CHAPTER I.
Omnipresence of Life—The Microscope—An Opalina and its wonders—The uses of Cilia—How our lungs are protected from dust and filings—Feeding without a mouth or stomach—What is an organ?—How a complex organism arises—Early stages of a frog and a philosopher—How the plants feed—Parasites of the frog—Metamorphoses and migrations of Parasites—Life within life—The budding of animals—A steady bore—Philosophy of the infinitely little.
Come with me, and lovingly study Nature, as she breathes, palpitates, and works under myriad forms of Life—forms unseen, unsuspected, or unheeded by the mass of ordinary men. Our course may be through park and meadow, garden and lane, over the swelling hills and spacious heaths, beside the running and sequestered streams, along the tawny coast, out on the dark and dangerous reefs, or under dripping caves and slippery ledges. It matters little where we go: everywhere—in the air above, the earth beneath, and waters under the earth—we are surrounded with Life. Avert your eyes awhile from our human world, with its ceaseless anxieties, its noble sorrow, poignant, yet sublime, of conscious imperfection aspiring to higher states, and contemplate the calmer activities of that other world with which we are so mysteriously related. I hear you exclaim,—
“The proper study of mankind is man;”
nor will I pretend, as some enthusiastic students seem to think, that
“The proper study of mankind is cells;”
but agreeing with you, that man is the noblest study, I would suggest that under the noblest there are other problems which we must not neglect. Man himself is imperfectly known, because the laws of universal Life are imperfectly known. His Life forms but one grand illustration of Biology—the science of Life,[21] as he forms but the apex of the animal world.
Our studies here will be of Life, and chiefly of those minuter, or obscurer forms, which seldom attract attention. In the air we breathe, in the water we drink, in the earth we tread on, Life is everywhere. Nature lives: every pore is bursting with Life; every death is only a new birth, every grave a cradle. And of this we know so little, think so little! Around us, above us, beneath us, that great mystic drama of creation is being enacted, and we will not even consent to be spectators! Unless animals are obviously useful, or obviously hurtful to us, we disregard them. Yet they are not alien, but akin. The Life that stirs within us, stirs within them. We are all “parts of one transcendent whole.” The scales fall from our eyes when we think of this; it is as if a new sense had been vouchsafed to us; and we learn to look at Nature with a more intimate and personal love.
Life everywhere! The air is crowded with birds—beautiful, tender, intelligent birds, to whom life is a song and a thrilling anxiety, the anxiety of love. The air is swarming with insects—those little animated miracles. The waters are peopled with innumerable forms, from the animalcule, so small that one hundred and fifty millions of them would not weigh a grain, to the whale, so large that it seems an island as it sleeps upon the waves. The bed of the seas is alive with polypes, crabs, star-fishes, and with sand-numerous shell-animalcules. The rugged face of rocks is scarred by the silent boring of soft creatures; and blackened with countless mussels, barnacles, and limpets.
Life everywhere! on the earth, in the earth, crawling, creeping, burrowing, boring, leaping, running. If the sequestered coolness of the wood tempt us to saunter into its chequered shade, we are saluted by the murmurous din of insects, the twitter of birds, the scrambling of squirrels, the startled rush of unseen beasts, all telling how populous is this seeming solitude. If we pause before a tree, or shrub, or plant, our cursory and half-abstracted glance detects a colony of various inhabitants. We pluck a flower, and in its bosom we see many a charming insect busy at its appointed labour. We pick up a fallen leaf, and if nothing is visible on it, there is probably the trace of an insect larva hidden in its tissue, and awaiting there development. The drop of dew upon this leaf will probably contain its animals, visible under the microscope. This same microscope reveals that the blood-rain suddenly appearing on bread, and awakening superstitious terrors, is nothing but a collection of minute animals (Monas prodigiosa); and that the vast tracts of snow which are reddened in a single night, owe their colour to the marvellous rapidity in reproduction of a minute plant (Protococcus nivalis). The very mould which covers our cheese, our bread, our jam, or our ink, and disfigures our damp walls; is nothing but a collection of plants. The many-coloured fire which sparkles on the surface of a summer sea at night, as the vessel ploughs her way, or which drips from the oars in lines of jewelled light, is produced by millions of minute animals.
