CLASSIFICATION OF FISHES
=Taxonomy.=--Classification, as Dr. Elliott Coues has well said,[147]
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 interdependencies 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 law.
It is therefore taxonomy (~taxis~, away; ~nomos~, 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 recognize all the evidence from the three ancestral documents,
palæontology, morphology, and ontogeny. It would consider structure
and form independently of adaptive or physiological or environmental
modifications. It would regard as most important those characters which
had existed longest unchanged in the history of the species or type.
It would regard as of first rank those characters which appear first
in the history of the embryo. It would regard as of minor importance
those which had arisen recently in response to natural selection or 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 a
perfect taxonomy or natural system of classification animals would not
be divided into groups nor ranged in linear series. We should imagine
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
interlocked with some earlier and otherwise diverging group.
=Defects in Taxonomy.=--A sound system of taxonomy of fishes should
be an exact record 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 cannot follow. A perfect taxonomy of fishes would be only
possible through the study, by some Artedi, Müller, Cuvier, Agassiz,
Traquair, Gill, or Woodward, of all the structures of all the fishes
which have ever lived. There are many fishes living in the sea which
are not yet known to any naturalist, many others are known from one or
two specimens, but not yet accessible to students 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 of species have perished in Palæozoic, 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
defects are the constant mistaking 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 evidence was
in. Because of these defects, the current system of classification is
always changing with each accession of knowledge.
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."
=Analogy and Homology.=--_Analogy_, says Dr. Coues, "is the apparent
resemblance between things really unlike--as the wing of a bird and
the wing of a butterfly, as the lungs of a bird and the gills of a
fish. _Homology_ is the real resemblance, or true relation between
things, however different they may appear to be--as the wing of a bird
and the foreleg of a horse, the lungs of a bird and the swim-bladder
of a fish. The former commonly rests upon mere functional, i.e.
physiological, modifications; the latter is grounded upon structural,
i.e., morphological, identity or unity. Analogy is the correlative of
physiology, homology of morphology; but the two may be coincident,
as when structures identical in morphology are used for the same
purposes, and are therefore physiologically identical. Physiological
diversity of structure is incessant, and continually interferes with
morphological identity of structure, to obscure or obliterate the
indications of affinity the latter would otherwise express clearly....
We must be on our guard against those physiological appearances which
are proverbially deceptive!"
"It is possible and conceivable that every animal should have been
constructed upon a plan of its own, having no resemblance whatever
to the plan of any other animal. For any reason we can discover
to the contrary, that combination of natural forces which we term
life might have resulted from, or been manifested by, a series of
infinitely diverse structures; nor would anything in the nature of
the case lead us to suspect a community of organization between
animals so different in habit and in appearance as a porpoise and a
gazelle, an eagle and a crocodile, or a butterfly and a lobster. Had
animals been thus independently organized, each working out its life
by a mechanism peculiar to itself, such a classification as that now
under contemplation would be obviously impossible; a morphological or
structural classification plainly implying morphological or structural
resemblances in the things classified.
"As a matter of fact, however, no such mutual independence of animal
forms exists in nature. On the contrary, the members of the animal
kingdom, from the highest to the lowest, are marvelously connected.
Every animal has something in common with all its fellows--much with
many of them, more with a few, and usually so much with several that it
differs but little from them.
"Now, a morphological classification is a statement of these
gradations of likeness which are observable in animal structures, and
its objects and uses are manifold. In the first place, it strives to
throw our knowledge of the facts which underlie, and are the cause
of, the similarities discerned into the fewest possible general
propositions, subordinated to one another, according to their greater
or less degree of generality; and in this way it answers the purpose of
a _memoria technica_, without which the mind would be incompetent to
grasp and retain the multifarious details of anatomical science."
=Coues on Classification.=--It is obvious that fishes like other
animals may be classified in numberless ways, and as a matter of fact
by numberless men they have been classified in all sorts of fashions.
"Systems," again quoting from Dr. Coues, "have been based on this and
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 creature, 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, not perversity of mind, is 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_ (~phylon~, tribe, race, stock),
as it is called, is rightly considered the only taxonomy worthy the
name--the only true or natural classification. In attempting this end,
we proceed upon the belief that, as explained above, all birds, like
all other animals and plants, are related to each other genetically, as
offspring are to parents, and that to discover their generic relations
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 upon considerations of structure or form
(~morphê~, form, _morphe_), and for the following reasons: Every
offspring tends to take on precisely the form or structure 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 modifications 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 the
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 affinities' which all classification aims to discover
and formulate."
