Kitabı oku: «The Atlantic Monthly, Volume 10, No. 57, July, 1862», sayfa 12

METHODS OF STUDY IN NATURAL HISTORY

It may seem to some of my readers that I have wandered from my subject and forgotten the title of these articles, which purport to be a series of papers on "Methods of Study in Natural History." But some idea of the progress of Natural History, of its growth as a science, of the gradual evolving of general principles out of a chaotic mass of facts, is a better aid to the student than direct instruction upon special modes of investigation; and it is with the intention of presenting the study of Natural History from this point of view that I have chosen my title.

I have endeavored thus far to show how scientific facts have been systematized so as to form a classification that daily grows more true to Nature, in proportion as its errors are corrected by a more intimate acquaintance with the facts; but I will now attempt a more difficult task, and try to give some idea of the mental process by which facts are transformed into scientific truth. I fear that the subject may seem very dry to my readers, and I would again ask their indulgence for details absolutely essential to my purpose, but which would indeed be very wearisome, did they not lead us up to an intelligent and most significant interpretation of their meaning.

I should be glad to remove the idea that science is the mere amassing of facts. It is true that scientific results grow out of facts, but not till they have been fertilized by thought The facts must be collected, but their mere accumulation will never advance the sum of human knowledge by one step;—it is the comparison of facts and their transformation into ideas that lead to a deeper insight into the significance of Nature. Stringing words together in incoherent succession does not make an intelligible sentence; facts are the words of God, and we may heap them together endlessly, but they will teach us little or nothing till we place them in their true relations and recognize the thought that binds them together as a consistent whole.

I have spoken of the plans that lie at the foundation of all the variety of the Animal Kingdom as so many structural ideas which must have had an intellectual existence in the Creative Conception independently of any special material expression of them. Difficult though it be to present these plans as pure abstract formulae, distinct from the animals that represent them, I would nevertheless attempt to do it, in order to show how the countless forms of animal life have been generalized into the few grand, but simple intellectual conceptions on which all the past populations of the earth as well as the present creation are founded. In such attempts to divest the thought of its material expression, especially when that expression is multiplied in such thousand-fold variety of form and color, our familiarity with living animals is almost an obstacle to our success. For I shall hardly be able to allude to the formula of the Radiates, for instance,—the abstract idea that includes all the structural possibilities of that division of the Animal Kingdom,—without recalling to my readers a Polyp or a Jelly-Fish, a Sea-Urchin or a Star-Fish. Neither can I present the structural elements of the Mollusk plan, without reminding them of an Oyster or a Clam, a Snail or a Cuttle-Fish,—or of the Articulate plan, without calling up at once the form of a Worm, a Lobster, or an Insect,—or of the Vertebrate plan, without giving it the special character of Fish, Reptile, Bird, or Mammal. Yet I insist that all living beings are but the different modes of expressing these formulae, and that all animals have, within the limits of their own branch of the Animal Kingdom, the same structural elements, though each branch is entirely distinct. If this be true, and if these organic formulae have the precision of mathematical formulae, with which I have compared them, they should be susceptible of the same tests.

The mathematician proves the identity of propositions that have the same mathematical value and significance by their convertibility. If they have the same mathematical quantities, it must be possible to transform them, one into another, without changing anything that is essential in either. The problem before us is of the same character. If, for instance, all Radiates, be they Sea-Anemones, Jelly-Fishes, Star-Fishes, or Sea-Urchins, are only various modes of expressing the same organic formula, each having the sum of all its structural elements, it should be possible to demonstrate that they are reciprocally convertible. This is actually the case, and I hope to be able to convince my readers that it is no fanciful theory, but may be demonstrated as clearly as the problems of the geometer. The naturalist has his mathematics, as well as the geometer and the astronomer; and if the mathematics of the Animal Kingdom have a greater flexibility than those of the positive sciences, and are therefore not so easily resolved into their invariable elements, it is because they have the freedom and pliability of life, and evade our efforts to bring all their external variety within the limits of the same structural law which nevertheless controls and includes them all.

I wish that I could take as the illustration of this statement animals with whose structure the least scientific of my readers might be presumed to be familiar; but such a comparison of the Vertebrates, showing the identity and relation of structural elements throughout the Branch, or even in any one of its Classes, would be too extensive and complicated, and I must resort to the Radiates,—that branch of the Animal Kingdom which, though less generally known, has the simplest structural elements.

I will take, then, for the further illustration of my subject, the Radiates, and especially the class of Echinoderms, Star-Fishes, Sea-Urchins, and the like, both in the fossil and the living types; and though some special description of these animals is absolutely essential, I will beg my readers to remember that the general idea, and not its special manifestations, is the thing I am aiming at, and that, if we analyze the special parts characteristic of these different groups, it is only that we may resolve them back again into the structural plan that includes them all.

