ISTHMUS BARRIERS SEPARATING FISH FAUNAS
=The Isthmus of Suez.=--In the study of the effect of the Isthmus of
Suez on the distribution of fishes we may first consider the alleged
resemblance between the fauna of the Mediterranean and that of Japan.
Dr. Günther claims that the actual identity of genera and species in
these two regions is such as to necessitate the hypothesis that they
have been in recent times joined by a continuous shore-line. This
shore-line, according to Prof. A. Ortmann and others, was not across
the Isthmus of Suez, but farther to the northward, probably across
Siberia.
=The Fish Fauna of Japan.=--For a better understanding of the problem
we may give a brief analysis of the fish fauna of Japan.
The group of islands which constitute the empire of Japan is remarkable
for the richness of its animal life. Its variety in climatic and other
conditions, its nearness to the great continent of Asia and to the
chief center of marine life, the East Indian Islands, its relation to
the warm Black Current or Kuro Shiwo from the south and to the cold
currents from the north, all tend to give variety and richness to the
fauna of its seas. Especially is this true in the group of fishes. In
spite of the political isolation of the Japanese Empire, this fact has
been long recognized and the characteristic types of Japanese fishes
have been well known to naturalists.
At present about 900 species of fishes are known from the four great
islands which constitute Japan proper--Hondo, Hokkaido, Kiusiu, and
Shikoku. About 200 others are known from the volcanic islands to the
north and south. Of these 1100 species, about fifty belong to the fresh
waters. These are all closely allied to forms found on the mainland of
Asia, from which region all of them were probably derived. In general
the same genera appear in China and with a larger range of species.
=Fresh-water Faunas of Japan.=--Two faunal areas of fresh waters may be
fairly distinguished, although broadly overlapping. The northern region
includes the island of Hokkaido and the middle and northern part of the
great island of Hondo. In a rough way, its southern boundary may be
defined by Fuji Yama, and the Bay of Matsushima. It is characterized by
the presence of salmon, trout, and sculpins, and northward by sturgeon
and brook lampreys. The southern area loses by degrees the trout
and other northern fishes, while in its clear waters abound various
minnows, gobies, and the famous ayu, or Japanese dwarf salmon, one of
the most delicate of food fishes. Sculpins and lampreys give place to
minnows, loaches, and chubs. Two genera, a sculpin[37] and a perch,[38]
besides certain minnows and catfishes, are confined to this region
and seem to have originated in it, but, like the other species, from
Chinese stock.
=Origin of Japanese Fresh-water Fishes.=--The question of the origin of
the Japanese river fauna seems very simple. All the types are Asiatic.
While most of the Japanese species are distinct, their ancestors must
have been estrays from the mainland. To what extent river fishes may
be carried from place to place by currents of salt water has never
been ascertained. One of the most widely distributed of Japanese river
fishes is the large hakone dace or chub.[39] This has been repeatedly
taken by us in the sea at a distance from any stream. It would
evidently survive a long journey in salt water. An allied species[40]
is found in the midway island of Tsushima, between Korea and Japan.
=Faunal Areas of Marine Fishes in Japan.=--The distribution of the
marine fishes of Japan is mainly controlled by the temperature of the
waters and the motion of the ocean currents. Five faunal areas may be
more or less clearly recognized, and these may receive names indicating
their scope--Kurile, Hokkaido, Nippon, Kiusiu, Kuro Shiwo, and Riu Kiu.
The first or Kurile district is frankly subarctic, containing species
characteristic of the Ochotsk Sea on the one hand, and of Alaska on
the other. The second or Hokkaido[41] district includes this northern
island and that part of the shore of the main island of Hondo[42]
which lies to the north of Matsushima and Noto. Here the cold northern
currents favor the development of a northern fauna. The herring and the
salmon occupy here the same economic relation as in Norway, Scotland,
Newfoundland, and British Columbia. Sculpins, blennies, rockfish, and
flounders abound of the rocky shores and are seen in all the markets.
South of Matsushima Bay and through the Island Sea as far as Kobe, the
Nippon fauna is distinctly one of the temperate zone. Most of the types
characteristically Japanese belong here, abounding in the sandy bays
and about the rocky islands.
About the islands of Kiusiu and Shikoku, the semi-tropical elements
increase in number and the Kiusiu fauna is less characteristically
Japanese, having much in common with the neighboring shores of China,
while some of the species range northward from India and Java. But
these faunal districts have no sharp barriers. Northern fishes[43]
unquestionably of Alaskan origin range as far south as Nagasaki,
while certain semi-tropical[44] types extend their range northward to
Hakodate and Volcano Bay. The Inland Sea, which in a sense bounds the
southern fauna, serves at the same time as a means of its extension.
While each species has a fairly definite northern or southern limit,
the boundaries of a faunal district as a whole must be stated in the
most general terms.
The well-known boundary called Blackiston's Line, which passes through
the Straits of Tsugaru, between the two great islands of Hondo and
Hokkaido, marks the northern boundary of monkeys, pheasants, and most
tropical and semi-tropical birds and mammals of Japan. But as to the
fishes, either marine or fresh water, this line has no significance.
The northern fresh-water species probably readily cross it; the
southern rarely reach it.
We may define as a fourth faunal area that of the Kuro
Shiwo district itself, which is distinctly tropical and contrasts
strongly with that of the inshore bays behind it. This warm "Black
Current," analogous to our Gulf Stream, has its origin in part from
a return current from the east which passes westward through Hawaii,
in part from a current which passes between Celebes and New Guinea.