=Species as Twigs of a Genealogical Tree.=--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 and 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 this 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 of
which the lowermost branches are the aquatic forms to which we give
the name of fishes. The fishes are here regarded as composed of six
classes or larger lines of descent. Each of these, again, is composed
of minor divisions called orders. The different species or ultimate
kinds of animals are 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,
often 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. Technical names are necessary because vernacular names fail.
Half a million kinds of animals are known, while not half a thousand
vernacular names exist in any language. And these are always loosely
used, half a dozen of them often for the same species, one name often
for a dozen species.
In the same way, whenever we undertake an exact description, we must
use names especially devised for that purpose. We cannot use the same
names for the bones of the head of a fish and those of the head of a
man, for a fish has a different series of bones, and this series is
different with different fishes.
=Nomenclature.=--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 super-family a group of allied families. Binomial 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 Linnæus, 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.
[Illustration: FIG. 239.--Horned Trunkfish, Cowfish, or Cuckold,
_Lactophrys tricornis_ (Linnæus). Charleston, S. C.]
=Nomenclature of Trunkfishes.=--Examples of such names are those of
the West Indian trunkfish, or cuckold (_Ostracion tricorne_, Linnæus).
Lister refers to a specimen in 1686 as "_Piscis triangularis capiti
cornutu cui e media cauda cutanea aculeus longus erigitus_." This
Artedi alters in 1738 to _Ostracion triangulatus aculeis duobus in
capite et unico longiore superne ad caudam_. This is more accurately
descriptive and it recognizes the existence of a generic type,
_Ostracion_, or trunkfish, to cover all similar fishes. French writers
transformed this into various phrases beginning "Coffre triangulaire
à trois cornes," or some similar descriptive epithet, and in English
or German it was likely to wander still farther from the original. But
Linnæus 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, 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 and Priority.=--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 are wrong names. In science, of necessity, a name is a
name without any necessary signification. For this reason and for the
further avoidance of confusion, it remains as it was originally spelled
by the author, obvious misprints aside, regardless of all possible
errors in classical form or meaning. 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 being
usually given the termination of the Latin genitive, as _Sebastodes
gillii, Liparis agassizi_. In recent custom all specific names 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_.
=The Conception of Genus.=--In the system of Linnæus, a genus
corresponds 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 disassociated 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 these characters are
undergoing constant change, and as different writers look upon them
from different points of view, or with different ideas of convenience,
we 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 Trunkfishes.=--The horned trunkfish, 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
trunkfishes in a group he called _Ostracion_. 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 trunkfish he thought different, having two spines under the tail,
while Lister's figure seemed to show one spine above. This Nagg's Head
specimen Artedi called "_Ostracion triangulatus 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
been _Ostracion tricorne_, as ~ostrakion~, a little box, is a neuter
diminutive. The Nagg's Head fish he named _Ostracion quadricornis_. The
right name now is _Ostracion tricornis_, because the name _tricornis_
stands first on the page in Linnæus' work, but _Ostracion quadricornis_
has been more often used by subsequent authors because it is more
truthful 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.
[Illustration: FIG. 240.--Horned Trunkfish, _Ostracion cornutum_
Linnæus. East Indies. (After Bleeker.)]
In 1818, Dr. Samuel Latham Mitchill, receiving a specimen from below
New Orleans, thought it different from _tricornis_ and _quadricornis_
and called it _Ostracion sexcornutus_; Dr. Holard, of Paris, 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, although my own judgment
is otherwise. This brings up the question of the generic name. Among
trunkfishes there are four-angled and three-angled kinds, and of each
form there are species with and without horns and spines. The original
_Ostracion_ of Linnæus we may interpret as being _Ostracion cubicus_ of
the coasts of Asia, a species similar to the _Ostracion rhinorhynchus_.
This species, _cubicus_, we call the type species of the genus, as the
Nagg's Head specimen of Artedi was the type specimen of the species
_quadricornus_, and the one that was used for Lister's figure the type
specimen of _tricornis_.
