THE GEOGRAPHICAL DISTRIBUTION OF FISHES
=Zoogeography.=--Under the head of distribution we consider the
facts of the actual location of species of organisms on the surface
of the earth and the laws by which their location is governed. This
constitutes the subject-matter of the science of zoogeography. In
physical geography we may prepare maps of the earth or of any part of
it, these bringing to prominence the physical features of its surface.
Such maps show here a sea, there a plateau, here a mountain chain,
there a desert, a prairie, a peninsula, or an island. In political
geography the maps show their physical features of the earth as
related to the people who inhabit them and the states or powers which
receive or claim their allegiance. In zoogeography the realms of the
earth are considered in relation to the species or tribes of animals
which inhabit them. Thus series of maps could be drawn representing
those parts of North America in which catfishes or trout or sunfishes
are found in the streams. In like manner the distribution of any
particular fish as the muskallonge or the yellow perch could be shown
on the map. The details of such a map are very instructive, and their
consideration at once raises a series of questions as to the cause
behind each fact. In science it must be supposed that no fact is
arbitrary or meaningless. In the case of fishes the details of the
method of diffusion of species afford matters of deep interest. These
are considered in a subsequent chapter.
The dispersion of animals may be described as a matter of space and
time, the movement being continuous but modified by barriers and
other conditions of environment. The tendency of recent studies in
zoogeography has been to consider the facts of present distribution
as the result of conditions in the past, thus correlating our present
knowledge with the past relations of land and water as shown through
paleontology. Dr. A. E. Ortmann well observes that "Any division of the
earth's surface into zoogeographical regions which starts exclusively
from the present distribution of animals without considering its
origin must always be unsatisfactory." We must therefore consider the
coast-lines and barriers of Tertiary and earlier times as well as those
of to-day to understand the present distribution of fishes.
=General Laws of Distribution.=--The general laws governing the
distribution of all animals are reducible to three very simple
propositions.
Each species of animal is found in every part of the earth having
conditions suitable for its maintenance, unless
(_a_) Its individuals have been unable to reach this region through
barriers of some sort; or,
(_b_) Having reached it, the species is unable to maintain itself,
through lack of capacity for adaptation, through severity of
competition with other forms, or through destructive conditions of
environment; or else,
(_c_) Having entered and maintained itself, it has become so altered
in the process of adaptation as to become a species distinct from the
original type.
=Species Absent through Barriers.=--The absence from the Japanese
fauna of most European or American species comes under the first head.
The pike has never reached the Japanese lakes, though the shade of
the-lotus leaf in the many clear ponds would suit its habits exactly.
The grunt[22] and porgies[23] of our West Indian waters have failed to
cross the ocean and therefore have no descendants in Europe or Asia.
=Species Absent through Failure to Maintain Foothold.=--Of species
under (_b_), those who have crossed the seas and not found lodgement,
we have, in the nature of things, no record. Of the existence of
multitudes of estrays we have abundant evidence. In the Gulf Stream
off Cape Cod are every year taken many young fishes belonging to
species at home in the Bahamas and which find no permanent place in
the New England fauna. In like fashion, young fishes from the tropics
drift northward in the Kuro Shiwo to the coasts of Japan, but never
finding a permanent breeding-place and never joining the ranks of the
Japanese fishes. But to this there have been, and will be, occasional
exceptions. Now and then one among thousands finds permanent lodgement,
and by such means a species from another region will be added to the
fauna. The rest disappear and leave no trace. A knowledge of these
currents and their influence is eventual to any detailed study of the
dispersion of fishes.
The occurrence of the young of many shore fishes of the Hawaiian
Islands as drifting plankton at a considerable distance from the shores
has been lately discovered by Dr. Gilbert. Each island is, in a sense,
a "sphere of influence," affecting the fauna of neighboring regions.
=Species Changed through Natural Selection.=--In the third class,
that of species changed in the process of adaptation, most insular
forms belong. As a matter of fact, at some time or another almost
every species must be in this category, for isolation is a source of
the most potent elements in the initiation and intensification of
the minor differences which separate related species. It is not the
preservation of the most useful features, but of those which actually
existed in the ancestral individuals, which distinguish such species.
Natural selection must include not only the process of the survival
of the fittest, but also the results of the survival of the existing.
