The Atlantic Monthly, Volume 09, No. 51, January, 1862

The Atlantic Monthly, Volume 09, No. 51, January, 1862
Various

Various

The Atlantic Monthly, Volume 09, No. 51, January, 1862 / A Magazine of Literature, Art, and Politics

METHODS OF STUDY IN NATURAL HISTORY

I

It is my intention, in this series of papers, to give the history of the progress in Natural History from the beginning,—to show how men first approached Nature,—how the facts of Natural History have been accumulated, and how those facts have been converted into science. In so doing, I shall present the methods employed in Natural History on a wider scale and with broader generalizations than if I limited myself to the study as it exists to-day. The history of humanity, in its efforts to understand the Creation, resembles the development of any individual mind engaged in the same direction. It has its infancy, with the first recognition of surrounding objects; and, indeed, the early observers seem to us like children in their first attempts to understand the world in which they live. But these efforts, that appear childish to us now, were the first steps in that field of knowledge which is so extensive that all our progress seems only to show us how much is left to do.

Aristotle is the representative of the learning of antiquity in Natural Science. The great mind of Greece in his day, and a leader in all the intellectual culture of his time, he was especially a naturalist, and his work on Natural History is a record not only of his own investigations, but of all preceding study in this department. It is evident that even then much had been done, and, in allusion to certain peculiarities of the human frame, which he does not describe in full, he refers his readers to familiar works, saying, that illustrations in point may be found in anatomical text-books.[1 - See Aristotle's Zoölogy, Book I., Chapter xiv.]

Strange that in Aristotle’s day, two thousand years ago, such books should have been in general use, and that in our time we are still in want of elementary text-books of Natural History, having special reference to the animals of our own country, and adapted to the use of schools. One fact in Aristotle’s “History of Animals” is very striking, and makes it difficult for us to understand much of its contents. It never occurs to him that a time may come when the Greek language—the language of all culture and science in his time—would not be the language of all cultivated men. He took, therefore, little pains to characterize the animals he alludes to, otherwise than by their current names; and of his descriptions of their habits and peculiarities, much is lost upon us from their local character and expression. There is also a total absence of systematic form, of any classification or framework to express the divisions of the animal kingdom into larger or lesser groups. His only divisions are genera and species: classes, orders, and families, as we understand them now, are quite foreign to the Greek conception of the animal kingdom. Fishes and birds, for instance, they considered as genera, and their different representatives as species. They grouped together quadrupeds also in contradistinction to animals with legs and wings, and they distinguished those that bring forth living young from those that lay eggs. But though a system of Nature was not familiar even to their great philosopher, and Aristotle had not arrived at the idea of a classification on general principles, he yet stimulated a search into the closer affinities among animals by the differences he pointed out. He divided the animal kingdom into two groups, which he called Enaima and Anaima, or animals with blood and animals without blood. We must remember, however, that by the word blood he designated only the red fluid circulating in the higher animals; whereas a fluid akin to blood exists in all animals, variously colored in some, but colorless in a large number of others.

After Aristotle, a long period elapsed without any addition to the information he left us. Rome and the Middle Ages gave us nothing, and even Pliny added hardly a fact to those that Aristotle recorded. And though the great naturalists of the sixteenth century gave a new impulse to this study, their investigations were chiefly directed towards a minute acquaintance with the animals they had an opportunity of observing, mingled with commentaries upon the ancients. Systematic Zoölogy was but little advanced by their efforts.

We must come down to the last century, to Linnæus, before we find the history taken up where Aristotle had left it, and some of his suggestions carried out with new vigor and vitality. Aristotle had distinguished only between genera and species; Linnæus took hold of this idea, and gave special names to other groups, of different weight and value. Besides species and genera, he gives us orders and classes,—considering classes the most comprehensive, then orders, then genera, then species. He did not, however, represent these groups as distinguished by their nature, but only by their range; they were still to him, as genera and species had been to Aristotle, only larger or smaller groups, not founded upon and limited by different categories of structure. He divided the animal kingdom into six classes, which I give here, as we shall have occasion to compare them with other classifications:—Mammalia, Birds, Reptiles, Fishes, Insects, Worms.

