection of her mate, the mastiff selecting the mastiff, the terrier the terrier, and so on.' And again, 'The merino sheep and the heath sheep of Scotland, if two flocks are mixed together, each will breed with its own variety.' Mr. Darwin has collected many facts illustrating this point.[CX] One of the chief pigeon-fanciers in England informed him that, if free to choose, each breed would prefer pairing with its own kind. Among the wild horses in Paraguay those of the same colour and size associate together; while in Circassia there are three races of horses which have received special names, and which, when living a free life, almost always refuse to mingle and cross, and will even attack one another. In one of the Faröe Islands, not more than half a mile in diameter, the half-wild native black sheep do not readily mix with imported white sheep. In the Forest of Dean and in the New Forest the dark and pale coloured herds of fallow deer have never been known to mingle; and even the curious ancon sheep, of quite modern origin, have been observed to keep together, separating themselves from the rest of the flock when put into enclosures with other sheep. The same rule applies to birds, for Darwin was informed by the Rev. W. D. Fox that his flocks of white and Chinese geese kept distinct. This constant preference of animals for their like, even in the case of slightly different varieties of the same species, is evidently a fact of great importance in considering the origin of species by natural selection, since it shows us that, so soon as a slight differentiation of form or colour has been effected, isolation will at once arise by the selective association of the animals themselves."

Mr. Wallace thus allows, nay, he lays no little stress on, preferential mating, and his name is associated with the hypothesis of recognition-marks. But he denies that preferential mating, acting on recognition-marks, has had any effect in furthering a differentiation of form or colour. He admits that so soon as a slight differentiation of form or colour has been effected, segregation will arise by the selective association of the animals themselves; but he does not admit that such selective association can carry the differentiation further.

Now, it is clear that mating preferences must be either fixed or variable. If fixed, how can differentiation occur in the same flock or herd? And how can selective association be a means of isolation? Or, granting that differentiation has occurred, if the mating preferences are then stereotyped, all further differentiation, so far as colour and form are concerned, will be rendered impossible; for divergent modifications, not meeting the stereotyped standard of taste, will for that reason fail to be perpetuated. We must admit, then, that these mating preferences are subject to variation. And now we come to the central question with regard to sexual selection by means of preferential mating. What guides the variation along special lines leading to heightened beauty? This, I take it, is the heart and centre of Mr. Wallace's criticism of Darwin's hypothesis. Sexual selection of preferential mating involves a standard of taste; that standard has advanced from what we consider a lower to what we consider a higher æsthetic level, not along one line, but along many lines. What has guided it along these lines?

Not as in any sense affording a direct answer to this question, but for illustrative purposes, we may here draw attention to what seems to be a somewhat parallel case, namely, the development of flowers through insect agency. In his "Origin of Species," Darwin contended that flowers had been rendered conspicuous and beautiful in order to attract insects, adding, "Hence we may conclude that, if insects had not been developed on the earth, our plants would not have been decked with beautiful flowers, but would have produced only such poor flowers as we see on our fir, oak, nut, and ash trees, on grasses, docks, and nettles, which are all fertilized through the agency of the wind." "The argument in favour of this view," says Mr. Wallace,[CY] who quotes this passage, "is now much stronger than when Mr. Darwin wrote;" and he cites with approval the following passage from Mr. Grant Allen's "Colour-Sense:" "While man has only tilled a few level plains, a few great river-valleys, a few peninsular mountain slopes, leaving the vast mass of earth untouched by his hand, the insect has spread himself over every land in a thousand shapes, and has made the whole flowering creation subservient to his daily wants. His buttercup, his dandelion, and his meadowsweet grow thick in every English field. His thyme clothes the hillside; his heather purples the bleak grey moorland. High up among the Alpine heights his gentian spreads its lakes of blue; amid the snows of the Himalayas his rhododendrons gleam with crimson light. Even the wayside pond yields him the white crowfoot and the arrowhead, while the broad expanses of Brazilian streams are beautified by his gorgeous water-lilies. The insect has thus turned the whole surface of the earth into a boundless flower-garden, which supplies him from year to year with pollen or honey, and itself in turn gains perpetuation by the baits that it offers to his allurement."[CZ]

Mr. Grant Allen is perfectly correct in stating that the insect has produced all this beauty. It is the result of insect choice, a genuine case of selection as contrasted with elimination. And when we ask in this case, as we asked in the case of the beautiful colours and forms of animals, what has guided their evolution along lines which lead to such rare beauty, we are given by Mr. Wallace himself the answer, "The preferential choice of insects." If these insects have been able to produce through preferential selection all this wealth of floral beauty (not, indeed, for the sake of the beauty, but incidentally in the practical business of their life), there would seem to be no a priori reason why the same class and birds and mammals should not have been able to produce, through preferential selection, all the wealth of animal beauty.