Nor does the vast procession end here. Our very mother-earth is formed of the débris of life. Plants and animals which have been, build up its solid fabric.[22] We dig downwards, thousands of feet below the surface, and discover with surprise the skeletons of strange, uncouth animals, which roamed the fens and struggled through the woods before man was. Our surprise is heightened when we learn that the very quarry itself is mainly composed of the skeletons of microscopic animals; the flints which grate beneath our carriage wheels are but the remains of countless skeletons. The Apennines and Cordilleras, the chalk cliffs so dear to homeward-nearing eyes—these are the pyramids of bygone generations of atomies. Ages ago, these tiny architects secreted the tiny shells, which were their palaces; from the ruins of these palaces we build our Parthenons, our St. Peters, and our Louvres. So revolves the luminous orb of Life! Generations follow generations; and the Present becomes the matrix of the Future, as the Past was of the Present: the Life of one epoch forming the prelude to a higher Life.
When we have thus ranged air, earth, and water, finding everywhere a prodigality of living forms, visible and invisible, it might seem as if the survey were complete. And yet it is not so. Life cradles within Life. The bodies of animals are little worlds, having their own animals and plants. A celebrated Frenchman has published a thick octavo volume devoted to the classification and description of “The Plants which grow on Men and Animals;”[23] and many Germans have described the immense variety of animals which grow on and in men and animals; so that science can now boast of a parasitic Flora and Fauna. In the fluids and tissues, in the eye, in the liver, in the stomach, in the brain, in the muscles, parasites are found; and these parasites have often their parasites living in them!
We have thus taken a bird’s-eye view of the field in which we may labour. It is truly inexhaustible. We may begin where we please, we shall never come to an end; our curiosity will never slacken.
“And whosoe’er in youth
Has thro’ ambition of his soul given way
To such desires, and grasp’d at such delights,
Shall feel congenial stirrings, late and long.”
As a beginning, get a microscope. If you cannot borrow, boldly buy one. Few purchases will yield you so much pleasure; and while you are about it, do, if possible, get a good one. Spend as little money as you can on accessory apparatus and expensive fittings, but get a good stand and good glasses. Having got your instrument, bear in mind these two important trifles—work by daylight, seldom or never by lamplight; and keep the unoccupied eye open. With these precautions you may work daily for hours without serious fatigue to the eye.
Now where shall we begin? Anywhere will do. This dead frog, for example, that has already been made the subject of experiments, and is now awaiting the removal of its spinal cord, will serve us as a text from which profitable lessons may be drawn. We snip out a portion of its digestive tube, which from its emptiness seems to promise little; but a drop of the liquid we find in it is placed on a glass slide, covered with a small piece of very thin glass, and brought under the microscope. Now look. There are several things which might occupy your attention; but disregard them now to watch that animalcule which you observe swimming about. What is it? It is one of the largest of the Infusoria, and is named Opalina. When I call this an Infusorium I am using the language of text-books; but there seems to be a growing belief among zoologists that the Opalina is not an Infusorium, but the infantile condition of some worm (Distoma?). However, it will not grow into a mature worm as long as it inhabits the frog; it waits till some pike, or bird, has devoured the frog, and then, in the stomach of its new captor, it will develop into its mature form: then, and not till then. This surprises you? And well it may; but thereby hangs a tale, which to unfold—for the present, however, it must be postponed, because the Opalina itself needs all our notice.