That this classification should have expressed all that was known in the last century of the most general relations among animals only shows how difficult it is to generalize on such a subject; nor should we expect to find it an easy task, when we remember the vast number of species (about a quarter of a million) already noticed by naturalists. Linnæus succeeded, however, in finding a common character on which to unite most of his classes; but the Mammalia, that group to which we ourselves belong, remained very imperfect. Indeed, in the earlier editions of his classification, he does not apply the name of Mammalia to this class, but calls the higher animals Quadrupedia, characterizing them as the animals with four legs and covered with fur or hair, that bring forth living young and nurse them with milk. In thus admitting external features as class characters, he excluded many animals which by their mode of reproduction, as well as by their respiration and circulation, belong to this class as much as the Quadrupeds,—as, for instance, all the Cetaceans, (Whales, Porpoises, and the like,) which, though they have not legs, nor are their bodies covered with hair or fur, yet bring forth living young, nurse them with milk, are warm-blooded and air-breathing. As more was learned of these animals, there arose serious discussion and criticism among contemporary naturalists respecting the classification of Linnæus, all of which led to a clearer insight into the true relations among animals. Linnæus himself, in his last edition of the “Systema Naturæ,” shows us what important progress he had made since he first announced his views; for he there substitutes for the name of Quadrupedia that of Mammalia, including among them the Whales, which he characterizes as air-breathing, warm-blooded, and bringing forth living young which they nurse with milk. Thus the very deficiencies of his classification stimulated naturalists to new criticism and investigation into the true limits of classes, and led to the recognition of one most important principle,—that such groups are founded, not on external appearance, but on internal structure, and that internal structure, therefore, is the thing to be studied. The group of Quadrupeds was not the only defective one in this classification of Linnæus; his class of Worms, also, was most heterogeneous, for he included among them Shell-Fishes, Slugs, Star-Fishes, Sea-Urchins, and other animals that bear no relation whatever to the class of Worms.

But whatever its defects, the classification of Linnæus was the first attempt at grouping animals together according to certain common structural characters. His followers and pupils engaged at once in a scrutiny of the differences and similarities among animals, which soon led to a great increase in the number of classes: instead of six, there were presently nine, twelve, and more. But till Cuvier’s time there was no great principle of classification. Facts were accumulated and more or less systematized, but they were not yet arranged according to law; the principle was still wanting by which to generalize them and give meaning and vitality to the whole. It was Cuvier who found the key. He himself tells us how he first began, in his investigations upon the internal organization of animals, to use his dissections with reference to finding the true relations between animals, and how, ever after, his knowledge of anatomy assisted him in his classifications, and his classifications threw new light again on his anatomical investigations,—each science thus helping to fertilize the other. He was not one of those superficial observers who are in haste to announce every new fact that they chance to find, and his first paper[2 - Sur un nouveau rapprochement à établir entre les Classes qui composent le Règne Animal. Ann. Mus., Vol. XIX.] specially devoted to classification gave to the world the ripe fruit of years of study. This was followed by his great work, “Le Règne Animal.” He said that animals were united in their most comprehensive groups, not on special characters, but on different plans of structure,—moulds, he called them, in which all animals had been cast. He tells us this in such admirable language that I must, to do justice to his thought, give it in his own words:—

“Si l’on considère le règne animal d’après les principes que nous venons de poser en se débarrassant des préjugés établis sur les divisions anciennement admises, en n’ayant égard qu’à l’organisation et à la nature des animaux, et non pas à leur grandeur, à leur utilité, au plus ou moins de connaissance que nous en avons, ni à toutes les autres circonstances accessoires, on trouvera qu’il existe quatre formes principales, quatre plans généraux, si l’on peut s’exprimer ainsi, d’après lesquels tous les animaux semblent avoir été modelés, et dont les divisions ultérieures, de quelque titre que les naturalistes les aient décorées, ne sont que des modifications assez légères, fondées sur le développement ou l’addition de quelques parties, qui ne changent rien à l’essence du plan.”

The value of this principle was soon tested by its application to facts already known, and it was found that animals whose affinities had been questionable before were now at once referred to their true relations with other animals by ascertaining whether they were built on one or another of these plans. Of such plans or structural conceptions Cuvier found in the whole animal kingdom only four, which he called Vertebrates, Mollusks, Articulates, and Radiates.

With this new principle as the basis of investigation, it was no longer enough for the naturalist to know a certain amount of features characteristic of a certain number of animals,—he must penetrate deep enough into their organization to find the secret of their internal structure. Till he can do this, he is like the traveller in a strange city, who looks on the exterior of edifices entirely new to him, but knows nothing of the plan of their internal architecture. To be able to read in the finished structure the plan on which the whole is built is now essential to every naturalist.

There have been many criticisms on this division of Cuvier’s, and many attempts to change it; but though some improvements have been made in the details of his classification, all departures from its great fundamental principle are errors, and do but lead us away from the recognition of the true affinities among animals.

Each of these plans may be stated in the most general terms. In the Vertebrates there is a vertebral column terminating in a prominent head; this column has an arch above and an arch below, forming a double internal cavity. The parts are symmetrically arranged on either side of the longitudinal axis of the body. In the Mollusks, also, the parts are arranged according to a bilateral symmetry on either side of the body, but the body has but one cavity, and is a soft, concentrated mass, without a distinct individualization of parts. In the Articulates there is but one cavity, and the parts are here again arranged on either side of the longitudinal axis, but in these animals the whole body is divided from end to end into transverse rings or joints movable upon each other. In the Radiates we lose sight of the bilateral symmetry so prevalent in the other three, except as a very subordinate element of structure; the plan of this lowest type is an organic sphere, in which all parts bear definite relations to a vertical axis.

