Chapter X: Preface (10)

The chief index or measure of the health of any locality is what is called “the death-rate” of that locality. Although there are several other important evidences as to the healthiness or unhealthiness of any given area, the “death-rate” is the chief and most obvious indication of the advantageous or disadvantageous action of the conditions of any given city or other chosen area upon human life. Its records are more easily kept with an approach to accuracy than are records of cases of sickness not terminating in death. The cause of death has to be certified in civilised communities by a medical man; the total number of deaths in a year is given by the number of burial certificates. The death-rate is stated at so many per thousand of the population per annum. Thus, in a city of 5 million inhabitants,—that is to say, 5 thousand thousands—a record of eighty thousand deaths in the year gives 16 deaths for every thousand persons living. That is called “an annual death-rate of 16.” The record for any single month may be stated (as it is stated at intervals in the newspapers) “as at the rate of so many in the thousand per annum,” by multiplying the actual monthly number per thousand by 12. Thus, in the case of the city just cited, if the death-rate were the same in every month of the year—namely, 16—it would mean that 6500 persons died regularly every month. But we should probably find that in some month or other as few as 5417 persons died. That would be reported “as at the rate of” 13 per thousand per annum; since, if every month gave only 5417 deaths, we should get 65,000 deaths a year, which works out at 13 in the thousand in a population of 5 millions. In other months it might run as high as 19 or 20 (representing over 8000 deaths a month), although, taking all the months together, the deaths are at the rate of 16 in the thousand for the year.

The bald statement of the death-rate, of course, admits of much analysis where proper records are kept. Thus the death-rate from different diseases and groups of diseases can be stated, and the death-rate in each group at different ages and for the two sexes can be given where proper records are kept. In this country the records of population in various areas and for the whole country, and of the deaths from various causes, and at different ages, are collected and tabulated by the Registrar-General and his officials. The annual reports issued by him show what an amazing progress has been made in increasing the security of life in our great cities within the last fifty years. Thus, in London, the death-rate was, fifty years ago, 24 in the thousand. In 1906 it was only 15.1 in the thousand—it has gradually fallen, year by year, so that now it is less than two-thirds of what it was half a century ago. In Manchester and Liverpool it was about 26 twenty years ago, and has fallen to 19 in Manchester and to a little over 20 in Liverpool. In the same period the improvement has been (omitting fractions) from 19 to 14 in Bristol; from 20 to 16 in Birmingham; from 20 to 14 in Leicester. This great diminution in the death-rate has been coincident with the expenditure of public funds on the improvement of the water supply and the sewage arrangements of those cities, as well as with the enforcement of regulations to prevent overcrowding, and with the demolition of the most insanitary houses. Rules as to the removal of filth from the neighbourhood of dwelling-houses have been obeyed, and sick persons suffering from infectious diseases have been removed from dwelling-houses and conveyed to special hospitals. There is no doubt that the diminished death-rate is due to the action thus taken, and more will be done in the future to the same end. The proper provision of pure milk (at a reasonable price) for the food of the youngest children, of regular meals for older children, and the protection of adults from the too frequent inducement to indulge in the use of distilled spirits, will be taken in hand by the municipalities, and lead to a further diminution in the death-rate.

We may, indeed, soon have to ask whether, in a population which has become so much less subject to diminution by death than was formerly the case, there is not too great an increase by birth—too great, that is to say, for the existing means of employment and food-production. A most serious, indeed, an alarming fact, has recently come to light in the study of this question, namely, that the increase of the population is due (as pointed out on p. 279) to the proportionately larger number of births amongst the poorer, and even destitute, sections of the community who have not the means of training and rearing their children satisfactorily, and are themselves likely to transmit incapacity of one kind and another to their offspring; whilst those who have valuable hereditary qualities and are prosperous have—it is clearly established—relatively few children—and, in fact, do not increase the population. Whether this condition of things constitutes a real danger, how it will ultimately work out if left alone, and how the difficulty is to be met, are problems for statesmen which cannot be solved off-hand, but require knowledge not only of the crude facts of statistics, but also of the causes at work. Scientific knowledge—that is to say, thorough and unassailable knowledge—of the laws of heredity, of psychology, and of the natural history of human populations, are among the essential qualifications for those who have to face and deal with this difficult matter. And who is there who has this knowledge or is even trying to obtain it? Not the State in this country or its officials: for in every department of government (however capable some of the subordinates may be) there is a determined opposition to and fear of Science on the part of the political and highly paid chiefs—the jealous fear due to complete and deadly ignorance.

