Читать книгу: «The Atlantic Monthly, Volume 09, No. 54, April, 1862», страница 7

VIII

After this digression, let us proceed to a careful examination of the natural groups of animals called Families by naturalists,—a subject already briefly alluded to in a previous chapter. Families are natural assemblages of animals of less extent than Orders, but, like Orders, Classes, and Branches, founded upon certain categories of structure, which are as distinct for this kind of group as for all the other divisions in the classification of the Animal Kingdom.

That we may understand the true meaning of these divisions, we must not be misled by the name given by naturalists to this kind of group. Here, as in so many other instances, a word already familiar, and that had become, as it were, identified with the special sense in which it had been used, has been adopted by science and has received a new signification. When naturalists speak of Families among animals, they do not allude to the progeny of a known stock, as we designate, in common parlance, the children or the descendants of known parents by the word family; they understand by Families natural groups of different kinds of animals, having no genetic relations so far as we know, but agreeing with one another closely enough to leave the impression of a more or less remote common parentage. The difficulty here consists in determining the natural limits of such groups, and in tracing the characteristic features by which they may be defined; for individual investigators differ greatly as to the degree of resemblance existing between the members of many Families, and there is no kind of group which presents greater diversity of circumscription in the classifications of animals proposed by different naturalists than these so-called Families.

It should be remembered, however, that, unless a sound criterion be applied to the limitation of Families, they, like all other groups introduced into zoölogical systems, must forever remain arbitrary divisions, as they have been hitherto. A retrospective glance at the progress of our science during the past century, in this connection, may perhaps help us to solve the difficulty. Linnaeus, in his System of Nature, does not admit Families; he has only four kinds of groups,—Classes, Orders, Genera, and Species. It was among plants that naturalists first perceived those general traits of resemblance which exist everywhere among the members of natural families, and added this kind of group to the framework of their system. In France, particularly, this method was pursued with success; and the improvements thus introduced by the French botanists were so great, and rendered their classification so superior to that of Linnaeus, that the botanical systems in which Families were introduced were called natural systems, in contradistinction especially to the botanical classification of Linnaeus, which was founded upon the organs of reproduction, and which received thenceforth the name of the sexual system of plants. The same method so successfully used by botanists was soon introduced into Zoölogy by the French naturalists of the beginning of this century,—Lamarck, Latreille, and Cuvier. But, to this day, the limitation of Families among animals has not yet reached the precision which it has among plants, and I see no other reason for the difference than the absence of a leading principle to guide us in Zoölogy.

Families, as they exist in Nature, are based upon peculiarities of form as related to structure; but though a very large number of them have been named and recorded, very few are characterized with anything like scientific accuracy. It has been a very simple matter to establish such groups according to the superficial method that has been pursued, for the fact that they are determined by external outline renders the recognition of them easy and in many instances almost instinctive; but it is very difficult to characterize them, or, in other words, to trace the connection between form and structure. Indeed, many naturalists do not admit that Families are based upon form; and it was in trying to account for the facility with which they detect these groups, while they find it so difficult to characterize them, that I perceived that they are always associated with peculiarities of form. Naturalists have established Families simply by bringing together a number of animals resembling each other more or less closely, and, taking usually the name of the Genus to which the best known among them belongs, they have given it a patronymic termination to designate the Family, and allowed the matter to rest there, sometimes without even attempting any description corresponding to those by which Genus and Species are commonly defined.