It should be noted that the answer to the question is in each case a manifestly incomplete one. For if we say that these forms of beauty, floral and animal, have been selected through animal preferences, there still remains behind the question—How and why have the preferences taken these æsthetic lines? To which I do not see my way to a satisfactory answer, though some suggestions in the matter will be made in a future chapter.[DA] At present all we can say is this—to be conspicuous was advantageous, since it furthered the mating of flowers and animals. To be diversely conspicuous was also advantageous. As Mr. Wallace says, "It is probably to assist the insects in keeping to one flower at a time, which is of vital importance to the perpetuation of the species, that the flowers which bloom intermingled at the same season are usually very distinct, both in form and colour."[DB] But conspicuousness is not beauty. And the question still remains—From what source comes this tendency to beauty?

Leaving this question on one side, we may state the argument in favour of sexual selection in the following form: The generally admitted doctrine of mimicry involves the belief that birds and other insect-eating animals have delicate and particular perceptual powers. The generally received doctrine of the origin of flowers involves the belief that their diverse forms and markings result from the selective choice of insects. There are a number of colour and form peculiarities in animals that cannot be explained by natural selection through elimination. There is some evidence in favour of preferential mating or selective association. It is, therefore, permissible to hold, as a provisional hypothesis, that just as the diverse forms of flowers result from the preferential choice of insects, so do the diverse secondary sexual characters of animals result, in part at least, from the preferential choice of animals through selective mating.

If this be admitted, then the elaborate display of their finery by male birds, which Mr. Wallace does admit, may fairly be held to have a value which he does not admit. For if preferential mating is à priori probable, such display may be regarded as the outcome of this mode of selection. At the same time, it may be freely admitted that more observations are required. In a recent paper, "On Sexual Selection in Spiders of the Family Attidæ,"[DC] by George W. and Elizabeth G. Peckham, a full, not to say elaborate, description is given of the courtship, as they regard it, of spiders. The "love-dances" and the display of special adornments are described in detail. And the observers, as the result, be it remembered, of long and patient investigation and systematic study, come to the conclusion that female spiders exercise selective choice in their mates. And courtship must be a serious matter for spiders, for if they fail to please, they run a very serious risk of being eaten by the object of their attentions. Some years ago I watched, on the Cape Flats, near Capetown, the courtship of a large spider (I do not know the species). In this case the antics were strange, and, to me, amusing; but they seemed to have no effect on the female spider, who merely watched him. Once or twice she darted forward towards him, but he, not liking, perhaps, the gleam in her eyes, retreated hastily. Eventually she seemed to chase him off the field.

We must remember how difficult it is to obtain really satisfactory evidence of mating preferences in animals. In most cases we must watch the animals undisturbed, and very rarely can we have an opportunity of determining whether one particular female selects her mate out of her various suitors. We watch the courtship in this, that, or the other case. In some we see that it is successful; in others that it is unsuccessful. How can we be sure that in the one case it was through fully attaining, in the other through failing to reach, the standard of taste? And yet it is evidence of this sort that Mr. Wallace demands. After noting the rejection by the hen of male birds which had lost their ornamental plumage, he says, "Such cases do not support the idea that males with the tail-feathers a trifle longer, or the colours a trifle brighter, are generally preferred, and that those which are only a little inferior are as generally rejected,—and this is what is absolutely needed to establish the theory of the development of these plumes by means of the choice of the female."[DD] If Mr. Wallace requires direct observational evidence of this kind, I do not suppose he is likely to get any large body of it. But one might fairly ask him what body of direct observational evidence he has of natural selection. The fact is that direct observational evidence is, from the nature of the processes involved, almost impossible to produce in either case. Natural selection is an explanation of organic phenomena reached by a process of logical inference and justified by its results. It is not claimed for the hypothesis of selective mating that it has a higher order of validity.