IV.
In presenting Classification as the subject of a series of papers in the "Atlantic Monthly," I am aware that I am drawing largely upon the patience of its readers; since the technical nature of the topic renders many details necessary which cannot be otherwise than dry to any but professional naturalists. Yet believing, as I do, that classification, rightly understood, means simply the creative plan of God as expressed in organic forms, I feel the importance of attempting at least to present it in a popular guise, divested, as far as possible, of technicalities, while I would ask the indulgence of my readers for such scientific terms and details as cannot well be dispensed with, begging them to remember that a long and tedious road may bring us suddenly upon a glorious prospect, and that a clearer mental atmosphere and a new intellectual sensation may well reward us for a little weariness in the outset. Besides, the time has come when scientific truth must cease to be the property of the few, when it must be woven into the common life of the world; for we have reached the point where the results of science touch the very problem of existence, and all men listen for the solving of that mystery. When it will come, and how, none can say; but this much at least is certain, that all our researches are leading up to that question, and mankind will never rest till it is answered. If, then, the results of science are of such general interest for the human race, if they are gradually interpreting the purposes of the Deity in creation, and the relation of man to all the past, then it is well that all should share in its teachings, and that it should not be kept, like the learning of the Egyptians, for an exclusive priesthood who may expound the oracle according to their own theories, but should make a part of all our intellectual culture and of our common educational systems. With this view, I will endeavor to simplify as far as may be my illustrations of the different groups of the Animal Kingdom, beginning with a more careful analysis of those structural features on which classes are founded.
I have said that the Radiates are the lowest type among animals, embodying, under an infinite variety of forms, that plan in which all parts bear definite relations to a vertical central axis. The three classes of Radiates are distinguished from each other by three distinct ways of executing that plan. I dwell upon this point; for we shall never arrive at a clear understanding of the different significance and value of the various divisions of the Animal Kingdom, till we appreciate the distinction between the structural conception and the material means by which it is expressed. A comparison will, perhaps, better explain my meaning. There are certain architectonic types, including edifices of different materials, with an infinite variety of architectural details and external ornaments; but the flat roof and the colonnade are typical of all Grecian temples, whether built of marble or granite or wood, whether Doric or Ionic or Corinthian, whether simple and massive or light and ornamented; and, in like manner, the steep roof and pointed arch are the typical characters of all Gothic cathedrals, whatever be the material or the details. The architectural conception remains the same in all its essential elements, however the more superficial features vary. Such relations as these edifices bear to the architectural idea that includes them all, do classes bear to the primary divisions or branches of the Animal Kingdom.
The three classes of Radiates, beginning with the lowest, and naming them in their relative order, are Polyps, Acalephs or Jelly-Fishes, and Echinoderms or Star-Fishes and Sea-Urchins. In the Polyps the plan is executed in the simplest manner by a sac, the sides of which are folded inward, at regular intervals from top to bottom, so as to divide it by vertical radiating partitions, converging from the periphery toward the centre. These folds or partitions do not meet in the centre, but leave an open space, which is the main cavity of the body. This open space, however, occupies only the lower part of the body; for in the upper there is a second sac hanging to a certain distance within the first. This inner sac has an aperture in the bottom, through which whatever enters it passes into the main cavity of the body. A central opening in the top forms a kind of mouth, around which are radiating tentacles connecting with the open chambers formed by the partitions within. Cutting such an animal across in a transverse section, we shall see the radiation of the partitions from the centre to the circumference, showing still more distinctly the typical structure of the division to which it belongs.
[Illustration: Vertical section of a Sea-Anemone of Actinia: o, mouth; t, tentacles; s, inner sac or stomach; b, main cavity; ff, reproductive organs; g, radiating partition; eee, radiating chambers; cc, circular openings in the partitions; aa, lower floor.]
[Illustration: Transverse section of a Sea-Anemone or Actinia.]
[Illustration: Staurophera seen in profile.]
[Illustration: Hippocrene seen in profile.]
[Illustration: Melicertum seen from above, with the tentacles spreading: oo, radiating tubes with ovaries; m, mouth; tttt, tentacles.]