XXXI

GOSSAMER

Fine as gossamer! Town-bred folks never see it, and do not believe in its existence; they think it is a poetical figment, like “honey-dew.” That, too, is nevertheless a real thing—a honey-like juice poured out by the little plant-lice or aphides. Gossamer is a very real and a most beautiful thing. You may see it on the hill-sides in fine October weather, when the sun is bright but low enough to illuminate the delicate threads and reveal the “veil of silk and silver thin” spread over Nature’s loveliness. The innumerable threads glisten, and are so fine that they shine with iridescent colours, as do the equally delicate soap-bubbles fabricated by men and boys, and from the same cause. When the eye gets accustomed to them and traces them—rippling and glimmering over acres and acres of grass-land—one feels disconcerted, almost awestruck, by the revelation of this vast network of threads. Sometimes the gentle currents of air break them loose from the herbage, and they float at a higher level and envelop the puzzled intruder in an almost invisible entanglement of fairy lines. Sometimes they become felted together in flakes and float or rest as incredibly delicate tissue, woven by unseen mysterious agency.

When the slopes of the new golf course at Wimbledon were covered last autumn with gossamer, my friends were asking what was its origin, some boldly asserting that it was impossible that such a vast acreage of threads could be produced, as others maintained, by tiny unseen spiders! Yet that is the true history of gossamer. Hundreds of thousands of minute spiders, young, and of a small kind, are present in grass fields in autumn, and throw out these marvellously fine threads from their little bodies (Fig. 47). Those who at first sight doubt this origin of gossamer are only in accordance with their forefathers. The French peasants call it _fil de la Vierge_; old English writers held it to be “dew evaporated.” A great discoverer and leader of science in his time, Robert Hook, who was elected with Nehemiah Grew as secretary of the Royal Society in 1677, and published a wonderful illustrated book called _Micrographia_ (see p. 173), wrote of gossamer. He was so far from recognising its true nature that he says: “It is not unlikely that those great white clouds which appear all the summer time may be of the same substance.” Yet it is now a simple and certain fact of observation that the countless threads in question are the work of minute spiders!

The pretty name “gossamer” has puzzled the etymologists and led to some far-fetched suggestions. That favoured by the authority of the great Oxford dictionary of the English language is that it is a corruption of “Go-summer,” because gossamer appears in autumn and is associated with St. Martin’s summer. This is like saying that the word “cray-fish” refers to fish that live in a “cray” or brook, instead of deriving it from the French word _écrevisse_. The Germans call gossamer _Sommerweben_. But the Latin word for cotton is _gossypium_; and there is an Italian word, _gossampino_, which occurs in an English form, _gossampine_, in the sixteenth century, and means a kind of silk or cotton obtained from the fluffy hairs of a plant called bombax. We also find “gossamer” spelt “gossamire” in English of that date; and it seems to me most likely that an Italian word _gossamira_, signifying “fairy-cotton” or “magic goose-down,” is the origin of our word.

There are 500 different kinds of spider carefully described as occurring in the British Islands, and about 2000 others from remoter regions. Precisely which of them forms the “gossamer” of our meadows it is difficult to say, as all have the habit of secreting a viscid fluid from one or two pairs of projecting spinning knobs or stalks, which are seen at the hinder end of the body (Figs. 48, 49, and 50). The viscid fluid is poured out by a great number of minute tubes, and hardens at once into a thread, which is wonderfully fine, yet strong. Different kinds of spiders make use of these threads for different purposes, hence their name “spinners.” Some make burrows in the ground and line them with a felt of these threads, others enclose their eggs in a case formed by winding them round the eggs, others form “snares” of the most marvellous mechanical ingenuity with them, by which insects are entangled and are then paralysed by the poisonous stab of the spider’s claws, and have their juices sucked out of them at the spider’s leisure. The snares of spiders are in some species merely irregular webs fastened and suspended by threads, in other cases they are gracefully-modelled funnels or cups, whilst a third kind, the disc-like webs made up of radiating and circularly-disposed threads fixed in a geometrical pattern, excel—in the mechanical precision of their workmanship and the masterly treatment of engineering difficulties—the constructions of any other kind of animal. It is amongst this kind of spiders that the formation by the spinning knobs of threads or lines and their use in various ways is most general and frequent. The smaller spiders expel the viscid thread, drawing it out from their bodies by their own movement away from the object to which it at first adhered. When it breaks loose from that support it is carried upwards by air-currents and drawn out from the spinners body to many yards’ length (Fig. 47). It then becomes a “flying-line,” and the spider may sail away on it or run up it and disappear. The celebrated story of the Indian juggler’s performance—traditional and even solemnly attested by witnesses, but failing to pass the test of photography—must have been suggested by this common, yet wonderful, proceeding of small spiders. The juggler, standing in an open place, surrounded by a ring of spectators, uncoils a rope, 50 feet long, from his waist, and holding one end, throws the other up into the air. The rope, without any support, remains stretched and upright. A small boy now enters the ring and climbs up the rope, draws it up after him, and disappears with it in the upper air! That is an illusion, but it is precisely what thousands of small spiders are continually doing. A big spider—the well-grown female of the common garden spider, for instance, cannot do this—her thread is not strong enough, and her weight is too great. But the male of the same species, who is much smaller, fortunately for him, can safely run on a hanging line—and thus can rapidly escape from the side of his mistress, who, after receiving his caresses, has an unpleasant habit of seizing, killing, and sucking the blood of the adventurous male, should he linger longer in her company, and fail in the agility and rapidity of his exit.