Observe how transparent it is, and with what easy, undulating grace it swims about; yet this swimmer has no arms, no legs, no tail, no backbone to serve as a fulcrum to moving muscles: nay, it has no muscles to move with. ’Tis a creature of the most absolute abnegations: sans eyes, sans teeth, sans everything;—no, not sans everything, for as we look attentively we see certain currents produced in the liquid, and on applying a higher magnifying power we detect how these currents are produced. All over the surface of the Opalina there are delicate hairs, in incessant vibration: these are the cilia.[24] They lash the water, and the animal is propelled by their strokes, as a galley by its hundred oars. This is your first sight of that ciliary action of which you have so often read, and which you will henceforth find performing some important service in almost every animal you examine. Sometimes the cilia act as instruments of locomotion; sometimes as instruments of respiration, by continually renewing the current of water; sometimes as the means of drawing in food—for which purpose they surround the mouth, and by their incessant action produce a small whirlpool into which the food is sucked. An example of this is seen in the Vorticella (Fig. 2).
Having studied the action of these cilia in microscopic animals, you will be prepared to understand their office in your own organism. The lining membrane of your air-passages is covered with cilia; which may be observed by following the directions of Professor Sharpey, to whom science is indebted for a very exhaustive description of these organs. “To see them in motion, a portion of the ciliated mucous membrane may be taken from a recently-killed quadruped. The piece of membrane is to be folded with its free, or ciliated, surface outwards, placed on a slip of glass, with a little water or serum of blood, and covered with thin glass or mica. When it is now viewed with a power of 200 diameters, or upwards, a very obvious agitation will be perceived on the edge of the fold, and this appearance is caused by the moving cilia with which the surface of the membrane is covered. Being set close together, and moving simultaneously or in quick succession, the cilia, when in brisk action, give rise to the appearance of a bright transparent fringe along the fold of the membrane, agitated by such a rapid and incessant motion that the single threads which compose it cannot be perceived. The motion here meant is that of the cilia themselves; but they also set in motion the adjoining fluid, driving it along the ciliated surface, as is indicated by the agitation of any little particles that may accidentally float in it. The fact of the conveyance of fluids and other matters along the ciliated surface, as well as the direction in which they are impelled, may also be made manifest by immersing the membrane in fluid, and dropping on it some finely-pulverized substance (such as charcoal in fine powder), which will be slowly but steadily carried along in a constant and determinate direction.”[25]
It is an interesting fact, that while the direction in which the cilia propel fluids and particles is generally towards the interior of the organism, it is sometimes reversed; and, instead of beating the particles inwards, the cilia energetically beat them back, if they attempt to enter. Fatal results would ensue if this were not so. Our air-passages would no longer protect the lungs from particles of sand, coal-dust, and filings, flying about the atmosphere; on the contrary, the lashing hairs which cover the surface of these passages would catch up every particle, and drive it onwards into the lungs. Fortunately for us, the direction of the cilia is reversed, and they act as vigilant janitors, driving back all vagrant particles with a stern “No admittance—even on business!” In vain does the whirlwind dash a column of dust in our faces—in vain does the air, darkened with coal-dust, impetuously rush up the nostrils: the air is allowed to pass on, but the dust is inexorably driven back. Were it not so, how could miners, millers, iron-workers, and all the modern Tubal Cains contrive to live in their loaded atmospheres? In a week, their lungs would be choked up.
Perhaps, you will tell me that this is the case: that manufacturers of iron and steel are very subject to consumption; and that there is a peculiar discoloration of the lungs which has often been observed in coal miners, examined after death.