For instance, from Canis, the Dog, Canidae has been formed, to designate the whole Family of Dogs, Wolves, Foxes, etc. Nothing can be more superficial than such a mode of classification; and if these groups actually exist in Nature, they must be based, like all the other divisions, upon some combination of structural characters peculiar to them. We have seen that Branches are founded upon the general plan of structure, Classes on the mode of executing the plan, Orders upon the greater or less complication of a given mode of execution, and we shall find that form, as determined by structure, characterizes Families. I would call attention to this qualification of my definition; since, of course, when speaking of form in this connection, I do not mean those superficial resemblances in external features already alluded to in my remarks upon Parallel or Collateral Types. I speak now of form as controlled by structural elements; and unless we analyze Families in this way, the mere distinguishing and naming them does not advance our science at all. Compare, for instance, the Dogs, the Seals, and the Bears. These are all members of one Order,—that of the Carnivorous Mammalia. Their dentition is peculiar and alike in all, (cutting teeth, canine teeth, and grinders,) adapted for tearing and chewing their food; and their internal structure bears a definite relation to their dentition. But look at these animals with reference to form. The Dog is comparatively slender, with legs adapted for running and hunting his prey; the Bear is heavier, with shorter limbs; while the Seal has a continuous uniform outline adapted for swimming. They form separate Families, and are easily recognized as such by the difference in their external outline; but what is the anatomical difference which produces the peculiarity of form in each, by which they have been thus distinguished? It lies in the structure of the limbs, and especially in that of the wrist and fingers. In the Seal the limbs are short, and the wrists are on one continuous line with them, so that it has no power of bending the wrist or the fingers, and the limbs, therefore, act like flappers or oars. The Bear has a well-developed paw with a flexible wrist, but it steps on the whole sole of the foot, from the wrist to the tip of the toe, giving it the heavy tread so characteristic of all the Bears. The Dogs, on the contrary, walk on tip-toe, and their step, though firm, is light, while the greater slenderness and flexibility of their legs add to their nimbleness and swiftness. By a more extensive investigation of the anatomical structure of the limbs in their connection with the whole body, it could easily be shown that the peculiarity of form in these animals is essentially determined by, or at least stands in the closest relation to, the peculiar structure of the wrist and fingers.

Take the Family of Owls as distinguished from the Falcons, Kites, etc. Here the difference of form is in the position of the eyes. In the Owl, the sides of the head are prominent and the eye-socket is brought forward. In the Falcons and Kites, on the contrary, the sides of the head are flattened and the eyes are set back. The difference in the appearance of the birds is evident to the most superficial observer; but to call the one Strigidae and the other Falconidae tells us nothing of the anatomical peculiarities on which this difference is founded.

These few examples, selected purposely among closely allied and universally known animals, may be sufficient to show, that, beyond the general complication of the structure which characterizes the Orders, there is a more limited element in the organization of animals, bearing chiefly upon their form, which, if it have any general application as a principle of classification, may well be considered as essentially characteristic of the Families. There are certainly closely allied natural groups of animals, belonging to the same Order, but including many Genera, which differ from each other chiefly in their form, while that form is determined by peculiarities of structure which do not influence the general structural complication upon which Orders are based, or relate to the minor details of structure on which Genera are founded. I am therefore convinced that form is the criterion by which Families may be determined. The great facility with which animals may be combined together in natural groups of this kind without any special investigation of their structure, a superficial method of classification in which zoölogists have lately indulged to a most unjustifiable degree, convinces me that it is the similarity of form which has unconsciously led such shallow investigators to correct results, since upon close examination it is found that a large number of the Families so determined, and to which no characters at all are assigned, nevertheless bear the severest criticism founded upon anatomical investigation.

The questions proposed to themselves by all students who would characterize Families should be these: What are, throughout the Animal Kingdom, the peculiar patterns of form by which Families are distinguished? and on what structural features are these patterns based? Only the most patient investigations can give us the answer, and it will be very long before we can write out the formulae of these patterns with mathematical precision, as I believe we shall be able to do in a more advanced stage of our science. But while the work is in progress, it ought to be remembered that a mere general similarity of outline is not yet in itself evidence of identity of form or pattern, and that, while seemingly very different forms may be derived from the same formula, the most similar forms may belong to entirely different systems, when their derivation is properly traced. Our great mathematician, in a lecture delivered at the Lowell Institute last winter, showed that in his science, also, similarity of outline does not always indicate identity of character. Compare the different circles,—the perfect circle, in which every point of the periphery is at the same distance from the centre, with an ellipse in which the variation from the true circle is so slight as to be almost imperceptible to the eye; yet the latter, like all ellipses, has its two foci by which it differs from a circle, and to refer it to the family of circles instead of the family of ellipses would be overlooking its true character on account of its external appearance; and yet ellipses may be so elongated, that, far from resembling a circle, they make the impression of parallel lines linked at their extremities. Or we may have an elastic curve in which the appearance of a circle is produced by the meeting of the two ends; nevertheless it belongs to the family of elastic curves, in which may even be included a line actually straight, and is formed by a process entirely different from that which produces the circle or the ellipse.