The threads of the garden spider (the _Porte-croix_ of the French, white-cross spider, _Epeira diadema_, Fig. 51) are fixed by astronomers in their telescopes for the purpose of giving fine lines in the field of view, by which the relative positions of stars may be accurately measured. For a century astronomers desired to make use of such lines of the greatest possible fineness, and procured at first silver wire drawn out to the extreme limit of tenuity attainable with that metal. They also tried hairs (1/500th of an inch thick) and threads of a silk-worm’s cocoon, which are split into two component threads each only 1/2000th of an inch thick. But in 1820 an English instrument maker named Troughton introduced the spider’s line. This can be readily obtained three or four times smaller in breadth than the silk-worm’s thread, and has also advantages in its strength and freedom from twist. In order to obtain the thread, the spider is carefully fixed on a miniature “rack,” and the thread, which at the moment of issue from the body is a viscid liquid, is made to adhere to a winder, by turning which the desired length of firm but elastic thread can be procured. It has been proposed to use spiders’ silk in manufactures as a substitute for silk-worms’ silk, and pioneers have woven gloves, stockings, and other articles from it. It appears that there are species of spider in other parts of the world whose thread is coarser and more suitable for this purpose than that of any of our British spiders. But it is estimated that the expense in feeding the spiders—which require insect food—would make the thread obtained from them far too costly to compete with silk-worm silk.

A number of different kinds of the lower animals besides spiders have the power of producing threads. The caterpillars of some moths are especially noted for this, since their thread is familiar to us all as “silk.” It is secreted as a viscid fluid by a pair of tubes opening at the mouth, and hardens on escape. Even some marine creatures—the mussels—produce threads, in this case from a gland or sac in the muscular foot, by means of which they fix themselves to rocks. A very big mussel—the _Pinna_—called _Capo lungo_ by the Mediterranean fishermen and _Capy longy_ at Plymouth, where they are also found, produces a sufficient quantity of fine horny threads to be used in weaving, and gloves have been made at Genoa from the shell-fish silk.

The threads produced by the hardening of the tenacious fluid exuded by these various animals were probably simply protective in origin. The curious caterpillar-like creature _Peripatus_ spits out a viscid fluid when it is disturbed, which hardens into threads, and hopelessly entangles any small enemy which may venture to attack it. Threads of a poisonous nature are thrown out by jelly-fishes, polyps, and sea anemones, and serve them both as defence and as means of paralysing and capturing prey. A later stage in the use of such threads is their “felting” to form a case or tube (as in the sea anemone called _Cerianthus_), and so their application has gradually developed to the formation of egg-cases, snares, and the wonderful web of the geometric spider, and the countless “flying-lines” of smaller spiders, which make up the mysterious thing we call “gossamer.”

As to the limits of the tenuity of the threads of gossamer there are no direct observations. Probably they are often as fine as the 1/16,000th or 1/20,000th of an inch in diameter. The condensation of a very minute quantity of moisture on gossamer threads and spiders’ webs no doubt helps to make them more readily visible to us in October weather than they are in full summer, when such moisture would not condense except in early morning or at sunset. It seems strange that man should have been unable to produce a thread so fine as that of the spider, but this reproach has now been removed. Spun glass is easily obtained 1/1000th of a inch in diameter; but Mr. C. V. Boys, F.R.S., has, by fusing quartz (rock-crystal) by the oxy-hydrogen flame, and drawing it out by means of a small arrow (a straw), discharged from a bow—the near end of the arrow being adherent to a fused droplet of quartz which is held fast—produced threads of great strength and of extraordinary tenuity. The fineness can be regulated by the rapidity with which the drawing is effected. The threads are prepared (for use in suspending swinging bars in delicate measurements of force) of a thickness of 1/10,000th of an inch. Some have been made so fine as to be not only invisible to the naked eye, but to be only vaguely indicated by the highest powers of the microscope. They are estimated to be only one-millionth of an inch in diameter. It is difficult to form any mental picture or conception of these finest quartz threads spun by Mr. Boys. But the following fact helps us to realise how delicate they are. A grain of sand just visible to the eye—that is to say, 1/100th an inch long, the same in breadth, and the same in height—would make twenty miles of such thread.

XXXII

THE JUMPING BEAN

One way of thinking of the six hundred thousand kinds or species of insects—those tiny, ubiquitous fellow-creatures of ours which inhabit nearly every corner and cranny of the earth’s surface—is to associate them with the plants upon which, either for food or protection, the greater number of them are dependent. This makes them appear less overwhelming in their astonishing and, at first sight, meaningless variety, than when one calls them to mind pinned out in long lines in innumerable drawers and cases, or assorted, like with like, in the wonderfully accurate and interminable pictures of them produced by those patient benefactors of mankind the systematic entomologists. Every plant of any size has a number of insects associated with it, living more or less completely on its substance, or making its home in some part of the plant. Some trees are known to have more than a hundred and fifty kinds or species of insects thus dependent on them, those which are vegetarian serving in their turn as food to a variety of carnivorous insects.