Not being a physician, and not intending to trouble you with medical questions, I must still place before you three considerations, which will show how untenable this notion is. First, although consumption may be frequent among the Sheffield workmen, the cause is not to be sought in their breathing filings, but in the sedentary and unwholesome confinement incidental to their occupation. Miners and coal-heavers are not troubled with consumption. Moreover, if the filings were the cause, all the artisans would suffer, when all breathe the same atmosphere. Secondly, while it is true that discoloured lungs have been observed in some miners, it has not been observed in all, or in many; whereas, it has been observed in men not miners, not exposed to any unusual amount of coal-dust. Thirdly, and most conclusively, experiment has shown that the coal-dust cannot penetrate to the lungs. Claude Bernard, the brilliant experimenter, tied a bladder, containing a quantity of powdered charcoal, to the muzzle of a rabbit. Whenever the animal breathed, the powder within the bladder was seen to be agitated. Except during feeding time, the bladder was kept constantly on, so that the animal breathed only this dusty air. If the powder could have escaped the vigilance of the cilia, and got into the lungs, this was a good occasion. But when the rabbit was killed and opened, many days afterwards, no powder whatever was found in the lungs, or bronchial tubes; several patches were collected about the nostrils and throat; but the cilia had acted as a strainer, keeping all particles from the air-tubes.
The swimming apparatus of the Opalina has led us far away from the little animal, who has been feeding while we have been lecturing. At the mention of feeding, you naturally look for the food that is eaten, the mouth and stomach that eat. But I hinted just now that this ethereal creature dispenses with a stomach, as too gross for its nature; and of course, by a similar refinement, dispenses with a mouth. Indeed, it has no organs whatever, except the cilia just spoken of. The same is true of several of the Infusoria; for you must know that naturalists no longer recognize the complex organization which Ehrenberg fancied he had detected in these microscopic beings. If it pains you to relinquish the piquant notion of a microscopic animalcule having a structure equal in complexity to that of the elephant, there will be ample compensation in the notion which replaces it, the notion of an ascending series of animal organisms, rising from the structureless amœba to the complex frame of a mammal. On a future occasion we shall see that, great as Ehrenberg’s services have been, his interpretations of what he saw have one by one been replaced by truer notions. His immense class of Infusoria has been, and is constantly being, diminished; many of his animals turn out to be plants; many of them embryos of worms; and some of them belong to the same divisions of the animal kingdom as the oyster and the shrimp: that is to say, they range with the Molluscs and Crustaceans. In these, of course, there is a complex organization; but in the Infusoria, as now understood, the organization is extremely simple. No one now believes the clear spaces visible in their substance to be stomachs, as Ehrenberg believed; and the idea of the Polygastrica, or many-stomached Infusoria, is abandoned. No one believes the coloured specs to be eyes; because, not to mention the difficulty of conceiving eyes where there is no nervous system, it has been found that even the spores of some plants have these coloured specs; and they are assuredly not eyes. If, then, we exclude the highly-organized Rotifera, or “Wheel Animalcules,” which are genuine Crustacea, we may say that all Infusoria, whether they be the young of worms or not, are of very simple organization.
And this leads us to consider what biologists mean by an organ: it is a particular portion of the body set apart for the performance of some particular function. The whole process of development is this setting apart for special purposes. The starting-point of Life is a single cell—that is to say, a microscopic sac, filled with liquid and granules, and having within it a nucleus, or smaller sac. Paley has somewhere remarked, that in the early stages, there is no difference discernible between a frog and a philosopher. It is very true; truer than he conceived. In the earliest stage of all, both the Batrachian and the Philosopher are nothing but single cells; although the one cell will develop into an Aristotle or a Newton, and the other will get no higher than the cold, damp, croaking animal which boys will pelt, anatomists dissect, and Frenchmen eat. From the starting-point of a single cell, this is the course taken: the cell divides itself into two, the two become four, the four eight, and so on, till a mass of cells is formed, not unlike the shape of a mulberry. This mulberry-mass then becomes a sac, with double envelopes, or walls: the inner wall, turned towards the yelk, or food, becomes the assimilating surface for the whole; the outer wall, turned towards the surrounding medium, becomes the surface which is to bring frog and philosopher into contact and relation with the external world—the Non-Ego, as the philosopher, in after life, will call it. Here we perceive the first grand “setting apart,” or differentiation, has taken place: the embryo having an assimilating surface, which has little to do with the external world; and a sensitive, contractile surface, which has little to do with the preparation and transport of food. The embryo is no longer a mass of similar cells; it is already become dissimilar, different, as respects its inner and outer envelope. But these envelopes are at present uniform; one part of each is exactly like the rest. Let us, therefore, follow the history of Development, and we shall find that the inner wall gradually becomes unlike itself in various parts; and that certain organs, constituting a very complex apparatus of Digestion, Secretion, and Excretion, are all one by one wrought out of it, by a series of metamorphoses, or differentiations. The inner wall thus passes from a simple assimilating surface to a complex apparatus serving the functions of vegetative life.
