Popular Science Monthly/Volume 63/May 1903/The Classification of Fishes

From Wikisource
Jump to navigation Jump to search

THE

POPULAR SCIENCE

MONTHLY

 

MAY, 1903.




THE CLASSIFICATION OF FISHES.
By President DAVID STARR JORDAN,

LELAND STANFORD JR. UNIVERSITY.

CLASSIFICATION, as Dr. Elliott Coues has well said, is a natural function of "the mind which always strives to make orderly disposition of its knowledge and so to discover the reciprocal relations and interdependences of the things it knows. Classification presupposes that there do exist such relations, according to which we may arrange objects in the manner which facilitates their comprehension, by bringing together what is like and separating what is unlike; and that such relations are the result of fixed, inevitable laws. It is, therefore, Taxonomy (τάςις, array; νόμος, law) or the rational, lawful disposition of observed facts."

A perfect taxonomy is one which would perfectly express all the facts in the evolution and development of the various forms. It would be based on morphology, the consideration of structure and form independent of adaptive, or physiological, or environmental modifications. It would regard those characters as most important which had existed longest unchanged in the history of the species or type, thus considering all knowledge derived from paleontology. It would regard as of minor importance those traits which had risen recently in response to natural selection or to the forced alteration through pressure of environment, while fundamental alterations as they appear one after another in geologic time would make the basal characters of corresponding groups in taxonomy. In greater or less degree, the life history of the individual, through the operation of the law of heredity, repeats the actual history of the group to which the individual belongs. For this reason the characters appearing first in the individual are likely to have greatest importance in classification.

In a perfect taxonomy, or natural system of classification, animals would not be divided into groups nor ranged in linear series. We should imagine a series variously and divergently branched, with each group at its earlier or lower end passing insensibly into the main or primitive stock. A very little alteration now and then in some structure is epoch-making and paves the way through specialization to a new class or order. But each class or order through its lowest types is intertangled with some earlier and otherwise diverging group. A sound system of taxonomy of fishes should be an exact reflex of the history of their evolution. But in the limitations of book making, this transcript must be made on a flat page, in linear series, while for centuries and perhaps forever whole chapters must be left vacant and others dotted everywhere with marks of doubt. For science demands that positive assertion should not go where certainty can not follow.

A perfect taxonomy of fishes would be only possible through the study, by some Artedi, Müller, Cuvier, Agassiz, Gill or Traquair, of all the structures of all the fishes which have ever lived. There are many fishes now living in the sea which are not yet known to any naturalist. Many others are known to one or two, but not yet accessible to those in other continents. Many are known externally from specimens in bottles, or drawings in books, but have not been studied thoroughly by any one, and the vast multitude even of the species have perished in Paleozoic, Mesozoic and Tertiary seas, without leaving a tooth or bone or fin behind them. With all this goes human fallibility, the marring of our records, such as they are, by carelessness, prejudice, dependence and error. Chief among these are the constant mistakes of analogy for homology, and the inability of men to trust their own eyes as against the opinion of the greater men who have had to form their opinions before all the evidence was in.

The result is, again to quote from Dr. Coues:

That the natural classification, like the elixir of life or the philosopher's stone, is a goal far distant.

It is obvious that fishes, like other animals, may be classified in numberless ways, and, as a matter of fact, by many different men they have been classified in all sorts of fashions.

