CHAPTER III.
A garden wall, and its traces of past life—Not a breath perishes—A bit of dry moss and its inhabitants—The “Wheel-bearers”—Resuscitation of Rotifers: drowned into life—Current belief that animals can be revived after complete desiccation—Experiments contradicting the belief—Spallanzani’s testimony—Value of biology as a means of culture—Classification of animals: the five great types—Criticism of Cuvier’s arrangement.
Pleasant, both to eye and mind, is an old garden wall, dark with age, gray with lichens, green with mosses of beautiful hues and fairy elegance of form: a wall shutting in some sequestered home, far from “the din of murmurous cities vast:” a home where, as we fondly, foolishly think, Life must needs throb placidly, and all its tragedies and pettinesses be unknown. As we pass alongside this wall, the sight of the overhanging branches suggests an image of some charming nook; or our thoughts wander about the wall itself, calling up the years during which it has been warmed by the sun, chilled by the night airs and the dews, and dashed against by the wild winds of March: all of which have made it quite another wall from what it was when the trowel first settled its bricks. The old wall has a past, a life, a story; as Wordsworth finely says of the mountain, it is “familiar with forgotten years.” Not only are there obvious traces of age in the crumbling mortar and the battered brick, but there are traces, not obvious, except to the inner eye, left by every ray of light, every raindrop, every gust. Nothing perishes. In the wondrous metamorphosis momently going on everywhere in the universe, there is change, but no loss.
Lest you should imagine this to be poetry, and not science, I will touch on the evidence that every beam of light, or every breath of air, which falls upon an object, permanently affects it. In photography we see the effect of light very strikingly exhibited; but perhaps you will object that this proves nothing more than that light acts upon an iodized surface. Yet in truth light acts upon, and more or less alters, the structure of every object on which it falls. Nor is this all. If a wafer be laid on a surface of polished metal, which is then breathed upon, and if, when the moisture of the breath has evaporated, the wafer be shaken off, we shall find that the whole polished surface is not as it was before, although our senses can detect no difference; for if we breathe again upon it, the surface will be moist everywhere except on the spot previously sheltered by the wafer, which will now appear as a spectral image on the surface. Again and again we breathe, and the moisture evaporates, but still the spectral wafer reappears. This experiment succeeds after a lapse of many months, if the metal be carefully put aside where its surface cannot be disturbed. If a sheet of paper, on which a key has been laid, be exposed for some minutes to the sunshine, and then instantaneously viewed in the dark, the key being removed, a fading spectre of the key will be visible. Let this paper be put aside for many months where nothing can disturb it, and then in darkness be laid on a plate of hot metal, the spectre of the key will again appear. In the case of bodies more highly phosphorescent than paper, the spectres of many different objects which may have been laid on in succession will, on warming, emerge in their proper order.[8]
This is equally true of our bodies, and our minds. We are involved in the universal metamorphosis. Nothing leaves us wholly as it found us. Every man we meet, every book we read, every picture or landscape we see, every word or tone we hear, mingles with our being and modifies it. There are cases on record of ignorant women, in states of insanity, uttering Greek and Hebrew phrases, which in past years they had heard their masters utter, without of course comprehending them. These tones had long been forgotten: the traces were so faint that under ordinary conditions they were invisible; but the traces were there, and in the intense light of cerebral excitement they started into prominence, just as the spectral image of the key started into sight on the application of heat. It is thus with all the influences to which we are subjected.
If a garden wall can lead our vagabond thoughts into such speculations as these, surely it may also furnish us with matter for our Studies in Animal Life? Those patches of moss must be colonies. Suppose we examine them? I pull away a small bit, which is so dry that the dust crumbles at a touch; this may be wrapped in a piece of paper—dirt and all—and carried home. Get the microscope ready, and now attend.
