Kitabı oku: «The Atlantic Monthly, Volume 09, No. 53, March, 1862», sayfa 8

According to Oken, the Radiates, the lowest type of the Animal Kingdom, embody digestion. They all represent a stomach, whether it is the simple sac of the Polyps, or the cavity of the Acalephs, with its radiating tubes traversing the gelatinous mass of the body, or the cavity and tubes of the Echinoderms, inclosed within walls of their own.

The Mollusks represent circulation; and his division of this type into classes, according to what he considers the higher or lower organization of the heart, agrees with the ordinary division into Acephala, Gasteropoda, and Cephalopoda.

The Articulates are the respiratory animals in this classification: they represent respiration. The Worms, breathing, as he asserts, through the whole surface of the skin, without special breathing organs, are the lowest; the Crustacea, with gills, or aquatic breathing organs, come next; and he places the Insects highest, with their branching tracheæ, admitting air to all parts of the body. The Vertebrates, or Flesh Animals, with their four classes, represent the Bones, the Muscles, the Nerves, and the Organs of Sense.

This theory, according to which there are as many great divisions as there are structural systems or combinations of systems in the Animal Kingdom, seemed natural and significant, and there was something attractive in the idea that man represents, as it were, the synthetic combination of all these different systems. Oken also, in his exposition of his mode of classification, showed an insight into the structure and relations of animals that commended it to the interest of all students of Nature, and entitles him to their everlasting gratitude. Nevertheless, his theory fails, when it is compared with facts. For instance, there are many Worms that have no respiration through the skin, while his appreciation of the whole class is founded on that feature; and in his type representing circulation, the Mollusks, there are those that have no heart at all. It would carry me too far into scientific details, were I to explain all the points at which this celebrated classification fails. Suffice it to say that there is no better proof of the discrepancy between the system and the facts than the constant changes in the different editions of Oken’s own works and in the publications of his followers founded upon his views, showing that they were themselves conscious of the shifting and unstable character of their scientific ground.

VI

What, then, is the relation of these larger groups to each other, if they do not stand in a connected series from the lowest to the highest? How far are each of the branches and each of the classes superior or inferior one to another? All agree, that, while Vertebrates stand at the head of the Animal Kingdom, Radiates are lowest. There can be no doubt upon this point; for, while the Vertebrate plan, founded upon a double symmetry, includes the highest possibilities of animal organization, there is a certain monotony of structure in the Radiate plan, in which the body is divided into a number of identical parts, bearing definite relations to a central vertical axis. But while all admit that Vertebrates are highest and Radiates lowest, how do the Articulates and Mollusks stand to these and to each other? To me it seems, that, while both are decidedly superior to the Radiates and inferior to the Vertebrates, we cannot predicate absolute superiority or inferiority of organization of either of these groups as compared with each other; they stand on one structural level, though with different tendencies,—the body in Mollusks having always a soft, massive, concentrated character, with great power of contraction and dilatation, while the body in Articulates has nothing of this compactness and concentration, but on the contrary is usually marked by a conspicuous external display of limbs and other appendages, and by a remarkable elongation of the body,—that feature characterized by Baer when he called them the Longitudinal type. There is in the Articulates an extraordinary tendency toward outward expression, singularly in contrast to the soft, contractile bodies of the Mollusks. We need only remember the numerous Insects with small bodies and enormously long wings, or the Spiders with little bodies and long legs, or the number and length of the claws in the Lobsters and Crabs, as illustrations of this statement for the Articulates, while the soft compact body of the Oyster or of the Snail is equally characteristic of the Mollusks; and though it may seem that this assertion cannot apply to the highest class of Mollusks, the Cephalopoda, including the Cuttle-Fishes with their long arms or feelers, yet even these conspicuous appendages have considerable power of contraction and dilatation, and in the Nautili may even be drawn completely within the shell. If this view be correct, these two types occupy an intermediate position between the highest and the lowest divisions of the Animal Kingdom, but are on equal ground when compared with each other.

