BARRIERS TO DISPERSION OF RIVER FISHES
=The Process of Natural Selection.=--We can say, in general, that in
all waters not absolutely uninhabitable there are fishes. The processes
of natural selection have given to each kind of river or lake species
of fishes adapted to the conditions of life which obtain there. There
is no condition of water, of bottom, of depth, of speed of current, but
finds some species with characters adjusted to it. These adjustments
are, for the most part, of long standing; and the fauna of any single
stream has as a rule been produced by immigration from other regions
or from other streams. Each species has an ascertainable range of
distribution, and within this range we may be reasonably certain to
find it in any suitable waters.
[Illustration: FIG. 189.--Slippery-dick or Doncella, _Halichoeres
bivittatus_ Bloch, a fish of the coral reefs, Key West. Family
_Labridæ_.]
But every species has beyond question some sort of limit to its
distribution, some sort of barrier which it has never passed in all
the years of its existence. That this is true becomes evident when we
compare the fish fauna of widely separated rivers. Thus the Sacramento,
Connecticut, Rio Grande, and St. John's Rivers have not a single
species in common; and with one or two exceptions, not a species is
common to any two of them. None of these[100] has any species peculiar
to itself, and each shares a large part of its fish fauna with the
water-basin next to it. It is probably true that the faunas of no two
distinct hydrographic basins are wholly identical, while on the other
hand there are very few species confined to a single one. The supposed
cases of this character, some twenty in number, occur chiefly in the
streams of the South Atlantic States and of Arizona. All of these need,
however, the confirmation of further exploration. It is certain that in
no case has an entire river fauna[101] originated independently from
the divergence into separate species of the descendants of a single
type.
The existence of boundaries to the range of species implies, therefore,
the existence of barriers to their diffusion. We may now consider these
barriers and in the same connection the degree to which they may be
overcome.
=Local Barriers.=--Least important to these are the barriers which may
exist within the limits of any single basin, and which tend to prevent
a free diffusion through its waters of species inhabiting any portion
of it. In streams flowing southward, or across different parallels of
latitude, the difference in climate becomes a matter of importance. The
distribution of species is governed very largely by the temperature
of the water. Each species has its range in this respect,--the
free-swimming fishes, notably the trout, being most affected by it;
the mud-loving or bottom fishes, like the catfishes, least. The
latter can reach the cool bottoms in hot weather, or the warm bottoms
in cold weather, thus keeping their own temperature more even than
that of the surface of the water. Although water communication is
perfectly free for most of the length of the Mississippi, there is a
material difference between the faunæ of the stream in Minnesota and
in Louisiana. This difference is caused chiefly by the difference
in temperature occupying the difference in latitude. That a similar
difference in longitude, with free water communication, has no
appreciable importance, is shown by the almost absolute identity of
the fish faunæ of Lake Winnebago and Lake Champlain. While many large
fishes range freely up and down the Mississippi, a majority of the
species do not do so, and the fauna of the upper Mississippi has more
in common with that of the tributaries of Lake Michigan than it has
with that of the Red River or the Arkansas. The influence of climate
is again shown in the paucity of the fauna of the cold waters of Lake
Superior, as compared with that of Lake Michigan. The majority of
our species cannot endure the cold. In general, therefore, cold or
Northern waters contain fewer species than Southern waters do, though
the number of individuals of any one kind may be greater. This is shown
in all waters, fresh or salt. The fisheries of the Northern seas are
more extensive than those of the tropics. There are more fishes there,
but are far less varied in kind. The writer once caught seventy-five
species of fishes in a single haul of the seine at Key West, while on
Cape Cod he obtained with the same net but forty-five species in the
course of a week's work. Thus it comes that the angler, contented with
many fishes of few kinds, goes to Northern streams to fish, while the
naturalist goes to the South.
[Illustration: FIG. 190.--_Peristedion miniatum_ Goode and Bean, a
deep-red colored fish of the depths of the Gulf Stream.]
But in most streams the difference in latitude is insignificant,
and the chief differences in temperature come from differences in
elevation, or from the distance of the waters from the colder source.