Now glance at the outer wall: from it also various organs have gradually been wrought: it has developed into muscles, nerves, bones, organs of sense, and brain: all these from a simple homogeneous membrane!
With this bird’s-eye view of the course of Development, you will be able to appreciate the grand law first clearly enunciated by Goethe and Von Baer, as the law of animal life, namely, that Development is always from the general to the special, from the simple to the complex, from the homogeneous to the heterogeneous; and this by a gradual series of differentiations.[26] Or to put it into the music of our deeply meditative Tennyson:—
“All nature widens upward. Evermore
The simpler essence lower lies:
More complex is more perfect—owning more
Discourse, more widely wise.”
You are now familiarized with the words “differentiation” and “development,” so often met with in modern writers; and have gained a distinct idea of what an “organ” is; so that on hearing of an animal without organs, you will at once conclude that in such an animal there has been no setting apart of any portion of the body for special purposes, but that all parts serve all purposes indiscriminately. Here is our Opalina, for example, without mouth, or stomach, or any other organ. It is an assimilating surface in every part; in every part a breathing, sensitive surface. Living on liquid food, it does not need a mouth to seize, or a stomach to digest, such food. The liquid, or gas, passes through the Opalina’s delicate skin, by a process which is called endosmosis; it there serves as food; and the refuse passes out again by a similar process, called exosmosis. This is the way in which many animals and all plants are nourished. The cell at the end of a rootlet, which the plant sends burrowing through the earth, has no mouth to seize, no open pores to admit the liquid that it needs; nevertheless the liquid passes into the cell, through its delicate cell-wall, and passes from this cell to other cells, upwards from the rootlet to the bud. It is in this way, also, that the Opalina feeds: it is all-mouth, no-mouth; all-stomach, no-stomach. Every part of its body performs the functions which in more complex animals are performed by organs specially set apart. It feeds without mouth, breathes without lungs, and moves without muscles.
The Opalina, as I said, is a parasite. It may be found in various animals, and almost always in the frog. You will, perhaps, ask why it should be considered a parasite; why may it not have been swallowed by the frog in a gulp of water? Certainly, nothing would have been easier. But to remove your doubts, I open the skull of this frog, and carefully remove a drop of the liquid found inside, which, on being brought under the microscope, we shall most probably find containing some animalcules, especially those named Monads. These were not swallowed. They live in the cerebro-spinal fluid, as the Opalina lives in the digestive tube. Nay, if we extend our researches, we shall find that various organs have their various parasites. Here, for instance, is a parasitic worm from the frog’s bladder. Place it under the microscope, with a high power, and behold! It is called Polystomum—many-mouthed, or, more properly, many-suckered. You are looking at the under side, and will observe six large suckers with their starlike clasps (e), and the horny instrument (f), with which the animal bores its way. At a there is another sucker, which serves also as a mouth; at b you perceive the rudiment of a gullet, and at d the reproductive organs. But pay attention to the pretty branchings of the digestive tube (c) which ramifies through the body like a blood-vessel.
This arrangement of the digestive tube is found in many animals, and is often mistaken for a system of blood-vessels. In one sense this is correct; for these branching tubes are carriers of nutriment, and the only circulating vessels such animals possess; but the nutriment is chyme, not blood: these simple animals have not arrived at the dignity of blood, which is a higher elaboration of the food, fitted for higher organisms.