_Ostracion cubicus_ is a four-angled species, and when the trunkfishes
were regarded as a family (_Ostraciidæ_), the three-angled ones
were set off as a separate genus. For this 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_. Most
recent American authors have placed the three-cornered species which
are mostly 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 _lætus_, smooth; Greek ~ophrys~, eyebrow; or else
from _lactoria_, a milk cow, and ~ophrys~) is either meaningless or
incorrectly written makes no difference with the necessity for its use.
[Illustration: FIG. 241.--Spotted Trunkfish, _Lactophrys bicaudalis_
(Linnæus). Cozumel Island, Yucatan.]
[Illustration: FIG. 242.--Spotted Trunkfish (face view), _Lactophrys
bicaudalis_ (Linnæus).]
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 four-angled 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.
[Illustration: FIG. 243.--Spineless Trunkfish, _Lactophrys triqueter_
(Linnæus). Tortugas.]
=Trinomial Nomenclature.=--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 many of the local forms correct and adequate definition such as
is awarded to similar variations among birds and mammals. Usually
varieties in ichthyology count as species or as nothing.
[Illustration: FIG. 244.--Hornless Trunkfish, _Lactophrys trigonus_
(Linnæus). Tortugas, Florida.]
[Illustration: FIG. 245.--Hornless Trunkfish (face-view), _Lactophrys
trigonus_ (Linnæus). Charleston, S. C.]
=Meaning of Species.=--Quoting once more from the admirable essay
of Dr. Coues on the taxonomy of birds: "The student cannot be too
well assured that no such things as species, in the old sense of
the word, exist in nature any more than have genera or families an
actual existence. Indeed they cannot be, if there is any truth in
the principles discussed in our earlier paragraphs. Species are
simply ulterior modifications, which once were, if they be not still,
inseparably linked together; and their nominal recognition is a
pure convention, like that of a genus. More practically hinges upon
the way we regard them than turns upon our establishment of higher
groups, simply because upon the way we decide in this case depends the
scientific labeling of specimens. If we are speaking of a robin, we do
not ordinarily concern ourselves with the family or order it belongs
to, but we do require a technical name for constant use. That name
is compounded of its genus, species, and variety. No infallible rule
can be laid down for determining what shall be held to be a species,
what a conspecies, subspecies, or variety. It is a matter of tact and
experience, like the appreciation of the value of any other group
in zoology. There is, however, a convention upon the subject, which
the present workers in ornithology in this country find available;
at any rate we have no better rule to go by. We treat as "specific"
any form, however little different from the next, that we do not
know or believe to intergrade with that next one, between which and
the next one no intermediate equivocal specimens are forthcoming,
and none, consequently, are supposed to exist. This is to imply that
differentiation is accomplished, the links are lost and the characters
actually become "specific." We treat as "varietal" of each other any
forms, however different in their extreme manifestation, which we know
to intergrade, having the intermediate specimens before us, or which we
believe with any good reason do intergrade. If the links still exist,
the differentiation is still incomplete, and the characters are not
specific, but only varietal, in the literal sense of these terms."
=Generalization and Specialization.=--A few terms in common 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 swordfish, which has a highly peculiar structure in 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.
=High and Low Forms.=--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. The teleost is adapted to its mode of life, and to that end
it is specialized in fin and skeleton rather than in brain and nerves.
All degeneration is associated with specialization. The degeneration
of the blindfish 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 globefish 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.
Referring to the position of Agassiz and his early friend and disciple,
Hugh Miller, Dr. Traquair (1900) uses these words in an address at
Bradford, England:
"It cannot but be acknowledged that the paleontology of fishes is not
less emphatic in the support of descent than that of any other division
of the animal kingdom. But in former days the evidence of fossil
ichthyology was by some read otherwise.
"It is now a little over forty years since Hugh Miller died: he who
was one of the first collectors of the fossil fishes of the Scottish
old red sandstone, and who knew these in some respects better than any
other man of his time, not excepting Agassiz himself. Yet his life was
spent in a fierce denunciation of the doctrine of evolution, then only
in its Lamarckian form, as Darwin had not yet electrified the world
with his 'Origin of Species.' Many a time I wonder greatly what Hugh
Miller would have thought had he lived a few years longer, so as to
have been able to see the remarkable revolution which was wrought by
the publication of that book.