Often the lowland waters are so different in character as to produce
a marked change in the quality of their fauna. These lowland waters
may form a barrier to the free movements of upland fishes; but
that this barrier is not impassable is shown by the identity of the
fishes in the streams[102] of the uplands of middle Tennessee with
those of the Holston and French Broad. Again, streams of the Ozark
Mountains, similar in character to the rivers of East Tennessee, have
an essentially similar fish fauna, although between the Ozarks and the
Cumberland range lies an area of lowland bayous, into which such fishes
are never known to penetrate. We can, however, imagine that these
upland fishes may be sometimes swept down from one side or the other
into the Mississippi, from which they might ascend on the other side.
But such transfers certainly do not often happen. This is apparent
from the fact that the two faunas[103] are not quite identical, and in
some cases the same species are represented by perceptibly different
varieties on one side and the other. The time of the commingling of
these faunæ is perhaps now past, and it may have occurred only when the
climate of the intervening regions was colder than at present.
The effect of waterfalls and cascades as a barrier to the diffusion
of most species is self-evident; but the importance of such obstacles
is less, in the course of time, than might be expected. In one way or
another very many species have passed these barriers. The falls of the
Cumberland limit the range of most of the larger fishes of the river,
but the streams above it have their quota of darters and minnows. It
is evident that the past history of the stream must enter as a factor
into this discussion, but this past history it is not always possible
to trace. Dams or artificial waterfalls now check the free movement of
many species, especially those of migratory habits; while conversely,
numerous other species have extended their range through the agency of
canals.[104]
Every year fishes are swept down the rivers by the winter's floods;
and in the spring, as the spawning season approaches, almost every
species is found working its way up the stream. In some cases, notably
the Quinnat salmon[105] and the blue-back salmon,[106] the length of
these migrations is surprisingly great. To some species rapids and
shallows have proved a sufficient barrier, and other kinds have been
kept back by unfavorable conditions of various sorts. Streams whose
waters are always charged with silt or sediment, as the Missouri,
Arkansas, or Brazos, do not invite fishes; and even the occasional
floods of red mud such as disfigure otherwise clear streams, like
the Red River or the Colorado (of Texas), are unfavorable. Extremely
unfavorable also is the condition which obtains in many rivers of the
Southwest, as, for example, the Red River, the Sabine, and the Trinity,
which are full from bank to bank in winter and spring, and which
dwindle to mere rivulets in the autumn droughts.
=Favorable Waters have Most Species.=--In general, those streams which
have conditions most favorable to fish life will be found to contain
the greatest number of species. Such streams invite immigration; and
in them the struggle for existence is individual against individual,
species against species, and not a mere struggle with hard conditions
of life. Some of the conditions most favorable to the existence in any
stream of a large number of species of fishes are the following, the
most important of which is the one mentioned first: Connection with
a large hydrographic basin; a warm climate; clear water; a moderate
current; a bottom of gravel (preferably covered by a growth of weeds);
little fluctuation during the year in the volume of the stream or in
the character of the water.
Limestone streams usually yield more species than streams flowing
over sandstone, and either more than the streams of regions having
metamorphic rocks. Sandy bottoms usually are not favorable to fishes.
In general, glacial drift makes a suitable river bottom, but the higher
temperature usual in regions beyond the limits of the drift gives
to certain Southern streams conditions still more favorable. These
conditions are all well realized in the Washita River in Arkansas, and
in various tributaries of the Tennessee, Cumberland, and Ohio; and in
these, among American streams, the greatest number of species has been
recorded.
The isolation and the low temperature of the rivers of New England have
given to them a very scanty fish fauna as compared with the rivers of
the South and West. This fact has been noticed by Professor Agassiz,
who has called New England a "zoological island."[107]
In spite of the fact that barriers of every sort are sometimes crossed
by fresh-water fishes, we must still regard the matter of freedom of
water communication as the essential one in determining the range of
most species. The larger the river basin, the greater the variety of
conditions likely to be offered in it, and the greater the number of
its species. In case of the divergence of new forms by the processes
called "natural selection," the greater the number of such forms which
may have spread through its waters; the more extended any river basin,
the greater are the chances that any given species may sometimes find
its way into it; hence the greater the number of species that actually
occur in it, and, freedom of movement being assumed, the greater the
number of species to be found in any one of its affluents.