But is there a transition from Radiates to Mollusks, or from Articulates to Vertebrates, or from any one of these divisions into any other? Let us first consider the classes as they stand within their divisions. We have seen that there are three classes of Radiates,—Polyps, Acalephs, and Echinoderms; three classes of Mollusks,—Acephala, Gasteropoda, and Cephalopoda; three classes of Articulates,—Worms, Crustacea, and Insects; and, according to the usually accepted classification, four classes of Vertebrates,—Fishes, Reptiles, Birds, and Mammalia. If there is indeed a transition between all these classes, it must become clear to us, when we have accurately interpreted their relative standing. Taking first the lowest branch, how do the classes stand within the limits of the type of Radiates? I think I have said enough of these different classes to show that Polyps as a whole are inferior to Acalephs as a whole, and that Acalephs as a whole are inferior to Echinoderms as a whole. But if they are linked together as a connected series, then the lowest Acaleph should stand next in structure above the highest Polyp, and the lowest Echinoderm next above the highest Acaleph. So far from this being the case, there are, on the contrary, many Acalephs which, in their specialization, are unquestionably lower in the scale of life than some Polyps, while there are some Echinoderms lower in the same sense than many Acalephs. This remark applies equally to the classes within the other types; they stand, as an average, relatively to each other, lower and higher, but considered in their diversified specification, there are some members of the higher classes that are inferior in organization to some members of the lower classes. The same is true of the great divisions as compared with each other. Instead of the highest Radiates being always lower in organization than the lowest Mollusks, there are many Star-Fishes and Sea-Urchins higher in organization than some Mollusks; and so when we pass from this branch to the Articulates, if we assume for the moment, as some naturalists believe, that the Mollusks are the inferior type, the Cuttle-Fishes are certainly very superior animals to most of the Worms; and passing from Articulates to Vertebrates, not only are there Insects of a more complex organization than the lowest Fishes, but we bring together two kinds of animals so remote from each other in structure that the wildest imagination can scarcely fancy a transition between them. A comparison may make my meaning clearer as to the relative standing of these groups. The Epic Poem is a higher order of composition than the Song,—yet we may have an Epic Poem which, from its inferior mode of execution, stands lower than a Song that is perfect of its kind. So the plan of certain branches is more comprehensive and includes higher possibilities than that of others, while at the same time there may be species in which the higher plan is executed in so simple a manner that it places their organization below some more highly developed being built on a lower plan. It is a poor comparison, because everything that God has made is perfect of its kind and in its place, though relatively lower or higher; yet it is only by comparison of what is after all akin,—of mind with mind,—even though so far apart as the works of the divine and the human reason, that we may arrive at some idea, however dim, of the mental operations of the Creative Intellect.

It is, then, in their whole bulk that any one of these groups is above any other. We may represent the relative positions of the classes by a diagram in which each successive class in every type starts at a lower point than that at which the preceding class closes. Taking the Polyps as the lowest class of Radiates, for instance, its highest animals rise above the lowest members of the Acalephs, but then the higher members of the class of Acalephs reach a point far above any of the Polyps,—and so on.

If this view be correct, it sets aside the possibility of any uninterrupted series based on absolute superiority or inferiority of structure, on which so much ingenuity and intellectual power have been wasted.

But it is not merely upon the structural relations established between these groups by anatomical features in the adult that we must decide this question. We must examine it also from the embryological point of view. Every animal in its growth undergoes a succession of changes: is there anything in these changes implying a transition of one type into another? Baer has given us the answer to this question. He has shown that there are four distinct modes of development, as well as four plans of structure; and though we have seen that higher animals of one class pass through phases of growth in which they transiently resemble lower animals of the same class, yet each one of these four modes of development is confined within the limits of the type, and a Vertebrate never resembles, at any stage of its growth, anything but a Vertebrate, or an Articulate anything but an Articulate, or a Mollusk anything but a Mollusk, or a Radiate anything but a Radiate.

Yet, although there is no embryological transition of one type into another, the gradations of growth within the limits of the same type and the same class, already alluded to, are very striking throughout the Animal Kingdom. There are periods in the development of the germs of the higher members of all the types, when they transiently resemble in their general outline the lower representatives of the same type, just as we have seen that the higher orders of one class pass through stages of development in which they transiently resemble lower orders of the same class. This gradation of growth corresponds to the gradation of rank in adult animals, as established upon comparative complication of structure. For instance, according to their structural character, all naturalists have placed Fishes lowest in the scale of Vertebrates. Now all the higher Vertebrates have a Fish-like character at first, and pass successively through phases in which they vaguely resemble other lower forms of the same type before they assume their own characteristic form; and this is equally true of the other great divisions, so that the history of the individual is, in some sort, the history of its type.