The ways in which insects are associated with plants may be briefly stated. It must be remembered that often, though not always, one particular species of plant, and that only, is capable of serving the needs of a given species of insect. Thus, the leaves of a given plant are the necessary food of the grubs of one or more insects which bite their food; its internal juices serve others which suck; its roots others; its nectar in the flower others, which in return serve the plant by carrying away its pollen and fertilising the other plants of the same species which they visit. Protection is sought and obtained from the same plant by insects which burrow in its leaves, or roll them up, or cut them into slices and carry them away, or hide in its bark, or in the flowers, or in other parts—or burrow for food and shelter into its wood. Others lay their eggs in the soft buds, producing or not producing according to their kind distorted growths, known as “galls” (one plant is known to have as many as thirty species of gall-flies which make use of it). Other insects lay their eggs in the flower-buds and immature fruits, or place them on the plant so that the young grubs, when hatched, can at once eat into those soft parts. Others bore into the wood or into hard or fleshy fruits expressly to lay their eggs, or into the ripe seeds. Certain ants live in chambers specially provided by the woody parts of the plant for them, and benefit both themselves and the plant by devouring other insects which seek the plant in order to devour it. In a museum of natural history there should be exhibited at least one plant with specimens and enlarged models of all the insects which depend upon it for food, protection, or nursery, and with accompanying illustrations of the way in which those purposes are served.

A curious product of the relationship of an insect and a plant is the so-called “jumping bean,” which is brought to this country from Mexico, and may be purchased in some of the London shops which deal in “miscellaneous” articles. They have been known for some years, but are becoming now a regular article of commerce. As one buys them (Fig. 52) they are segments of a globular fruit which has divided into three, comparable to the familiar segments of an orange, but less numerous. They are about one-third of an inch long, light, quite dry, and apparently hollow, without any visible opening. Two sides of the little capsule are flat, and the third side is bulged and rounded, so that the capsule easily rocks when resting on that side. When these dry fruits or segments of a fruit are brought into a warm room or placed near a fire so as to make them as warm as the hand, they commence to rock and move with curious little jerks. They jump as much as one-eighth of an inch from the ground, and advance as much as a quarter of an inch at a time, though by rolling they may progress a good deal more. They will often move seven or eight times in the same direction so as to make a progress of a couple of inches on a flat surface, and I have found that if a cool surface or protection from warmth is within reach they will in the course of time arrive at that cool area and come to rest. When the plate on which they are placed becomes cool or the temperature of the room falls to what we should call “chilly,” they cease to move, but can be roused again by renewed warmth.

How and why do these “beans,” or, rather, fruit-segments (for they are not beans), move in this determined purposeful manner? The whole proceeding has a mysterious and uncanny aspect. They have no legs, no spring; they are simple little smooth capsules, and yet they jump and seemingly “walk” about. The explanation is that there is a grub inside each so-called “bean.” Cut one of the beans or capsules open, and you find that it is a very thin-walled and hollow case, but coiled on itself in the cavity you open, and about half filling it, is a yellowish white grub (Figs. 52 and 53). It is not a “maggot,” but a “caterpillar,” that is to say, it is not legless, but has eight pairs of legs—namely, three pairs of short walking legs in front, four pairs of sucker-like legs, and a hinder pair of larger size called “claspers.” It has a hard brown plate on its head, and possesses hard jaws. It refuses to leave the opened capsule, and crawls back again if forcibly removed, and in the course of a few hours spins a silken cover to replace the piece of “shell” you have cut away. Mr. Rollo has lately succeeded in getting the caterpillar to patch up its injured residence with a thin piece of glass, such as is used by microscopists, which he put in place of a side of the capsule removed by a knife. He was thus able subsequently to watch through the glass the movements of the little creature when it causes the mended capsule or “bean” to jump. It rears itself from the lower surface of the capsule, and gives a series of sharp blows to the roof, projecting its body with each blow, and thus overbalances the capsule, or, if the flat side is lying downwards, jerks it along much as one may sit with one’s feet on the rail of a chair and cause it to jerk along the floor by the swinging movements of the body. The caterpillar does not die at once when removed from the capsule; it has been kept alive in a glass tube for a month.

So far so good. The next questions are: What Mexican plant is it that forms the capsule or tripartite fruit in which the caterpillar is found? How did the caterpillar get there? What kind of an insect does it turn into, and when? I will answer the last question first. The caterpillar turns into a chrysalis in the early part of the year, having first cut a perfectly circular ring in the shell of the capsule. The circular plate thus within the ring is not disturbed, and cannot be observed without very close inspection. The making of this perfectly circular cut without removing the piece marked out must be effected by a rotation of the caterpillar’s head and jaws as a centre-bit—an astonishing performance. But when the moth emerges from the chrysalis, a gentle push is enough to cause the little circular plate to fall out, and the moth creeps through the hole to the outer world. The moth, which comes out of the chrysalis-coat, is a very pretty little creature (see Fig. 54), measuring two-thirds of an inch across the opened wings, which are marked with dark and reddish-brown-coloured bands. It is a close ally of the British codling moth, the caterpillar of which eats its way into the core of apples, and is familiar to all growers and eaters of that fruit. The codling moth and the Mexican “jumper” belong to a group of small moths called _Tortricinæ_, and they are named respectively _Carpocapsa saltitans_ (the one whose grub or caterpillar inhabits the “jumping bean”) and _Carpocapsa pomonana_, the codling moth. There are other British species of _Carpocapsa_, the grubs of which eat into the acorn, the walnut, the chestnut, and the beechnut—a distinct kind or species for each. None of these grubs cause the nuts they attack to “jump.”