I have already in a previous article named the different Orders of this Class in their relative rank, and have compared the standing of the living ones, according to the greater or less complication of their structure, with the succession of the fossil ones. Of the five Orders, Beches-de-Mer, Sea-Urchins, Star-Fishes, Ophiurans, and Crinoids,—or, to name them all according to their scientific nomenclature, Holothurians, Echinoids, Asteroids, Ophiurans, and Crinoids,—the last-named are lowest in structure and earliest in time. Cuvier was the first naturalist who detected the true nature of the Crinoids, and placed them where they belong in the classification of the Animal Kingdom. They had been observed before, and long and laborious investigations had been undertaken upon them, but they were especially baffling to the student, because they were known only in the fossil condition from incomplete specimens; and though they still have their representatives among the type of Echinoderms as it exists at present, yet, partly owing to the rarity of the living specimens and partly to the imperfect condition of the fossil ones, the relation between them was not recognized. The errors about them certainly did not arise from any want of interest in the subject among naturalists, for no less than three hundred and eighty different authors have published their investigations upon the Crinoids, and the books that have been printed about these animals, many of which were written long before their animal nature was suspected, would furnish a library in themselves. The ancients knew little about them. The only one to be found in the European seas resembles the Star-Fish closely, and they called it Asterias; but even Aristotle was ignorant of its true structural relations, and alludes only to its motion and general appearance. Some account of the gradual steps by which naturalists have deciphered the true nature of these lowest Echinoderms and their history in past times may not be without interest, and is very instructive as showing bow such problems may be solved.

In the sixteenth century some stones were found bearing the impression of a star on their surface. They received the name of Trochites, and gave rise to much discussion. Naturalists puzzled their brains about them, called them star-shaped crystals, aquatic plants, corals; and to these last Linnaeus himself, the great authority of the time on all such questions, referred them. Beside these stony stars, which were found in great quantities when attention was once called to them, impressions of a peculiar kind had been observed in the rocks, resembling flowers on long stems, and called "stone lilies" naturally enough, for their long, graceful stems, terminating either in a branching crown or a closer cup, recall the lily tribe among flowers. The long stems of these seeming lilies are divided transversely at regular intervals;—the stem is easily broken at any of these natural divisions, and on each such fragment is stamped a star-like impression resembling those found upon the loose stones or Trochites.

About a century ago, Guettard the naturalist described a curious specimen from Porto Rico, so similar to these fossil lilies of the rocks that he believed they must have some relation to each other. He did not detect its animal nature, but from its long stem and branching crown he called it a marine palm. Thus far neither the true nature of the living specimen, nor of the Trochites, nor of the fossil lilies was understood, but it was nevertheless an important step to have found that there was a relation between them. A century passed away, and Guettard's specimen, preserved at the Jardin des Plantes, waited with Sphinx-like patience for the man who should solve its riddle.

Cuvier, who held the key to so many of the secrets of Nature, detected at last its true structure; he pronounced it to be a Star-Fish with a stem, and at once the three series of facts respecting the Trochites, the fossil lilies, and Guettard's marine palm assumed their true relation to each other. The Troehites were recognized as simply the broken portions of the stem of some of these old fossil Crinoids, and the Crinoids themselves were seen to be the ancient representatives of the present Comatulae and Star-Fishes with stems. So is it often with the study of Nature; many scattered links are collected before the man comes who sees the connection between them and speaks the word that reconstructs the broken chain.

I will begin my comparison of all Echinoderms with an analysis of the Star-Fishes and Sea-Urchins, because I think I can best show the identity of parts between them, notwithstanding the difference in their external form; the Sea-Urchins having always a spherical body, while the Star-Fishes are always star-shaped, though in some the star is only hinted at, sketched out, as it were, in a simply pentagonal outline, while in others the indentations between the rays are very deep, and the rays themselves so intricate in their ramifications as to be broken up into a complete net-work of branches. But under all this variety of outline, our problem remains always the same: to build with the same number of pieces a star and a sphere, having the liberty, however, of cutting the pieces differently and changing their relative proportions. Let us take first the Sea-Urchin and examine in detail all parts of its external structure. I shall say nothing of the internal structure of any of these animals, because it does not affect the comparison of their different forms and the external arrangement of parts, which is the subject of the present article.

