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Snakes. Such organs are found in the Sting-rays, the tail of which is
armed with one or more powerful barbed spines. Although they lack
a special organ secreting poison, or a canal in or on the spine by
which the venomous fluid is conducted, the symptoms caused by a
wound from the spine of a Sting-ray are such as cannot be
accounted for merely by the mechanical laceration, the pain being
intense, and the subsequent inflammation and swelling of the
wounded part terminating not rarely in gangrene. The mucus
secreted from the surface of the fish and inoculated by the jagged
spine evidently possesses venomous properties. This is also the
case in many Scorpænoids, and in the Weaver (Trachinis), in which
the dorsal and opercular spines have the same function as the
caudal spines of the Sting-rays; however, in the Weavers the spines
are deeply grooved, the groove being charged with a fluid mucus. In
Synanceia the poison-organ (Fig. 99,) is still more developed: each
dorsal spine is in its terminal half provided with a deep groove on
each side, at the lower end of which lies a pear-shaped bag
containing the milky poison; it is prolonged into a membranous duct,
lying in the groove of the spine, and open at its point. The native
fishermen, well acquainted with the dangerous nature of these
fishes, carefully avoid handling them; but it often happens that
persons wading with naked feet in the sea, step upon the fish, which
generally lies hidden in the sand. One or more of the erected spines
penetrate the skin, and the poison is injected into the wound by the
pressure of the foot on the poison-bags. Death has not rarely been
the result.
Fig. 99.—A dorsal spine, with
poison-bags, of Synanceia
verrucosa. Indian Ocean.
 Fig. 100.—Opercular part of the
Poison-apparatus of
Thalassophryne (Panama).
1. Hinder half of the head, with the
venom-sac* in situ. a, Lateral
line and its branches; b, Gill-
opening; c, Ventral fin; d, Base
of Pectoral fin; e, Base of
dorsal.
2. Operculum with the
perforated spine.

The most perfect poison-organs hitherto discovered in fishes are

those of Thalassophryne, a Batrachoid genus of fishes from the
coasts of Central America. In these fishes the operculum again and
the two dorsal spines are the weapons. The former (Fig. 100, 2) is
very narrow, vertically styliform and very mobile; it is armed behind
with a spine, eight lines long, and of the same form as the hollow
venom-fang of a snake, being perforated at its base and at its
extremity. A sac covering the base of the spine discharges its
contents through the apertures and the canal in the interior of the
spine. The structure of the dorsal spines is similar. There are no
secretory glands imbedded in the membranes of the sacs; and the
fluid must be secreted by their mucous membrane. The sacs are
without an external muscular layer, and situated immediately below
the thick loose skin which envelops the spines to their extremity; the
ejection of the poison into a living animal, therefore, can only be
effected, as in Synanceia, by the pressure to which the sac is
subjected the moment the spine enters another body.
Finally, a singular apparatus found in many Siluroids may be
mentioned in connection with the poison-organs, although its
function is still problematical. Some of these fishes are armed with
powerful pectoral spines and justly feared on account of the
dangerous wounds they inflict; not a few of them possess, in addition
to the pectoral spines, a sac with a more or less wide opening in the
axil of the pectoral fin; and it does not seem improbable that it
contains a fluid which may be introduced into a wound by means of
the pectoral spine, which would be covered with it, like the barbed
arrow-head of an Indian. However, whether this secretion is equally
poisonous in all the species provided with that axillary sac, or
whether it has poisonous qualities at all, is a question which can be
decided by experiments only made with the living fishes.
 CHAPTER XV.

DISTRIBUTION OF FISHES IN TIME.

Of what kind the fishes were which were the first to make their
appearance on the globe; whether or not they were identical with, or
similar to, any of the principal types existing at present; are
questions which probably will for ever remain hidden in mystery and
uncertainty. The supposition that the Leptocardii and Cyclostomes,
the lowest of the vertebrate series, must have preceded the other
sub-classes, is an idea which has been held by many Zoologists:
and as the horny teeth of the Cyclostomes are the only parts of their
body which under favourable circumstances might have been
preserved, Palæontologists have ever been searching for this
evidence.