I moisten a fragment of this moss with distilled water. Any water will do as well, but the use of distilled water prevents your supposing that the animals you are about to watch were brought in it, and were not already in the moss. I now squeeze the bit between my fingers, and a drop of the contained water—somewhat turbid with dirt—falls on the glass slide, which we may now put on the microscope stage. A rapid survey assures us that there is no animal visible. The moss is squeezed again; and this time little yellowish bodies of an irregular oval are noticeable among the particles of dust and moss. Watch one of these, and presently you will observe a slow bulging at one end, and then a bulging at the other end. The oval has elongated itself into a form not unlike that of a fat caterpillar, except that there is a tapering at one end. Now a forked tail is visible; this fixes on to the glass, while the body sways to and fro. Now the head is drawn in—as if it were swallowed—and, suddenly, in its place are unfolded two broad membranes, having each a circle of waving cilia. The lifeless oval has become a living animal! You have assisted at a resuscitation, not from death by drowning, but by drying: the animal has been drowned into life! The unfolded membranes, with their cilia, have so much the appearance of wheels that the name of “Wheel-bearer” (Rotifera) or “Wheel Animalcule” has been given to the animal.
The Rotifera (also—and more correctly—called Rotatoria) form an interesting study. Let us glance at their organization:—
Fig. 16.
Rotifer Vulgaris. A, with the wheels drawn in (at c). B, with wheels expanded; b, eye spots; c, jaws and teeth; f, alimentary canal; g, embryo; h, embryo further developed; i, water-vascular system; k, vent.
There are many different kinds of Rotifers, varying very materially in size and shape; the males, as was stated in the last chapter, being more imperfectly organized than the females. They may be seen either swimming rapidly through the water by means of the vibratile cilia called “wheels,” because the optical effect is very much that of a toothed-wheel; or crawling along the side of the glass, fastening to it by the head, and then curving the body till the tail is brought up to the spot, which is then fastened on by the tail, and the head is set free. They may also be seen fastened to a weed, or the glass, by the tail, the body waving to and fro, or thrusting itself straight out, and setting the wheels in active motion. In this attitude the aspect of the jaws is very striking. Leuwenhoek mistook it for the pulsation of a heart, which its incessant rhythm much resembles. The tail, and the upper part of the body, have a singular power of being drawn out, or drawn in, like the tube of a telescope. There is sometimes a shell, or carapace, but often the body is covered only with a smooth firm skin, which, however, presents decided indications of being segmented.
The first person who described these Rotifers was the excellent old Leuwenhoek;[9] and his animals were got from the gutter of a house-top. Since then, they have been minutely studied, and have been shown to be, not Infusoria, as Ehrenberg imagined, but Crustacea.[10] Your attention is requested to the one point which has most contributed to the celebrity of these creatures—their power of resuscitation. Leuwenhoek described—what you have just witnessed, namely—the slow resuscitation of the animal (which seemed as dry as dust, and might have been blown about like any particle of dust,) directly a little moisture was brought to it. Spallanzani startled the world with the announcement that this process of drying and moistening—of killing and reviving—could be repeated fifteen times in succession; so that the Rotifer, whose natural term of life is about eighteen days, might, it was said, be dried and kept for years, and at any time revived by moisture. That which seems now no better than a grain of dust will suddenly awaken to the energetic life of a complex organism, and may again be made as dust by evaporation of the water.
This is very marvellous: so marvellous that a mind, trained in the cultivated caution of science, will demand the evidence on which it is based. Two months ago I should have dismissed the doubt with the assurance that the evidence was ample and rigorous, and the fact indisputable. For not only had the fact been confirmed by the united experience of several investigators: it had stood the test of very severe experiment. Thus in 1842, M. Doyère published experiments which seemed to place it beyond scepticism. Under the air-pump he set some moss, together with vessels containing sulphuric acid, which would absorb every trace of moisture. After leaving the moss thus for a week, he removed it into an oven, the temperature of which was raised to 300° Fahrenheit. Yet even this treatment did not prevent the animals from resuscitating when water was added.
