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.
[Illustration: Fig. 103.--Ganoid scales of _Tetragonolepis_.]