This means the preservation through heredity of the traits not of the
species alone, but those of the actual individuals set apart to be the
first in the line of descent in a new environment. In hosts of cases
the persistence of characters rests not on any special usefulness or
fitness, but on the fact that individuals possessing these characters
have, at one time or another, invaded a certain area and populated it.
The principle of utility explains survivals among competing structures.
It rarely accounts for qualities associated with geographical
distribution.
=Extinction of Species.=--The extinction of species may be noted here
in connection with their extension of range. Prof. Herbert Osborn has
recognized five different types of elimination.
1. That extinction which comes from modification or progressive
evolution, a relegation to the past as the result of a transmutation
into more advanced forms. 2. Extinction from changes of physical
environment which outrun the powers of adaptation. 3. The extinction
which results from competition. 4. The extinction from extreme
specialization and limitation to special conditions the loss of which
means extinction. 5. Extinction as a result of exhaustion. As an
illustration of No. 1, we may take almost any species which has a
cognate species on the further side of some barrier or in the tertiary
seas. Thus the trout of the Twin Lakes in Colorado has acquired its
present characters in the place of those brought into the lake by its
actual ancestors. No. 2 is illustrated by the disappearance of East
Indian types (_Zanclus_, _Platax_, _Toxotes_, etc.) in Italy at the
end of the Eocene, perhaps for climatic reasons. Extinction through
competition is shown in the gradual disappearance of the Sacramento
perch (_Archoplitis interruptus_) after the invasion of the river
by catfish and carp. From extreme specialization certain forms have
doubtless disappeared, but no certain case of this kind has been
pointed out among fishes, unless this be the cause of the disappearance
of the Devonian mailed _Ostracophores_ and _Arthrodires_. It is not
likely that any group of fishes has perished through exhaustion of the
stock of vigor.
=Barriers Checking Movement of Marine Fishes.=--The limits of the
distribution of individual species or genera must be found in some sort
of barrier, past or present. The chief barriers which limit marine
fishes are the presence of land, the presence of great oceans, the
differences of temperature arising from differences in latitude, the
nature of the sea bottom, and the direction of oceanic currents. That
which is a barrier to one species may be an agent in distribution to
another. The common shore fishes would perish in deep waters almost as
surely as on land, while the open Pacific is a broad highway to the
albacore or the swordfish.
Again, that which is a barrier to rapid distribution may become an
agent in the slow extension of the range of a species. The great
continent of Asia is undoubtedly one of the greatest of barriers to
the wide movement of species of fish, yet its long shore-line enables
species to creep, as it were, from bay to bay, or from rock to rock,
till, in many cases, the same species is found in the Red Sea and in
the tide-pools or sand-reaches of Japan. In the North Pacific, the
presence of a range of half-submerged volcanoes, known as the Aleutian
and the Kurile Islands, has greatly aided the slow movement of the
fishes of the tide-pools and the kelp. To a school of mackerel or of
flying-fishes these rough islands with their narrow channels might form
an insuperable barrier.
[Illustration: FIG. 173.--Japanese filefish, _Rudarius ercodes_ Jordan
and Snyder. Wakanoura, Japan. Family _Monacanthidæ_.]
=Temperature the Central Fact in Distribution.=--It has long been
recognized that the matter of temperature is the central fact in all
problems of geographical distribution. Few species in any group freely
cross the frost-line, and except as borne by oceanic currents, not
many extend their range far into waters colder than those in which
the species is distinctively at home. Knowing the average temperature
of the water in a given region we know in general the types of fishes
which must inhabit it. It is the similarity in temperature and physical
conditions which chiefly explains the resemblance of the Japanese fauna
to that of the Mediterranean or the Antilles. This fact alone must
explain the resemblance of the Arctic and Antarctic faunæ, there being
in no case a barrier in the sea that may not some time be crossed. Like
forms lodge in like places.
=Agency of Ocean Currents.=--We may consider again for a moment the
movements of the great currents in the Pacific as agencies in the
distribution of species.
A great current sets to the eastward, crossing the ocean just south
of the equator. It extends past Samoa and passes on nearly to the
coast of Mexico, touching the Galapagos Islands, Clipperton Island,
and especially the Revillagigedos. This may account for the number of
Polynesian species found on these islands, about which they are freely
mixed with immigrants from the mainland of Mexico.
