Oken, the great German naturalist, also attempted to establish a connected chain throughout the Animal Kingdom, but on an entirely different principle; and I cannot allude to this most original investigator, so condemned by some, so praised by others, so powerful in his influence on science in Germany, without attempting to give some analysis of his peculiar philosophy. For twenty years his classification was accepted by his countrymen without question; and though I believe it to be wrong, yet, by the ingenuity with which he maintained it, he has shed a flood of light upon science, and has stimulated other naturalists to most important and interesting investigations. This famous classification was founded upon the idea that the system of man, the most perfect created being, is the measure for the whole Animal Kingdom, and that in analyzing his organization we have the clue to all organized beings. The structure of man includes two systems of organs: those which maintain the body in its integrity, and which he shares in some sort with the lower animals,—the organs of digestion, circulation, respiration, and reproduction; and that higher system of organs, the brain, spinal marrow, and nerves, with the organs of sense, on which all the manifestations of the intelligent faculties depend, and by which his relations to the external world are established and controlled: the whole being surrounded by flesh, muscles, and skin. On account of this fleshy envelope of the hard parts in all the higher animals, Oken divided the Animal Kingdom into two groups, the Vertebrates and Invertebrates, or, as he called them, the “Eingeweide und Fleisch Thiere”—which we may translate as the Intestinal Animals, or those that represent the intestinal systems of organs, and the Flesh Animals, or those that combine all the systems of organs under one envelope of flesh. Let us examine a little more closely this singular theory, by which each branch of the Invertebrates becomes, as it were, the exponent of a special system of organs, while the Vertebrates, with man at their head, include all these systems.

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.

RADIATES.MOLLUSKS.ARTICULATES.VERTEBRATES.
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||||Mammalia.
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||Echinoderms.||Cephalopoda.||Insects.||Birds.
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|Acalephs.|Gasteropoda.|Crustacea.|Reptiles.
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Polyps.Acephala.Worms.Fishes.