On the lower side is the mouth, and we may call that side and all the parts that radiate from it the oral region. On the upper side is a small area to which the parts converge, and which, from its position just opposite the so-called mouth or oral opening, we may call the ab-oral region. I prefer these more general terms, because, if we speak of the mouth, we are at once reminded of the mouth in the higher animals, and in this sense the word, as applied to the aperture through which the Sea-Urchins receive their food, is a misnomer. Very naturally the habit has become prevalent of naming the different parts of animals from their function, and not from their structure; and in all animals the aperture through which food enters the body is called the mouth, though there is not the least structural relation between the organs so designated, except within the limits of each different branch or division. To speak of these opposite regions in the Sea-Urchin as the upper and lower sides would equally mislead us, since, as we have seen, there is, properly speaking, no above and below, no right and left sides, no front and hind extremities in these animals, all parts being evenly distributed around a vertical axis. I will, therefore, although it has been my wish to avoid technicalities as much as possible in these papers, make use of the unfamiliar terms oral and ab-oral regions, to indicate the mouth with the parts diverging from it and the opposite area towards which all these parts converge.²⁸

[Illustration: Sea-Urchin seen from the oral side, showing the zones with the spines and suckers; for the ab-oral side, on the summit of which the zones unite, see February Number, p. 216.]

The whole surface of the animal is divided by zones,—ten in number, five broader ones alternating with five narrower ones. The five broad zones are composed of large plates on which are the most prominent spines, attached to tubercles that remain on the surface even when the spines drop off after death, and mark the places where the spines have been. The five small zones are perforated with regular rows of holes, and through these perforations pass the suckers or water-tubes which are their locomotive appendages. For this reason these narrower zones are called the ambulacra, while the broader zones intervening between them and supporting the spines are called the interambulacra. Motion, however, is not the only function of these suckers; they are subservient also to respiration and circulation, taking in water, which is conveyed through them into various parts of the body.

[Illustration: Portion of Sea-Urchin representing one narrow zone with a part of the broad zones on either side and the ab-oral area on the summit.]

The oral aperture is occupied by five plates, which may be called jaws, remembering always that here again this word signifies the function, and not the structure usually associated with the presence of jaws in the higher animals; and each of these jaws or plates terminates in a tooth. Even the mode of eating in these animals is controlled by their radiate structure; for these jaws, evenly distributed about the circular oral aperture, open to receive the prey and then are brought together to crush it, the points meeting in the centre, thus working concentrically, instead of moving up and down or from right to left, as in other animals. From the oral opening the ten zones diverge, spreading over the whole surface, like the ribs on a melon, and converging in the opposite direction till they meet in the small space which we have called the ab-oral region opposite the starting-point.

Here the broad zones terminate in five large plates differing somewhat from those that form the zones in other parts of the body, and called ovarian plates, because the eggs pass out through certain openings in them; while the five narrow zones terminate in five small plates on each of which is an eye, making thus five eyes alternating with five ovarian plates. The centre of this area containing the ovarian plates and the visual plates is filled up with small movable plates closing the space between them. I should add that one of the five ovarian plates is larger than the other four, and has a peculiar structure, long a puzzle to naturalists. It is perforated with minute holes, forming an exceedingly delicate sieve, and this is actually the purpose it serves. It is, as it were, a filter, and opens into a canal which conducts water through the interior of the body; closed by this sieve on the outside, all the water that passes into it is purified from all foreign substances that might be injurious to the animal, and is thus fitted to pass into the water-system, from which arise the main branches leading to the minute suckers which project through the holes in the narrow zones of plates.

[Illustration: Star-Fish from the ab-oral side.]

Now in order to transform theoretically our Sea-Urchin into a Star-Fish, what have we to do? Let the reader imagine for a moment that the small ab-oral area closing the space between the ovarian plates and the eye-plates is elastic and may be stretched out indefinitely; then split the five broad zones along the centre and draw them down to the same level with the mouth, carrying the ovarian plates between them. We have then a star, just as, dividing, for instance, the peel of an orange into five compartments, leaving them, of course, united at the base, then stripping it off and spreading it out flat, we should have a five-rayed star.

[Illustration: One arm of Star-Fish from the oral side.]