From the Revillagigedos[24] the current moves northward and westward,
passing the Hawaiian Islands and thence onward to the Ladrones. The
absence in Hawaii of most of the characteristic fishes of Polynesia
and Micronesia may be in part due to the long detour made by these
currents, as the conditions of life in these groups of islands are
not very different. Northeast of Hawaii is a great spiral current,
moving with the hands of the watch, forming what is called Fleurieu's
Whirlpool. This does not reach the coast of California. This fact may
help to account for the almost complete distinction in the shore fishes
of Hawaii and California.[25]
No other group of islands in the tropics has a fish fauna so isolated
as that of Hawaii. The genera are largely the ordinary tropical types.
The species are largely peculiar to these islands.
The westward current from Hawaii reaches Luzon and Formosa. It is
deflected to the northward and, joining a northward current from
Celebes, it forms the Kuro Shiwo or Black Stream of Japan, which strews
its tropical species in the rock pools along the Japanese promontories
as far as Tokio. Then, turning into the open sea, it passes northward
to the Aleutian Islands, across to Sitka. Thence it moves southward as
a cold current, bearing Ochotsk-Alaskan types southward as far as the
Santa Barbara Islands, to which region it is accompanied by species of
Aleutian origin. A cold return current seems to extend southward in
Japan, along the east shore perhaps as far as Matsushima. A similar
current in the sea to the west of Japan extends still further to the
southward, to Noto, or beyond.
It is, of course, not necessary that the movements of a species in an
oceanic current should coincide with the direction of the current.
Young fishes, or fresh-water fishes, would be borne along with the
water. Those that dwell within floating bodies of seaweed would go
whither the waters carry the drifting mass. But free-swimming fishes,
as the mackerel or flying-fishes, might as readily choose the reverse
direction. To a free-swimming fish the temperature of the water would
be the only consideration. It is thus evident that a current which to
certain forms would prove a barrier to distribution, to others would be
a mere convenience in movement.
In comparing the Japanese fauna with that of Australia, we find some
trace of both these conditions. Certain forms are perhaps excluded by
cross-currents, while certain others seem to have been influenced only
by the warmth of the water. A few Australian types on the coast of
Chile seem to have been carried over by the cross-currents of the South
Atlantic.
It is fair to say that the part taken by oceanic currents in the
distribution of shore fishes is far from completely demonstrated. The
evidence that they assist in such distribution is, in brief, as follows:
1. The young of shore fishes often swim at the surface.
2. The young of very many tropical fishes drift northward in the Gulf
Stream and the Japanese Kuro Shiwo.
3. The faunal isolation of Hawaii may be correlated with the direction
of the oceanic currents.
=Centers of Distribution.=--We may assume, in regard to any species,
that it has had its origin in or near that region in which it is most
abundant and characteristic. Such an assumption must involve a very
large percentage of error or of doubt, but in considering the mass
of species, it may represent essential truth. In the same fashion we
may regard a genus as being autochthonous or first developed in the
region where it shows the greatest range or variety of species. Those
regions where the greatest number of genera are thus autochthonous may
be regarded as centers of distribution. So far as the marine fishes
are concerned, the most important of these supposed centers are found
in the Pacific Ocean. First of these in importance is the East-Indian
Archipelago, with the neighboring shores of India. Next would come
the Arctic Pacific and its bounding islands, from Japan to British
Columbia. Third in importance in this regard is Australia. Important
centers are found in temperate Japan, in California, the Panama region,
and in New Zealand, Chili, and Patagonia. The fauna of Polynesia is
almost entirely derived from the Indies; and the shore fauna of the Red
Sea, the Bay of Bengal, and Madagascar, so far as genera are concerned,
seems to be not really separable from the Indian fauna generally.
[Illustration: FIG. 174.--Globefish, _Tetraodon setosus_ Rosa Smith.
Clarion Island, Mexico.]
I know of but six genera which may be regarded as autochthonous in the
Red Sea, and nearly all of these are of doubtful value or of uncertain
relation. The many peculiar genera described by Dr. Alcock, from the
dredgings of the _Investigator_ in the Bay of Bengal, belong to the
bathybial or deep-water series, and will all, doubtless, prove to be
forms of wide distribution.