It moves northward by way of Luzon and Formosa, touching the east
shores of the Japanese islands Kiusiu and Shikoku, to the main island
of Hondo, flooding the bays of Kagoshima and Kochi, of Waka, Suruga,
and Sagami. The projecting headlands reach out into it and the fauna
of their rock-pools is distinctly tropical as far to the northward as
Tokio.
[Illustration: FIG. 178.--Sacramento Perch, _Archoplites interruptus_
Girard. Family _Centrarchidæ_. Sacramento River.]
These promontories of Hondo, Waka, Ise, Izu, Misaki, and Awa have
essentially the same types of fishes as are found on the reefs of
tropical Polynesia. The warmth of the off-shore currents gives the
fauna of Misaki its astonishing richness, and the wealth of life is
by no means confined to the fishes. Corals, crustaceans, worms, and
mollusks show the same generous profusion of species.
A fifth faunal area, closely related to that of the Black Current, is
formed by the volcanic and coral reefs of the Riu Kiu Archipelago. This
fauna, so far as known, is essentially East Indian, the genera and most
of the species being entirely identical with those of the islands about
Java and Celebes.
=Resemblance of the Japanese and Mediterranean Fish Faunas.=--It has
been noted by Dr. Günther that the fish fauna of Japan bears a marked
resemblance to that of the Mediterranean. This likeness is shown in the
actual identity of genera and species, and in their relation to each
other. This resemblance he proposes to explain by the hypothesis that
at some recent period the two regions, Japan and the Mediterranean,
have been united by a continuous shore-line. The far-reaching character
of this hypothesis demands a careful examination of the data on which
it rests.
The resemblance of the two faunal areas, so far as fishes are
concerned, may be stated as follows: There are certain genera[45] of
shore fishes, tropical or semi-tropical, common to the Mediterranean
and Japan, and wanting to California, Panama, and the West Indies, and
in most cases to Polynesia also. Besides these, certain others found in
deeper water (100 to 200 fathoms) are common to the two areas,[46] and
have been rarely taken elsewhere.
=Significance of Resemblance.=--The significance of these facts can be
shown only by a fuller analysis of the fauna in question, and those
of other tropical and semi-tropical waters. If the resemblances are
merely casual, or if the resemblances are shown by other regions, the
hypothesis of shore continuity would be unnecessary or untenable. It is
tenable if the resemblances are so great as to be accounted for in no
other way.
Of the genera regarded as common, only two[47] or three are represented
in the two regions by identical species, and these have a very wide
distribution in the warm seas. Of the others, nearly all range to
India, to the Cape of Good Hope, to Australia, or to Brazil. They
may have ranged farther in the past; they may even range farther at
present. Not one is confined to the two districts in question. As
equally great resemblances exist between Japan and Australia or Japan
and the West Indies, the case is not self-evident without fuller
comparison. I shall therefore undertake a somewhat fuller analysis
of the evidence bearing on this and similar problems with a view to
the conclusions which may be legitimately drawn from the facts of fish
distribution.
=Differences between Japanese and Mediterranean Fish Faunas.=--We
may first, after admitting the alleged resemblances and others, note
that differences are equally marked. In each region are a certain
number of genera which we may consider as autochthonous. These
genera are represented by many species or by many individuals in the
region of their supposed origin, but are more scantily developed
elsewhere. Such genera in Mediterranean waters are _Crenilabrus_,
_Labrus_, _Spicara_, _Pagellus_, _Mullus_, _Boops_, _Spondyliosoma_,
_Oblata_. None of these occurs in Japan, nor have they any near
relatives there. Japanese autochthonous types, as _Pseudoblennius_,
_Vellitor_, _Duymæria_, _Anoplus_, _Histiopterus_, _Monocentrus_,
_Oplegnathus_, _Plecoglossus_, range southward to the Indies or to
Australia, but all of them are totally unknown to the Mediterranean.
The multifarious genera of Gobies of Japan show very little resemblance
to the Mediterranean fishes of this family, while blennies, labroids,
scaroids, and scorpænoids are equally diverse in their forms and
alliances. To the same extent that likeness in faunas is produced by
continuity of means of dispersion is it true that unlikeness is due to
breaks in continuity. Such a break in continuity of coast-line, in the
present case, is the Isthmus of Suez, and the unlikeness in the faunas
is about what we might conceive that such a barrier should produce.
=Sources of Faunal Resemblances.=--There are two main sources of faunal
resemblances: first, the absence of any barriers permitting the actual
mingling of the species; second, the likeness of temperature and shore
configuration on either side of an imperfect barrier. Absolute barriers
do not exist and apparently never have existed in the sea. If the fish
faunas of different regions have mingled in recent times, the fact
would be shown by the presence of the same species in each region. If
the union were of a remote date, the species would be changed, but the
genera might remain identical.
In case of close physical resemblances in different regions, as in the
East Indies and West Indies, like conditions would favor the final
lodgement of like types, but the resemblance would be general, the
genera and species being unlike. Without doubt part of the resemblance
between Japan and the Mediterranean is due to similarity of temperature
and shores. Is that which remains sufficient to demand the hypothesis
of a former shore-line connection?
=Effects of Direction of Shore-line.=--We may first note that a
continuous shore-line produces a mingling of fish faunas only when not
interrupted by barriers due to climate. A north and south coast-line,
like that of the East Pacific, however unbroken, permits great faunal
differences. It is crossed by the different zones of temperature. An
east and west shore-line lies in the same temperature. In all cases
of the kind which now exist on the earth (the Mediterranean, the Gulf
of Mexico, the Caribbean Sea, the shores of India), even species will
extend their range as far as the shore-line goes. The obvious reason
is because such a shore-line rarely offers any important barrier to
distribution, checking dispersion of species. We may, therefore,
consider the age and nature of the Isthmus of Suez and the character of
the faunas it separates.