In presence of testimony like this, doubt will seem next to impossible. Nevertheless, my own experiments leave me no choice but to doubt. Not having witnessed M. Doyère’s experiment, I am not prepared to say wherein its fallacy lies; but that there is a fallacy, seems to me capable of decisive proof. In M. Pouchet’s recent work[11] I first read a distinct denial of the pretended resuscitation of the Rotifers; this denial was the more startling to me, because I had myself often witnessed the reawakening of these dried animals. Nevertheless, whenever a doubt is fairly started, we have not done justice to it until we have brought it to the test of experiment; accordingly I tested this, and quickly came upon what seems to me the source of the general misconception. Day after day experiments were repeated, varied, and controlled, and with results so unvarying that hesitation vanished; and as some of these experiments are of extreme simplicity, you may verify what I say with little trouble. Squeeze a drop from the moss, taking care that there is scarcely any dirt in it; and, having ascertained that it contains Rotifers, or Tardigrades,[12] alive and moving, place the glass-slide under a bell-glass, to shield it from currents of air, and there allow the water to evaporate slowly, but completely, by means of chloride of calcium, or sulphuric acid, placed under the bell-glass; or, what is still simpler, place a slide with the live animals on the mantelpiece when a fire is burning in the grate. If on the day following you examine this perfectly dry glass, you will see the contracted bodies of the Rotifers, presenting the aspect of yellowish oval bodies; but attempt to resuscitate them by the addition of a little fresh water, and you will find that they do not revive, as they revived when dried in the moss: they sometimes swell a little, and elongate themselves, and you imagine this is a commencement of resuscitation; but continue watching for two or three days, and you will find it goes no further. Never do these oval bodies become active crawling Rotifers; never do they expand their wheels, and set the œsophagus at work. No: the Rotifer once dried is dead, and dead for ever.
But if, like a cautious experimenter, you vary and control the experiment, and beside the glass-slide place a watch-glass containing Rotifers with dirt, or moss, you will find that the addition of water to the contents of the watch-glass will often (not always) revive the animals. What you cannot effect on a glass-slide without dirt, or with very little, you easily effect in a watch-glass with dirt, or moss; and if you give due attention you will find that in each case the result depends upon the quantity of the dirt. And this leads to a clear understanding of the whole mystery; this reconciles the conflicting statements. The reason why Rotifers ever revive is, because they have not been dried—they have not lost by evaporation that small quantity of water which forms an integral constituent of their tissues; and it is the presence of dirt, or moss, which prevents this complete evaporation. No one, I suppose, believes that the Rotifer actually revives after once being dead. If it has a power of remaining in a state of suspended animation, like that of a frozen frog, it can do so only on the condition that its organism is not destroyed; and destroyed it would be, if the water were removed from its tissues: for, strange as it may seem, water is not an accessory, but a constituent element of every tissue; and this cannot be replaced mechanically—it can only be replaced by vital processes. Every one who has made microscopic preparations must be aware that when once a tissue is desiccated, it is spoiled: it will not recover its form and properties on the application of water; because the water was not originally worked into the web by a mere process of imbibition—like water in a sponge—but by a molecular process of assimilation, like albumen in a muscle. Therefore, I say, that desiccation is necessarily death; and the Rotifer which revives cannot have been desiccated. This being granted, we have only to ask, What prevents the Rotifer from becoming completely dried? Experiment shows that it is the presence of dirt, or moss, which does this. The whole marvel of the Rotifer’s resuscitation, therefore, amounts to this:—that if the water in which it lives be evaporated, the animal passes into a state of suspended animation, and remains so, as long as its own water is protected from evaporation.
I am aware that this is not easily to be reconciled with M. Doyère’s experiments, since the application of a temperature so high as 300° Fahr. (nearly a hundred degrees above boiling water) must, one would imagine, have completely desiccated the animals, in spite of any amount of protecting dirt. It is possible that M. Doyère may have mistaken that previously-noted swelling-up of the bodies, on the application of water, for a return to vital activity. If not, I am at a loss to explain the contradiction; for certainly in my experience a much more moderate desiccation—namely, that obtained by simple evaporation over a mantelpiece, or under a large bell-glass—always destroyed the animals, if little or no dirt were present.