Fig. 101. Right

dental plate of
Myxine affinis.
Indeed, in deposits belonging to the Lower Silurian and
Devonian, in Russia, England, and North America, minute, slender,
pointed horny bodies, bent like a hook, with sharp opposite margins,
have been found and described under the name of Conodonts. More
frequently they possess an elongated basal portion, in which there is
generally a larger tooth with rows of similar but smaller denticles on
one or both sides of the larger tooth, according as this is central or at
one end of the base. In other examples there is no prominent central
tooth, but a series of more or less similar teeth is implanted on a
straight or curved base. Modifications of these arrangements are
very numerous, and many Palæontologists entertain still doubts
whether the origin of these remains is not rather from Annelids and
Mollusks than from Fishes.
 [See G. J. Hinde, in “Quarterly Journal of the Geological Society,”
1879.]
The first undeniable evidence of a fish, or, indeed, of a vertebrate
animal, occurs in the Upper Silurian Rocks, in a bone-bed of the
Downton sandstone, near Ludlow. It consists of compressed, slightly
curved, ribbed spines, of less than two inches in length (Onchus); of
small shagreen-scales (Thelodus); the fragment of a jaw-like bar with
pluricuspid teeth (Plectrodus); the cephalic bucklers of what seems
to be a species of Pteraspis; and, finally, the coprolitic bodies of
phosphate and carbonate of lime, including recognisable remains of
the Mollusks and Crinoids inhabiting the same waters. But no
vertebra or other part of the skeleton has been found. The spines
and scales seem to have belonged to the same kind of fish, which
probably was a Plagiostome. It is quite uncertain whether or not the
jaw (if it be the jaw of a fish[16]) belonged to the buckler-bearing
Pteraspis, the position of which among Ganoids, with which it is
generally associated, is open to doubt.
No detached undoubted tooth of a Plagiostome or Ganoid scale
has been discovered in the Ludlow deposits: but so much is certain
that those earliest remains in Palæozoic rocks belonged to fishes
closely allied to forms occurring in greater abundance in the
succeeding formation, the Devonian, where they are associated with
undoubted Palæichthyes, Plagiostomes as well as Ganoids.

These fish-remains of the Devonian or Old Red Sandstone, can

be determined with greater certainty. They consist of spines or the
so-called Ichthyodorulites, which show sufficiently distinctive
characters to be referred to several genera, one of them, Onchus,
still surviving from the Silurian epoch. All these spines are believed
to be those of Chondropterygians, to which order some pluricuspid
teeth (Cladodus) from the Old Red Sandstone in the vicinity of St.
Petersburg have been referred likewise.
 The remains of the Ganoid fishes are in a much more perfect
state of preservation, so that it is even possible to obtain a tolerably
certain idea of the general appearance and habits of some of them,
especially of such as were provided with hard carapaces, solid
scales, and ordinary or bony fin-rays. A certain proportion of them,
as might have been expected, remind us, with regard to external
form, of Teleosteous fishes rather than of any of the few still existing
Ganoid types; but it is contrary to all analogy and to all
palæontological evidence to suppose that those fishes were, with
regard to their internal structure, more nearly allied to Teleosteans
than to Ganoids. If they were not true Ganoids, they may be justly
supposed to have had the essential characters of Palæichthyes.
Other forms exhibit even at that remote geological epoch so
unmistakably the characteristics of existing Ganoids, that no one can
entertain any doubt with regard to their place in the system. In none
of these fishes is there any trace of vertebral segmentation.
The Palæichthyes of the Old Red Sandstone, the systematic
position of which is still obscure, are the Cephalaspidæ from the
Lower Old Red Sandstone of Great Britain and Eastern Canada;
Pterichthys, Coccosteus, and Dinichthys: genera which have been
combined in one group—Placodermi; and Acanthodes and allied
genera, which combined numerous branchiostegals with
chondropterygian spines and a shagreen-like dermal covering.
Among the other Devonian fishes (and they formed the majority)
two types may be recognised, both of which are unmistakably
Ganoids. The first approaches the still living Polypterus, with which
some of the genera like Diplopterus singularly agree in the form and
armature of the head, the lepidosis of the body, the lobate pectoral
fins, and the termination of the vertebral column. Other genera, as
Holoptychius, have cycloid scales; many have two dorsal fins
(Holoptychius), and, instead of branchiostegals, jugular scutes;
others one long dorsal confluent with the caudal (Phaneropleuron).
In the second type the principal characters of the Dipnoi are
manifest, and some of them, for example Dipterus, Palædaphus,
Holodus, approach so closely the Dipnoi which still survive, that the
differences existing between them warrant a separation into families
only.
Devonian fishes are frequently found under peculiar
circumstances, enclosed in the so-called nodules. These bodies are
elliptical flattened pebbles, which have resisted the action of water in
consequence of their greater hardness, whilst the surrounding rock
has been reduced to detritus by that agency. Their greater density is
due to the dispersion in their substance of the fat of the animal which
decomposed in them. Frequently, on cleaving one of these nodules
with the stroke of the hammer, a fish is found embedded in the
centre. At certain localities of the Devonian, fossil fishes are so
abundant that the whole of the stratum is affected by the
decomposing remains emitting a peculiar smell when newly opened,
and acquiring a density and durability not possessed by strata
without fishes. The flagstones of Caithness are a remarkable
instance of this.