Systems have been based on this or that set of characters, and erected from this or that preconception in the mind of the systematist. . . . The mental point of view was that every species of bird (or of fish) was a separate creation, and as much of a fixture in nature's museum as any specimen in a naturalist's cabinet. Crops of classifications have been sown in the fruitful soil of such blind error, but no lasting harvest has been reaped. . . . The genius of modern taxonomy seems to be so certainly right, to be tending so surely, even if slowly in the direction of the desired consummation, that all differences of opinion, we hope, will soon be settled, and defect of knowledge, no perversity of mind will be the only obstacle in the way of success. The taxonomic goal is not now to find the way in which birds (or other animals) may be most conveniently arranged, but to discover their pedigree, and so construct their family tree. Such a genealogical table or phylum (λῡΦον, tribe, race, stock) as it is called, is rightly considered the only sound basis of taxonomy. In attempting this end, we proceed upon the belief. . . that all birds, like all other animals and plants, are related to each other genetically, as offsprings are to parents; and that to discover their genetic relationships is to bring out their true affinities—in other words, to reconstruct the actual taxonomy of nature. In this view there can be but one 'natural' classification, to the perfecting of which all increase in our knowledge of the structure of birds infallibly and inevitably tends. The classification now in use, or coming into use, is the result of our best endeavors to accomplish this purpose, and represents what approach we have made to this end. It is one of the great corollaries of that theorem of Evolution which most naturalists are satisfied has been demonstrated. It is necessarily a—

Morphological Classification; that is, one based solely on consideration of structure or form (μοΦρή, morphe, form); and for the following reasons: Every offspring tends to take on precisely the structure or form of its parents, as its natural physical heritage; and the principle involved, or the law of heredity, would, if nothing interfered, keep the descendants perfectly true to the physical characters of their progenitors; they would breed true and be exactly alike. But counter influences are incessantly operative, in consequence of constantly varying external conditions of environment; the plasticity of organization of all creatures rendering them more or less susceptible of modification by such means, they become unlike their ancestors in various ways and to different degrees. On a large scale is thus accomplished, by natural selection and other natural agencies, just what man does in a small way in producing and maintaining different breeds of domestic animals. Obviously amidst such ceaselessly shifting scenes, degrees of likeness or unlikeness of physical structure indicate with the greatest exactitude the nearness or remoteness of organisms in kinship. Morphological characters derived from examination of structure are therefore the surest guides we can have to the blood-relationships we desire to establish; and such relationships are the 'natural afiinities' which all classification aims to discover and formulate. (Coues.)

A few terms in general use may receive a moment's discussion. A type or group is said to be specialized when it has a relatively large number of peculiarities, or when some one peculiarity is carried to an extreme. A sculpin is a specialized fish, having many unusual phases of development, as is also a sword-fish, which has a highly peculiar structure of the snout. A generalized type is one with fewer peculiarities, as the herring in comparison with the sculpin. In the process of evolution, generalized types usually give place to specialized ones. Generalized types are therefore as a rule archaic types.

The terms high and low are also relative; a high type being one with varied structure and functions. Low types may be primitively generalized, as the lancelet in comparison with all other fishes, or the herring in comparison with the perch; or they may be due to degradation, a loss of structures which have been elaborately specialized in their ancestry.

The sea-snail (Liparis), an ally of the sculpin, with scales lost and fins deteriorated, is an example of a low type which is specialized as well as degraded.

In the earlier history of ichthyology, much confusion resulted from the misconception of the terms 'high' and 'low.' Because sharks appeared earlier than bony fishes, it was assumed that they should be lower than any of their subsequent descendants. That the brain and muscular system in sharks was more highly developed than in most bony fishes seemed also certain. Therefore, it was thought that the Teleost series could not have had a common origin with the series of sharks. It is now understood that evolution means chiefly adaptation, and adaptation among fishes is almost as often degradation as advance. The bony fish is adapted to its mode of life, and to that end it is specialized in fin and skeleton rather than in brain and nerves as compared with its ancestors. All degeneration is associated with specialization. The degeneration of the blind fish is a specialization for better adaptation to life in the darkness of caves; the degeneration of the deep-sea fish meets the demands of the depths; the degeneration of the globe fish means the sinking of one line of functions in the extension of some other.

Referring to his own work on the fossil fishes in the early forties, Professor Agassiz once said to the writer:

At that time I was on the verge of anticipating the views of Darwin, but it seemed to me that the facts were contrary to the theories of evolution; we had the highest fishes first.