"The main argument on which Miller rested was the 'high' state of
organization of the ancient fishes of the Paleozoic formations, and
this was apparently combined with a confident assumption of the
completeness of the geological record. As to the first idea, we know
of course that evolution means the passage from the more general to
the more special, and that as the general result an onward advance has
taken place; yet 'specialization' does not always or necessarily mean
'highness' of organization in the sense in which the term is usually
employed. As to the idea of the perfection of the geological record,
that of course is absurd.
"We do not and cannot know the oldest fishes, as they would not have
had hard parts for preservation, but we may hope to come to know
many more old ones, and older ones still than we do at present. My
experience on the subject of fossil ichthyology is that it is not
likely to become exhausted in our day.
"We are introduced at a period far back in geological history to
certain groups of fishes, some of which certainly are high in
organization as animals, but yet of generalized type, being fishes
and yet having the potentiality of higher forms. But because their
ancestors are unknown to us, that it is no evidence that they did
not exist, and cannot overthrow the morphological testimony in favor
of evolution with which the record actually does furnish us. We may
therefore feel very sure that fishes or 'fish-like vertebrates' lived
long ages before the oldest forms with which we are acquainted came
into existence.
"The modern type of bony fishes, though not so 'high' in many
anatomical points as that of the Selachii, Crossopterygii, Dipnoi,
Acipenseroidei, and Lepidosteoidei of the Palæozoic and Mesozoic
eras, is more specialized in the direction of the fish proper, and,
as already indicated, specialization and 'highness' in the ordinary
sense of the word are not necessarily coincident. But ideas about these
things have undergone a wonderful change since those pre-Darwinian
days, and though we shall never be able fully to unravel the problems
concerning the descent of animals, we see many things a great deal more
clearly now than we did then."
Dr. Gill observes: "Perhaps there are no words in science that have
been productive of more mischief and more retarded the progress of
biological taxonomy than those words pregnant with confusion, High and
Low, and it were to be wished that they might be erased from scientific
terminology. They deceive the person to whom they are addressed. They
insensibly mislead the one who uses them. Psychological prejudices
and fancies are so inextricably associated with these words that the
use of them is provocative of such ideas. The words, generalized and
specialized, having become almost limited to the expression of the
ideas which the scientific biologist wishes to unfold by the others,
can with great gain be employed in their stead." ("Families of Fishes,"
1872.)
=The Problem of the Highest Fishes.=--As to which fishes should be
ranked highest and which lowest, Dr. Gill gives ("Families of Fishes,"
1872) the following useful discussion: "While among the mammals there
is almost universal concurrence as to the forms entitled to the first
as well as the last places, naturalists differ much as to the 'highest'
of the ichthyoid vertebrates, but are all of one accord respecting the
form to be designated as the 'lowest.' With that admitted lowest form
as a starting-point, inquiry may be made respecting the forms which are
successively _most nearly related_.
"No dissent has ever been expressed from the proposition that the
Leptocardians (_Branchiostoma_) are the lowest of the vertebrates;
while they have doubtless deviated much from the representatives of the
immediate line of descent of the higher vertebrates, and are probably
specialized considerably, in some respects, in comparison with those
vertebrates from which they (in common with the higher forms) have
descended, they undoubtedly have diverged far less, and furnish a
better hint as to the protovertebrates than any other form.
"Equally undisputed it is that most nearly related to the Leptocardians
are the Marsipobranchiates (_Lampreys_, etc.), and the tendency has
been rather to overlook the fundamental differences between the two,
and to approximate them too closely, than the reverse.
"But here unanimity ends, and much difference of opinion has prevailed
with respect to the succession in the system of the several
subclasses (by whatever name called) of true fishes: (1) Some (e.g.,
Cuvier, J. Müller, Owen, Lütken, Cope) arranging next to the lowest
the Elasmobranchiates, and, as successive forms, the Ganoids and
Teleosteans; (2) while others (e.g., Agassiz, Dana, Duméril, Günther)
adopt the sequence Leptocardians, Marsipobranchiates, Teleosteans,
Ganoids, and Elasmobranchiates. The source of this difference
of opinion is evident and results partly from metaphysical or
psychological considerations, and partly from those based (in the case
of the Ganoids) on real similarities and affinities.