=Numbers of Genera in Different Faunas.=--For our purposes the
genera must be rigidly defined, a separate name being used in case
of each definable difference in structure. The wide-ranging genera
of the earlier systematists were practically cosmopolitan, and their
geographical distribution teaches us little. On the other hand, when
we come to the study of geological distribution, the broad definition
of the genus is the only one usually available. The fossil specimens
are always defective. Minor characters may be lost past even the
possibility of a guess, and only along broad lines can we achieve the
classification of the individual fossil.
Using the modern definition of genus, we find in Japan 483 genera of
marine fishes; in the Red Sea, 225; in the Mediterranean, 231. In New
Zealand 150 are recorded; in Hawaii, 171; 357 from the West Indies, 187
from the Pacific coast of tropical America, 300 from India, 450 from
the East-Indian islands, and 227 from Australia.
Of the 483 genera ascribed to Japan, 156 are common to the
Mediterranean also, 188 to the West Indies and Japan, 169 to the
Pacific coast of the United States and Mexico. With Hawaii Japan shares
90 genera, with New Zealand 62; 204 are common to Japan and India, 148
to Japan and the Red Sea, most of these being found in India also. Two
hundred genera are common to Japan and Australia.
From this it is evident that Japan and the Mediterranean have much in
common, but apparently not more than Japan shares with other tropical
regions. Japan naturally shows most likeness to India, and next to this
to the Red Sea. Proportionately less is the resemblance to Australia,
and the likeness to the Mediterranean seems much the same as that to
the West Indies or to the Pacific coast of America.
But, to make these comparisons just and effective, we should consider
not the fish fauna as a whole; we should limit our discussion solely
to the forms of equatorial origin. From the fauna of Japan we may
eliminate all the genera of Alaskan-Aleutian origin, as these could not
be found in the other regions under comparison. We should eliminate
all pelagic and all deep-sea forms, for the laws which govern the
distribution of these are very different from those controlling the
shore fishes, and most of the genera have reached a kind of equilibrium
over the world.
=Significance of Rare Forms.=--We may note also, as a source of
confusion in our investigation, that numerous forms found in Japan
and elsewhere are very rarely taken, and their real distribution is
unknown. Some of these will be found to have, in some unexpected
quarter, their real center of dispersion. In fact, since these pages
were written, I have taken in Hawaii representatives of three[48]
genera which I had enumerated as belonging chiefly to Japan and the
West Indies. Numerous other genera common to the two regions have
since been obtained by Dr. Gilbert. Such species may inhabit oceanic
plateaus, and find many halting places in their circuit of the tropical
oceans. We have already discovered that Madeira, St. Helena, Ascension,
and other volcanic islands constitute such halting places. We shall
find many more such, when the deeper shore regions are explored,
the region between market-fishing and the deep-sea dredgings of the
_Challenger_ and the _Albatross_. In some cases, no doubt, these forms
are verging on extinction and a former wide distribution has given
place to isolated colonies.
The following table shows the contents, so far as genera are
concerned, of those equatorial areas in which trustworthy catalogues
of species are accessible. It includes only those fishes of stationary
habit living in less than 200 fathoms. It goes without saying that
considerable latitude must be given to these figures, to allow for
errors, omissions, uncertainties, and differences of opinion.
=Distribution of Shore Fishes.=--
_A. Japan and the Mediterranean._
Genera[49] chiefly confined to these regions 2
Genera of wide distribution 77
----
Total of common genera 79
Total in both regions 399
Genera above included, found in all equatorial regions 55
Genera[50] found in most equatorial regions 11
Genera more or less restricted 13
----
79
_B. Japan and the Red Sea._
Genera[51] chiefly confined to these two regions 2
Genera of wide distribution 109
----
Total genera common 111
Total in both regions 424
_C. Japan and Hawaii._
Genera chiefly confined to these regions 3
Genera of wide distribution 79
----
Total genera common 82
Total in both regions 396
_D. Japan and Australia._
Genera chiefly confined to these regions 13
Genera of wide distribution (chiefly East Indian) 122
----
Total genera common 135
Total in both regions 533
_E. Japan and Panama._
Genera chiefly confined to these regions 2
Genera of wide distribution 89
----
Total genera common 91
Total in both regions 499
_F. Japan and the West Indies._
Genera chiefly confined to these regions 5
Genera of wide distribution 108
----
Total genera common 113
Total in both regions 520
_G. The Mediterranean and the Red Sea._
Genera confined to the Suez region 0
Genera of wide distribution (chiefly Indian) 40
----
Total genera common 40
Total in both regions 295
_H. West Indies and the Mediterranean._
Genera chiefly confined to the equatorial Atlantic 11
Genera of wide distribution 59
----
Total 70
Total in both regions 373
_I. West Indies and Panama._
Genera chiefly confined to equatorial America 68
Genera of wide distribution 101
----
Total genera common 169
Total in equatorial America 376
_J. Hawaii and Panama._
Genera chiefly confined to the regions in question 3
Genera of wide distribution 74
----
Total genera common 77
Total in both regions 323
_K. Hawaii and the East Indies._
Genera chiefly confined to Hawaii 4
Genera of wide distribution in the equatorial Pacific 123
Genera confined to Hawaii and the West Indies 1
_Summary._
Genera (shore fishes only) in the Mediterranean
Sea. 144
Genera in the Red Sea 191
Genera in India 280
Genera in Japan (exclusive of northern forms) 334
Genera in Australia 344
Genera in New Zealand 108
Genera in Hawaii 144
Genera about Panama 256
Genera in West Indies 299
=Extension of Indian Fauna.=--From the above tables it is evident that
the warm-water fauna of Japan, as well as that of Hawaii, is derived
from the great body of the fauna of the East Indies and Hindostan; that
the fauna of the Red Sea is derived in the same way; that the fauna
of the Mediterranean bears no especial resemblance to that of Japan,
rather than to other elements of the East Asiatic fauna in similar
conditions of temperature, and no greater than is borne by either to
the West Indies; that the faunas of the sides of the Isthmus of Suez
have relatively little in common, while those of the two sides of the
Isthmus of Panama show large identity of genera, although few species
are common to the two sides. Of the 255 genera recorded from the Panama
region, 179, or over 70 per cent., are also in the West Indies, while
68, or more than 30 per cent. of the number, are limited to the two
regions in question.