This statement leads us to consider what is meant by 'high' and 'low.' Undoubtedly the sharks are higher than the bony fishes in the sense of being nearer to the higher vertebrates. In brain, muscle, teeth and reproductive structures, they are also more highly developed. In all skeletal and cranial characters the sharks stand distinctly lower. But the essential fact, so far as evolution is concerned, is not that the sharks are high or low. They are in almost all respects distinctly generalized and primitive. The bony fishes are specialized in various ways through adaptation to the various modes of life they lead. Much of this specialization involves corresponding degeneration of organs whose functions have ceased to be important. As a broad proposition, it is not true that 'we had our highest fishes first,' for in a complete definition of 'high' and 'low,' the specialized perch or bass stands higher. But whether true or not, it does not touch the question of evolution which is throughout a process of adaptation to conditions of life.

In another essay. Dr. Coues has compared species of animals to "the twigs of a tree separated from the parent stem. We name and arrange them arbitrarily in default of a means of reconstructing the whole tree according to nature's ramifications."

If one had a tree, all in fragments, pieces of twig and stem, some of them lost, some destroyed, and some not yet separated from the mass not yet picked over, and wished to place each part where he could find it, he would be forced to adopt some system of natural classification. In such a scheme he would lay those parts together which grew from the same branch. If he were compelled to arrange all the fragments in a linear series, he would place together those of one branch, and when these were finished, he would begin with another. If all this were a matter of great importance, extending over years or over many lifetimes, with many errors to be made and corrected, a set of names would be adopted—for the main trunk, for the chief branches, the lesser branches, and on down to the twigs and buds.

A task of tills sort on a world-wide scale is the problem of systematic zoology. There is reason to believe that all animals and plants sprang from a single stock. There is reasonable certainty that all vertebrate animals are derived from a single origin. These vertebrate animals stand related to each other, like the twigs of a gigantic tree, the lowermost branches are the aquatic forms to which we give the name of fishes, with their still more primitive fish-like relatives.

The aquatic vertebrates, reasonably called by the names of fishes, constitute about three classes, or larger lines of descent. There are lampreys, sharks and true fishes. If we include the extinct forms, we may perhaps add two more, but this is uncertain, while below the fishes are the protochordate classes of Enteropneustans, Tunicates and Lancelets, which stand nearer to fishes than to anything else. Each of these groups differs from the others in varying degree.

Each of these again is composed of minor divisions called orders, each containing many species. The different species, or ultimate kinds of animals are again grouped in genera. A genus is an assemblage of closely related species grouped around a central species as type. The type of a genus is, in common usage, that species with which the name of the genus was first associated. The name of the genus, as a noun, taken with that of the species, which is an adjective in signification, if not in form, constitutes the scientific name of the species. Thus Petromyzon is the genus of the common large lamprey; marinus is its species, and the scientific name of the species is Petromyzon marinus. Petromyzon means stone-sucker; marinus of the sea; thus distinguishing it from a species called fluviatilis, of the river.

In like fashion all animals and plants are named in scientific record or taxonomy.

A family in zoology is an assemblage of related genera. The name of a family, for convenience, always ends in the patronymic idæ, and it is always derived from the leading genus, that is, the one best known or earliest studied. Thus all lampreys constitute the family Petromyzonidæ.

An order may contain one or more families. An order is a division of a larger group; a family, an assemblage of related smaller groups. Intermediate groups are often recognized by the prefixes sub or super. A subgenus is a division of a genus. A subspecies is a geographic race or variation within a species; a superfamily, a group of allied families.