Thus may our frog, besides its own marvels, afford us many “authentic tidings of invisible things,” and is itself a little colony of Life. Nature is economic as well as prodigal of space. She fills the illimitable heavens with planetary and starry grandeurs, and the tiny atoms moving over the crust of earth she makes the homes of the infinitely little. Far as the mightiest telescope can reach, it detects worlds in clusters, like pebbles on the shore of Infinitude; deep as the microscope can penetrate, it detects Life within Life, generation within generation; as if the very Universe itself were not vast enough for the energies of Life!
That phrase, generation within generation, was not a careless phrase; it is exact. Take the tiny insect (Aphis) which, with its companions, crowds your rose-tree; open it, in a solution of sugar-water, under your microscope, and you will find in it a young insect nearly formed; open that young insect with care, and you will find in it, also, another young one, less advanced in its development, but perfectly recognizable to the experienced eye; and beside this embryo you will find many eggs, which would in time become insects!
Or take that lazy water-snail (Paludina vivipara), first made known to science by the great Swammerdamm, the incarnation of patience and exactness, and you will find, as he found, forty or fifty young snails, in various stages of development; and you will also find, as he found, some tiny worms, which, if you cut them open, will suffer three or four infusoria to escape from the opening.[27] In your astonishment you will ask, Where is this to end?
The observation recorded by Swammerdamm, like so many others of this noble worker, fell into neglect; but modern investigators have made it the starting-point of a very curious inquiry. The worms he found within the snail are now called Cercaria-sacs, because they contain the Cercariæ, once classed as Infusoria, and which are now known to be the early forms of parasitic worms inhabiting the digestive tube, and other cavities, of higher animals. These Cercariæ have vigorous tails, with which they swim through the water like tadpoles, and like tadpoles, they lose their tails in after life. But how, think you, did these sacs containing Cercariæ get into the water-snails? “By spontaneous generation,” formerly said the upholders of that hypothesis; and those who condemned the hypothesis were forced to admit they had no better explanation. It was a mystery, which they preferred leaving unexplained, rather than fly to spontaneous generation. And they were right. The mystery has at length been cleared up.[28] I will endeavour to bring together the scattered details, and narrate the curious story.
Under the eyelids of geese and ducks may be constantly found a parasitic worm (of the Trematode order), which naturalists have christened Monostomum mutabile—Single-mouth, Changeable. This worm brings forth living young, in the likeness of active Infusoria, which, being covered with cilia, swim about in the water, as we saw the Opalina swim. Here is a portrait of one. (Fig. 4.)
Each of these animalcules develops a sac in its interior. The sac you may notice in the engraving. Having managed to get into the body of the water-snail, the animalcule’s part in the drama is at an end. It dies, and in dying liberates the sac, which is very comfortably housed and fed by the snail. If you examine this sac (Fig. 5), you will observe that it has a mouth and digestive tube, and is, therefore, very far from being, what its name imports, a mere receptacle; it is an independent animal, and lives an independent life. It feeds generously on the juices of the snail, and having fed, thinks generously of the coming generations. It was born inside the animalcule; why should it not in turn give birth to children of its own? To found a dynasty, to scatter progeny over the bounteous earth, is a worthy ambition. The mysterious agency of Reproduction begins in this sac-animal; and in a short while a brood of Cercariæ move within it. The sac bursts, and the brood escapes. But how is this? The children are by no means the “very image” of their parent. They are not sacs, nor in the least resembling sacs, as you see. (Fig. 6.)
They have tails, and suckers, and sharp boring instruments, with other organs which their parent was without. To look at them you would as soon suspect a shrimp to be the progeny of an oyster, as these to be the progeny of the sac-animal. And what makes the paradox more paradoxical is, that not only are the Cercariæ unlike their parent, but their parent was equally unlike its parent the embryo of Monostomum (compare fig. 4). However, if we pursue this family history, we shall find the genealogy rights itself at last, and that this Cercaria will develop in the body of some bird into a Monostomum mutabile like its ancestor. Thus the worm produces an animalcule, which produces a sac-animal, which produces a Cercaria, which becomes a worm exactly resembling its great-grandfather.