=The Isthmus of Suez as a Barrier to Distribution.=--With the aid of
the above table we may examine further the relation of the fauna of
Japan to that of the Mediterranean. If a continuity of shore-line
once existed, it would involve the obliteration of the Isthmus. With
free connection across this isthmus the fauna of the Red Sea must
have been once practically the same as that of the Mediterranean.
The present differences must be due to later immigrations to one or
the other region, or to the extinction of species in one locality
or the other, through some kind of unfitness. In neither region is
there evidence of extensive immigration from the outside. The present
conditions of water and temperature differ a little, but not enough
to explain the difference in faunæ. The Red Sea is frankly tropical
and its fauna is essentially Indian, much the same, so far as genera
are concerned, as that of southern Japan. The Mediterranean is at most
not more than semi-tropical and its fishes are characteristically
European. Its tropical forms belong rather to Guinea than to the East
Indies. With the Red Sea the Mediterranean has very little in common,
not so much, for example, as has Hawaii. Forty genera of shore fishes
(and only fifty of all fishes) are identical in the two regions, the
Mediterranean and the Red Sea. Of those, every one is a genus of wide
distribution, found in nearly all warm seas. Of shore fishes, only one
genus in seven is common to the two regions. Apparently, therefore, we
cannot assume a passage across the Isthmus of Suez within the lifetime
of the present genera. Not one of the types alleged to be peculiar to
Japan and the Mediterranean is thus far known in the Red Sea. Not one
of the characteristically abundant Mediterranean types[52] crosses the
Isthmus of Suez, and the distinctive Red Sea and Indian types[53] are
equally wanting in the Mediterranean. The only genera which could have
crossed the Isthmus are certain shallow-water or brackish-water forms,
sting-rays, torpedoes, sardines, eels, and mullets, widely diffused
through the East Indies and found also in the Mediterranean. The former
channel, if one ever existed, had, therefore, much the same value in
distribution of species as the present Suez Canal.
=Geological Evidence of Submergence of the Isthmus of Suez.=--Yet, from
geological data, there is strong evidence that the Isthmus of Suez was
submerged in relatively recent times. The recognized geological maps
of the Isthmus show that a broad area of post-Pliocene or Pliocene
deposits constitutes the Isthmus and separates the nummulitic hills
of Suez from their fellows about thirty miles to the eastward. The
northern part of the Isthmus is alluvium from the Nile, and its western
part is covered with drifting sands. The Red Sea once extended farther
north than now and the Mediterranean farther to the southeast. Assuming
the maps to be correct, the Isthmus must have been open water in the
late Pliocene or post-Pliocene times.
Admitting this as a fact, the difference in the fish fauna would seem
to show that the waters over the submerged area were so shallow that
the rock-loving forms did not and could not cross it. Moreover, the
region was very likely overspread with silt-bearing fresh waters from
the Nile. To such fishes as _Chætodon_, _Holocentrus_, _Thalassoma_
of the Red Sea, or to _Crenilabrus_, _Boops_, and _Zeus_ of the
Mediterranean, such waters would form a barrier as effective as the
sand-dunes of to-day.
=Conclusions as to the Isthmus of Suez.=--We are led, therefore, to
these conclusions:
1. There is no evidence derivable from the fishes of the recent
submergence of the Isthmus of Suez.
2. If the Isthmus was submerged in Pliocene or post-Pliocene times,
the resultant channel was shallow and muddy, so that ordinary marine
fishes or fishes of rock bottoms or of deep waters did not cross it.
3. It formed an open water to brackish-water fishes only.
4. The types common to Japan and the Mediterranean did not enter either
region from the other by way of the Red Sea.
5. As most of these are found also in India or Australia or both, their
dispersion was probably around the south coast of Africa or by the Cape
of Good Hope.
6. In view of the fact that numerous East Indian genera, as _Zanclus_,
_Enoplosus_, _Toxotes_, _Ephippus_, _Platax_, _Teuthis_, _Acanthurus_
(_Monoceros_), _Myripristis_ occur in the Eocene rocks of Tuscany,
Syria, and Switzerland, we may well suppose that an open waterway
across Africa then existed. Perhaps these forms were destroyed
in European waters by a wave of glacial cold, perhaps after the
Miocene. As our knowledge of the Miocene fish faunæ of Europe is
still imperfect, we cannot locate accurately the period of their
disappearance. About half the species found in the Eocene of Italy
belong to existing genera, and these genera are almost all now
represented in the Indian fauna, and those named above with others are
confined to it.
The study of fishes alone furnishes no adequate basis for mapping
the continental masses of Tertiary times. The known facts in regard
to their distribution agree fairly with the provisional maps lately
published by Dr. Ortmann (Bull. Philos. Soc., XLI). In the Eocene
map (Fig. 179) the Mediterranean extends to the northward of Arabia,
across to the mouth of the Ganges. This extension would account for the
tropical, Eocene, and Miocene fish fauna of Southern Europe.