The subject has recently been brought before the French Academy of Sciences by M. Davaine, whose experiments[13] lead him to the conclusion that those Rotifers which habitually live in ponds will not revive after desiccation: whereas those which live in moss always do so. I believe the explanation to be this: the Rotifers living in ponds are dried without any protecting dirt, or moss, and that is the reason they do not revive.
After having satisfied myself on this point, I did what perhaps would have saved me some trouble if thought of before. I took down Spallanzani, and read his account of his celebrated experiments. To my surprise and satisfaction, it appeared that he had accurately observed the same facts, but curiously missed their real significance. Nothing can be plainer than the following passage: “But there is one condition indispensable to the resurrection of wheel-animals: it is absolutely necessary that there should be a certain quantity of sand; without it they will not revive. One day I had two wheel-animals traversing a drop of water about to evaporate, which contained very little sand. Three quarters of an hour after evaporation, they were dry and motionless. I moistened them with water to revive them; but in vain, notwithstanding that they were immersed in water many hours. Their members swelled to thrice the original size, but they remained motionless. To ascertain whether the fact was accidental, I spread a portion of sand, containing animals, on a glass slide, and waited till it became dry in order to wet it anew. The sand was carelessly scattered on the glass, so as to be a thin covering on some parts, and on others in a very small quantity: here the animals did not revive: but all that were in those parts with abundance of sand revived.”[14] He further says that if sand be spread out in considerable quantities in some places, much less in others, and very little in the rest, on moistening it the revived animals will be numerous in the first, less numerous in the second, and none at all in the third.
It is not a little remarkable that observations so precise as these should have for many years passed unregarded, and not led to the true explanation of the mystery. Perhaps an inherent love of the marvellous made men greedily accept the idea of resuscitation, and indisposed them to attempt an explanation of it. Spallanzani’s own attempt is certainly not felicitous. He supposes that the dust prevents the lacerating influence of the air from irritating and injuring the animals. And this explanation is accepted by his Translator.
[Since the foregoing remarks were in type, M. Gavarret has published (Annales des Sciences Naturelles, 1859, xi. p. 315) the account of his experiments on Rotifers and Tardigrades, in which he found that after subjecting the moss to a desiccation the most complete according to our present means, the animals revived after twenty-four hours’ immersion of the moss in water. This result seems flatly to contradict the result I arrived at; but only seems to contradict it, for in my experiments the animals, not the moss, were subjected to desiccation. Nevertheless, I confess that my confidence was shaken by experiments so precise, and performed by so distinguished an investigator, and I once more resumed the experiments, feeling persuaded that the detection of the fallacy, wherever it might be, would be well worth the trouble. The results of these controlling experiments are all I can find room for here:—Whenever the animals were completely separated from the dirt, they perished; in two cases there was a very little dirt—a mere film, so to speak—in the watch-glass, and glass-cell, and this, slight as it was, sufficed to protect two out of eight, and three out of ten Rotifers, which revived on the second day; the others did not revive even on the third day after their immersion. In one instance, a thin covering-glass was placed over the water on the slide, and the evaporation of the water seemed complete, yet this glass-cover sufficed to protect a Rotifer, which revived in three hours.
If we compare these results with those obtained by M. Davaine, we can scarcely avoid the conclusion that it is only when the desiccation of the Rotifers is prevented by the presence of a small quantity of moss, or of dirt—between the particles of which they find shelter—that they revive on the application of water. And even in the severe experiments of M. Doyère and M. Gavarret, some of the animals must have been thus protected; and I call particular attention to the fact that, although some animals revived, others always perished. But if the organization of the Rotifer, or Tardigrade, is such that it can withstand desiccation—if it only needs the fresh applications of moisture to restore its activity—all, or almost all, the animals experimented on ought to revive; and the fact that only some revive leads us to suspect that these have not been desiccated—a suspicion which is warranted by direct experiments. I believe, then, that the discrepancy amounts to this: investigators who have desiccated the moss containing animals, find some of the animals revive on the application of moisture; but those who desiccate the animals themselves, will find no instances of revival.]