The second class is that of Jelly-Fishes or Acalephs; and here the same plan is carried out in the form of a hemispherical gelatinous disk, the digestive cavity being hollowed, or, as it were, scooped, out of the substance of the body, which is traversed by tubes that radiate from the centre to the periphery. Cutting it across transversely, or looking through its transparent mass, the same radiation of the internal structure is seen again; only that in this instance the radiating lines are not produced by vertical partition-walls, with open spaces between, as in the Polyps, but by radiating tubes passing through the gelatinous mass of the body. At the periphery is a circular tube connecting them all, and the tentacles, which hang down when the animal is in its natural position, connect at their base with the radiating tubes, while numerous smaller tentacles may form a kind of fringe all round the margin.
The third and highest class includes the Star-Fishes, Sea-Urchins, and Holothurians or Beches-de-Mer. The radiation is equally distinct in each of these; but here again the mode of execution differs from that of the two other classes. The internal cavity and the radiating tubes, instead of being connected with the outer wall of the body as in Polyps, or hollowed out of the substance of the body as in Jelly-Fishes, are here inclosed within independent walls of their own, quite distinct from the wall of the body. But notwithstanding this difference, a transverse section shows in these animals, as distinctly as in all the rest, the radiating structure typical of the whole branch. In these three classes we have no difference of plan, nor even any modification of the same plan,—for either one of them expresses it as clearly as any other,—but simply three different ways of executing one structural idea.
[Illustration: Common Sea-Urchin, Echinus, seen from above]
[Illustration: Echinarachnius, opened by a transverse or horizontal section, and showing the internal arrangement: c, mouth; eeeee, ambulacra, with their ramifications cmcmcm; wwww, interambulacra.]
I have mentioned only three classes of Radiates. Cuvier had five in his classification; for he placed among them the Intestinal Worms and the Infusoria or Animalcules. The Intestinal Worms are much better known now than they were in his day. Their anatomy and embryology have been traced, and it has been shown that the essential features of these parasites are the same as those of all Articulates, their whole body being divided into successive, movable joints or rings. Cuvier was misled by the circular arrangement of certain parts around the mouth, and by the presence of a wreath of feelers around the head of some of these Worms, resembling the tentacles of many Radiates. This is, however, no indication of radiate structure, but a superficial feature in no way related to the internal organization.
We must carefully distinguish between affinity and analogy among animals. The former is founded on identity of plan; the latter only upon external resemblance, produced by similar features, which, when they are intimately connected with the whole internal organization, as in some groups, may be considered as typical characters, but when only grafted, as it were, in a superficial manner on animals of another type, have no relation to the essential elements of structure, and become at once subordinate and unimportant. Such is the difference between the tentacles in a Radiate and the wreath of feelers in a Worm;—the external effect may be much the same; but in the former every tentacle opens into one of the chambers as in a Polyp, or connects with one of the radiating tubes as in Acalephs, or with the locomotive suckers as in Star-Fishes, and is therefore closely linked with the whole internal organization; whereas the feelers in the latter are only external appendages, in no way connected with the essential structural elements. We have a striking illustration of this superficial resemblance in the wings of Birds and Insects. In Birds, wings are a typical feature, corresponding to the front limbs in all Vertebrates, which are constructed in the same way, whether they are arms as in Man, or forelegs as in Quadrupeds, or pectoral fins as in Fishes, or wings as in Birds. The wing in an Insect, on the contrary, is a flattened, dried-up gill, having no structural relation whatever to the wing of a Bird. They are analogous only because they resemble each other in function, being in the same way subservient to flight; but as organs they are entirely different.