There is still another aspect of this question,—that of time. If neither the gradation of structural rank among adult animals, nor the gradation of growth in their embryological development gives us any evidence of a transition between types, does not the sequence of animals in their successive introduction upon the globe afford any proof of such a connection? In this relation, I must briefly allude to the succession of geological formations that compose the crust of our globe. The limits of this article will not allow me to enter at any length into the geological details connected with this question; but I will, in the most cursory manner, give a sketch of the great geological periods, as generally accepted now by geologists. The first of these periods has been called the Azoic or lifeless period, because it is the only one that contains no remains of organic life, and it is therefore supposed that at that early stage of the world’s history the necessary conditions for the maintenance of animals and plants were not yet established. After this, every great geological period that follows has been found to be characterized by a special set of animals and plants, differing from all that follow and all that precede it, till we arrive at our own period, when Man, with the animals and plants that accompany him on earth, was introduced.

There is, then, an order of succession in time among animals; and if there has been any transition between types and classes, any growth of higher out of lower forms, it is here that we should look for the evidence of it. According to this view, we should expect to find in the first period in which organic remains are found at all only the lowest type, and of that type only the lowest class, and, indeed, if we push the theory to its logical consequences, only the lowest forms of the lowest class. What are now the facts? This continent affords admirable opportunities for the investigation of this succession, because, in consequence of its mode of formation, we have, in the State of New York, a direct, unbroken sequence of all the earliest geological deposits.

The ridge of low hills, called the Laurentian Hills, along the line of division between Canada and the States was the first American land lifted above the ocean. That land belongs to the Azoic period, and contains no trace of life. Along the base of that range of hills lie the deposits of the next great geological period, the Silurian; and the State of New York, geologically speaking, belongs almost entirely to this Silurian period, with its lowest Taconic division, and the Devonian period, the third in succession of these great epochs. I need hardly remind those of my readers who have travelled through New York, and have visited Niagara or Trenton, or, indeed, any of the localities where the broken edges of the strata expose the buried life within them, how numerous this early population of the earth must have been. No one who has held in his hand one of the crowded slabs of sand—or lime-stone, full of Crustacea, Shells, and Corals, from any of the old Silurian or Devonian beaches which follow each other from north to south across the State of New York, can suppose that the manifestation of life was less multitudinous then than now. Now, what does this fossil creation tell us? It says this: that, in the Silurian period, the first in which organic life is found at all, there were the three classes of Radiates, the three classes of Mollusks, two of the classes of Articulates, and one class of Vertebrates. In other words, at the dawn of life on earth, the plan of the animal creation with its four fundamental ideas was laid out,—Radiates, Mollusks, Articulates, and Vertebrates were present at that first representation of life upon our globe. If, then, all the primary types appeared simultaneously, one cannot have grown out of another,—they could not be at once contemporaries and descendants of each other.

The diagram on the opposite page represents the geological periods in their regular succession, and the approximate time at which all the types and all the classes of the Animal Kingdom were introduced; for there is still some doubt as to the exact period of the introduction of several of the classes, though all geologists are agreed respecting them, within certain limits, not very remote from each other, according to geological estimates of time.

If such discussions were not inappropriate here from their technical character, I think I could show upon combined geological and zoological evidence that the classes which are not present with the others at the beginning, such as Insects among Articulates, or Reptiles, Birds, and Mammalia among Vertebrates, are always introduced at the time when the conditions essential to their existence are established,—as, for instance, Reptiles, at the period when the earth was not fully redeemed from the waste of waters, and extensive marshes afforded means for the half-aquatic, half-terrestrial life even now characteristic of all our larger Reptiles, while Insects, so dependent on vegetable growth, make their appearance with the first forests; so that we need not infer, because these and other classes come in after the earlier ones, that they are therefore a growth out of them, since it is altogether probable that they would not be created till the conditions necessary for their maintenance on earth were established. From a merely speculative point of view it seems to me natural to suppose that the physical and the organic world have progressed together, and that there is a direct relation between the successive creations and the condition of the earth at the time of those creations. We know that all the beings of the Silurian and Devonian periods were marine; the land, so far as it existed in their time, was a great beach, and along those shores, wherever any part of the continents was lifted above the level of the waters, the Silurian and Devonian animals lived. Later, in the marshes and the fern-forests of the Carboniferous period, Reptiles and Insects found their place; and only when the earth was more extensive, when marshes had become dry land, when islands had united to form continents, when mountain-chains had been thrown up to make the inequalities of the surface, were the larger quadrupeds introduced, to whose mode of existence all these circumstances are important accessories.