The fish-remains of the Carboniferous formation show a great

similarity to those of the preceding. They occur throughout the
series, but are very irregularly distributed, being extremely scarce in
some countries, whilst in others entire beds (the so-called bone-
beds) are composed of ichthyolites. In the ironstones they frequently
form the nuclei of nodules, as in the Devonian.
Of Chondropterygians the spines of Onchus and others still
occur, with the addition of teeth indicative of the existence of fishes
allied to the Cestracion-type (Cochliodus, Psammodus): a type which
henceforth plays an important part in the composition of the extinct
marine fish faunæ. Another extinct Selachian family, that of
Hybodontes, makes its appearance, but is known from the teeth
only.
Of the Ganoid fishes, the family Palæoniscidæ (Traquair) is
numerously represented; others are Cœlacanths (Cœlocanthus,
Rhizodus), and Saurodipteridæ (Megalichthys). None of these fishes
have an ossified vertebral column, but in some (Megalichthys) the
outer surface of the vertebræ is ossified into a ring; the termination of
their tail is heterocercal. The carboniferous Uronemus and the
Devonian Phaneropleuron are probably generically the same; and
the Devonian Dipnoi are continued as, and well represented by,
Ctenodus.

The fishes of the Permian group are very similar to those of the
Carboniferous. A type which in the latter was but very scantily
represented, namely the Platysomidæ, is much developed. They
were deep-bodied fish, covered with hard rhomboid scales
possessing a strong anterior rib, and provided with a heterocercal
caudal, long dorsal and anal, short non-lobate paired fins (when
present), and branchiostegals. The Palæoniscidæ are represented
by many species of Palæoniscus, Pygopterus and Acrolepis, and
Cestracionts by Janassa and Strophodus.