One peculiarity in this history is that while the Monostomum produces its young in the usual way, the two intermediate forms are produced by a process of budding, analogous to that observed in plants. Plants, as you know, are reproduced in two ways, from the seed, and from the bud. For seed-reproduction, peculiar organs are necessary; for bud-reproduction, there is no such differentiation needed: it is simply an out-growth. The same is true of many animals: they also bud like plants, and produce seeds (eggs) like plants. I have elsewhere argued that the two processes are essentially identical; and that both are but special forms of growth.[29] Not, however, to discuss so abstruse a question here, let us merely note that the Monostomum, into which the Cercaria will develop, produces eggs, from which young will issue; the second generation is not produced from eggs, but by internal budding; the third generation is likewise budded internally; but it, on acquiring maturity, will produce eggs. For this maturity, it is indispensable that the Cercaria should be swallowed by some bird or animal; only in the digestive tube can it acquire its egg-producing condition. How is it to get there? The ways are many; let us witness one:—
In this watchglass of water we have several Cercariæ swimming about. To them we add three or four of those darting, twittering insects which you have seen in every vase of pond-water, and have learned to be the larvæ, or early forms, of the Ephemeron. The Cercariæ cease flapping the water with their impatient tails, and commence a severe scrutiny of the strangers. When Odry, in the riotous farce, Les Saltimbanques, finds a portmanteau, he exclaims, “Une malle! ce doit être à moi!” (“Surely this must belong to me!”) This seems to be the theory of property adopted by the Cercaria: “An insect! surely this belongs to me!” Accordingly every one begins creeping over the bodies of the Ephemera, giving an interrogatory poke with the spine, which will pierce the first soft place it can detect. Between the segments of the insect’s armour a soft and pierceable spot is found; and now, lads, to work! Onwards they bore, never relaxing in their efforts till a hole is made large enough for them to slip in by elongating their bodies. Once in, they dismiss their tails as useless appendages; and begin what is called the process of encysting—that is, of rolling themselves up into a ball, and secreting a mucus from their surface, which hardens round them like a shell. Thus they remain snugly ensconced in the body of the insect, which in time develops into a fly, hovers over the pond, and is swallowed by some bird. The fly is digested, and the liberated Cercaria finds itself in comfortable quarters, its shell is broken, and its progress to maturity is rapid.
Von Siebold’s description of another form of emigration he has observed in parasites will be read with interest. “For a long time,” he says, “the origin of the threadworm, known as Filaria insectorum, that lives in the cavity of the bodies of adult and larval insects, could not be accounted for. Shut up within the abdominal cavity of caterpillars, grasshoppers, beetles, and other insects, these parasites were supposed to originate by spontaneous generation, under the influence of wet weather or from decayed food. Helminthologists (students of parasitic worms) were obliged to content themselves with this explanation, since they were unable to find a better. Those who dissected these threadworms and submitted them to a careful inspection, could not deny the probability, since it was clear that they contained no trace of sexual organs. But on directing my attention to these entozoa, I became aware of the fact that they were not true Filariæ at all, but belonged to a peculiar family of threadworms, embracing the genera of Gordius and Mermis. Furthermore, I convinced myself that these parasites wander away when full-grown, boring their way from within through any soft place in the body of their host, and creeping out through the opening. These parasites do not emigrate because they are uneasy, or because the caterpillar is sickly; but from that same internal necessity which constrains the horsefly to leave the stomach of the horse where he has been reared, or which moves the gadfly to work its way out through the skin of the oxen. The larvæ of both these insects creep forth in order to become chrysalises, and thence to proceed to their higher and perfect condition. I have demonstrated that the perfect, full-grown, but sexless threadworms of insects are in like manner moved by their desire to wander out of their previous homes, in order to enter upon a new period of their lives, which ends in the development of their sex. As they leave the bodies of their hosts they fall to the