Читать книгу The Atlantic Monthly, Volume 09, No. 51, January, 1862 - Various - Страница 2

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.