The “jumping bean” of Mexico is a segment of the triply divided fruit of a large spurge, which is called _Sebastiana palmeri_. The spurges are known in England as little green-leaved annuals, with yellow-green flowers and a milky juice. Botanists call them the _Euphorbiaceæ_, and in that “natural order” are included the boxwood tree and some tropical trees of great value and importance. None other than the Brazilian indiarubber tree, Hevea, of which we hear so much nowadays, its rubber to the value of £14,000,000 being exported every year from Brazil, is one of them. So also is the Chinese candle-tree, which furnishes a tallow-like fat, made into candles in China. Others are the croton oil and the castor oil shrubs, natives of India, and the manihot or tapioca plant. The fruits of _Sebastiana_ (the jumping bean) are very much like those of the croton; and as there are crotons (though not the one of the purgative oil) in abundance in Mexico, it has taken some time to make sure that the “jumping bean” is not the fruit of a croton, but that of the allied plant _Sebastiana_. It appears that there is no commercial value for this plant, and that those capsules which happen to contain a grub and move are collected from the ground by the native Mexican boys and sold as curiosities.

The moth (_Carpocapsa saltitans_) lays its eggs on the Sebastian shrub, and the young grub, on hatching, eats its way into the young fruit when the latter is still quite soft and the seed unformed, and so leaves no hole to mark its entrance. As the fruit swells the grub eats out the seed and surrounding pulp of the segment of the fruit into which it entered early in life. By the time the fruits are dry and fall to the ground the caterpillar is fully grown. Of course, it is only a very few of the capsules which are thus invaded by a grub.

The question very naturally arises, “Why should the caterpillar put itself to the great muscular effort of making the little capsule in which it is contained jump and move over the ground?” It seems probable that these movements are made in order to bring the capsule from an exposed position when it falls on to the ground—where it might be crushed or eaten by some animal—into a position of shelter, either into a hole, or under some stone or fallen wood. The warmth of the sun in an exposed position excites the caterpillar to activity, which ceases when it has reached the shade offered by some protecting cranny. In the same way I have applied artificial heat and, alternatively, shelter from heat, so as to cause the movements or the resting of the jumping bean in a London sitting-room.

These things and others of absorbing interest may be seen in the truly wonderful museum of Kew Gardens, where perhaps the visitor will be disposed to spend more time in cold weather than in the summer. The park at Kew Gardens, with its splendid forest and lakes, and its Italian tower, is one of the beautiful things of England, and it has a special quality even in this season of mist and veiled sunshine. I found there recently, under the trees, as I did fifty years ago, a rare and strange-looking fungus, the _Phallus impudicus_ of botanists,—a furtive denizen of the glades which in late spring are purple with wild hyacinths. The same spot in June presents within a few minutes’ journey from the smoke and smell and noise of Piccadilly a perfect sample of what is, perhaps, the most beautiful sight in Nature—bright sunlight breaking through the young green leaves of a forest on to green herbage. And close by are the azaleas!

XXXIII

PROTECTIVE COLOURING IN ANIMALS

Every one is familiar with some of the instances in which the natural colour of an animal helps to hide it from view. Green caterpillars, for instance, are less visible when among the green leaves which they eat than they would be were they brown, blue, red, yellow, or black. The little green tree-frog is difficult to see when he is clinging to a leaf, because his colour is the same as that of the leaf. Sandy-brown-coloured animals, birds, reptiles, and beasts of prey, are found on the sands of the desert; white birds, foxes, hares, and bears on the Arctic snow. The similarity of the colouring of these animals to that of the ground on which they live results in their escaping the observation of man’s eye, and we are entitled to believe that they escape for the same reason the observation of other animals. They are thus in many cases protected from the attacks of enemies searching for them as prey, or in other cases they may themselves be enabled the more easily in consequence of their concealing colour to creep upon other animals and seize them as food. Some of the simpler cases of this resemblance between an animal and its surroundings are easy to observe, and the value of the resemblance as protection, or as a means of secret attack, is plain enough.

But there are far more numerous cases in which the significance of colour as concealment, is not so immediately obvious. There are the curious stick insects, with long bodies and delicate long legs, sometimes with bud-like knobs on the body which look like bits of the branches of trees, not merely on account of their colour, but on account of their shape. Shape or modelling has a great deal to do with the effective concealment of an animal. Then, too, there is the curious fact that some insects (and also some birds) when at rest on the stems of trees, are practically invisible, but if they spread their wings are conspicuous. The beech-leaf butterfly of Assam and Africa is of a purple colour, marked with a great orange-coloured bar on each fore-wing when the wings are open, and it is obvious enough. But when the wings are closed and the insect is at rest, the undersides only are seen, and are coloured so as to represent the veining and fungus marks of a dry brown leaf, so that not even a human observer, let alone a bird or a lizard, can distinguish at two-feet distance the butterfly from dried leaves placed near it.