But it is sometimes exceedingly difficult to find the relation between structure and form in Families, and I remember a case which I had taken as a test of the accuracy of the views I entertained upon this subject, and which perplexed and baffled me for years. It was that of our fresh-water Mussels, the Family of Unios. There is a great variety of outline among them,—some being oblong and very slender, others broad with seemingly square outlines, others having a nearly triangular form, while others again are almost circular; and I could not detect among them all any feature of form that was connected with any essential element of their structure. At last, however, I found this test-character, and since that time I have had no doubt left in my mind that form, determined by structure, is the true criterion of Families. In the Unios it consists of the rounded outline of the anterior end of the body reflected in a more or less open curve of the shell, bending more abruptly along the lower side with an inflection followed by a bulging, corresponding to the most prominent part of the gills, to which alone, in a large number of American Species of this Family, the eggs are transferred, giving to this part of the shell a prominence which it has not in any of the European Species. At the posterior end of the body this curve then bends upwards and backwards again, the outline meeting the side occupied by the hinge and ligament, which, when very short, may determine a triangular form of the whole shell, or, when equal to the lower side and connected with a great height of the body, gives it a quadrangular form, or, if the height is reduced, produces an elongated form, or, finally, a rounded form, if the passage from one side to the other is gradual. A comparison of the position of the internal organs of different Species of Unios with the outlines of their shells will leave no doubt that their form is determined by the structure of the animal.

A few other and more familiar examples may complete this discussion. Among Climbing Birds, for instance, which are held together as a more comprehensive group by the structure of their feet and by other anatomical features, there are two Families so widely different in their form that they may well serve as examples of this principle. The Woodpeckers (Picidae) and the Parrots (Psittacidae), once considered as two Genera only, have both been subdivided, in consequence of a more intimate knowledge of their generic characters, into a large number of Genera; but all the Genera of Woodpeckers and all the Genera of the Parrots are still held together by their form as Families, corresponding as such to the two old Genera of Picus and Psittacus. They are now known as the Families of Woodpeckers and Parrots; and though each group includes a number of Genera combined upon a variety of details in the finish of special parts of the structure, such as the number of toes, the peculiarities of the bill, etc., it is impossible to overlook the peculiar form which is characteristic of each. No one who is familiar with the outline of the Parrot will fail to recognize any member of that Family by a general form which is equally common to the diminutive Nonpareil, the gorgeous Ara, and the high-crested Cockatoo. Neither will any one, who has ever observed the small head, the straight bill, the flat back, and stiff tail of the Woodpecker, hesitate to identify the family form in any of the numerous Genera into which this group is now divided. The family characters are even more invariable than the generic ones; for there are Woodpeckers which, instead of the four toes, two turning forward and two backward, which form an essential generic character, have three toes only, while the family form is always maintained, whatever variations there may be in the characters of the more limited groups it includes.

The Turtles and Terrapins form another good illustration of family characters. They constitute together a natural Order, but are distinguished from each other as two Families very distinct in general form and outline. Among Fishes I may mention the Family of Pickerels, with their flat, long snout, and slender, almost cylindrical body, as contrasted with the plump, compressed body and tapering tail of the Trout Family. Or compare, among Insects, the Hawk-Moths with the Diurnal Butterfly, or with the so-called Miller,—or, among Crustacea, the common Crab with the Sea-Spider, or the Lobsters with the Shrimps,—or, among Worms, the Leeches with the Earth-Worms,—or, among Mollusks, the Squids with the Cuttle-Fishes, or the Snails with the Slugs, or the Periwinkles with the Limpets and Conchs, or the Clam with the so-called Venus, or the Oyster with the Mother-of-Pearl shell,—everywhere, throughout the Animal Kingdom, difference of form points at difference of Families.