Binominal nomenclature, or the use of the name of genus and species as a scientific name was introduced into science as a systematic method by Linnæus. In the tenth edition of his 'Systema Naturæ' published in 1758, this method was first consistently applied to animals. By common consent, the scientific naming of animals begins with this year, and no account is taken of names given earlier, as these are, except by accident, never binomial. Those authors who wrote before the adoption of the rule of binomials and those who neglected it are alike ruled out of court. The idea of genus and species was well understood before Linnaeus, but the specific name used was not one word but a descriptive phrase, and this phrase was changed at the whim of the different authors. Examples of such names are these of the West Indian trunk-fish, or Cuckold: Ostracion tricornis of Linnæus. Lister refers to a specimen in 1686 as Piscis triangularis capiti cormitis cui e media cauda cutanea oculeus longus erigitus. This Aretdi alters in 1738 to Ostracion triangulatus aculeis duobus in capiti et unico longioro superne ad caudam. This is more accurately descriptive and it recognizes the existence of a generic type, Ostracion, or trunk fish, to cover all similar fishes. French writers transformed this into various phrases beginning: Coffre triangulaire a trois cornes or some similar descriptive epithet, and in English or German it was likely to wander still farther from the original. But Linnaeus condenses it all in the word tricornis, which although not fully descriptive, is still a name which all future observers can use and recognize.

It is true that common consent fixes the date of the beginning of nomenclature at 1758, but to this there are many exceptions. Some writers date genera from the first recognition of a collective idea under a single name. Others follow even species back through the occasional accidental binomials. Most British writers have chosen the final and completed edition of the 'Systema Naturæ,' the last work of Linnæus' hand in 1766, in preference to the earlier volume. But all things considered, justice and convenience alike seem best served by the use of the edition of 1758.

Synonymy is the record of the names applied at different times to the same group or species. With characteristic pungency Dr. Coues defines synonymy as 'a burden and a disgrace to science.' It has been found that the only way to prevent utter confusion is to use for each genus or species the first name applied to it and no other. The first name, once properly given is sacred because it is the right name. All other later names, whatever their appropriateness in meaning, are wrong names in taxonomy. In science, of necessity, a name is a name without any necessary signification. For this reason and for the further avoidance of confusion, it should remain as it was originally spelled by the author, obvious misprints aside, regardless of all possible errors in classical form or meaning. This rule is now generally adopted in America, because attempts at classical purism have simply produced confusion. The names in use are properly written in Latin or in latinized Greek, the Greek forms being usually preferred as generic names, the Latin adjectives for names of species. Many species are named in honor of individuals, these names bearing usually the termination of the Latin genitive, as Sebastodes gillii, Liparis agassizi. In recent custom all specific names in zoology are written with the small initial; all generic names with the capital.

One class of exceptions must be made to the law of priority. No generic name can be used twice among animals, and no specific name twice in the same genus. Thus the name Diabasis has to be set aside in favor of the next name, Hæmulon, because Diabasis was earlier used for a genus of beetles. The specific name, Pristipoma humile, is abandoned, because there was already a humile in the genus Pristipoma.

In the system of Linnæus, a genus corresponded roughly to the modern conception of a family. Most of the primitive genera contained a great variety of forms, as well as usually some species belonging to other groups dissociated from their real relationships.

As greater numbers of species have become known, the earlier genera have undergone subdivision until in the modern systems almost any structural character not subject to intergradation and capable of exact definition is held to distinguish a genus. As the views of the value of characters are undergoing constant change, and as different writers look upon them from different points of view, or with different ideas of convenience, we must have constant changes in the boundaries of genera. This brings constant changes in the scientific names, although the same specific name should be used whatever the generic name to which it may be attached. We may illustrate these changes and the 'burden of synonymy' as well by a concrete example. The horned trunk-fish or cuckold of the West Indies was first recorded by Lister in 1686 in the descriptive phrase above quoted. Artedi in 1738 recognized that it belonged with other trunk-fishes in a group he called Ostracion treating the word as a Latin masculine although derived from a Greek neuter diminutive (ὀδτραχἱον, a small box). This, to be strictly classic, he should have written Ostracium, but he preferred a partly Greek form to the Latin one. In the Nagg's Head Inn in London, Artedi saw a trunk-fish he thought different, having two spines on the tail, while Lister's figure seemed to show one spine ahove it. This Nagg's Head specimen Artedi called Ostracion triangulabus duobus aculeis in fronte et totidem in imo ventre subcaudalesque binis."