A well-known little moth, with pale green mottled wings, is the only case in which I have myself watched the protection afforded by colour at work. It was on a summer’s evening, when I saw this little moth zigzagging up and down with the most extraordinarily irregular flight, and a bird pursuing it. Twice the bird swooped and just missed his prey owing to a sudden turn and drop on the part of the moth. And then to my great delight the moth flopped against the stem of a tree on which was growing a greenish-grey lichen. The bird swooped again close to the tree, but failed to see the insect, and quitted the chase. It took me an appreciable time to detect the little moth resting against the lichen, and closely matching it in colour. There are endless examples known of such “protective resemblances,” some of them (such as that of the buff-tip moth, which, with its wings closed, looks like a broken birch twig) being most unexpected and fascinating. In the forests of Madagascar, the whitish-grey tree lichens are imitated by thread-like growths on beetles, tree-bugs, locusts, and even lizards, with a wonderful concealing effect, and some other flat membrane-like insects are so much like the greenish and yellowish bark of trees, that we actually lost a specimen for some time in the case labelled “Mimicry,” in which a series of these things was arranged by me for the edification of visitors to the Natural History Museum. It was found, after a day or two, to have been present all the time with other specimens on a piece of bark, from which it was indistinguishable.

Some eight years ago a distinguished American painter, Mr. Abbott Thayer, was able to add very importantly to our knowledge of the ways in which colour serves to conceal animals when in their natural surroundings. Mr. Thayer was able to do this owing to the fact that he was a devoted student of woodland life. This, however, alone was not enough. Mr. Thayer had the special ability to deal with this subject which comes from the trained eye of an artist. He had, above all, the knowledge of “tone values” and of the illusive and delusive effects of false shading and of colour-spots and bars, and of complementary colours and “irradiation”—which only a painter who deals every day in the most practical way with these matters can attain to. Mr. Thayer showed eight years ago—and demonstrated conclusively by means of models, one of which he presented to the Natural History Museum at my request—that in very many cases it is of no use for an animal to be of the same colour as its surroundings, since if the animal (a bird, or a quadruped, or a fish) is of plump and rounded shape and is observed under the open canopy of heaven, a deep shadow will exist on its lower surface and make it as obvious as a shaded charcoal drawing on a piece of light-brown paper. But if the back of the animal is of a dark tint and its belly white or whitish, then the effect of light and shade is (Mr. Thayer showed) completely counteracted and the animal becomes totally invisible in its natural surroundings.

Mr. Thayer’s model demonstrating this consists of two life-size wooden models of ducks seated on a stick—one to the left, the other to the right. The stick, with the two models on it, is fixed horizontally in a box, which is open above (that is, has no lid) and is also open in front. The box is, in fact, a little stage, lit from above by the light of the sky, and its three remaining sides are sufficiently high to form a complete background to the model ducks, whose perch runs across the “scene” at some 7 in. or 8 in. from the floor of the box. The box itself is lined with a pale purplish-brown flannel, and each bird is tightly covered with the same material. When so prepared the box is placed on a table under a skylight (where it is to stay), the table being high enough to bring the ducks just below the line of sight. Of course, deep shadows are formed by the top-light on the under side of the beak, head, and body of the models, and in spite of their colour being itself identical with that of the walls of the box, they are as obvious as it is possible for anything to be. Now Mr. Thayer takes his paints and very carefully darkens the back of one of the ducks and whitens its belly and the under side of its head and beak. The light and dark regions merge into one another along the side of the bird by skilful gradation. When this shading and whitening is finished (and, of course, the perfection of the result depends on the continuance of the right amount of sunlight, which is not a thing one can always ensure in a London museum) the duck-model so treated is absolutely invisible at a distance of 10 ft. or 15 ft.—and even when one is nearer escapes notice—looking like a haze or vague shadow of a bird even to an observer who knows nevertheless that it is there and is really as solid and large as the untreated model by its side. If now some one stretches out his hand so as to cut off the top-light falling on the painted model, it immediately becomes as solid to the eye as the untreated one, and when the hand is withdrawn it melts away again like Banquo’s ghost. The models made by Mr. Thayer were, so long as I was director, exhibited in the small room between the fish gallery and the central hall of the Natural History Museum, and, if they have not yet been removed, are well worth a visit.