In adding Infusoria to the Radiates, Cuvier was false to his own principle of founding all classification on plan. He was influenced by their seeming simplicity of structure, and placed them in the lowest division of the Animal Kingdom on that account. But even this simplicity was only apparent in many of them. At certain seasons of the year myriads of these little Animalcules may be seen in every brook and road-side pool. They are like transparent little globules, without any special organization, apparently; and were it not that they are in constant rotation, exhibiting thus a motion of their own, one would hardly suspect that they were endowed with life. To the superficial observer they all look alike, and it is not strange, that, before they had been more carefully investigated, they should have been associated together as the lowest division of the Animal Kingdom, representing, as it were, a border-land between animal and vegetable life. But since the modern improvements in the microscope, Ehrenberg, the great master in microscopic investigation, has shown that many of these little globules have an extraordinary complication of structure. Subsequent investigations have proved that they include a great variety of beings: some of them belonging to the type of Mollusks; others to the type of Articulates, being in fact little Shrimps; while many others are the locomotive germs of plants, and so far from forming a class by themselves, as a distinct group in the Animal Kingdom, they seem to comprise representatives of all types except Vertebrates, and to belong in part to the Vegetable Kingdom, Siebold, Leuckart, and other modern zoölogists, have considered them as a primary type, and called them Protozoa; but this is as great a mistake as the other. The rotatory motion in them all is produced by an apparatus that exists not only in all animals, but in plants also, and is a most important agent in sustaining the freshness and vitality of their circulating fluids and of the surrounding medium in which they live. It consists of soft fringes, called Vibratile Cilia. Such fringes cover the whole surface of these little living beings, and by their unceasing play they maintain the rotating motion that carries them along in the water.
The Mollusks, the next great division of the Animal Kingdom, also include three classes. With them is introduced that character of bilateral symmetry, or division of parts on either side of a longitudinal axis, that prevails throughout the Animal Kingdom, with the exception of the Radiates. The lowest class of Mollusks has been named Acephala, to signify the absence of any distinct head; for though their whole organization is based upon the principle of bilateral symmetry, it is nevertheless very difficult to determine which is the right side and which the left in these animals, because there is so little prominence in the two ends of the body that the anterior and posterior extremities are hardly to be distinguished. Take the Oyster as an example. It has, like most Acephala, a shell with two valves united by a hinge on the back, one of these valves being thick and swollen, while the other is nearly flat. If we lift the shell, we find beneath a soft lining-skin covering the whole animal and called by naturalists the mantle, from the inner surface of which arise a double row of gills, forming two pendent folds on the sides of the body; but at one end of the body these folds do not meet, but leave an open space, where is the aperture we call the mouth. This is the only indication of an anterior extremity; but it is enough to establish a difference between the front and hind ends of the body, and to serve as a guide in distinguishing the right and left sides. If now we lift the mantle and gills, we find beneath the principal organs: the stomach, with a winding alimentary canal; the heart and liver; the blood-vessels, branching from either side of the heart to join the gills; and a fleshy muscle passing from one valve of the shell to the other, enabling the animal by its dilatation or contraction to open and close its shell at will. A cut across an animal of this class will show us better the bilateral arrangement of the parts. In such a section we see the edge of the two shells on either side; within these the edge of the mantle; then the double rows of gills; and in the middle the alimentary canal, the heart, and the blood-vessels branching right and left. Some of these animals have eye-specks on the edge of the mantle; but this is not a constant feature. This class of Acephala includes all the Oysters, Clams, Mussels, and the like. When named with reference to their double shells, they are called Bivalves; and with them are associated a host of less conspicuous animals, known as Ascidians, Brachiopods, and Bryozoa.
[Illustration: Common Mussel, Unio, cut transversely: a, foot; bb, gills; c, mantle; d, shell; e, heart; f, main cavity, with intestines.]
The second class in this type is that of Gasteropoda, so named from the fleshy muscular expansion on which they move, and which is therefore called a foot: a very inappropriate name; since it has no relation or resemblance to a foot, though it is used as a locomotive organ. This class includes all the Snails, Slugs, Cockles, Conchs, Periwinkles, Whelks, Limpets, and the like. Some of them have no solid covering; but the greater part are protected by a single shell, and on this account they are called Univalves, in contradistinction to the Acephala or Bivalves. These shells, though always single, differ from each other by an endless variety of form and color,—from the flat simple shell of the Limpet to the elaborate spiral and brilliant hues of the Cones and Cowries. Different as is their external covering, however, if we examine the internal structure of a Gasteropod, we find the same general arrangement of parts that prevails in the Acephala, showing that both belong to the same great division of the Animal Kingdom. The mantle envelops the animal, and lines its single shell as it lined the double shell of the Oyster; the gills are placed on either side of it; the stomach, with the winding alimentary canal, is in the centre of the body; the heart and liver are placed in the same relation to it as in the Acephala; and though the so-called foot would seem to be a new feature, it is but a muscular expansion of the ventral side of the body. There is an evident superiority in this class over the preceding one, in the greater prominence of the anterior extremity, where there are two or more feelers, with which eyes more or less developed are connected; and though there is nothing that can be properly called a head, yet there can be no hesitation as to the distinction between the front and hind ends of the body.