There is a chapter in the Natural History of Animals that has hardly been touched upon as yet, and that will be especially interesting with reference to Families. The voices of animals have a family character not to be mistaken. All the Canidae bark and howl: the Fox, the Wolf, the Dog have the same kind of utterance, though on a somewhat different pitch. All the Bears growl, from the White Bear of the Arctic snows to the small Black Bear of the Andes. All the Cats miau, from our quiet fireside companion to the Lions and Tigers and Panthers of the forest and jungle. This last may seem a strange assertion; but to any one who has listened critically to their sounds and analyzed their voices, the roar of the Lion is but a gigantic miau, bearing about the same proportion to that of a Cat as its stately and majestic form does to the smaller, softer, more peaceful aspect of the Cat. Yet, notwithstanding the difference in their size, who can look at the Lion, whether in his more sleepy mood as he lies curled up in the corner of his cage, or in his fiercer moments of hunger or of rage, without being reminded of a Cat? And this is not merely the resemblance of one carnivorous animal to another; for no one was ever reminded of a Dog or Wolf by a Lion. Again, all the Horses and Donkeys neigh; for the bray of the Donkey is only a harsher neigh, pitched on a different key, it is true, but a sound of the same character,—as the Donkey himself is but a clumsy and dwarfish Horse. All the Cows low, from the Buffalo roaming the prairie, the Musk-Ox of the Arctic ice-fields, or the Jack of Asia, to the Cattle feeding in our pastures. Among the Birds, this similarity of voice in Families is still more marked. We need only recall the harsh and noisy Parrots, so similar in their peculiar utterance. Or take as an example the web-footed Family,—do not all the Geese and the innumerable host of Ducks quack? Does not every member of the Crow Family caw, whether it be the Jackdaw, the Jay, the Magpie, the Rook in some green rookery of the Old World, or the Crow of our woods, with its long, melancholy caw that seems to make the silence and solitude deeper? Compare all the sweet warblers of the Songster Family,—the Nightingales, the Thrushes, the Mocking-Birds, the Robins; they differ in the greater or less perfection of their note, but the same kind of voice runs through the whole group. These affinities of the vocal systems among animals form a subject well worthy of the deepest study, not only as another character by which to classify the Animal Kingdom correctly, but as bearing indirectly also on the question of the origin of animals. Can we suppose that characteristics like these have been communicated from one animal to another? When we find that all the members of one zoological Family, however widely scattered over the surface of the earth, inhabiting different continents and even different hemispheres, speak with one voice, must we not believe that they have originated in the places where they now occur with all their distinctive peculiarities? Who taught the American Thrush to sing like his European relative? He surely did not learn it from his cousin over the waters. Those who would have us believe that all animals have originated from common centres and single pairs, and have been distributed from such common centres over the world, will find it difficult to explain the tenacity of such characters and their recurrence and repetition under circumstances that seem to preclude the possibility of any communication, on any other supposition than that of their creation in the different regions where they are now found. We have much yet to learn in this kind of investigation, with reference not only to Families among animals, but to nationalities among men also. I trust that the nature of languages will teach us as much about the origin of the races as the vocal systems of the animals may one day teach us about the origin of the different groups of animals. At all events, similarity of vocal utterance among animals is not indicative of identity of Species; I doubt, therefore, whether similarity of speech proves community of origin among men.

The similarity of motion in Families is another subject well worth the consideration of the naturalist: the soaring of the Birds of Prey,—the heavy flapping of the wings in the Gallinaceous Birds,—the floating of the Swallows, with their short cuts and angular turns,—the hopping of the Sparrows,—the deliberate walk of the Hens and the strut of the Cocks,—the waddle of the Ducks and Geese,—the slow, heavy creeping of the Land-Turtle,—the graceful flight of the Sea-Turtle under the water,—the leaping and swimming of the Frog,—the swift run of the Lizard, like a flash of green or red light in the sunshine,—the lateral undulation of the Serpent,—the dart of the Pickerel,—the leap of the Trout,—the rush of the Hawk-Moth through the air,—the fluttering flight of the Butterfly,—the quivering poise of the Humming-Bird,—the arrow-like shooting of the Squid through the water, —the slow crawling of the Snail on the land,—the sideway movement of the Sand-Crab,—the backward walk of the Crawfish,—the almost imperceptible gliding of the Sea-Anemone over the rock,—the graceful, rapid motion of the Pleurobrachia, with its endless change of curve and spiral. In short, every Family of animals has its characteristic action and its peculiar voice; and yet so little is this endless variety of rhythm and cadence both of motion and sound in the organic world understood, that we lack words to express one-half its richness and beauty.