Mr. Thayer’s models work perfectly, and astonish every one who sees them. The great point of interest about them, however, is, that the bird with dark back and light belly is really in the condition which is quite common in a number of birds, especially ducks and wading birds, where it must act as a means of concealing the bird—just as it does in the painted model. Of course, there are vast numbers of birds not so shaded, but it is possible to explain the darker and lighter colouring, in various arrangements seen in birds, as helping to produce concealment or disappearance from view, when the habits and natural surroundings of the bird are known. So, too, with many hairy quadrupeds (mammals, or “animals,” or “beasts,” as they are often called). The white hair under the tail and about the rump, helps a running animal to escape the vision of its pursuer—blending, as Mr. Thayer shows that it does—with the white colour of the sky-line. In the case of fish—especially fresh-water fish—the dark back and light belly are very common, and although they do not help to conceal the fish when seen from above, swimming over a light-coloured river-bed, yet when looked at by other fishes or by otters in the water, the effect of the light from above on this disposition of dark and light tints on the fish’s body must be the same as that demonstrated by Mr. Thayer’s “disappearing duck,” and must often render the fish absolutely invisible, even at close quarters.

Mr. Thayer has pursued this subject during the past seven years, and last autumn he gave some interesting demonstrations in the Zoological Gardens in London. He showed a model of a white egret, which was but little noticeable when standing up clear against a bright, white-clouded sky. The long plumes on the wings, developed in the breeding season, were shown (by putting them on and taking them off) to assist in causing invisibility, since they made the side of the body flat and concealed the shadow on its rounded underside. A similar bird-model marked with strong black on the neck and legs—the rest being white—refused (so to speak) to shape itself as a bird at all, and looked at a distance of twenty yards like a bit of rock or stump of wood with a twig and dead leaf attached. The effect of different tones of brown cardboard cut into the form of a butterfly, when seen on different backgrounds, was shown; but the most interesting experiment was made with a black-green piece of cloth cut to the shape of a butterfly and fastened on to a sheet of dead-black cloth in the open air, in the presence of white cloud light of moderate brilliancy. At five yards one could see the outline of the dark-green butterfly-shaped piece; at fifteen yards one could just distinguish the edge separating the dark-green piece from the black cloth. Now Mr. Thayer stuck in the middle of the dark-green butterfly-wing a small circle of pure white (about one-third of an inch across). The effect was entirely to obliterate the previously visible edge; one could no longer see the dark-green area at all—one only saw a white spot on a continuous dark ground, the dark-green and the black were merged into one. That is no doubt due to the powerful stimulation of the sensitive “retina” of the eye by the white light of the spot; the feeble stimulation by the dark-green and black, though these remain physically as distinct from one another as before, ceases to affect the brain, which is, as it were, entirely occupied with the strong white spot. This, according to Mr. Thayer, is the value to butterflies and other animals of a violently contrasted white spot or band on a dark general colouring. The fringe of white dots and connected white flakes nearer the centre of the wing—common on the wings of butterflies—has, similarly, the result of rendering the wing-outline imperceptible and the butterfly invisible. Many such relations of colour spots and bands, as well as of dark and light markings, have been elucidated by Mr. Thayer, and will be illustrated by coloured drawings in the book which he is preparing on the subject.

While it is the fact that Mr. Thayer has thrown new light on the colour-protection and invisibility of animals, it must be remembered that there are other explanations of certain cases of brilliant colouring in animals besides that which he has so well illustrated. “Warning” colours, recognition marks, and sexually attractive colouring all certainly and demonstrably exist in well-known and well-studied kinds of animals. It is very possible that some of these colour-markings have been produced by a slight change in what were previously “concealing” patterns or colour-markings. The tendency of the human observer is to regard any colour, spot, or pattern on a bird, fish, beast, or insect as a “mark” or distinguishing “sign.” We examine these things at close quarters, and do not, unless we reflect a good deal on the matter and experiment with the object, realise that what is a mark of distinction or recognition when seen at a few inches’ distance may be an illusive and obscuring colour-scheme when seen at a distance of some feet, and in natural and habitual surroundings. It is not unlikely that we shall arrive at definite knowledge of the psychological “sight interpretations” of animals by a further study of this subject. It is in the highest degree probable that the retinal picture produced in an animal’s eye by certain spots of colour, shade, and light exhibited by another animal, are not interpreted by the receptive animal in the same way as they would be by a scrutinising, inquiring, reasoning man, even one who is what we call a “savage.” Moreover, though many English naturalists have travelled and seen “life and light” in the sunny regions of the earth, there are few students of the colour-markings of animals in our museums, especially in great cities, who have adequate experience of what colour-markings really can effect in the way of concealment and illusion when light and surrounding objects are as they are, in the tropics or sub-tropical regions. It is a fashion nowadays in the best-provided museums of natural history to exhibit stuffed beasts, birds, and insects in what are called “their natural surroundings.” The fatal objection to such exhibitions is that were the beasts, birds, and insects placed in their most usual “natural surroundings,” they would be invisible!