The time spent on these Rotifers will not have been misspent if it has taught us the necessity of caution in all experimental inquiries. Although Experiment is valuable—nay, indispensable—as a means of interrogating Nature, it is constantly liable to mislead us into the idea that we have rightly interrogated, and rightly interpreted the replies; and this danger arises from the complexity of the cases with which we are dealing, and our proneness to overlook, or disregard, some seemingly trifling condition—a trifle which may turn out of the utmost importance. The one reason why the study of Science is valuable as a means of culture, over and above its own immediate objects, is that in it the mind learns to submit to realities, instead of thrusting its figments in the place of realities—endeavours to ascertain accurately what the order of Nature is, and not what it ought to be, or might be. The one reason why, of all sciences, Biology is pre-eminent as a means of culture, is, that owing to the great complexity of all the cases it investigates, it familiarizes the mind with the necessity of attending to all the conditions, and it thus keeps the mind alert. It cultivates caution, which, considering the tendency there is in men to “anticipate Nature,” is a mental tonic of inestimable worth. I am far from asserting that biologists are more accurate reasoners than other men; indeed, the mass of crude hypothesis which passes unchallenged by them, is against such an idea. But whether its advantages be used or neglected, the truth nevertheless is, that Biology, from the complexity of its problems, and the necessity of incessant verification of its details, offers greater advantages for culture than any other branch of science.
I have once or twice mentioned the words Mollusc and Crustacean, to which the reader unfamiliar with the language of Natural History will have attached but vague ideas; and although I wanted to explain these, and convey a distinct conception of the general facts of Classification, it would have then been too great an interruption. So I will here make an opportunity, and finish the chapter with an indication of the five Types, or plans of structure, under one of which every animal is classed. Without being versed in science, you discern at once whether the book before you is mathematical, physical, chemical, botanical, or physiological. In like manner, without being versed in Natural History, you ought to know whether the animal before you belongs to the Vertebrata, Mollusca, Articulata, Radiata, or Protozoa.
Fig. 17.
Male Triton, or Water-Newt.
A glance at the contents of our glass vases will yield us samples of each of these five divisions of the animal kingdom. We begin with this Triton. It is a representative of the Vertebrate division, or sub-kingdom. You have merely to remember that it possesses a backbone and an internal skeleton, and you will at once recognize the cardinal character which makes this Triton range under the same general head as men, elephants, whales, birds, reptiles, or fishes. All these, in spite of their manifold differences, have this one character in common:—they are all backboned; they have all an internal skeleton; they are all formed according to one general type. In all vertebrate animals the skeleton is found to be identical in plan. Every bone in the body of a triton has its corresponding bone in the body of a man, or of a mouse; and every bone preserves the same connection with other bones, no matter how unlike may be the various limbs in which we detect its presence. Thus, widely as the arm of a man differs from the fin of a whale, or the wing of a bird, or the wing of a bat, or the leg of a horse, the same number of bones, and the same connections of the bones, are found in each. A fin is one modified form of the typical limb; an arm is another; a wing another. That which is true of the limbs, is also true of all the organs; and it is on this ground that we speak of the vertebrate type. From fish to man one common plan of structure prevails; and the presence of a backbone is the index by which to recognize this plan.