It is not upon any special features, then, that these largest divisions of the animal kingdom are based, but simply upon the general structural idea. Striking as this statement was, it was coldly received at first by contemporary naturalists: they could hardly grasp Cuvier’s wide generalizations, and perhaps there was also some jealousy of the grandeur of his views. Whatever the cause, his principle of classification was not fully appreciated; but it opened a new road for study, and gave us the keynote to the natural affinities among animals. Lamarck, his contemporary, not recognizing the truth of this principle, distributed the animal kingdom into two great divisions, which he calls Vertebrates and Invertebrates. Ehrenberg also, at a later period, announced another division under two heads,—those with a continuous solid nervous centre, and those with merely scattered nervous swellings.[3 - For more details upon the different systems of Zoölogy, see Agassiz's Essay on Classification in his Contributions to the Natural History of the United States, Vol. I.]

But there was no real progress in either of these latter classifications, so far as the primary divisions are concerned; for they correspond to the old division of Aristotle, under the head of animals with or without blood, the Enaima and Anaima. This coincidence between systems based on different foundations may teach us that every structural combination includes certain inherent necessities which will bring animals together on whatever set of features we try to classify them; so that the division of Aristotle, founded on the circulating fluids, or that of Lamarck, on the absence or presence of a backbone, or that of Ehrenberg, on the differences of the nervous system, cover the same ground. Lamarck attempted also to use the faculties of animals as a groundwork for division among them. But our knowledge of the psychology of animals is still too imperfect to justify any such use of it. His divisions into Apathetic, Sensitive, and Intelligent animals are entirely theoretical. He places, for instance, Fishes and Reptiles among the Intelligent animals, as distinguished from Crustacea and Insects, which he refers to the second division. But one would be puzzled to say how the former manifest more intelligence than the latter, or why the latter should be placed among the Sensitive animals. Again, some of the animals that he calls Apathetic have been proved by later investigators to show an affection and care for their young, seemingly quite inconsistent with the epithet he has applied to them. In fact, we know so little of the faculties of animals that any classification based upon our present information about them must be very imperfect.

Many modifications of Cuvier’s great divisions have been attempted. Some naturalists, for instance, have divided off a part of the Radiates and Articulates, insisting upon some special features of structure, and mistaking these for the more important and general characteristics of their respective plans. All subsequent investigations of such would-be improvements show them to be retrograde movements, only proving more clearly that Cuvier detected in his four plans all the great structural ideas on which the vast variety of animals is founded. This result is of greater importance than may at first appear. Upon it depends the question, whether all such classifications represent merely individual impressions and opinions of men, or whether there is really something in Nature that presses upon us certain divisions among animals, certain affinities, certain limitations, founded upon essential principles of organization. Are our systems the inventions of naturalists, or only their reading of the Book of Nature? and can that book have more than one reading? If these classifications are not mere inventions, if they are not an attempt to classify for our own convenience the objects we study, then they are thoughts which, whether we detect them or not, are expressed in Nature,—then Nature is the work of thought, the production of intelligence carried out according to plan, therefore premeditated,—and in our study of natural objects we are approaching the thoughts of the Creator, reading His conceptions, interpreting a system that is His and not ours.

All the divergence from the simplicity and grandeur of this division of the animal kingdom arises from an inability to distinguish between a plan and the execution, of a plan. We allow the details to shut out the plan itself, which exists quite independent of special forms. I hope we shall find a meaning in all these plans that will prove them to be the parts of one great conception and the work of one Mind.

II

Proceeding upon the view that there is a close analogy between the way in which every individual student penetrates into Nature and the progress of science as a whole in the history of humanity, I continue my sketch of the successive steps that have led to our present state of knowledge. I began with Aristotle, and showed that this great philosopher, though he prepared a digest of all the knowledge belonging to his time, yet did not feel the necessity of any system or of any scientific language differing from the common mode of expression of his day. He presents his information as a man with his eyes open narrates in a familiar style what he sees. As civilization spread and science had its representatives in other countries besides Greece, it became indispensable to have a common scientific language, a technical nomenclature, combining many objects under common names, and enabling every naturalist to express the results of his observations readily and simply in a manner intelligible to all other students of Natural History.

Linnæus devised such a system, and to him we owe a most simple and comprehensive scientific mode of designating animals and plants. It may at first seem no advantage to give up the common names of the vernacular and adopt the unfamiliar ones, but a word of explanation will make the object clear. Perceiving, for instance, the close relations between certain members of the larger groups, Linnæus gave to them names that should be common to all, and which are called generic names,—as we speak of Ducks, when we would designate in one word the Mallard, the Widgeon, the Canvas-Back, etc.; but to these generic names he added qualifying epithets, called specific names, to indicate the different kinds in each group. For example, the Lion, the Tiger, the Panther, the Domestic Cat constitute such a natural group, which Linnæus called Felis, Cat, indicating the whole genus; but the species he designates as Felis catus, the Domestic Cat,—Felis leo, the Lion,—Felis tigris, the Tiger,—Felis panthera, the Panther. So he called all the Dogs Canis; but for the different kinds we have Canis familiaris, the Domestic Dog,—Canis lupus, the Wolf,—Canis vulpes, the Fox, etc.