The passage from the Palæozoic into the Mesozoic era is not
indicated by any marked change as far as fishes are concerned. The
more remarkable forms of the Trias are Shark-like fishes represented
by ichthyodorulithes like Nemacanthus, Liacanthus, and Hybodus;
and Cestracionts represented by species of Acrodus and
Strophodus. Of the Ganoid genera Cœlacanthus, Amblypterus
(Palæoniscidæ), Saurichthys persist from the Carboniferous epoch.
Ceratodus appears for the first time (Muschel-Kalk of Germany).
Thanks to the researches of Agassiz, and especially Sir P.
Egerton, the ichthyological fauna of the Lias is, perhaps, the best
known of the Mesozoic era, 152 species having been described. Of
the various localities, Lyme Regis has yielded more than any other,
nearly all the Liassic genera being represented there by not less
than seventy-nine species. The Hybodonts and Cestracionts
continue in their fullest development. Holocephales (Ischyodus), true
Sharks (Palæoscyllium), Rays (Squaloraja, Arthropterus), and
Sturgeons (Chondrosteus) make their first appearance; but they are
sufficiently distinct from living types to be classed in separate
genera, or even families. The Ganoids, especially Lepidosteoids,
predominate over all the other fishes: Lepidotus, Semionotus,
Pholidophorus, Pachycormus, Eugnathus, Tetragonolepis, are
represented by numerous species; other remarkable genera are
Aspidorhynchus, Belonostomus, Saurostomus, Sauropsis,
Thrissonotus, Conodus, Ptycholepis, Endactis, Centrolepis,
Legnonotus, Oxygnathus, Heterolepidotus, Isocolum, Osteorhachis,
Mesodon. These genera offer evidence of a great change since the
preceding period, the majority not being represented in older strata,
whilst, on the other hand, many are continued into the succeeding
oolithic formations. The homocercal termination of the vertebral
column commences to supersede the heterocercal, and many of the
genera have well ossified and distinctly segmented spinal columns.
Also the cycloid form of scales becomes more common: one genus
(Leptolepis) being, with regard to the preserved hard portions of its
organisation, so similar to the Teleosteous type that some
Palæontologists refer it (with much reason) to that sub-class.
[See E. Sauvage, Essai sur la Faune Ichthyologique de la période
Liasique. In “Bibl. de l’école des hautes études,” xiii. art. 5. Paris 1875.
8o.]
As already mentioned, the Oolithic formations show a great
similarity of their fish-fauna to that of the Lias; but still more apparent
is its approach to the existing fauna. Teeth have been found which
cannot even generically be distinguished from Notidanus. The Rays
are represented by genera like Spathobatis, Belemnobatis,
Thaumas; the Holocephali are more numerous than in the Lias
(Ischyodus, Ganodus). The most common Ganoid genera are
Caturus, Pycnodus, Pholidophorus, Lepidotus, Leptolepis, all of
which had been more or less fully represented in the Lias. Also
Ceratodus is continued into it.
 The Cretaceous group offers clear evidence of the further
advance towards the existing fauna. Teeth of Sharks of existing
genera Carcharias (Corax), Scyllium, Notidanus, and Galeocerdo,
are common in some of the marine strata, whilst Hybodonts and
Cestracionts are represented by a small number of species only; of
the latter one new genus, Ptychodus, appears and disappears. A
very characteristic Ganoid genus, Macropoma, comprises
homocercal fishes with rounded ganoid scales sculptured externally
and pierced by prominent mucous tubes. Caturus becomes extinct.
Teeth and scales of Lepidotus (with Sphærodus as subgenus),
clearly a freshwater fish, are widely distributed in the Wealden, and
finally disappear in the chalk; its body was covered with large
rhomboidal ganoid scales. Gyrodus and Aspidorhynchus occur in the
beds of Voirons, Coelodus and Amiopsis (allied to Amra), in those of
Comen, in Istria. But the Palæichthyes are now in the minority;
undoubted Teleosteans have appeared, for the first time, on the
stage of life in numerous genera, many of which are identical with
still existing fishes. The majority are Acanthopterygians, but
Physostomes and Plectognaths are likewise well represented, most
of them being marine. Of Acanthopterygian families the first to
appear are the Berycidæ, represented by several very distinct
genera: Beryx; Pseudoberyx with abdominal ventral fins; Berycopsis
with cycloid scales; Homonotus, Stenostoma, Sphenocephalus,
Acanus, Hoplopteryx, Platycornus with granular scales; Podocys
with a dorsal extending to the neck; Acrogaster, Macrolepis,
Rhacolepis from the chalk of Brazil. The position of Pycnosterynx is
uncertain, it approaches certain Pharyngognaths. True Percidæ are
absent, whilst the Carangidæ, Sphyrænidæ, Cataphracti, Gobiidæ,
Cottidæ, and Sparidæ are represented by one or more genera.
Somewhat less diversified are the Physostomes, which belong
principally to the Clupeidæ and Dercetidæ, most of the genera being
extinct; Clupea is abundant in some localities. Scopelidæ
(Hemisaurida and Saurocephalus) occur in the chalk of Comen in
Istria, and of Mæstricht. Of all cretaceous deposits none surpass
those of the Lebanon with regard to the number of genera, species,
and individuals; the forms are exclusively marine, and the remains in
the most perfect condition.
 In the Tertiary epoch the Teleosteans have almost entirely
replaced the Ganoids; a few species only of the latter make their
appearance, and they belong to existing genera, or, at least, very
closely allied forms (Lepidosteus, Amia, Hypamia, Acipenser). The
Chondropterygians merge more and more into recent forms;
Holocephali continue, and still are better represented than in the
present fauna. The Teleosteans show even in the Eocene a large
proportion of existing genera, and the fauna of some localities of the
Miocene (Oeningen) is almost wholly composed of them. On the
whole, hitherto more than one-half have been found to belong to
existing genera, and there is no doubt that the number of seemingly
distinct extinct genera will be lessened as the fossils will be
examined with a better knowledge of the living forms. The
distribution of the fishes differed widely from that of our period, many
of our tropical genera occurring in localities which are now included
within our temperate zone, and being mixed with others, which
nowadays are restricted to a colder climate: a mixture which
continues throughout the Pliocene.

A few families of fishes, like the freshwater Salmonidæ, seem to

have put in their appearance in Post-pliocene times; however, not
much attention has been paid to fish-remains of these deposits; and
such as have been incidentally examined offer evidence of the fact
that the distribution of fishes has not undergone any further essential
change down to the present period.
[See E. Sauvage, Mémoire sur la Faune Ichthyologique de la
période Tertiaire. Paris 1873. 8°.]
Fig. 102.—Pycnodus rhombus, a Ganoid from the Upper
Oolite.
 CHAPTER XVI.