=The Cape of Good Hope as a Barrier to Fishes.=--The fishes of the Cape
of Good Hope are not well enough known for close comparison with those
of other regions. Enough is known of the Cape fauna to show its general
relation to those of India and Australia. The Cape of Good Hope lies in
the South Temperate Zone. It offers no absolutely impassable barrier
to the tropical fishes from either side. It bears a closer relation to
either the Red Sea or the Mediterranean than they bear to each other.
It is, therefore, reasonable to conclude that the transfer of tropical
shore fishes of the Old World between the Atlantic and Pacific, in
recent times, has taken place mainly around the southern point of
Africa. To pelagic and deep-sea fishes the Cape of Good Hope has
offered no barrier whatever. To ordinary fishes it is an obstacle, but
not an impassable one. This the fauna itself shows. It has, however,
not been passed by many tropical species, and by these only as the
result of thousands of years of struggle and point-to-point migration.
=Relations of Japan to Mediterranean Explainable by Present
Conditions.=--We may conclude that the resemblance of the Mediterranean
fish fauna to that of Japan or India is no more than might be expected,
even had the present contour of the continents been permanent for the
period of duration of the present genera and species. An open channel
in recent times would have produced much greater resemblances than
actually exist.
=The Isthmus of Panama as a Barrier to Distribution.=--Conditions in
some regards parallel with those of the Isthmus of Suez exist in but
one other region--the Isthmus of Panama. Here the first observers were
very strongly impressed by the resemblance of forms. Nearly half the
genera found on the two sides of this isthmus are common to both sides.
Taking those of the Pacific shore for first consideration, we find
that three-fourths of the genera of the Panama fauna occur in the West
Indies as well.
This identity is many times greater than that existing at the Isthmus
of Suez. Moreover, while the Cape of Good Hope offers no impassable
barrier to distribution, the same is not true of the southern part of
South America. The subarctic climate of Cape Horn has doubtless formed
a complete check to the movements of tropical fishes for a vast period
of geologic time.
=Unlikeness of Species on the Shores of the Isthmus of Panama.=--But,
curiously enough, this marked resemblance is confined chiefly to the
genera and does not extend to the species on the two shores.
Of 1400 species of fishes recorded from tropical America north of the
Equator, only about 70 are common to the two coasts. The number of
shore fishes common is still less. In this 70 are included a certain
number of cosmopolitan types which might have reached either shore from
the Old World.
[Illustration: FIG. 179.--Map of the Continents, Eocene time. (After
Ortmann.)]
A few others invade brackish or fresh waters and may possibly have
found their way, in one way or another, across the Isthmus of
Nicaragua. Of fishes strictly marine, strictly littoral, and not known
from Asia or Polynesia, scarcely any species are left as common to the
two sides. This seems to show that no waterway has existed across the
Isthmus within the lifetime, whatever that may be, of the existing
species. The close resemblance of genera shows apparently with almost
equal certainty that such a waterway has existed, and within the
period of existence of the groups called genera. How long a species
of fish may endure unchanged no one knows, but we know that in this
regard great differences must exist in different groups. Assuming that
different species crossed the Isthmus of Panama in Miocene times, we
should not be surprised to find that a few remain to all appearances
unchanged; that a much larger number have become "representative"
species, closely related forms retaining relations to the environment
to those of the parent form, and, finally, that a few species have been
radically altered.
This is exactly what has taken place at the Isthmus of Panama with
the marine shore fishes. Curiously enough, the movement of genera
seems to have been chiefly from the Atlantic to the Pacific. Certain
characteristic genera[54] of the Panama region have not passed over to
the Pacific. On the other hand, most of the common genera[55] show a
much larger number of species on the Atlantic side. This may be held to
show their Atlantic origin.
Of the relatively small number of genera which Panama has received from
Polynesia[56] few have crossed the Isthmus to appear in the West Indian
fauna.
=Views of Earlier Writers on the Fishes of the Isthmus of Panama.=--The
elements of the problem at Panama may be better understood by a glance
at the results of previous investigations.
In 1869 Dr. Günther, after enumerating the species examined by him
from Panama, reaches the conclusion that nearly one-third of the marine
fishes on the two shores of tropical America will be found to be
identical. He enumerates 193 such species as found on the two coasts;
59 of these, or 31 per cent. of the total, being actually identical.
From this he infers that there must have been, at a comparatively
recent date, a depression of the Isthmus and intermingling of the two
faunas.[57]
=Catalogue of Fishes of Panama.=--In an enumeration of the fishes of
the Pacific coast in 1885,[58] the present writer showed that Dr.
Günther's conclusions were based on inadequate data.
In my list 407 species were recorded from the Pacific coast of tropical
America--twice the number enumerated by Dr. Günther. Of these 71
species, or 17-1/2 per cent., were found also in the Atlantic. About
800 species are known from the Caribbean and adjacent shores, so
that out of the total number of 1,136 species but 71, or 6 per cent.
of the whole, are common to the two coasts. This number does not
greatly exceed that of the species common to the West Indies and the
Mediterranean, or even the West Indies and Japan. It is to be noted
also that the number 71 is not very definitely ascertained, as there
must be considerable difference of opinion as to the boundaries of
species, and the actual identity in several cases is open to doubt.
This discrepancy arises from the comparatively limited representation
of the two faunas at the disposal of Dr. Günther. He enumerates 193
marine or brackish-water species as found on the two coasts, 59 of
which are regarded by him as specifically identical, this being 31 per
cent. of the whole. But in 30 of these 59 cases I regard the assumption
of complete identity as erroneous, so that taking the number 193 as
given I would reduce the percentage to 15. But these 193 species form
but a fragment of the total fauna, and any conclusion based on such
narrow data is certain to be misleading.