In the Atlantic, the chief center of distribution is the West Indies;
the second is the Mediterranean. On the shores to the northward or
southward of these regions occasional genera have found their origin.
This is true especially of the New England region, the North Sea, the
Gulf of Guinea, and the coast of Argentina. The fish fauna of the North
Atlantic is derived mainly from the North Pacific, the differences
lying mainly in the relative paucity of the North Atlantic. But in
certain groups common to the two regions the migration must have been
in the opposite direction, exceptions that prove the rule.
=Distribution of Marine Fishes.=--The distribution of marine fishes
must be indicated in a different way from that of the fresh-water
forms. The barriers which limit their range furnish also their means
of dispersion. In some cases proximity overbalances the influence of
temperature; with most forms questions of temperature are all-important.
=Pelagic Fishes.=--Before consideration of the coast-lines we may
glance at the differences in vertical distribution. Many species,
especially those in groups allied to the mackerel family, are
pelagic--that is, inhabiting the open sea and ranging widely within
limits of temperature. In this series some species are practically
cosmopolitan. In other cases the genera are so. Each school or group of
individuals has its breeding place, and from the isolation of breeding
districts new species may be conceived to arise. The pelagic types have
reached a species of equilibrium in distribution. Each type may be
found where suitable conditions exist, and the distribution of species
throws little light on questions of distribution of shore fishes. Yet
among these species are all degrees of localization. The pelagic fishes
shade into the shore fishes on the one hand and into the deep-sea
fishes on the other.
=Bassalian Fishes.=--The vast group of bassalian or deep-sea fishes
includes those forms which live below the line of adequate light. These
too are localized in their distribution, and to a much greater extent
than was formerly supposed. Yet as they dwell below the influence
of the sun's rays, zones and surface temperatures are nearly alike
to them, and the same forms may be found in the Arctic or under the
equator. Their differences in distribution are largely vertical, some
living at greater depths than others, and they shade off by degrees
from bathybial into semi-bathybial, and finally into ordinary pelagic
and ordinary shore types. Apparently all of the bassalian fishes are
derived from littoral types, the changes in structure being due to
degeneration of the osseous and muscular systems and of structures not
needed in deep-sea life.
[Illustration: FIG. 175.--Sting-ray, _Dasyatis sabina_ Le Sueur.
Galveston.]
The fishes of the great depths are soft in substance, some of them
blind, some of them with very large eyes, all black in color, and very
many are provided with luminous spots or areas. A large body of species
of fishes are semi-bathybial, inhabiting depths of 20 to 100 fathoms,
showing many of the characters of shore fishes, but far more widely
distributed. Many of the remarkable cases of wide distribution of type
belong to this class. In moderate depths red colors are very common,
corresponding to the zone of red algæ, and the colors in both cases are
perhaps determined from the fact that the red rays of light are the
least refrangible.
A certain number of species are both marine and fresh water, inhabiting
estuaries and brackish waters, while some more strictly marine ascend
the rivers to spawn. In none of these cases can any hard and fast line
be drawn, and some groups which are shore fishes in one region will be
represented by semi-bathybial or fluviatile forms in another.[26]
=Littoral Fishes.=--The shore fishes are in general the most highly
specialized in their respective groups, because exposed to the greatest
variety of selecting conditions and of competition. Their distribution
in space is more definite than that of the pelagic and bassalian types,
and they may be more definitely assigned to geographical areas.
=Distribution of Littoral Fishes by Coast-lines.=--Their distribution
is best indicated, not by realms or areas, but as forming four parallel
series corresponding to the four great north and south continental
outlines. Each of these series may be represented as beginning at
the north in the Arctic fauna, practically identical in each of the
four series, actually identical in the two Pacific series. Passing
southward, forms are arranged according to temperature. One by one
in each series, the Arctic types disappear; subarctic, temperate,
and semi-tropical types take their places, giving way in turn to
south-temperate and Antarctic forms. The distribution of these is
modified by barriers and by currents, yet though genera and species may
be different, each isotherm is represented in each series by certain
general types of fishes.
[Illustration: FIG. 176.--Green-sided Darter, _Diplesion blennioides_
Rafinesque. Clinch River. Family _Percidæ_.]