"The evidence in favor of the title of the Elasmobranchiates to the
'highest' rank is based upon (1) the superior development of the brain;
(2) the development of the egg, and the ovulation; (3) the possession
of a placenta; and (4) the complexity of the organs of generation.
"(1) It has not been definitely stated wherein the superior development
of the brain consists, and as it is not evident to the author, the
vague claim can only be met by this simple statement; it may be added,
however, that the brains comparable in essentials and most similar as
a whole to those of the Marsipobranchiates are those of the sharks. In
answer to the statement that the sharks exhibit superior intelligence,
and thus confirm the indications of cerebral structure, it may be
replied that the impression is a subjective one, and the author has
not been thus influenced by his own observations of their habits.
Psychological manifestations, at any rate, furnish too vague criteria
to be available in exact taxonomy.
"(2) If the development of the eggs, their small number, and their
investment in cases are arguments in favor of the high rank of the
Elasmobranchiates, they are also for the Marsipobranchiates, and
thus prove too much or too little for the advocates of the views
discussed. The variation in number of progeny among true fishes (e.g.,
Cyprinodonts, _Embiotocids_) also demonstrates the unreliability of
those modifications _per se_.
"(3) The so-called placenta of some Elasmobranchiates may be
_analogous_ to that of mammals, but that it is not _homologous_
(i.e., homogenetic) is demonstrable from the fact that all the forms
intervening between them and the specialized placental mammals are
devoid of a placenta, and by the variation (presence or want) among the
Elasmobranchiates themselves.
"(4) The organs of generation in the Elasmobranchiates are certainly
more complex than in most other fishes, but as the complexity results
from specialization of parts _sui generis_ and different from those of
the higher (quadruped) vertebrates, it is not evident what bearing the
argument has. If it is claimed simply on the ground of specialization,
irrespective of homological agreement with admitted higher forms,
then are we equally entitled to claim any specialization of parts
as evidence of high rank, or at least we have not been told within
what limits we should be confined. The Cetaceans, for example, are
excessively specialized mammals, and, on similar grounds, would rank
above the other mammals and man; the aye-aye exhibits in its dentition
excessive specialization and deviation from the primitive type (as
exhibited in its own milk teeth) of the Primates, and should thus also
rank above man. It is true that in other respects the higher primates
(even including man) may be more specialized, but the specialization is
not as obvious as in the cases referred to, and it is not evident how
we are to balance _irrelative_ specializations against each other, or
even how we shall subordinate such cases. We are thus compelled by the
_reductio ad absurdum_ to the confession that irrelative specialization
of single organs is untrustworthy, and are fain to return to that
better method of testing affinities by the equation of agreement in
whole and after the elimination of special teleological modifications.
"The question then recurs, What forms are the most _nearly allied_
to the Marsipobranchiates, and what show the closest approach in
_characteristic_ features? And in response thereto the evidence is
not undecisive. Wide as is the gap between Marsipobranchiates and
fishes, and comparatively limited as is the range of the latter among
themselves, the Elasmobranchiates are very appreciably more like, and
share more characters in common with them, than any other; so much is
this the case that some eminent naturalists (e.g., Pallas, Geoffroy,
St. Hilaire, Latreille, Agassiz, formerly Lütken) have combined the
two forms in a peculiar group, contradistinguished from the other
fishes. The most earnest and extended argument in English, in favor of
this combination has been published by Professor Agassiz in his 'Lake
Superior,' but that eminent naturalist subsequently arrived at the
opposite conclusions already indicated.
"The evidences of the closer affinity of the Elasmobranchiates (than
of any other fishes) with the Marsipobranchiates are furnished by (1)
the cartilaginous condition of the skeleton; (2) the post-cephalic
position of the branchiæ; (3) the development of the branchiæ and their
restriction to special chambers; (4) the larger number of the branchiæ;
(5) the imperfect development of the skull; (6) the mode of attachment
of the teeth; (7) the slight degree of specialization of the rays of
the fins; and (8) the rudimentary condition of the shoulder-girdle."
FOOTNOTES:
[147] Key to North American Birds.