[Illustration: Limpet, Patella, cut transversely: a, foot; b, gills; c, mantle; d, shell; e, heart; f, main cavity, with intestines.]
The third and highest class of Mollusks has been called Cephalopoda, in reference again to a special feature of their structure. They have long arms or feelers around the head, serving as organs of locomotion, by which they propel themselves through the water with a velocity that is quite extraordinary, when compared with the sluggishness of the other Mollusks. In these animals the head is distinctly marked,—being separated, by a contraction or depression behind it, from the rest of the body. The feelers, so prominent on the anterior extremity of the Gasteropoda, are suppressed in Cephalopoda, and the eyes are consequently brought immediately on the side of the head, and are very large in proportion to the size of the animal. A skin corresponding to the mantle envelops the body, and the gills are on either side of it;—the stomach with its winding canal, the liver, and heart occupy the centre of the body, as in the two other classes. This class includes all the Cuttle-Fishes, Squids, and Nautili, and has a vast number of fossil representatives. Many of these animals are destitute of any shell; and where they have a shell, it is not coiled from right to left or from left to right as in the spiral of the Gasteropoda, but from behind forwards as in the Nautilus. These shells are usually divided into a number of chambers,—the animal, as it grows, building a wall behind it at regular intervals, and always occupying the external chamber, retaining, however, a connection with his past home by a siphon that runs through the whole succession of chambers. The readers of the "Atlantic Monthly" cannot fail to remember the exquisite poem suggested to the Autocrat of the Breakfast-Table by this singular feature in the structure of the so-called Chambered Shells.
[Illustration: Common Squid, Loligo, cut transversely: a, foot or siphon; b, gills; c, mantle; d, shell; e, heart; f, main cavity, with intestines.]
Cuvier divided the Mollusks also into a larger number of classes than are now admitted. He placed the Barnacles with them on account of their shells; and it is only since an investigation of the germs born from these animals has shown them to be Articulates that their true position is understood. They give birth to little Shrimps that afterwards become attached to the rocks and assume the shelly covering that has misled naturalists about them. Brachiopods formed another of his classes; but these differ from the other Bivalves only in having a net-work of blood-vessels in the place of the free gills, and this is merely a complication of structure, not a difference in the general mode of execution, for their position and relation to the rest of the organization are exactly the same in both. Pteropods constituted another class in his division of the type of Mollusks; but these animals, again, form only an order in the class of Gasteropoda, as Brachiopods form an order in the class of Acephala.
In the third division of the Animal Kingdom, the Articulates, we have again three classes: Worms, Crustacea, and Insects. The lowest of these three classes, the Worms, presents the typical structure of that branch in the most uniform manner, with little individualization of parts. The body is a long cylinder divided through its whole length by movable joints, while the head is indicated only by a difference in the front-joint. There is here no concentration of vitality in special parts of the structure, as in the higher animals, but the nervous force is scattered through the whole body,—every ring having, on its lower side, either two nervous swellings, one on the right, the other on the left side, connected by nervous threads with those that precede and those that follow them, or these swellings being united in the median line. It is this equal distribution of nervous force through the whole system that gives to these animals such an extraordinary power of repairing any injured part, so that, if cut in two, the front part may even reconstruct a tail for itself, while the hind part produces a new head, and both continue to live as distinct animals. This facility of self-repair, after a separation of the parts, which is even a normal mode of multiplication in some of them, does not indicate, as may at first appear, a greater intensity of vital energy, but, on the contrary, arises from an absence of any one nervous centre such as exists in all the higher animals, and is the key to their whole organization. A serious injury to the brain of a Vertebrate destroys vitality at once, for it holds the very essence of its life; whereas in many of the lower animals any part of the body may be destroyed without injury to the rest. The digestive cavity in the Worms runs the whole length of the body; and the respiratory organs, wherever they are specialized, appear as little vesicles or gill-like appendages either along the back or below the sides, connected with the locomotive appendages.