Passing southward the two American series, the East Atlantic and the
East Pacific, pass on gradually through temperate to Antarctic types.
These are analogous to those of the Arctic, and in a few cases they
are generally identical. The West Pacific (East Asian) series is not
a continuous line on account of the presence of Australia, the East
Indies, and Polynesia. The irregularities of these regions make a
number of subseries, which break up the simplicity expressed in the
idea of four parallel series. Yet the fauna of Polynesia is strictly
East Indian, modified by the omission or alteration of species,
and that of Australia is Indian at the north, and changes to the
southward much as that of Africa does. In its marine fishes, it does
not constitute a distinct "realm." The East Atlantic (Europe-African)
series follows the same general lines of change as that of the West
Atlantic. It extends, however, only to the South Temperate Zone,
developing no Antarctic elements. The relative shortness of Africa
explains in large degree, as already shown, the similarity between the
tropical elements in the two Old-World series, as the similarity in
tropical elements in the two American series must be due to a former
depression of the connecting Isthmus. The practical unity of the Arctic
marine fauna needs no explanation in view of the present shore lines of
the Arctic Ocean.
=Minor Faunal Areas.=--The minor faunal areas of shore fishes may be
grouped as follows:
East Atlantic.
Icelandic,
British,
Mediterranean,
Guinean,
Cape.
West Atlantic.
Greenlandic,
New England,
Virginian,
Austroriparian,
Floridian,
Antillæan,
Caribbean,
Brazilian,
Argentinan,
Patagonian.
East Pacific.
Arctic,
Aleutian,
Sitkan,
Californian,
San Diegan,
Sinaloan,
Panamanian,
Peruvian,
Revillagigedan,
Galapagan,
Chilian,
Patagonian.
West Pacific.
Arctic,
Aleutian,
Kurile,
Hokkaido,
Nippon,
Chinese,
East Indian,
Polynesian,
Hawaiian,
Indian,
Arabian,
Madagascarian,
Cape,
North Australian,
Tasmanian,
New Zealand,
Antarctic.
=Equatorial Fishes Most Specialized.=--In general, the different
types are most highly specialized in equatorial waters. The processes
of specific change, through natural selection or other causes, if
other causes exist, take place most rapidly there and produce most
far-reaching modification. As elsewhere stated, the coral reefs of
the tropics are the centers of fish-life, the cities in fish economy.
The fresh waters, the arctic waters, the deep sea and the open sea
represent forms of ichthyic backwoods, regions where change goes on
more slowly, and in them we find survivals of archaic or generalized
types. For this reason the study in detail of the distribution
of marine fishes of equatorial regions is in the highest degree
instructive.
=Realms of Distribution of Fresh-water Fishes.=--If we consider the
fresh-water fishes alone we may divide the land areas of the earth
into districts and zones not differing fundamentally with those marked
out for mammals and birds. The river basin, bounded by its shores and
the sea at its mouth, shows many resemblances, from the point of view
of a fish, to an island considered as the home of an animal. It is
evident that with fishes the differences in latitude outweigh those of
continental areas, and a primary division into Old World and New World
would not be tenable.
The chief areas of distribution of fresh-water fishes we may indicate
as follows, following essentially the grouping proposed by Dr.
Günther:[27]
=Northern Zone.=--With Dr. Günther we may recognize first the _Northern
Zone_, characterized familiarly by the presence of sturgeon, salmon,
trout, whitefish, pike, lamprey, stickleback, and other species of
which the genera and often the species are identical in Europe,
Siberia, Canada, Alaska, and most of the United States, Japan, and
China. This is subject to cross-division into two great districts,
the first Europe-Asiatic, the second North American. These two agree
very closely to the northward, but diverge widely to the southward,
developing a variety of specialized genera and species, and both of
them passing finally by degrees into the Equatorial Zone.
Still another line of division is made by the Ural Mountains in the
Old World and by the Rocky Mountains in the New. In both cases the
Eastern region is vastly richer in genera and species, as well as in
autochthonous forms, than the Western. The reason for this lies in the
vastly greater extent of the river basins of China and the Eastern
United States, as compared with those of Europe or the Californian
region.
[Illustration: FIG. 177.--Japanese Sea-horse, _Hippocampus mohnikei_
Bleeker. Misaki, Japan.]