In some families of the vegetable kingdom we can appreciate better the application of this nomenclature, because we have something corresponding to it in the vernacular. We have, for instance, one name for all the Oaks, but we call the different kinds Swamp Oak, Red Oak, White Oak, Chestnut Oak, etc. So Linnæus, in his botanical nomenclature, called all the Oaks by the generic name Quercus, (characterizing them by their fruit, the acorn, common to all,) and qualified them as Quercus bicolor, Quercus rubra, Quercus alba, Quercus castanea, etc., etc. His nomenclature, being so easy of application, became at once exceedingly popular and made him the great scientific legislator of his century. He insisted on Latin names, because, if every naturalist should use his own language, it must lead to great confusion, and this Latin nomenclature of double significance was adopted by all. Another advantage of this binominal Latin nomenclature consists in preventing the confusion frequently arising from the use of the same name to designate different animals in different parts of the world,—as, for instance, the name of Robin, used in America to designate a bird of the Thrush family, entirely different from the Robin of the Old World,—or of different names for the same animal, as Perch or Chogset or Burgall for our Cunner. Nothing is more to be deprecated than an over-appreciation of technicalities, valuing the name more highly than the thing; but some knowledge of this nomenclature is necessary to every student of Nature.

The improvements in science thus far were chiefly verbal. Cuvier now came forward and added a principle. He showed that all animals are built upon a certain number of definite plans. This momentous step, the significance of which is not yet appreciated to its full extent; for, had its importance been understood, the efforts of naturalists would have been directed toward a further illustration of the distinctive characteristics of all the plans,—instead of which, the division of the animal kingdom into larger and smaller groups chiefly attracted their attention, and has been carried too far by some of them. Linnæus began with six classes, Cuvier brought them up to nineteen, and at last the animal kingdom was subdivided by subsequent investigators into twenty-eight classes. This multiplication of divisions, however, soon suggested an important question: How far are these divisions natural or inherent in the objects themselves, and not dependent on individual views?

While Linnæus pointed out classes, orders, genera, and species, other naturalists had detected other divisions among animals, called families. Lamarck, who had been a distinguished botanist before he began his study of the animal kingdom, brought to his zoölogical researches his previous methods of investigation. Families in the vegetable kingdom had long been distinguished by French botanists; and one cannot examine the groups they call by this name, without perceiving, that, though they bring them together and describe them according to other characters, they have been unconsciously led to unite them from the general similarity of their port and bearing. Take, for instance, the families of Pines, Oaks, Beeches, Maples, etc., and you feel at once, that, besides the common characters given in the technical descriptions of these trees, there is also a general resemblance among them that would naturally lead us to associate them together, even if we knew nothing of the other features of their structure. By an instinctive recognition of this family likeness between plants, botanists have been led to seek for structural characters on which to unite them, and the groups so founded generally correspond with the combinations suggested by their appearance.

By a like process Lamarck combined animals into families. His method was adopted by French naturalists generally, and found favor especially with Cuvier, who was particularly successful in limiting families among animals, and in naming them happily, generally selecting names expressive of the features on which the groups were founded, or borrowing them from familiar animals. Much, indeed, depends upon the pleasant sound and the significance of a name; for an idea reaches the mind more easily when well expressed, and Cuvier’s names were both simple and significant. His descriptions are also remarkable for their graphic precision,—giving all that is essential, omitting all that is merely accessory. He has given us the key-note to his progress in his own expressive language:—

“Je dus donc, et cette obligation me prit un temps considérable, je dus faire marcher de front l’anatomie et la zoologie, les dissections et le classement; chercher dans mes premières remarques sur l’organisation des distributions meilleures; m’en servir pour arriver à des remarques nouvelles; employer encore ces remarques à perfectionner les distributions; faire sortir enfin de cette fécondation mutuelle des deux sciences, l’une par l’autre, un système zoologique propre à servir d’introducteur et de guide dans le champ de l’anatomie, et un corps de doctrine anatomique propre à servir de développement et d’explication au système zoologique.”

It is deeply to be lamented that so many naturalists have entirely overlooked this significant advice of Cuvier’s, to combine zoölogical and anatomical studies in order to arrive at a clearer perception of the true affinities among animals. To sum it up in one word, he tells us that the secret of his method is “comparison,”—ever comparing and comparing throughout the enormous range of his knowledge of the organization of animals, and founding upon the differences as well as the similarities those broad generalizations under which he has included all animal structures. And this method, so prolific in his hands, has also a lesson for us all. In this country there is a growing interest in the study of Nature; but while there exist hundreds of elementary works illustrating the native animals of Europe, there are few such books here to satisfy the demand for information respecting the animals of our land and water. We are thus forced to turn more and more to our own investigations and less to authority; and the true method of obtaining independent knowledge is this very method of Cuvier’s,—comparison.