This class includes animals of various degrees of complication of structure, from those with highly developed organizations to the lowest Worms that float like long threads in the water and hardly seem to be animals. Yet even these creatures, so low in the scale of life, are not devoid of some instincts, however dim, of feeling and affection. I remember a case in point that excited my own wonder at the time, and may not be uninteresting to my readers. A gentleman from Detroit had had the kindness to send me one of those long thread-like Worms (Gordius) found often in brooks and called Horse-Hairs by the common people. When I first received it, it was coiled up in a close roll at the bottom of the bottle, filled with fresh water, that contained it, and looked more like a little tangle of black sewing-silk than anything else. Wishing to unwind it, that I might examine its entire length, I placed it in a large china basin filled with water, and proceeded very gently to disentangle its coils, when I perceived that the animal had twisted itself around a bundle of its eggs, holding them fast in a close embrace. In the process of unwinding, the eggs dropped away and floated to a little distance. Having finally stretched it out to its full length, perhaps half a yard, I sat watching to see if this singular being that looked like a long black thread in the water would give any signs of life. Almost immediately it moved towards the bundle of eggs, and, having reached it, began to sew itself through and through the little white mass, passing one end of its body through it, and then returning to make another stitch, as it were, till the eggs were at last completely entangled again in an intricate net-work of coils. It seemed to me almost impossible that this care of copying could be the result of any instinct of affection in a creature of so low an organization, and I again separated it from the eggs, and placed them at a greater distance, when the same action was repeated. On trying the experiment a third time, the bundle of eggs had become loosened, and a few of them dropped off singly into the water. The efforts which the animal then made to recover the missing ones, winding itself round and round them, but failing to bring them into the fold with the rest, because they were too small, and evaded all efforts to secure them, when once parted from the first little compact mass, convinced me that there was a definite purpose in its attempts, and that even a being so low in the scale of animal existence has some dim consciousness of a relation to its offspring. I afterwards unwound also the mass of eggs, which, when coiled up as I first saw it, made a roll of white substance about the size of a coffee-bean, and found that it consisted of a string of eggs, measuring more than twelve feet in length, the eggs being held together by some gelatinous substance that cemented them and prevented them from falling apart. Cutting this string across, and placing a small section under the microscope, I counted on one surface of such a cut from seventy to seventy-five eggs; and estimating the entire number of eggs according to the number contained on such a surface, I found that there were not less than eight millions of eggs in the whole string. The fertility of these lower animals is truly amazing, and is no doubt a provision of Nature against the many chances of destruction to which these germs, so delicate and often microscopically small, must be exposed. The higher we rise in the Animal Kingdom, the more limited do we find the number of progeny, and the care bestowed upon them by the parents is in proportion to this diminution.
The next class in the type of Articulates is that of Crustacea, including Lobsters, Crabs, and Shrimps. It may seem at first that nothing can be more unlike a Worm than a Lobster; but a comparison of the class-characters shows that the same general plan controls the organization in both. The body of the Lobster is divided into a succession of joints or rings, like that of the Worm; and the fact that the front rings in the Lobster are soldered together, so as to make a stiff front region of the body, inclosing the head and chest, while only the hind rings remain movable, thus forming a flexible tail, does not alter in the least the general structure, which consists in both of a body built of articulated rings. The nervous swellings, which were evenly distributed through the whole body in the Worm, are more concentrated here, in accordance with the prevalent combination of the rings in two distinct regions of the body, the larger ones corresponding to the more important organs; but their relation to the rest of the organization, and their connection by nervous threads with each other, remain the same. The respiratory organs, which in most of the Worms were mere vesicles on the lower part of the sides of the body, are here more highly organized gills; but their general character and relation to other parts of the structure are unchanged, and in this respect the connection of the gills of Crustacea with their legs is quite significant. The alimentary canal consists of a single digestive cavity passing through the whole body, as in Worms, the anterior part of which is surrounded by a large liver. What is true of the Lobsters is true also, so far as class-characters are concerned, of all the Crustacea.