Minor divisions are those which separate the Great Lake region from
the streams tributary to the Gulf of Mexico; and in Asia, those which
separate China from tributaries of the Caspian, the Black, and the
Mediterranean.
=Equatorial Zone.=--The Equatorial Zone is roughly indicated by the
tropics of Cancer and Capricorn. Its essential feature is that of the
temperature, and the peculiarities of its divisions are caused by
barriers of sea or mountains.
Dr. Günther finds the best line of separation into two divisions to lie
in the presence or absence of the great group of dace or minnows,[28]
to which nearly half of the species of fresh-water fishes the world
over belong. The entire group, now spread everywhere except in the
Arctic, South America, Australia, and the islands of the Pacific, seems
to have had its origin in India, from which region its genera have
radiated in every direction.
The Cyprinoid division of the Equatorial Zone forms two districts,
the Indian and the African. The Acyprinoid division includes South
America, south of Mexico, and all the islands of the tropical Pacific
lying to the east of Wallace's line. This line, separating Borneo from
Celebes and Bali from Lompoe, marks in the Pacific the western limit
of Cyprinoid fishes, as well as that of monkeys and other important
groups of land animals. This line, recognized as very important in
the distribution of land animals, coincides in general with the ocean
current between Celebes and Papua, which is one of the sources of the
Kuro Shiwo.
In Australia, Hawaii, and Polynesia generally, the fresh-water fishes
are derived from marine types by modification of one sort or another.
In no case, so far as I know, in any island to the eastward of Borneo,
is found any species derived from fresh-water families of either the
Eastern or the Western Continent. Of course, minor subdivisions in
these districts are formed by the contour lines of river basins. The
fishes of the Nile differ from those of the Niger or the Congo, or of
the streams of Madagascar or Cape Colony, but in all these regions
the essential character of the fish fauna remains the same.
=Southern Zone.=--The third great region, the Southern Zone, is
scantily supplied with fresh-water fishes, and the few it possesses
are chiefly derived from modifications of the marine fauna or from the
Equatorial Zone to the north. Three districts are recognized--Tasmania,
New Zealand, and Patagonia.
=Origin of the New Zealand Fauna.=--The fact that certain peculiar
groups are common to these three regions has attracted the notice of
naturalists. In a critical study of the fish fauna of New Zealand,[29]
Dr. Gill discusses the origin of the four genera and seven species
of fresh-water fishes found in these islands, the principal of these
genera (_Galaxias_) being represented by nearly related species in
South Australia, in Patagonia,[30] the Falkland Islands, and in South
Africa.
According to Dr. Gill, we can account for this anomaly of distribution
only by supposing, on the one hand, that their ancestors were carried
for long distances in some unnatural manner, as (_a_) having been
carried across entombed in ice, or (_b_) being swept by ocean currents,
surviving their long stay in salt water, or else that they were derived
(_c_) from some widely distributed marine type now extinct, its
descendants restricted to fresh water.
On the other hand, Dr. Gill suggests that as "community of type must
be the expression of community of origin," the presence of fishes of
long-established fresh-water types must imply continuity or at least
contiguity of land. The objections raised by geologists to the supposed
land connection of New Zealand and Tasmania do not appear to Dr. Gill
insuperable. It is well known, he says, "that the highest mountain
chains are of comparatively recent geological age. It remains, then,
to consider which is the more probable, (1) that the types now common
in distant regions were distributed in some unnatural manner by the
means referred to, or (2) that they are descendants of forms once
wide-ranging over lands now submerged." After considering questions as
to change of type in other groups, Dr. Gill is inclined to postulate,
from the occurrence of species of the trout-like genus _Galaxias_, in
New Zealand, South Australia, and South America, that "there existed
some terrestrial passage-way between the several regions at a time
as late as the close of the Mesozoic period. The evidence of such a
connection afforded by congeneric fishes is fortified by analogous
representatives among insects, mollusca, and even amphibians. The
separation of the several areas must have occurred little later than
the late Tertiary, inasmuch as the salt-water fishes of corresponding
isotherms found along the coast of the now widely separated lands are
to such a large extent specifically different. In general, change seems
to have taken place more rapidly among marine animals than fresh-water
representatives of the same class."