Let us make the most common application of it to natural objects. Suppose we see together a Dog, a Cat, a Bear, a Horse, a Cow, and a Deer. The first feature that strikes us as common to any two of them is the horn in the Cow and Deer. But how shall we associate either of the others with these? We examine the teeth, and find those of the Dog, the Cat, and the Bear sharp and cutting, while those of the Cow, the Deer, and the Horse have flat surfaces, adapted to grinding and chewing, rather than cutting and tearing. We compare these features of their structure with the habits of these animals, and find that the first are carnivorous, that they seize and tear their prey, while the others are herbivorous or grazing animals, living only on vegetable substances, which they chew and grind. We compare farther the Horse and Cow, and find that the Horse has front teeth both in the upper and lower jaw, while the Cow has them only in the lower; and going still farther and comparing the internal with the external features, we find this arrangement of the teeth in direct relation to the different structure of the stomach in the two animals,—the Cow having a stomach with four pouches, adapted to a mode of digestion by which the food is prepared for the second mastication, while the Horse has a simple stomach. Comparing the Cow and the Deer, we find that the digestive apparatus is the same in both; but though they both have horns, in the Cow the horn is hollow, and remains through life firmly attached to the bone, while in the Deer it is solid and is shed every year. With these facts before us, we cannot hesitate to place the Dog, the Cat, and the Bear in one division, as carnivorous animals, and the other three in another division as herbivorous animals,—and looking a little farther, we perceive, that, in common with the Cow and the Deer, the Goat and the Sheep have cloven feet, and that they are all ruminants, while the Horse has a single hoof, does not ruminate, and must therefore be separated from them, even though, like them, he is herbivorous.

This is but the simplest illustration, taken from the most familiar objects, of this comparative method; but the same process is equally applicable to the most intricate problems in animal structures, and will give us the clue to all true affinities between animals. The education of a naturalist, now, consists chiefly in learning how to compare. If he have any power of generalization, when he has collected his facts, this habit of mental comparison will lead him up to principles, to the great laws of combination. It must not discourage us, that the process is a slow and laborious one, and the results of one lifetime after all very small. It might seem invidious, were I to show here how small is the sum total of the work accomplished even by the great exceptional men, whose names are known throughout the civilized world. But I may at least be permitted to speak of my own efforts, and to sum up in the fewest words the result of my life’s work. I have devoted my whole life to the study of Nature, and yet a single sentence may express all that I have done. I have shown that there is a correspondence between the succession of Fishes in geological times and the different stages of their growth in the egg,—this is all. It chanced to be a result that was found to apply to other groups and has led to other conclusions of a like nature. But, such as it is, it has been reached by this system of comparison, which, though I speak of it now in its application to the study of Natural History, is equally important in every other branch of knowledge. By the same process the most mature results of scientific research in Philology, in Ethnology, and in Physical Science are reached. And let me say that the community should foster the purely intellectual efforts of scientific men as carefully as they do their elementary schools and their practical institutions, generally considered so much more useful and important to the public. For from what other source shall we derive the higher results that are gradually woven into the practical resources of our life, except from the researches of those very men who study science not for its uses, but for its truth? It is this that gives it its noblest interest: it must be for truth’s sake, and not even for the sake of its usefulness to humanity, that the scientific man studies Nature. The application of science to the useful arts requires other abilities, other qualities, other tools than his; and therefore I say that the man of science who follows his studies into their practical application is false to his calling. The practical man stands ever ready to take up the work where the scientific man leaves it, and to adapt it to the material wants and uses of daily life.

The publication of Cuvier’s proposition, that the animal kingdom is built on four plans, created an extraordinary excitement throughout the scientific world. All naturalists proceeded to test it, and many soon recognized in it a great scientific truth,—while others, who thought more of making themselves prominent than of advancing science, proposed poor amendments, that were sure to be rejected on farther investigation. There were, however, some of these criticisms and additions that were truly improvements, and touched upon points overlooked by Cuvier. Blainville, especially, took up the element of form among animals,—whether divided on two sides, whether radiated, whether irregular, etc. He, however, made the mistake of giving very elaborate names to animals already known under simpler ones. Why, for instance, call all animals with parts radiating in every direction Actinomorpha or Actinozoaria, when they had received the significant name of Radiates? It seemed, to be a new system, when in fact it was only a new name. Ehrenberg, likewise, made an important distinction, when he united the animals according to the difference in their nervous systems; but he also incumbered the nomenclature unnecessarily, when he added to the names Anaima and Enaima of Aristotle those of Myeloneura and Ganglioneura.

But it is not my object to give all the classifications of different authors here, and I will therefore pass over many noted ones, as those of Burmeister, Milne, Edwards, Siebold and Stannius, Owen, Leuckart, Vogt, Van Beneden, and others, and proceed to give some account of one investigator who did as much for the progress of Zoölogy as Cuvier, though he is comparatively little known among us. Karl Ernst von Baer proposed a classification based, like Cuvier’s, upon plan; but he recognized what Cuvier failed to perceive,—namely, the importance of distinguishing between type (by which he means exactly what Cuvier means by plan) and complication of structure,—in other words, between plan and the execution of the plan. He recognized four types, which correspond exactly to Cuvier’s four plans, though he calls them by different names. Let us compare them.