ground, and crawl away into the deeper and moister parts of the soil. Threadworms found in the damp earth, in digging up gardens and cutting ditches, have often been brought to me, which presented no external distinctions from the threadworms of insects. This suggested to me that the wandering threadworms of insects might instinctively bury themselves in damp ground, and I therefore instituted a series of experiments by placing the newly-emigrated worms in flower-pots filled with damp earth. To my delight I soon perceived that they began to bore with their heads into the earth, and by degrees drew themselves entirely in. For many months I kept the earth in the flower-pots moderately moist, and on examining the worms from time to time I found they had gradually attained their sex-development, and eggs were deposited in hundreds. Towards the conclusion of winter I could succeed in detecting the commencing development of the embryos in these eggs. By the end of spring they were fully formed, and many of them having left their shells were to be seen creeping about the earth. I now conjectured that these young worms would be impelled by their instincts to pursue a parasitic existence, and to seek out an animal to inhabit and to grow to maturity in; and it seemed not improbable that the brood I had reared would, like their parents, thrive best in the caterpillar. In order, therefore, to induce my young brood to immigrate, I procured a number of very small caterpillars which the first spring sunshine had just called into life. For the purpose of my experiment I filled a watch-glass with damp earth, taking it from amongst the flower-pots where the threadworms had wintered. Upon this I placed several of the young caterpillars.” The result was as he expected; the caterpillars were soon bored into by the worms, and served them at once as food and home.[30]
Frogs and parasites, worms and infusoria—are these worth the attention of a serious man? They have a less imposing appearance than planets and asteroids, I admit, but they are nearer to us, and admit of being more intimately known; and because they are thus accessible, they become more important to us. The life that stirs within us is also the life within them. It is for this reason, as I said at the outset, that although man’s noblest study must always be man, there are other studies less noble, yet not therefore ignoble, which must be pursued, even if only with a view to the perfection of the noblest. Many men, and those not always the ignorant, whose scorn of what they do not understand is always ready, despise the labours which do not obviously and directly tend to moral or political advancement. Others there are, who, fascinated by the grandeur of Astronomy and Geology, or by the immediate practical results of Physics and Chemistry, disregard all microscopic research as little better than dilettante curiosity. But I cannot think any serious study is without its serious value to the human race; and I know that the great problem of Life can never be solved while we are in ignorance of its simpler forms. Nor can anything be more unwise than the attempt to limit the sphere of human inquiry, especially by applying the test of immediate utility. All truths are related; and however remote from our daily needs some particular truth may seem, the time will surely come when its value will be felt. To the majority of our countrymen during the Revolution, when the conduct of James seemed of incalculable importance, there would have seemed something ludicrously absurd in the assertion that the newly-discovered differential calculus was infinitely more important to England and to Europe than the fate of all the dynasties; and few things could have seemed more remote from any useful end than this product of mathematical genius; yet it is now clear to every one that the conduct of James was supremely insignificant in comparison with this discovery. I do not say that men were unwise to throw themselves body and soul into the Revolution; I only say they would have been unwise to condemn the researches of mathematicians.
Let all who have a longing to study Nature in any of her manifold aspects, do so without regard to the sneers or objections of men whose tastes and faculties are directed elsewhere. From the illumination of many minds on many points, Truth must finally emerge. Man is, in Bacon’s noble phrase, the minister and interpreter of Nature; let him be careful lest he suffer this ministry to sink into a priesthood, and this interpretation to degenerate into an immovable dogma. The suggestions of apathy, and the prejudices of ignorance, have at all times inspired the wish to close the temple against new comers. Let us be vigilant against such suggestions, and keep the door of the temple ever open.