THE DISTRIBUTION OF EXISTING FISHES OVER THE EARTH’S SURFACE—GENERAL

REMARKS.

In an account of the geographical distribution of fishes the

Freshwater forms are to be kept separate from the Marine. However,
when we attempt to draw a line between these two kinds of fishes,
we meet with a great number of species and of facts which would
seem to render that distinction very vague. There are not only
species which can gradually accommodate themselves to a sojourn
in either salt or fresh water, but there are also such as seem to be
quite indifferent to a rapid change from one into the other: so that
individuals of one and the same species (Gastrosteus, Gobius,
Blennius, Osmerus, Retropinna, Clupea, Syngnathus, etc.), may be
found at some distance out at sea, whilst others live in rivers far
beyond the influence of the tide, or even in inland fresh waters
without outlet to the sea. The majority of these fishes belong to forms
of the fauna of the brackish water, and as they are not an
insignificant portion of the fauna of almost every coast, we shall have
to treat of them in a separate chapter.
Almost every large river offers instances of truly marine fishes
(such as Serranus, Sciænidæ, Pleuronectes, Clupeidæ, Tetrodon,
Carcharias, Trygonidæ), ascending for hundreds of miles of their
course; and not periodically, or from any apparent physiological
necessity, but sporadically throughout the year, just like the various
kinds of marine Porpoises which are found all along the lower course
of the Ganges, Yang-tseKiang, the Amazons, the Congo, etc. This is
evidently the commencement of a change in a fish’s habits, and,
indeed, not a few of such fishes have actually taken up their
permanent residence in fresh waters (as species of Ambassis,
Apogon Dules, Therapon, Sciæna, Blennius, Gobius, Atherina,
Mugil, Myxus, Hemirhamphus, Clupea, Anguilla, Tetrodon, Trygon):
all forms originally marine.
On the other hand, we find fishes belonging to freshwater genera
descending rivers and sojourning in the sea for a more or less limited
period; but these instances are much less in number than those in
which the reverse obtains. We may mention species of Salmo (the
Common Trout, the Northern Charr), and Siluroids (as Arius,
Plotosus). Coregonus, a genus so characteristic of the inland lakes
of Europe, Northern Asia, and North America, nevertheless offers
some instances of species wandering by the effluents into the sea,
and taking up their residence in salt water, apparently by preference,
as Coregonus oxyrhynchus. But of all the Freshwater families none
exhibit so great a capability of surviving the change from fresh into
salt water, as the Gastrosteidæ (Sticklebacks), of the northern
Hemisphere, and the equally diminutive Cyprinodontidæ of the
tropics; not only do they enter into, and live freely in, the sea, but
many species of the latter family inhabit inland waters, which, not
having an outlet, have become briny, or impregnated with a larger
proportion of salts than pure sea water. During the voyage of the
“Challenger” a species of Fundulus, F. nigrofasciatus, which inhabits
the fresh and brackish waters of the Atlantic States of North
America, was obtained, with Scopelids and other pelagic forms, in
the tow-net, midway between St. Thomas and Teneriffe.
Some fishes annually or periodically ascend rivers for the
purpose of spawning, passing the rest of the year in the sea, as
Sturgeons, many Salmonoids, some Clupeoids, Lampreys, etc. The
two former evidently belonged originally to the freshwater series, and
it was only in the course of their existence that they acquired the
habit of descending to the sea, perhaps because their freshwater
home did not offer a sufficient supply of food. These migrations of
freshwater fishes have been compared with the migrations of birds;
but they are much more limited in extent, and do not impart an
additional element to the fauna of the place to which they migrate, as
is the case with the distant countries to which birds migrate.
The distinction between freshwater and marine fishes is further
obscured by geological changes, in consequence of which the salt
water is gradually being changed into fresh, or vice versa. These
changes are so gradual and spread over so long a time, that many of
the fishes inhabiting such localities accommodate themselves to the
new conditions. One of the most remarkable and best studied
instances of such an alteration is the Baltic, which, during the second
half of the Glacial period, was in open and wide communication with
the Arctic Ocean, and evidently had the same marine fauna as the
White Sea. Since then, by the rising of the land of Northern
Scandinavia and Finland, this great gulf of the Arctic Ocean has
become an inland sea, with a narrow outlet into the North Sea, and
its water, in consequence of the excess of the fresh water pouring
into it over the loss by evaporation, has been so much diluted as to
be nearly fresh at its northern extremities: and yet nine species, the
origin of which from the Arctic Ocean can be proved, have survived