Of the 71 identical species admitted in our list, several (_e.g._,
_Mola_, _Thunnus_) are pelagic fishes common to most warm seas.
Still others (_e.g._, _Trachurus_, _Carangus_, _Diodon_ sp.) are
cosmopolitan in the tropical waters. Most of the others (_e.g._,
_Gobius_, _Gerres_, _Centropomus_, _Galeichthys_ sp., etc.) often
ascend the rivers of the tropics, and we may account for their
diffusion, perhaps, as we account for the dispersion of fresh-water
fishes on the Isthmus, on the supposition that they may have crossed
from marsh to marsh at some time in the rainy season.
In very few cases are representatives of any species from opposite
sides of the Isthmus exactly alike in all respects. These differences
in some cases seem worthy of specific value, giving us "representative
species" on the two sides. In other cases the distinctions are very
trivial, but in most cases they are appreciable, especially in fresh
specimens.
Further, I expressed the belief that "fuller investigation will not
increase the proportion of common species. If it does not, the two
faunas show no greater resemblance than the similarity of physical
conditions on the two sides would lead us to expect." This similarity
causes the same types of fishes to persist on either side of the
Isthmus while through isolation or otherwise these have become
different as species.
This conclusion must hold so far as species are concerned, but the
resemblance of the genera on the sides has a significance of its own.
In 1880[59] Dr Günther expressed his views in still stronger language,
claiming a still larger proportion of the fishes of tropical America to
be identical on the two sides of the continent. He concluded that "with
scarcely any exceptions the genera are identical, and of the species
found on the Pacific side, nearly one-half have proved to be the same
as those of the Atlantic. The explanation of this fact has been found
in the existence of communications between the two oceans by channels
and straits which must have been open till within a recent period. The
isthmus of Central America was then partially submerged, and appeared
as a chain of islands similar to that of the Antilles; but as the
reef-building corals flourished chiefly north and east of these islands
and were absent south and west of them, reef fishes were excluded
from the Pacific shores when the communications were destroyed by the
upheaval of land."
=Conclusions of Evermann and Jenkins.=--This remark led to a further
discussion of the subject on the part of Dr. B. W. Evermann and Dr. O.
P. Jenkins. From their paper on the fishes of Guaymas[60] I make the
following quotations:
"The explorations since 1885 have resulted (1) in an addition of about
100 species to one or other of the two faunas; (2) in showing that at
least two species that were regarded as identical on the two shores[61]
are probably distinct; and (3) in the addition of but two species to
those common to both coasts.[62]
"All this reduces still further the percentage of common species.
"Of the 110 species obtained by us, 24, or less than 21 per cent.,
appear to be common to both coasts. Of these 24 species, at least 16,
from their wide distribution, would need no hypothesis of a former
waterway through the Isthmus to account for their presence on both
sides. They are species fully able to arrive at the Pacific shores of
the Americas from the warm seas west. It thus appears that not more
than eight species, less than 8 per cent. of our collection, all of
which are marine species, require any such hypothesis to account for
their occurrence on both coasts of America. This gives us, then, 1,307
species that should properly be taken into account when considering
this question, not more than 72 of which, or 5.5 per cent., seem to be
identical on the two coasts. This is very different from the figures
given by Dr. Günther in his 'Study of Fishes.'
"Now, if from these 72 species, admitted to be common to both coasts,
we subtract the 16 species of wide distribution--so wide as to keep
them from being a factor in this problem--we have left but 56 species
common to the two coasts that bear very closely upon the waterway
hypothesis. _This is less than 4.3 per cent. of the whole number._
"But the evidence obtained from a study of other marine life of that
region points to the same conclusion.
"In 1881, Dr. Paul Fischer discussed the same question in his 'Manual
de Conchyliologie,' pp. 168, 169, in a section on the Molluscan Fauna
of the Panamic Province, and reached the same general conclusions. He
says: 'Les naturalistes Américians se sont beaucoup preéoccupés des
espèces de Panama qui paraissent identiques avec celles des Antilles,
ou qui sont représentatives. P. Carpenter estime qu'il en existe 35.
Dans la plupart des cas, l'identite absolue n'a pu être constantée
et on a trouvé quelques caractères distinctifs, ce qui n'a rien
d'ètonnant, puisque dans l'hypothèse d'une origine commune, les deux
races pacifique et atlantique sont séparée depuis la periode Miocène.
Voici un liste de ces espèces représentatives ou identiques.' Here
follows a list of 20 species. 'Mais ces formes semblables,' he says,
'constituent un infime minorité (3 per cent.).'
"These facts have a very important bearing upon certain geological
questions, particularly upon the one concerning the cold of the Glacial
period.
"In Dr. G. Frederick Wright's recent book, 'The Ice Age in North
America,' eight different theories as to the cause of the cold are
discussed. The particular theory which seems to him quite reasonable
is that one which attributes the cold as due to a change of different
parts of the country, and a depression of the Isthmus of Panama is
one of the important changes he considers. He says: 'Should a portion
of the Gulf Stream be driven through a depression across the Isthmus
of Panama into the Pacific, and an equal portion be diverted from the
Atlantic coast of the United States by an elevation of the sea-bottom
between Florida and Cuba, the consequences would necessarily be
incalculably great, so that the mere existence of such a possible cause
for great changes in the distribution of moisture over the northern
hemisphere is sufficient to make one hesitate before committing himself
unreservedly to any other theory; at any rate, to one which has not for
itself independent and adequate proof.'