But while all the types and most of the classes were introduced upon the earth simultaneously at the beginning, these types and classes have nevertheless been represented in every great geological period by different sets or species of animals. In this sense, then, there has been a gradation in time among animals, and every successive epoch of the world’s physical history has had its characteristic population. We have found that there is a correspondence between the gradation of structural complication among adult animals as known to us to-day, which we may call the Series of Rank, and the gradation of embryological changes in the same animals, which we may call the Series of Growth; and there is also a correspondence between these two series and the order of succession in time, that establishes a certain gradation in the introduction of animals upon earth, and which we may call the Series of Time. Take as an illustration the class of Echinoderms. The first representatives of this class were a sort of Star-Fishes on stems; then were introduced animals of the same order without stems; in later periods come in the true Star-Fishes and Sea-Urchins; and the highest order of the class, the Holothurians, are introduced only in the present geological epoch. Compare now with this the ordinal division of the class as it exists today. The present representative of those earliest Echinoderms on stems is an animal that upon structural evidence stands lowest in the class; next above it are the Comatulæ, corresponding to the early Echinoderms without stems; next in our classification are the Star-Fishes and Sea-Urchins; and the Holothurians stand highest, on account of certain structural features that place them at the head of their class. The Series of Time and the Series of Rank, then, accord perfectly, and investigations of the embryological development of these animals have shown that the higher Echinoderms pass through changes in the egg that indicate the same kind of gradation, for the young in some of them have a stem which is gradually dropped, and their successive phases of development recall the adult forms of the lower orders. Take as another illustration the class of Polyps. First in time we find a kind of Polyp Coral, one among the early Reef-Builders, who built their myriad lives into the solid crust of our globe then as their successors do now. These old Corals have their representatives among the present Polyps, and from their structure they are placed lowest in their class, while the embryological development of the higher ones recalls in the younger condition of the germ the same peculiar character. I might multiply examples, and draw equally striking illustrations from the other classes; and though these correspondences cannot be fully established while our knowledge of the embryological growth of animals is so scanty, and information about their geological succession, yet wherever we have been able to trace the connected history of any group of animals in time, and to compare it with the history of their embryological development and their structural relations as they exist to-day, the correspondence is found to be so complete that we are justified in believing that it will not fail in other instances. I may add that a gradation of exactly the same character controls the geographical distribution of animals over the surface of the globe. Here again I must beg my readers to take much of the evidence, which, if expanded, would fill a volume, for granted, since it would be entirely inappropriate here. But I may briefly state that animals are not scattered over the surface of our globe at random, but that they are associated together in what are called faunæ, and that these faunæ have their homes within certain districts—called by naturalists zoölogical provinces. The limits of these provinces are absolutely fixed, in the ocean as well as on the land, by certain physical conditions connected with climate, with altitude, with the pressure of the atmosphere, the weight of the water, etc.; and this is true even for animals of migratory habits, for all such migrations are periodical, and have boundaries as definite and impassable as those that limit the permanent homes of animals. There is a certain series established by the relations between different kinds of animals, as thus distributed over the globe, which agrees with the gradation in their rank, their growth, and their succession in time;—the law which distributes animals in successive faunæ, and in accordance both with their relative superiority or inferiority, and with the physical conditions essential to their existence, being the same as that which controls their structural relations, their embryological development, and their succession in time.

What, then, does this correspondence between the Series of Rank, the Series of Growth, the Series of Time, and the Series of Geographical Distribution in the life of animals teach us? Surely not that the connection between animals is a material one; for the same kind of relation exists between lower and higher animals of one type or one class to-day, in their structural features, in their embryological growth, and in their geographical distribution, as we trace in their order of succession in time; and therefore, if this kind of evidence proves that the later animals are the descendants of the earlier in any genealogical sense, it should also prove that the animals living in one part of the earth at present grow out of animals living in another part, and that the higher animals of one class as it exists now are developed out of the lower ones. The first of these propositions needs no refutation; and with regard to the second, all our investigations go to show that every being born into the world to-day adheres to its individual law of life, and though it passes through transient phases of growth that resemble other beings of its own kind, never pauses at a lower stage of development, or passes on to a higher condition than the one it is bound to fill. If, then, this connection is not a material one, what is it?—for that such a connection does exist throughout the Animal Kingdom, as intimate, as continuous, as complex as any series which the development theorists have ever contended for, is not to be denied. What can it be but an intellectual one? These correspondences are correspondences of thought,—of a thought that is always the same, whether it is expressed in the history of the type through all time, or in the life of the individuals that represent the type at the present moment, or in the growth of the germ of every being born into that type to-day. In other words, the same thought that spans the whole succession of geological ages controls the structural relations of all living beings as well as their distribution over the surface of the earth, and is repeated within the narrow compass of the smallest egg in which any being undergoes its growth.