Though perhaps less felicitous, the names of Baer express the same ideas as those of Cuvier. By the Peripheric he signified those in which all the parts converge from the periphery or circumference of the animal to its centre. Cuvier only reverses this definition in his name of Radiates, signifying the animals in which all parts radiate from the centre to the circumference. By Massive, Baer indicated those animals in which the structure is soft and concentrated, without a very distinct individualization of parts,—exactly the animals included by Cuvier under his name of Mollusks, or soft-bodied animals. In his selection of the epithet Longitudinal, Baer was less fortunate; for all animals have a longitudinal diameter, and this word was not, therefore, sufficiently special. Yet his Longitudinal type answers exactly to Cuvier’s Articulates,—animals in which all parts are arranged in a succession of articulated joints along a longitudinal axis. Cuvier has expressed this jointed structure in the name Articulates; whereas Baer, in his name of Longitudinal, referred only to the arrangement of joints in longitudinal succession, in a continuous string, as it were, one after another. For the Doubly Symmetrical type his name is the better of the two; for Cuvier’s name of Vertebrates alludes only to the backbone,—while Baer, who is an embryologist, signifies in his their mode of growth also. He knew what Cuvier did not know, that in its first formation the germ of the Vertebrate divides in two folds: one turning up above the backbone, to inclose all the sensitive Organs,—the spinal marrow, the organs of sense, all those organs by which life is expressed; the other turning down below the backbone, and inclosing all those organs by which life is maintained,—the organs of digestion, of respiration, of circulation, of reproduction, etc. So there is in this type not only an equal division of parts on either side, but also a division above and below, making thus a double symmetry in the plan, expressed by Baer in the name he gave it. Baer was perfectly original in his conception of these four types, for his paper was published in the very same year with that of Cuvier. But even in Germany, his native land, his ideas were not fully appreciated: strange that it should be so,—for, had his countrymen recognized his genius, they might have claimed him as the compeer of the great French naturalist.

Baer also founded the science of Embryology, under the guidance of his teacher, Dollinger. His researches in this direction showed him that animals were not only built on four plans, but that they grew according to four modes of development. The Vertebrate arises from the egg differently from the Articulate,—the Articulate differently from the Mollusk,—the Mollusk differently from the Radiate. Cuvier only showed us the four plans as they exist in the adult; Baer went a step farther, and showed us the four plans in the process of formation. But his greatest scientific achievement is perhaps the discovery that all animals originate in eggs, and that all these eggs are at first identical in substance and structure. The wonderful and untiring research condensed into this simple statement, that all animals arise from eggs and that all those eggs are identical in the beginning, may well excite our admiration. This egg consists of an outer envelope, the vitelline membrane, containing a fluid more or less dense, the yolk; within this is a second envelope, the so-called germinative vesicle, containing a somewhat different and more transparent fluid, and in the fluid of this second envelope float one or more so-called germinative specks. At this stage of their growth all eggs are microsopically small, yet each one has such tenacity of its individual principle of life that no egg was ever known to swerve from the pattern of the parent animal that gave it birth.

III

From the time that Linnæus showed us the necessity of a scientific system as a framework for the arrangement of scientific facts in Natural History, the number of divisions adopted by zoölogists and botanists increased steadily. Not only were families, orders, and classes added to genera and species, but these were further multiplied by subdivisions of the different groups. But as the number of divisions increased, they lost in precise meaning, and it became more and more doubtful how far they were true to Nature. Moreover, these divisions were not taken in the same sense by all naturalists: what were called families by some were called orders by others, while the orders of some were the classes of others, till it began to be doubted whether these scientific systems had any foundation in Nature, or signified anything more than that it had pleased Linnæus, for instance, to call certain groups of animals by one name, while Cuvier had chosen to call them by another.

These divisions are, first, the most comprehensive groups, the primary divisions, called branches by some, types by others, and divided by some naturalists into so-called sub-types, meaning only a more limited circumscription of the same kind of group; next we have classes, and these also have been divided into sub-classes, then orders and sub-orders, families, sub-families, and tribes; then genera, species, and varieties. With reference to the question, whether these groups really exist in Nature or are merely the expression of individual theories and opinions, it is worth while to study the works of the early naturalists, in order to trace the natural process by which scientific classification has been reached; for in this, as in other departments of learning, practice has always preceded theory. We do the thing before we understand why we do it: speech precedes grammar, reason precedes logic; and so a division of animals into groups, upon an instinctive perception of their differences, has preceded all our scientific creeds and doctrines. Let us, therefore, proceed to examine the meaning of these names as adopted by naturalists.