The Triton has been wriggling grotesquely in our grasp while we have made him our text, and, now he is restored to his vase, plunges to the bottom with great satisfaction at his escape. This water-snail, crawling slowly up the side of the vase, and cleaning it of the green growth of microscopic plants, which he devours, shall be our representative of the second great division—the Mollusca. I cannot suggest any obvious character so distinctive as a backbone, by which the word Mollusc may at once call up an idea of the type which prevails in the group. It won’t do to say “shell-fish,” because many molluscs have no shells, and many animals which have shells are not molluscs. The name was originally bestowed on account of the softness of the animals. But they are not softer than worms, and much less so than jelly-fish. You may know that snails and slugs, oysters and cuttlefish, are molluscs; but if you want some one character by which the type may be remembered, you must fix on the imperfect symmetry of the mollusc’s organs. I say imperfect symmetry, because it is an error, though a common one, to speak of the mollusc’s body not being bilateral—that is to say, of its not being composed of two symmetrical halves. A vertebrate animal may be divided lengthwise, and each half will closely resemble the other; the backbone forms, as it were, an axis, on either side of which the organs are disposed; but the mollusc is said to have no such axis, no such symmetry. I admit the absence of an axis, but I deny the total absence of symmetry. Many of its organs are as symmetrical as those of a vertebrate animal—i.e. the eyes, the feelers, the jaws—and the gills in Cuttlefish, Eolids, and Pteropods; while, on the other hand, several organs in the vertebrate animal are as unsymmetrical as any of those in the mollusc, i.e. the liver, spleen, pancreas, stomach, and intestines.[15] As regards bilateral structure, therefore, it is only a question of degree. The vertebrate animal is not entirely symmetrical, nor is the mollusc entirely unsymmetrical. But there is a characteristic disposition of the nervous system peculiar to molluscs: it neither forms an axis for the body—as it does in the Vertebrata and Articulata—nor a centre—as it does in the Radiata—but is altogether irregular and unsymmetrical. This will be intelligible from the following diagram of the nervous systems of a Mollusc and an insect, with which that of a Star-fish may be compared (Fig. 18). Here you perceive how the nervous centres, and the nerves which issue from them, are irregularly disposed in the molluscs, and symmetrically in the insect.
But the recognition of a mollusc will be easier when you have learned to distinguish it from one of the Articulata, forming the third great division,—the third animal Type. Of these, our vases present numerous representatives: prawns, beetles, water-spiders, insect-larvæ, entomostraca, and worms. There is a very obvious character by which these may be recognized: they have all bodies composed of numerous segments, and their limbs are jointed, and they have mostly an external skeleton from which their limbs are developed. Sometimes the segments of their bodies are numerous, as in the centipede, lobster, &c.; sometimes several segments are fused together, as in the crab; and sometimes, as in worms, they are indicated by slight markings or depressions of the skin, which give the appearance of little rings, and hence the worms have been named Annelida, or Annulata, or Annulosa. In these last-named cases the segmental nature of the type is detected in the fact that the worms grow, segment by segment; and also in the fact that in most of them each segment has its own nerves, heart, stomach, &c.—each segment is, in fact, a zöoid.[16]
Fig. 18.
Nervous System of Sea-Hare (A) and Centipede (B).
Just as we recognize a vertebrate by the presence of a backbone and internal skeleton, we recognize an articulate by its jointed body and external skeleton. In both, the nervous system forms the axis of the body. The Mollusc, on the contrary, has no skeleton, internal or external;[17] and its nervous system does not form an axis. As a rule, both vertebrates and articulates have limbs—although there are exceptions in serpents, fishes, and worms. The Molluscs have no limbs. Backboned,—jointed,—and non-jointed,—therefore, are the three leading characteristics of the three types.
Let us now glance at the fourth division—the Radiata,—so called because of the disposition of the organs round a centre, which is the mouth. Our fresh-water vases afford us only one representative of this type—the Hydra, or fresh-water Polype, whose capture was recorded in the last chapter. Is it not strange that while all the Radiata are aquatic, not a single terrestrial representative having been discovered, only one should be found in fresh water? Think of the richness of the seas, with their hosts of Polypes, Actiniæ, Jelly-fish, Star-fishes, Sea-urchins, Sea-pens (Pennatulæ), Lily-stars (Comatulæ), and Sea-cucumbers (Holothuriæ), and then compare the poverty of rivers, lakes, and ponds, reduced to their single representative, the Hydra. The radiate structure may best be exhibited by this diagram of the nervous system of the Star-fish.[18]
Fig. 19.