But in thus dividing the broad zones of the Sea-Urchins, we leave the narrow zones in their original relation to them, except that every narrow zone, instead of being placed between two broad zones, has now one-half of each of the zones with which it alternated in the Sea-Urchin on either side of it and lies between them. The adjoining wood-cut represents a single ray of a Star-Fish, drawn from what we call its lower side or the oral side. Along the centre of every such ray, diverging from the central opening or the mouth, we have a furrow, corresponding exactly to the narrower zones of the Sea-Urchin. It is composed of comparatively small perforated plates through which pass the suckers or locomotive appendages. On either side of the furrows are other plates corresponding to the plates of the broad zones in the Sea-Urchin. Where shall we look for the five eyes? Of course, at the tip of every ray; exactly where they were when the rays were drawn up to form the summit of a sphere, so that the eyes, which are now at their extremities, were clustered together at their point of meeting. Where shall we look for the ovarian plates? At each angle of the five rays, because, when the broad zones of which they formed the summit were divided, they followed the split, and now occupy the place which, though it seems so different on the surface of the Star-Fish, is nevertheless, relatively to the rest of the body, the same as they occupied in the Sea-Urchin. Assuming, as we premised, that the central area of the ab-oral region, forming the space between the plates at the summit of the zones in the Sea-Urchin, is elastic, it has stretched with the spreading out of the zones, following the indentation between the rays, and now forms the whole upper surface of the body. All the internal organs of the animal lie between the oral and ab-oral regions, just as they did in the Sea-Urchin, only that in the Star- Fish these regions are coequal in extent, while in the Sea-Urchin the ab-oral region is very contracted, and the oral region with the parts belonging to it occupies the greater part of its surface.

Such being the identity of parts between a Star-Fish and a Sea-Urchin, let us see now how the Star-Fish may be transformed into the Pedunculated Crinoid, the earliest representative of its Class, or into a Comatula, one of the free animals that represent the Crinoids in our day.

[Illustration: Crinoid with branching crown; oral side turned upward.]

We have seen that in the Sea-Urchins the ab-oral region is very contracted, the oral region and the parts radiating from it and forming the sides being the predominant features in the structure; and we shall find, as we proceed in our comparison, that the different proportion of these three parts, the oral and ab-oral regions and the sides, determines the different outlines of the various Orders in this Class. In the Sea-Urchin the oral region and the sides are predominant, while the ab-oral region is very small. In the Star-Fish, the oral and ab-oral regions are brought into equal relations, neither preponderating over the other, and the sides are compressed, so that, seen in profile, the outline of the Star-Fish is that of a slightly convex disk, instead of a sphere, as in the Sea-Urchin. But when we come to the Crinoids, we find that the great preponderance of the ab-oral region determines all that peculiarity of form that distinguishes them from the other Echinoderms, while the oral region is comparatively insignificant. The ab-oral region in the Crinoid rises to form a sort of cup-like or calyx-like projection. The plates forming it, which in the Star-Fish or the Sea-Urchin are movable, are soldered together so as to be perfectly immovable in the Crinoid. Let this seeming calyx be now prolonged into a stem, and we see at once how striking is the resemblance to a flower; turn it downwards, an attitude which is natural to these Crinoids, and the likeness to a drooping lily is still more remarkable The oral region, with the radiating ambulacra, is now limited to the small flat area opposite the juncture of the stem with the calyx; and whether it stretches out to form long arms, or is more compact, so as to close the calyx like a cup, it seems in either case to form a flower-like crown. In these groups of Echinoderms the interambulacral plates are absent; there are no rows of plates of a different kind alternating with the ambulacral ones, as in the Sea-Urchins and the Star-Fishes, but the ab-oral region closes immediately upon the ambulacra.

It seems a contradiction to say, that, though these Crinoids were the only representatives of their Class in the early geological ages, while it includes five Orders at the present time, Echinoderms were as numerous and various then as now. But, paradoxical as it may seem, this is nevertheless true, not only for this Class, but for many others in the Animal Kingdom. The same numerical proportions, the same richness and vividness of conception were manifested in the early creation as now; and though many of the groups were wanting that are most prominent in modern geological periods, those that existed were expressed in such endless variety that the Animal Kingdom seems to have been as full then as it is to-day. The Class of the Echinoderms is one of the most remarkable instances of this. In the Silurian period, the Crinoids stood alone; there were neither Ophiurans, Asteroids, Echinoids, nor Holothurians; and yet in one single locality, Lockport, in the State of New York, over an area of not more than a few square miles, where the Silurian deposits have been carefully examined, there have been found more different Species of Echinoderms than are living now along our whole Atlantic coast from Maine to Florida.