When Cuvier proposed his four primary divisions of the animal kingdom, he added his argument for their adoption,—because, he said, they are constructed on four different plans. All the progress in our science since his time confirms this result; and I shall attempt to show that there are really four, and only four, such structural ideas at the foundation of the animal kingdom, and that all animals are included under one or another of them. But it does not follow, that, because we have arrived at a sound principle, we are therefore unerring in our practice. From ignorance we may misplace animals, and include them under the wrong division. This is a mistake, however, which a better insight into their organization rectifies; and experience constantly proves, that, whenever the structure of an animal is perfectly understood, there is no hesitation as to the head under which it belongs. We may consequently test the merits of these four primary groups on the evidence furnished by investigation. It has already been seen that these plans may be presented in the most abstract manner without any reference to special animals. Radiation expresses in one word the idea on which the lowest of these types is based. In Radiates we have no prominent bilateral symmetry, as in all other animals, but an all-sided symmetry, in which there is no right and left, no anterior and posterior extremity, no above and below. They are spheroidal bodies; yet, though many of them remind us of a sphere, they are by no means to be compared to a mathematical sphere, but rather to an organic sphere, so loaded with life, as it were, as to produce an infinite variety of radiate symmetry. The whole organization is arranged around a centre toward which all the parts converge, or, in a reverse sense, from which all the parts radiate. In Mollusks there is a longitudinal axis and a bilateral symmetry; but the longitudinal axis in these soft concentrated bodies is not very prominent; and though the two ends of this axis are distinct from each other, the difference is not so marked that we can say at once, for all of them, which is the anterior and which the posterior extremity. In this type, right and left have the preponderance over the other diameters of the body. The sides are the prominent parts,—they are charged with the important organs, loaded with those peculiarities of the structure that give it character. The Oyster is a good instance of this, with its double valve, so swollen on one side, so flat on the other. There is an unconscious recognition of this in the arrangement of all collections of Mollusks; for, though the collectors do not put up their specimens with any intention of illustrating this peculiarity, they instinctively give them the position best calculated to display their distinctive characteristics, and to accomplish this they necessarily place them in such a manner as to show the sides. In Articulates there is also a longitudinal axis of the body and a bilateral symmetry in the arrangement of parts; the head and tail are marked, and the right and left sides are distinct. But the prominent tendency in this type is the development of the dorsal and ventral region; here above and below prevail over right and left. It is the back and the lower side that have the preponderance over any other part of the structure in Articulates. The body is divided from end to end by a succession of transverse constrictions, forming movable rings; but the character of the animal, its striking features, are always above or below, and especially developed on the back. Any collection of Insects or Crustacea is an evidence of this; being always instinctively arranged in such a manner as to show the predominant features, they uniformly exhibit the back of the animal. The profile view of an Articulate has no significance; whereas in a Mollusk, on the contrary, the profile view is the most illustrative of the structural character. In the highest division, the Vertebrates, so characteristically called by Baer the Doubly Symmetrical type, a solid column runs through the body with an arch above and an arch below, thus forming a double internal cavity. In this type, the head is the prominent feature; it is, as it were, the loaded end of the longitudinal axis, so charged with vitality as to form an intelligent brain, and rising in man to such predominance as to command and control the whole organism. The structure is arranged above and below this axis, the upper cavity containing all the sensitive organs, and the lower cavity containing all those by which life is maintained.

While Cuvier and his followers traced these four distinct plans, as shown in the adult animal, Baer opened to us a new field of investigation in the embryology of the four types, showing that for each there was a special mode of growth in the egg. Looking at them from this point of view, we shall see that these four types, with their four modes of growth, seem to fill out completely the plan or outline of the animal kingdom, and leave no reason to expect any further development or any other plan of animal life within these limits. The eggs of all animals are spheres, such as I have described them; but in the Radiate the whole periphery is transformed into the germ, so that it becomes, by the liquefying of the yolk, a hollow sphere. In the Mollusks, the germ lies above the yolk, absorbing its whole substance through the under side, thus forming a massive close body instead of a hollow one. In the Articulate, the germ is turned in a position exactly opposite to that of the Mollusk, and absorbs the yolk upon the back. In the Vertebrate, the germ divides in two folds, one turning upward, the other turning downward, above and below the central backbone. These four modes of development seem to exhaust the possibilities of the primitive sphere, which is the foundation of all animal life, and therefore I believe that Cuvier and Baer were right in saying that the whole animal kingdom is included under these four structural ideas.

Leuckart proposed to subdivide the Radiates into two groups: the Coelenterata, including Polyps and Acalephs or Jelly-Fishes,—and Echinoderms, including Star-Fishes, Sea-Urchins, and Holothurians. His reason for this distinction is the fact that in the latter the organs are inclosed within walls of their own, distinct from the body-wall; whereas in the former the organs are formed by internal folds of the outer wall of the body, as in the Polyps, or are hollowed out of the substance of the body, as in Jelly-Fishes. This implies no difference in the plan, but merely a difference in the execution of the plan. Both are equally radiate in their structure; and when Leuckart separated them as distinct primary types, he mistook a difference in the material expression of the plan for a difference in the plan itself. So some naturalists have distinguished Worms from the other Articulates as a separate division. But the structural plan of this type is a body divided by transverse constrictions or joints; and whether those joints are uniformly arranged from one end of the body to the other, as in the Worms, or whether the front joints are soldered together so as to form two regions of the body, as in Crustacea, or divided so as to form three regions of the body, as in winged Insects, does not in the least affect the typical character of the structure, which remains the same in all. Branches or types, then, are natural groups of the animal kingdom, founded on plans of structure or structural ideas.