It is the merit of Mr. Thayer to have drawn attention to these considerations, and to have carried out some interesting demonstrations of the more frequent significance of colour-markings as means of concealment and illusion than had been recognised before his work. At the same time, it is not possible to consider the yellow and black livery of wasps, of certain evil-tasting grubs, and of poisonous salamanders as anything but a “danger-flag,” a warning to other animals that the yellow and black animal had better not be bitten and tasted. So the previous experience of animals who have bitten yellow and black creatures is appealed to, and ensures the safety of the yellow and black gentry from tentative bites which would kill them. Other recognition marks by which ill-tasting, nauseous butterflies are distinguished, and in consequence of which they escape attack, and, not only that, but are “mimicked” (as the yellow and black poisonous wasp is mimicked by some innocuous flies which thus escape attack) by other pleasant-tasting butterflies which fly with them, are considered by Mr. Thayer to be wrongly interpreted as recognition or “warning” marks. He shows, with more or less success, that the markings of the butterflies known as Heliconiæ are effective as concealment, and is therefore inclined to deny their value as “warning” marks, serving to indicate a noxious quarry best left untasted.

It is, of course, quite possible that what are “concealment markings” when viewed by an aggressive bird or lizard at a distance, may be recognised as “warning marks” when seen by the same observers at close quarters, and it is also possible that the latter may have become the more important or only important result of the colour marks of a given butterfly which were once useful as “concealment.” The possible change of significance of colour spots and markings in wild animals may be illustrated by the effect on human beings of the burglar’s crêpe mask. At the present moment probably the most prominent result of the appearance in a house full of people in the dead of night of a man with a crêpe mask over his face would be terror to those who saw him. The mask would be interpreted as a “mark” or “sign” of evil, not to say violent intentions on the part of the masked man. It would be a “warning colour,” and most unathletic individuals would severely avoid it; in fact, retire from it in alarm. But actually, the burglar’s mask—as possibly some noxious insects’ distinctive markings—was not invented for the purpose of causing alarm. Far from it! The burglar, or nocturnal malefactor, dons his crêpe mask in order to cover the white glitter of his face, and so to escape observation. In origin it is a protective coloration leading to invisibility, and only secondarily has it become a “warning colour” or “mark” at close quarters. There will be much more ascertained, and much instructive discussion as to the colours and markings of not only animals, but also of flowers and foliage, before this wonderful subject is thrashed out. I have only been able here to indicate its outlines.

XXXIV

HOP-BLIGHT

Hops have for many years now been a very uncertain investment for those who, in England, devote capital to the growing, drying, and marketing of this crop. In some years a fortune may be made, in some years a dead loss, in many a bare return of expenditure. Hence, it is not surprising that English hop-growers should wish for legislation which shall make their business more secure by taxing the hops produced in other countries, and imported by our brewers. The whole subject of “hops” is a very complicated one. It is the fact that every plant and animal cultivated by civilised man has led to the accumulation of an astonishing amount of detailed knowledge and experience in each case, and that there are increasing difficulties and surprises in regard to varieties, and the competition of new supplies brought from all quarters of the globe. New areas of cultivation, new methods of transport, new fashion and taste continually disturb, and even destroy, old-established industries. It is for statesmen to consider how far the remorseless current of unforeseen changes should be checked and manipulated, so as to prevent disaster in the old-established and flourishing industries of the countryside.

The hop (called _Humulus lupulus_ by botanists) is a native of this country, and of the more temperate parts of Europe. The Greeks and Romans never made “beer,” and were unacquainted with the use of the hop. More than a thousand years ago the German and Scandinavian peoples made use of various fragrant herbs (sweet gale, bark of tamarisk and oak) to flavour the sweet beer which they brewed from malted grain, just as borage, cucumber, and other plants are still used to flavour “cups.” Wild hops were used, amongst other herbs, for this purpose, and gradually—but only gradually—became the favourite source of flavour. The hop owes its selection not merely to its bitter tonic quality, but also to its wonderful and most delicate perfume. Not only that, but the hop is found to be effective in checking continued fermentation and souring—and also to have a narcotic sleep-producing quality, for which it is still used medicinally. Distinct chemical compounds are found in hops to which these several properties are due. A warm “hop-pillow”—a pillow stuffed with dried hop-flowers—has given, and still gives, sleep to many a wakeful countryman. The older use of other fragrant plants in the making of beer survives in some foreign beers, such as the Norwegian ale, the beer of Louvain, and the “green” spruce-beer of Jena.

Hops were first cultivated with a view to obtaining varieties which would furnish abundant and large, well-flavoured flower-heads. The flower-heads are “cones,” consisting of numerous minute flowers, protected by overlapping green-coloured scales or bracts. The cultivated hop was brought to this country in the time of Henry VIII, and the cultivation of hops in hop-gardens and the skilful drying of the flower-heads in large bulk was commenced, and regulated by law. The male or pollen-producing hop-vine is distinct from the female seed-bearing hop-vine; it is the female flower-cone which carries the valuable fragrant and resinous products which the brewer desires. Hops are artificially propagated by root-cuttings, and it is interesting to note that the hop-grower finds that it is not desirable to allow the female flowers to be fertilised, since, although the hops weigh more after the setting of the seed, the valuable extractive substances contained in the flower are diminished, used up in the growth of the seed. Hence, often only one male hop-vine to every 200 female hop-vines is allowed in a hop-garden.