"In the appendix to the same volume Mr. Warren Upham, in discussing
the probable causes of glaciation, says: 'The quaternary uplifts of
the Andes and Rocky Mountains and of the West Indies make it nearly
certain that the Isthmus of Panama has been similarly elevated during
the recent epoch.... It may be true, therefore, that the submergence
of this isthmus was one of the causes of the Glacial period, the
continuation of the equatorial oceanic currents westward into the
Pacific having greatly diminished or wholly diverted the Gulf Stream,
which carries warmth from the tropics to the northern Atlantic and
northwestern Europe.'
[Illustration: FIG. 180.--_Caulophryne jordani_ Goode and Bean, a
deep-sea fish of the Gulf Stream. Family _Ceratiidæ_.]
[Illustration: FIG. 181.--_Exerpes asper_ Jenkins and Evermann, a fish
of the rock-pools, Guaymas, Mexico. Family _Blenniidæ_.]
"Any _very_ recent means by which the fishes could have passed readily
from one side to the other would have resulted in making the fish
faunas of the two shores practically identical; but the time that has
elapsed since such a waterway could have existed has been long enough
to allow the fishes of the two sides to become _practically distinct_.
That the mollusks of the two shores are almost wholly distinct, as
shown by Dr. Fischer, is even stronger evidence of the remoteness of
the time when the means of communication between the two oceans could
have existed, for 'species' among the mollusks are probably more
persistent than among fishes.
"Our present knowledge, therefore, of the fishes of tropical America
justifies us in regarding the fish faunas of the two coasts as being
essentially distinct, and believing that there has not been, at any
comparatively recent time, any waterway through the Isthmus of Panama."
It is thus shown, I think, conclusively, that the Isthmus of Panama
could not have been depressed for any great length of time in a recent
geological period.
=Conclusions of Dr. Hill.=--These writers have not, however, considered
the question of generic identity. To this we may find a clue in the
geological investigations of Dr. Robert T. Hill.
In a study of "The Geological History of the Isthmus of Panama and
Portions of Costa Rica," Dr. Hill uses the following language:
"By elimination we have concluded that the only period of time since
the Mesozoic within which communication between the seas could have
taken place is the Tertiary period, and this must be restricted to
the Eocene and Oligocene epochs of that period. The paleontologic
evidence upon which such an opening can be surmised at this period
is the occurrence of a few California Eocene types in the Atlantic
sides of the tropical American barrier, within the ranges of latitude
between Galveston (Texas) and Colon, which are similar to others found
in California. There are no known structural data upon which to locate
the site of this passage, but we must bear in mind, however, that this
structure has not been completely explored.
"Even though it was granted that the coincidence of the occurrence of a
few identical forms on both sides of the tropical American region, out
of the thousands which are not common, indicates a connection between
the two seas, there is still an absence of any reason for placing this
connection at the Isthmus of Panama, and we could just as well maintain
that the locus thereof might have been at some other point in the
Central American region.
"The reported fossil and living species common to both oceans are
littoral forms, which indicate that if a passage existed it must have
been of a shallow and ephemeral character.
"There is no evidence from either a geologic or a biologic standpoint
for believing that the oceans have ever communicated across the
Isthmian regions since Tertiary time. In other words, there is no
evidence for these later passages which have been established upon
hypothetical data, especially those of Pleistocene time.
"The numerous assertions, so frequently found in literature, that the
two oceans have been frequently and recently connected across the
Isthmus, and that the low passes indicative of this connection still
exist, may be dismissed at once and forever and relegated to the domain
of the apocryphal. A few species common to the waters of both oceans in
a predominantly Caribbean fauna of the age of the Claiborne epoch of
the Eocene Tertiary is the only paleontologic evidence in any time upon
which such a connection may be hypothesized.
"There has been a tendency in literature to underestimate the true
altitude of the isthmian passes, which, while probably not intentional,
has given encouragement to those who think that this Pleistocene
passage may have existed. Maack has erroneously given the pass at 186
feet. Dr. J. W. Gregory states 'that the summit of the Isthmus at one
locality is 154 feet and in another 287 feet in height.' The lowest
isthmian pass, which is not a summit, but a drainage col, is 287-295
feet above the ocean.
"If we could lower the isthmian region 300 feet at present, the waters
of the two oceans would certainly commingle through the narrow Culebra
Pass. But the Culebra Pass is clearly the headwater col of two streams,
the Obispo flowing into the Chagres, and the Rio Grande flowing into
the Pacific, and has been cut by fluviatile action, and not by marine
erosion, out of a land mass which has existed since Miocene time. Those
who attempt to establish Pleistocene interoceanic channels through this
pass on account of its present low altitude must not omit from their
calculations the restoration of former rock masses which have been
removed by the general levelling of the surface by erosion."
[Illustration: FIG. 182.--_Xenocys jessiæ_ Jordan and Bollman.
Galapagos Islands. Family _Lutianidæ_.]
In conclusion, Dr. Hill asserts that "there is considerable evidence
that a land barrier in the tropical region separated the two oceans as
far back in geologic history as Jurassic time, and that that barrier
continued throughout the Cretaceous period. The geological structure
of the Isthmus and Central American regions, so far as investigated,
when considered aside from the paleontology, presents no evidence by
which the former existence of a free communication of oceanic waters
across the present tropical land barriers can be established. The
paleontologic evidence indicates the ephemeral existence of a passage
at the close of the Eocene period. All lines of inquiry--geologic,
paleontologic, and biologic--give evidence that no connection has
existed between the two oceans since the close of the Oligocene. This
structural geology is decidedly opposed to any hypothesis by which the
waters of the two oceans could have been connected across the regions
in Miocene, Pliocene, Pleistocene, or recent times."
=Final Hypothesis as to Panama.=--If we assume the correctness of Dr.