Highest in this type are the Insects, and among these I include Spiders and Centipedes as well as Winged Insects. It is true that the Centipedes have a long uniform body like Worms, and the Spiders have the body divided into two regions like the Crustacea, while the body in true Insects has three distinct regions, head, chest, and hind body; but notwithstanding this difference, both the former share in the peculiar class-character that places them with the Winged Insects in a separate group, distinct from all the other Articulates. We have seen that in the Worms the respiratory organs are mere vesicles, while in the Crustacea they are more highly organized gills; but in Centipedes, Spiders, and Winged Insects, the breathing-apparatus is aerial, consisting of air-holes on the sides of the body, connected with a system of tubes and vessels extending into the body and admitting air to all parts of it. In the Winged Insects this system is very elaborate, filling the body with air to such a degree as to render it exceedingly light and adapted to easy and rapid flight. The general arrangement of parts is the same in this class as in the two others, the typical character being alike in all.
We come now to the highest branch of the Animal Kingdom, that to which we ourselves belong,—the Vertebrates. This type is usually divided into four classes, Fishes, Reptiles, Birds, and Mammalia; and though many naturalists believe that it includes more, and I am myself of that opinion, I shall allude here only to the four generally admitted classes, as they are sufficient for my present purpose, and will serve to show the characters upon which classes are based. In a former paper I have explained in general terms the plan of structure of this type,—a backbone, with a bony arch above and a bony arch below, forming two cavities that contain all the systems of organs, the whole being surrounded by the flesh and skin. Now whether a body so constructed lie prone in the water, like a Fish,—or be lifted on imperfect legs, like a Reptile,—or be balanced on two legs, while the front limbs become wings, as in Birds,—or be raised upon four strong limbs terminating in paws or feet, as in Quadrupeds,—or stand upright with head erect, while the limbs consist of a pair of arms and a pair of legs, as in Man,—does not in the least affect that structural conception under which they are all included. Every Vertebrate has a backbone; every Vertebrate has a bony arch above that backbone and a bony arch below it, forming two cavities,—no matter whether these arches be of hard bone, or of cartilage, or even of a softer substance; every Vertebrate has the brain, the spinal marrow or spinal cord, and the organs of the senses in the upper cavity, and the organs of digestion, respiration, circulation, and reproduction in the lower one; every Vertebrate has four locomotive appendages built of the same bones and bearing the same relation to the rest of the organization, whether they be called pectoral and ventral fins, or legs, or wings and legs, or arms and legs. Notwithstanding the rudimentary condition of these limbs in some Vertebrates and their difference of external appearance in the different groups, they are all built of the same structural elements. These are the typical characters of the whole branch, and exist in all its representatives.
What now are the different modes of expressing this structural plan that lead us to associate certain Vertebrates together in distinct classes? Beginning with the lowest class,—the Fishes are cold-blooded, they breathe through gills, and they are egg-laying; in other words, though they have the same general structure as the other Vertebrates, they have a special mode of circulation, respiration, and reproduction. The Reptiles are also cold-blooded, though their system of circulation is somewhat more complicated than that of the Fishes; they breathe through lungs, though part of them retain their gills through life; and they lay eggs, but larger and fewer ones than the Fishes, diminishing in number in proportion to their own higher or lower position in their class. They also bestow greater care upon their offspring than most of the Fishes. The Birds are warm-blooded and air-breathing, having a double circulation; they are egg-laying like the two other classes, but their eggs are comparatively few in number, and the young are hatched by the mother and fed by the parent birds till they can provide for themselves.
The Mammalia are also warm-blooded and breathe through lungs; but they differ from all other Vertebrates in their mode of reproduction, bringing forth living young which they nurse with milk. Even in the lowest members of this highest group of the Vertebrates, at the head of which stands Man himself, looking heavenward it is true, but nevertheless rooted deeply in the Animal Kingdom, we have the dawning of those family relations, those intimate ties between parents and children, on which the whole social organization of the human race is based. Man is the crowning work of God on earth; but though so nobly endowed, we must not forget that we are the lofty children of a race whose lowest forms lie prostrate within the water, having no higher aspiration than the desire for food; and we cannot understand the possible degradation and moral wretchedness of Man, without knowing that his physical nature is rooted in all the material characteristics that belong to his type and link him even with the Fish. The moral and intellectual gifts that distinguish him from them are his to use or to abuse; he may, if he will, abjure his better nature and be Vertebrate more than Man. He may sink as low as the lowest of his type, or he may rise to a spiritual height that will make that which distinguishes him from the rest far more the controlling element of his being than that which unites him with them.