In this case, when one guess is set against another, it seems to me
that the hypothesis first suggested, rather than the other, lies in
the line of least logical resistance. I think it better to adopt
provisionally some theory not involving the existence of a South
Pacific Antarctic Continent, to account for the distribution of
_Galaxias_. For this view I may give five reasons:
1. There are many other cases of the sort equally remarkable and
equally hard to explain. Among these is the presence of species of
paddle-fish and shovel-nosed sturgeon,[31] types characteristic of
the Mississippi Valley, in Central Asia. The presence of one and only
one of the five or six American species of pike[32] in Europe; of one
of the three species of mud-minnow in Austria,[33] the others being
American. Still another curious case of distribution is that of the
large pike-like trout of the genus _Hucho_, one species (_Hucho hucho_)
inhabiting the Danube, the other (_Hucho blackistoni_) the rivers of
northern Japan. Many such cases occur in different parts of the globe
and at present admit of no plausible explanation.
2. The supposed continental extension should show permanent traces in
greater similarity in the present fauna, both of rivers and of sea. The
other fresh-water genera of the regions in question are different, and
the marine fishes are more different than they could be if we imagine
an ancient shore connection. If New Zealand and Patagonia were once
united other genera than _Galaxias_ would be left to show it.
3. We know nothing of the power of _Galaxias_ to survive submergence in
salt water, if carried in a marine current. As already noticed, I found
young and old in abundance of the commonest of Japanese fresh-water
fishes in the open sea, at a distance from any river. Thus far, this
species, the hakone[34] dace, has not been recorded outside of Japan,
but it might well be swept to Korea or China. Two fresh-water fishes of
Japanese origin now inhabit the island of Tsushima in the Straits of
Korea.
4. The fresh-water fishes of Polynesia show a remarkably wide
distribution and are doubtless carried alive in currents. One
river-goby[35] ranges from Tahiti to the Riu Kiu Islands. Another
species,[36] originally perhaps from Brazil through Mexico, shows an
equally broad distribution.
5. We know that _Galaxias_ with its relatives must have been derived
from a marine type. It has no affinity with any of the fresh-water
families of either continent, unless it be with the Salmonidæ. The
original type of this group was marine, and most of the larger species
still live in the sea, ascending streams only to spawn.
When the investigations of geologists show reason for believing in
radical changes in the forms of continents, we may accept their
conclusions. That geological evidence exists which seems to favor
the existence of a former continent, Antarctica, is claimed on high
authority. If this becomes well established we may well explain the
distribution of _Galaxias_ with reference to it. But we cannot, on
the other hand, regard the anomalous distribution of _Galaxias_ alone
constituting proof of shore connection. There can be no doubt that
almost every case of anomalies in the distribution of fishes admits of
a possible explanation through "the slow action of existing causes."
Real causes are always simple when they are once known. All anomalies
in distribution cease to be such when the facts necessary to understand
them are at our disposal.
FOOTNOTES:
[22] _Hæmulon._
[23] _Calamus._
[24] Clarion Island and Socorro Island.
[25] A few Mexican shore fishes, _Chætodon humeralis_, _Galeichthys
dasycephalus_, _Hypsoblennius parvipinnis_, have been wrongly
accredited to Hawaii by some misplacement of labels.
[26] The dragonets (_Callionymus_) are shore fishes of the shallowest
waters in Europe and Asia, but inhabit considerable depths in
tropical America. The sea-robins (_Prionotus_) are shore fishes in
Massachusetts, semi-bathybial fishes at Panama. Often Arctic shore
fishes become semi-bathybial in the Temperate Zone, living in water of
a given temperature. A long period of cold weather will sometimes bring
such to the surface.
[27] "Introduction to the Study of Fishes."
[28] Cyprinidæ.
[29] "A Comparison of Antipodal Faunæ," 1887.
[30] _Galaxias_, _Neochanna_, _Prototroctes_, and _Retropinna_.
[31] The shovel-nosed sturgeon (_Scaphirynchus_ and _Kessleria_) and
the paddle-fish (_Polyodon_ and _Psephurus_).
[32] _Esox lucius._
[33] _Umrba_, the mud-minnow.
[34] _Leuciscus hakuensis._
[35] _Eleotris fusca._
[36] _Awaous genivittatus._