Hill's conclusions, they may accord in a remarkable degree with the
actual facts of the distribution of the fishes about the Isthmus. To
account for the remarkable identity of genera and divergence of species
I may suggest the following hypothesis:
During the lifetime of most of the present species, the Isthmus has not
been depressed. It was depressed in or before Miocene time, during the
lifetime of most of the present genera. We learn from other sources
that few of the extant species of fishes are older than the Pliocene.
Relatively few genera go back to the Eocene, and most of the modern
families appear to begin in the Eocene or later Cretaceous. In general
the Miocene may be taken as the date of the origin of modern genera.
The channel formed across the Isthmus was relatively shallow, excluding
forms inhabiting rocky bottoms at considerable depths. It was wide
enough to permit the infiltration from the Caribbean Sea of numerous
species, especially of shore fishes of sandy bays, tide pools, and
brackish estuaries. The currents set chiefly to the westward, favoring
the transfer of Atlantic rather than Pacific types.
[Illustration: FIG. 183.--Channel Catfish, _Ictalurus punctatus_
(Rafinesque). Illinois River. Family _Siluridæ_.]
Since the date of the closing of this channel the species left on the
two sides have been altered in varying degrees by the processes of
natural selection and isolation. The cases of actual specific identity
are few, and the date of the establishment as species, of the existing
forms, is subsequent to the date of the last depression of the Isthmus.
We may be certain that none of the common genera ever found their way
around Cape Horn. Most of them disappear to the southward, along the
coasts of Brazil and Peru.
While local oscillations, involving changes in coast-lines, have
doubtless frequently taken place and are still going on, the past and
present distribution of fishes does not alone give adequate data for
their investigation.
Further, it goes without saying that we have no knowledge of the
period of time necessary to work specific changes in a body of species
isolated in an alien sea. Nor have we any data as to the effect on
a given fish fauna of the infiltration of many species and genera
belonging to another. All such forces and results must be matters of
inference.
The present writer does not wish to deny that great changes have
taken place in the outlines of continents in relatively recent times.
He would, however, insist that the theory of such changes must be
confirmed by geological evidence, and evidence from groups other
than fishes, and that likeness in separated fish faunas may not be
conclusive.
[Illustration: FIG. 184.--Drawing the net on the beach of Hilo, Hawaii.
Photograph by Henry W. Henshaw.]
FOOTNOTES:
[37] _Rheopresbe._
[38] _Bryttosus._
[39] _Leuciscus hakuensis_ Günther.
[40] _Leuciscus jouyi._
[41] Formerly, but no longer, called Yeso in Japan.
[42] Called Nippon on foreign maps, but not so in Japan, where Nippon
means the whole empire.
[43] _Pleuronichthys cornutus_, _Hexogrammos otakii_, etc.
[44] As _Halichoeres_, _Tetrapturus_, _Callionymus_, _Ariscopus_, etc.
[45] Of these, the principal ones are _Oxystomus_, _Myrus_, _Pagrus_,
_Sparus_, _Macrorhamphosus_, _Cepola_, _Callionymus_, _Zeus_,
_Uranoscopus_, _Lepidotrigla_, _Chelidonichthys_.
[46] Among these are _Beryx_, _Helicolenus_, _Lotella_, _Nettastoma_,
_Centrolophus_, _Hoplostethus_, _Aulopus_, _Chlorophthalmus_,
_Lophotes_.
[47] _Beryx_, _Hoplostethus_.
[48] _Antigonia_, _Etelis_, _Emmelichthys_.
[49] _Lepadogaster_, _Myrus_; _Lophotes_, thus far recorded from
Japan, the Mediterranean, and the Cape of Good Hope, is bassalian and
of unknown range. _Beryx_, _Trachichthys_, _Hoplostethus_, etc., are
virtually cosmopolitan as well as semi-bassalian.
[50] In this group we must place _Cepola_, _Callionymus_, _Pagrus_,
_Sparus_, _Beryx_, _Zeus_, all of which have a very wide range in
Indian waters.
[51] _Cryptocentrus_, _Asterropteryx_. The range of neither of these
genera of small shore fishes is yet well known.
[52] As _Crenilabrus_, _Labrus_, _Symphodus_, _Pagellus_,
_Spondyliosoma_, _Sparisoma_.
[53] As _Chætodon_, _Lethrinus_, _Monotaxis_, _Glyphisodon_, etc.
[54] _Hoplopagrus_, _Xenichthys_, _Xenistius_, _Xenocys_,
_Microdesmus_, _Cerdale_, _Cratinus_, _Azevia_, _Microlepidotus_,
_Orthostoechus_, _Isaciella_, etc.
[55] _Hæmulon_, _Anisotremus_, _Gerres_, _Centropomus_, _Galeichthys_,
_Hypoplectrus_, _Mycteroperca_, _Ulæma_, _Stellifer_, _Micropogon_,
_Bodianus_, _Microspathodon_.
[56] Among these are perhaps _Teuthis_ (_Acanthurus_), _Ilisha_,
_Salarias_, _Myripristis_, _Thalassoma_. Some such which have not
crossed the Isthmus are _Cirrhitus_, _Sectator_, _Sebastopsis_, and
_Lophiomus_.
[57] "Fishes of Central America," 1869, 397.
[58] _Proc. U. S. Nat. Mus._, 1885, 393.
[59] Introduction to the "Study of Fishes," 1880, p. 280.
[60] _Proc. U. S. Nat. Mus._, 1891, pp. 124-126.
[61] _Citharichthys spilopterus_ and _C. gilberti_.
[62] _Hæmulon steindachneri_ and _Gymnothorax castaneus_ of the west
coast probably being identical with _H. schranki_ and _Gymnothorax
funebris_ of the east coast.