There is nothing more striking in these early populations of the earth than the richness of the types. It would seem as if, before the world was prepared for the manifold existences that find their home here now, when organic life was limited by the absence of many of the present physical conditions, the whole wealth of the Creative Thought lavished itself upon the forms already introduced upon the globe. After thirty years' study of the fossil Crinoids, I am every day astonished by some new evidence of the ingenuity, the invention, the skill, if I may so speak, shown in varying this single pattern of animal life. When one has become, by long study of Nature, in some sense intimate with the animal creation, it is impossible not to recognize in it the immediate action of thought, and even to specialize the intellectual faculties it reveals. It speaks of an infinite power of combination and analysis, of reminiscence and prophecy, of that which has been in eternal harmony with that which is to be; and while we stand in reverence before the grandeur of the Creative Conception as a whole, there breaks from it such lightness of fancy, such richness of invention, such variety and vividness of color, nay, even the ripple of mirthfulness,—for Nature has its humorous side also,—that we lose our grasp of its completeness in wonder at its details, and our sense of its unity is clouded by its marvellous fertility. There may seem to be an irreverence in thus characterizing the Creative Thought by epithets which we derive from the exercise of our own mental faculties; but it is nevertheless true, that, the nearer we come to Nature, the more does it seem to us that all our intellectual endowments are merely the echo of the Almighty Mind, and that the eternal archetypes of all manifestations of thought in man are found in the Creation of which he is the crowning work.

In no group of the Animal Kingdom is the fertility of invention more striking than in the Crinoids. They seem like the productions of one who handles his work with an infinite ease and delight, taking pleasure in presenting the same thought under a thousand different aspects. Some new cut of the plates, some slight change in their relative position is constantly varying their outlines, from a close cup to an open crown, from the long pear-shaped oval of the calyx in some to its circular or square or pentagonal form in others. An angle that is simple in one projects by a fold of the surface and becomes a fluted column in another; a plate that was smooth but now has here a symmetrical figure upon it drawn in beaded lines; the stem which is perfectly unbroken in one, except by the transverse divisions common to them all, in the next puts out feathery plumes at every such transverse break. In some the plates of the stem are all rigid and firmly soldered together; in others they are articulated upon each other in such a manner as to give it the greatest flexibility, and allow the seeming flower to wave and bend upon its stalk. It would require an endless number of illustrations to give even a faint idea of the variety of these fossil Crinoids. There is no change that the fancy can suggest within the limits of the same structure that does not find expression among them. Since I have become intimate with their wonderful complications, I have sometimes amused myself with anticipating some new variation of the theme, by the introduction of some undescribed structural complication, and then seeking for it among the specimens at my command, and I have never failed to find it in one or other of these ever-changing forms.

The modern Crinoid without stem, or the Comatula, though agreeing with the ancient in all the essential elements of structure, differs from it in some specific features. It drops its stem when full-grown, though the ab-oral region still remains the predominant part of the body and retains its cup-like or calyx-like form. The Comatulae are not abundant, and though represented by a number of Species, yet the type as it exists at present is meagre in comparison to its richness in former times. Indeed, this group of Echinoderms, which in the earliest periods was the exponent of all its kind, has dwindled gradually, in proportion as other representatives of the Class have come in, and there exists only one species now, the Pentacrinus of the West Indies, which retains its stem in its adult condition. It is a singular fact, to which I have before alluded, and which would seem to have especial reference to the maintenance of the same numeric proportions in all times, that, while a Class is represented by few types, those types are wonderfully rich and varied, but in proportion as other expressions of the same structure are introduced, the first dwindle, and, if they do not entirely disappear, become at least much less prominent than before.

[Illustration: Ophiuran; showing one ray from the oral side.]

There remain only two other Orders to be considered, the Ophiurans and the Holothurians. The Ophiurans approach the Crinoids more nearly than any other group of Echinoderms, and in our classifications are placed next above them. In them the ab-oral region, which has such a remarkable predominance in the Crinoid, has become depressed; it no longer extends into a stem, nor does it even rise into the calyx-like or cup-like projection so characteristic of the Crinoids,—though, when the animal is living, the ab-oral side of the disk is still quite convex. The disk in the Ophiurans is small in comparison to the length of the arms, and perfectly circular; it does not merge gradually in the arms as in the Star-Fish, but the arms start abruptly from its periphery. In these, as in the Crinoids, the interambulacral plates are absent, and the interambulacral spaces are filled by an encroachment of the ab-oral region upon them. There is an infinite variety and beauty both of form and color in these Sea-Stars. The arms frequently measure many times the diameter of the whole disk, and are so different in size and ornamentation in the different Species that at first sight one might take them for animals entirely distinct from each other. In some the arms are comparatively short and quite simple,—in others they are very long, and may be either stretched to their full length or partly contracted to form a variety of graceful curves; in some they are fringed all along the edges,—in others they are so ramified that every arm seems like a little bush, as it were, and, intertwining with each other, they make a thick network all around the animal. In the geological succession, these Ophiurans follow the Crinoids, being introduced at about the Carboniferous period, and perhaps earlier. They have had their representatives in all succeeding times, and are still very numerous in the present epoch.