Next came Linnæus, 1758, who named Lister's figure and the species it represented Ostracion tricornis, which should in strictness have heen Ostracion tricorne, as ὀστραχἱον, is a neuter diminutive. The Kagg's Head fish he named Ostracion quadricornis. The right name is Ostracion tricornis, because the name tricornis stands first on the page; but Ostracion quadricornis has been most used by subsequent authors, it being nearer correct as a descriptive phrase.

In 1798, Lacépède changed the name of Lister's fish to Ostracion listeri, a needless alteration which could only make confusion.

In 1818, Professor Mitchill, receiving a specimen from below New Orleans, thought it different from tricornis and quadricornis and called it Ostracion sex-cornutus. Hollard in 1857 named a specimen Ostracion maculatus, and at about the same time Bleeker named two others from Africa which seem to be the same thing, Ostracion guineensis and Ostracion gronovii. Lastly Poey calls a specimen from Cuba Acanthostracion polygonius, thinking it different from all the rest, which it may be, though the chances are to the contrary.

This brings up the question of the generic name. Among trunkfishes there are four-angled and three-angled kinds, and in each form species with and without horns and spines. The original Ostracion of Linnæus we may interpret as being based on Ostracion culicus of the coasts of Asia. This we call the type species of the genus, as the Nagg's Head specimen of Artedi was the type specimen of the species quadricornis, or the one that was used for Lister's figure, the type specimen of tricornis.

Cubicus is a four-angled species, and when the trunk-fishes were regarded as a family, Ostraciidæ, the three-angled ones, were set off as a separate genus. For these forms two names were offered, both by Swainson in 1839. For trigonus, a species without horns before the eyes, he gave the name Lactophrys, and for triqueter, a species without spines anywhere, the name of Rhinesomus. Several recent American authors have placed the three-cornered species, which are all American, in one genus, which must therefore be called Lactophrys. Of this name Rhinesomus is a synonym, and our species should stand as Lactophrys tricornis. The fact that Lactophrys, as a word (from Latin lactus, smooth, Greek ὀφρὐς, eyebrow; or else from lactoria, a milk cow and ὀφρὐς) is either meaningless or incorrect makes no difference with the necessity for its use.

In 1862, Bleeker undertook to divide these fishes differently. Placing all the hornless species, whether three-angled or four-angled in Ostracion, he proposed the name Acanthostracion, for the species with horns, tricornis being the type. But Acanthostracion has not been usually adopted except as the name of a section under Lactophrys. The three-angled American species are usually set apart from the fourangled species of Asia, and our cuckold is called Lactophrys tricornis. But it may be with perfect correctness called Ostracion tricorne in the spirit called conservative. Or with the radical systematists we may accept the finer definition and again correctly call it Acanthostracion tricorne. But to call it quadricornis, or listeri, or maculatus, with any generic name, whatever, would be to violate the law of priority.

By trinomial nomenclature we mean the use of a second, subordinate specific name to designate a geographic subspecies, variety or other intergrading race. Thus Salmo clarki virginalis indicates the variety of Clark's trout, or the cut-throat trout, found in the lakes and streams of the Great Basin of Utah, as distinguished from the genuine Salmo clarkii of the Columbia.

Trinomials are not much used among fishes, as we are not yet able to give most of these local forms correct and adequate definitions such as is awarded to similar variations among birds and mammals. Some of these forms will turn out to be real species, while others represent slight individual variations. It will take long study of much material to define these two sorts of subspecies and to separate one from the other. It is easier to preserve and to study birds than fishes and more people are engaged in it. For this reason the fine discrimination of variant forms has been possible much earlier in ornithology than in ichthyology.