What now are classes? Are they lesser divisions, differing only in extent, or are they founded on special characters? I believe the latter view to be the true one, and that class characters have a significance quite different from that of their mere range or extent. These divisions are founded on certain categories of structure; and were there but one animal of a class in the world, if it had those characters on which a class is founded, it would be as distinct from all other animals as if its kind were counted by thousands. Baer approached the idea of the classes when he discriminated between plan of structure or type and the degree of perfection in the structure. But while he understands the distinction between a plan and its execution, his ideas respecting the different features of structure are not quite so precise. He does not, for instance, distinguish between the complication of a given structure and the mode of execution of a plan, both of which are combined in what he calls degrees of perfection. And yet, without this distinction, the difference between classes and orders cannot be understood; for classes and orders rest upon a just appreciation of these two categories, which are quite distinct from each other, and have by no means the same significance. Again, quite distinct from both of these is the character of form, not to be confounded either with complication of structure, on which orders are based, or with the execution of the plan, on which classes rest. An example will show that form is no guide for the determination of classes or orders. Take, for instance, a Beche-de-Mer, a member of the highest class of Radiates, and compare it with a Worm. They are both long cylindrical bodies; but one has parallel divisions along the length of the body, the other has the body divided by transverse rings. Though in external form they resemble each other, the one is a worm-like Radiate, the other is a worm-like Articulate, each having the structure of its own type; so that they do not even belong to the same great division of the animal kingdom, much less to the same class. We have a similar instance in the Whales and Fishes,—the Whales having been for a long time considered as Fishes, on account of their form, while their structural complication shows them to be a low order of the class of Mammalia, to which we ourselves belong, that class being founded upon a particular mode of execution of the plan characteristic of the Vertebrates, while the order to which the Whales belong depends upon their complication of structure, as compared with other members of the same class. We may therefore say that neither form nor complication of structure distinguishes classes, but simply the mode of execution of a plan. In Vertebrates, for instance, how do we distinguish the class of Mammalia from the other classes of the type? By the peculiar development of the brain, by their breathing through lungs, by their double circulation, by their bringing forth living young and nursing them with milk. In this class the beasts of prey form a distinct order, superior to the Whales or the herbivorous animals, on account of the higher complication of their structure; and for the same reason we place the Monkeys above them all. But among the beasts of prey we distinguish the Bears, as a family, from the family of Dogs, Wolves, and Cats, on account of their different form, which does not imply a difference either in the complication of their structure or in the mode of execution of their plan.

AGNES OF SORRENTO

CHAPTER XVIII.

THE PENANCE

The course of our story requires us to return to the Capuchin convent, and to the struggles and trials of its Superior; for in his hands is the irresistible authority which must direct the future life of Agnes.

From no guilty compliances, no heedless running into temptation, had he come to love her. The temptation had met him in the direct path of duty; the poison had been breathed in with the perfume of sweetest and most life-giving flowers: nor could he shun that temptation, nor cease to inhale that fatal sweetness, without confessing himself vanquished in a point where, in his view, to yield was to be lost. The subtle and deceitful visit of Father Johannes to his cell had the effect of thoroughly rousing him to a complete sense of his position, and making him feel the immediate, absolute necessity of bringing all the energy of his will, all the resources of his nature to bear on its present difficulties. For he felt, by a fine intuition, that already he was watched and suspected;—any faltering step now, any wavering, any change in his mode of treating his female penitents, would be maliciously noted. The military education of his early days had still left in his mind a strong residuum of personal courage and honor, which made him regard it as dastardly to flee when he ought to conquer, and therefore he set his face as a flint for victory.

But reviewing his interior world, and taking a survey of the work before him, he felt that sense of a divided personality which often becomes so vivid in the history of individuals of strong will and passion. It seemed to him that there were two men within him: the one turbulent, passionate, demented; the other vainly endeavoring by authority, reason, and conscience to bring the rebel to subjection. The discipline of conventual life, the extraordinary austerities to which he had condemned himself, the monotonous solitude of his existence, all tended to exalt the vivacity of the nervous system, which, in the Italian constitution, is at all times disproportionately developed; and when those weird harp-strings of the nerves are once thoroughly unstrung, the fury and tempest of the discord sometimes utterly bewilders the most practised self-government.

But he felt that something must be done with himself, and done immediately; for in a few days he must again meet Agnes at the confessional. He must meet her, not with weak tremblings and passionate fears, but calm as Fate, inexorable as the Judgment-Day. He must hear her confession, not as man, but as God; he must pronounce his judgments with a divine dispassionateness. He must dive into the recesses of her secret heart, and, following with subtile analysis all the fine courses of those fibres which were feeling their blind way towards an earthly love, must tear them remorselessly away. Well could he warn her of the insidiousness of earthly affections; better than any one else he could show her how a name that was blended with her prayers and borne before the sacred shrine in her most retired and solemn hours might at last come to fill all her heart with a presence too dangerously dear. He must direct her gaze up those mystical heights where an unearthly marriage awaited her, its sealed and spiritual bride; he must hurry her footsteps onward to the irrevocable issue.