the changes, propagating their species, agreeing with their brethren
in the Arctic Ocean in every point, but remaining comparatively
smaller. On the other hand, fishes which we must regard as true
freshwater fishes, like the Rudd, Roach, Pike, Perch, enter freely the
brackish water of the Baltic.[17] Instances of marine fishes being
permanently retained in fresh water in consequence of geological
changes are well known: thus Cottus quadricornis in the large lakes
of Scandinavia; species of Gobius, Blennius, and Atherina in the
lakes of Northern Italy; Comephorus, of the depths of the Lake of
Baikal, which seems to be a dwarfed Gadoid. Carcharias gangeticus
in inland lakes of the Fiji Islands, is another instance of a marine fish
which has permanently established itself in fresh water.
In the miocene formation of Licata in Sicily, in which fish remains
abound, numerous Cyprinoids are mixed with littoral and pelagic
forms. Sauvage found in 450 specimens from that locality, not less
than 266, which were Leucisci, Alburni, or Rhodei. Now, although it
is quite possible that in consequence of a sudden catastrophe the
bodies of those Cyprinoids were carried by a freshwater current into,
and deposited on the bottom of, the sea, the surmise that they lived
together with the littoral fishes in the brackish water of a large
estuary, which was not rarely entered by pelagic forms, is equally
admissible. And, if confirmed by other similar observations, this
instance of a mixture of forms which are now strictly freshwater or
marine, may have an important bearing on the question to what
extent fishes have in time changed their original habitat.
Thus there is a constant exchange of species in progress
between the freshwater and marine faunæ, and in not a few cases it
would seem almost arbitrary to refer a genus or even larger group of
fishes to one or the other; yet there are certain groups of fishes
which entirely, or with but few exceptions are, and, apparently, during
the whole period of their existence have been, inhabitants either of
the sea or of fresh water; and as the agencies operating upon the
distribution of marine fishes differ greatly from those influencing the
dispersal of freshwater fishes, the two series must be treated
separately. The most obvious fact that dry land, which intervenes
between river systems, offers to the rapid spreading of a freshwater
fish an obstacle which can be surmounted only exceptionally or by a
most circuitous route, whilst marine fishes may readily and
voluntarily extend their original limits, could be illustrated by a great
number of instances. Without entering into details, it may suffice to
state as the general result, that no species or genus of freshwater
fishes has anything like the immense range of the corresponding
categories of marine fishes; and that, with the exception of the
Siluroids, no other freshwater family is so widely spread as the
families of marine fishes. Surface temperature or climate which is, if
not the most, one of the most important physical factors in the
limitation of freshwater fishes, similarly affects the distribution of
marine fishes, but in a less degree, and only those which live near to
the shore or the surface of the ocean; whilst it ceases to exercise its
influence in proportion to the depth, the true deep-sea forms being
entirely exempt from its operation. Light, which is pretty equally
distributed over the localities inhabited by freshwater fishes, cannot
be considered as an important factor in their distribution, but it
contributes towards constituting the impassable barrier between the
surface and abyssal forms of marine fishes. Altitude has stamped
the fishes of the various Alpine provinces of the globe with a certain
character, and limited their distribution; but the number of these
Alpine forms is comparatively small, ichthyic life being extinguished
at great elevations even before the mean temperature equals that of
the high latitudes of the Arctic region, in which some freshwater
fishes flourish. On the other hand, the depths of the ocean, far
exceeding the altitude of the highest mountains, still swarm with
forms specially adapted for abyssal life. That other physical
conditions of minor and local importance, under which fresh water
fishes live, and by which their dispersal is regulated, are more
complicated than similar ones of the ocean, is probable, though
perhaps less so than is generally supposed: for the fact is that the
former are more accessible to observation than the latter, and are,
therefore, more generally and more readily comprehended and
acknowledged. Thus, not only because many of the most
characteristic forms of the marine and freshwater series are found,
on taking a broader view of the subject, to be sufficiently distinct, but
also because their distribution depends on causes different in their
nature as well as the degree of their action, it will be necessary to
treat of the two series separately. Whether the oceanic areas
correspond in any way to the terrestrial will be seen in the sequel.

Fig. 103.—Ganoid scales of Tetragonolepis.