Nervous System of Star-Fish.
Cuvier, to whom we owe this classification of the animal kingdom into four great divisions, would have been the first to recognize the chaotic condition in which he left this last division, and would have acquiesced in the separation of the Protozoa, which has since been made. This fifth division includes many of the microscopic animals known as Infusoria; and receives its name from the idea that these simplest of all animals represent, as it were, the beginnings of life.[19]
But Cuvier’s arrangement is open to a more serious objection. The state of science in his day excused the imperfection of classing the Infusoria and parasites under the Radiata; but it was owing, I conceive, to an unphilosophical view of morphology, that he placed the molluscs next to the Vertebrata, instead of placing the Articulata in that position. He was secretly determined by the desire to show that there are four very distinct types, or plans of structure, which cannot by any transitions be brought under one law of development. Lamarck and Geoffroy St. Hilaire maintained the idea of unity of composition throughout the animal kingdom;—in other words, that all the varieties of animal forms were produced by successive modifications: and several of the German naturalists maintained that the vertebrata in their embryonic stages passed through forms which were permanent in the lower animals. This idea Cuvier always opposed. He held that the four types were altogether distinct; and by his arrangement of them, their distinctness certainly appears much greater than would be the case on another arrangement. But without discussing this question here, it is enough to point out the fact of the enormous superiority in intelligence, in sociality, and in complexity of animal functions, which insects and spiders exhibit, when compared with the highest of the molluscs, to justify the removal of the mollusca, and the elevation of the articulata to the second place in the animal hierarchy. Nor is this all. If we divide animals into four groups, these four naturally dispose themselves into two larger groups: the first of these, comprising Vertebrata and Articulata, is characterized by a nervous axis and a skeleton; the second, comprising Mollusca and Radiata, is characterized by the absence of both nervous axis and skeleton. It is obvious that a bee much more closely resembles a bird, than any mollusc resembles any vertebrate. If there are many and important differences between the vertebrate and articulate types, there are also many and important resemblances; if the nervous axis is above the viscera, and forms the dorsal line of the vertebrate, whereas it is underneath the viscera, and forms the ventral line in the articulate, it is, nevertheless, in both, the axis of the body, and in both it sends off nerves to supply symmetrical limbs; in both it has similar functions. And while the articulata thus approach in structure the vertebrate type, the mollusca are not only removed from that type by many diversities, but a number of them have such affinities with the Radiate type, that it is only in quite recent days that the whole class of Polyzoa (or Bryozoa, as they are also called) has been removed from the Radiata, and ranged under the Mollusca.
To quit this topic, and recur once more to the five divisions, we have only the broad outlines of the picture in Vertebrata, Mollusca, Articulata, Radiata, and Protozoa; but this is a good beginning, and we can now proceed to the further sub-divisions. Each of these five sub-kingdoms is divided into Classes; these again into Orders; these into Families; these into Genera; these into Species; and these finally into Varieties. Thus suppose a dwarf terrier is presented to us with a request that we should indicate its various titles in the scheme of classification: we begin by calling it a vertebrate; we proceed to assign its Class as the mammalian; its Order is obviously that of the carnivora; its Family is that of the fox, wolf, jackal, &c., named Canidæ; its Genus is, of course, that of Canis; its Species, terrier; its Variety, dwarf-terrier. Inasmuch as all these denominations are the expressions of scientific research, and not at all arbitrary or fanciful, they imply an immense amount of labour and sagacity in their establishment; and when we remember that naturalists have thus classed upwards of half a million of distinct species, it becomes an interesting inquiry,—What has been the guiding principle of this successful labour? on what basis is so large a superstructure raised? This question we shall answer in the next chapter.