Of the six hundred species of fishes found in the rivers of the United
States, about two hundred have been recorded from the basin of the
Mississippi. From fifty to one hundred of these species can be found in
any one of the tributary streams of the size, say, of the Housatonic
River or the Charles. In the Connecticut River there are but about
eighteen species permanently resident; and the number found in the
streams of Texas is not much larger, the best known of these, the Rio
Colorado, having yielded but twenty-four species.
The waters of the Great Basin are not rich in fishes, the
[Illustration: FIG. 191.--Ancient Outlet of Lake Bonneville, Great Salt
Lake, in Idaho. (Photograph by Prof. J. M. Aldrich.)] species now
found being evidently an overflow from the Snake River when in late
glacial times it drained Lake Bonneville. This postglacial lake once
filled the present basin of the Great Salt Lake and Utah Lake, its
outlet flowing northwest from Ogden into Snake River. The same fishes
are now found in the upper Snake River and the basins of Utah Lake and
of Sevier Lake. In the same fashion Lake Lahontan once occupied the
basin of Nevada, the Humboldt and Carson sinks, with Pyramid Lake.
Its drainage fell also into the Snake River, and its former limits
are shown in the present range of species. These have almost nothing
in common with the group of species inhabiting the former drainage of
Lake Bonneville. Another postglacial body of water, Lake Idaho, once
united the lakes of Southeastern Oregon. The fauna of Lake Idaho, and
of the lakes Malheur, Warner, Goose, etc., which have replaced it, is
also isolated and distinctive. The number of species now known from
this region of these ancient lobes is about 125. This list is composed
almost entirely of a few genera of suckers,[108] minnows,[109] and
trout.[110] None of the catfishes, perch, darters, or sunfishes,
moon-eyes, pike, killifishes, and none of the ordinary Eastern types of
minnows[111] have passed the barrier of the Rocky Mountains.
West of the Sierra Nevada the fauna is still more scanty, only about
seventy species being enumerated. This fauna, except for certain
immigrants[112] from the sea, is of the same general character as that
of the Great Basin, though most of the species are different. This
latter fact would indicate a considerable change, or "evolution,"
since the contents of the two faunæ were last mingled. There is a
considerable difference between the fauna of the Columbia and that
of the Sacramento. The species which these two basins have in common
are chiefly those which at times pass out into the sea. The rivers of
Alaska contain but few species, barely a dozen in all, most of these
being found also in Siberia and Kamchatka. In the scantiness of its
faunal list, the Yukon agrees with the Mackenzie River, and with Arctic
rivers generally.
There can be no doubt that the general tendency is for each species to
extend its range more and more widely until all localities suitable
for its growth are included. The various agencies of dispersal which
have existed in the past are still in operation. There is apparently
no limit to their action. It is probable that new "colonies" of one
species or another may be planted each year in waters not heretofore
inhabited by such species. But such colonies become permanent only
where the conditions are so favorable that the species can hold
its own in the struggle for food and subsistence. That the various
modifications in the habitat of certain species have been caused by
human agencies is of course too well known to need discussion here.
=Watersheds.=--We may next consider the question of watersheds, or
barriers which separate one river basin from another.
Of such barriers in the United States, the most important and most
effective is unquestionably that of the main chain of the Rocky
Mountains. This is due in part to its great height, still more to its
great breadth, and most of all, perhaps, to the fact that it is nowhere
broken by the passage of a river. But two species--the red-throated or
Rocky Mountain trout[113] and the Rocky Mountain whitefish[114]--are
found on both sides of it, at least within the limits of the United
States; while many genera, and even several families, find in it either
an eastern or a western limit to their range. In a few instances
representative species, probably modifications or separated branches
of the same stock, occur on opposite sides of the range, but there
are not many cases of correspondence even thus close. The two faunas
are practically distinct. Even the widely distributed red-spotted or
"dolly varden" trout[115] of the Columbia River and its affluents does
not cross to the east side of the mountains, nor does the Montana
grayling[116] ever make its way to the West. In Northern Mexico,
however, numerous Eastern river fishes have crossed the main chain of
the Sierra Madre.
=How Fishes Cross Watersheds.=--It is easy to account for this
separation of the faunæ; but how shall we explain the almost universal
diffusion of the whitefish and the trout in suitable waters on both
sides of the dividing ridge? We may notice that these two are the
species which ascend highest in the mountains, the whitefish inhabiting
the mountain pools and lakes, the trout ascending all brooks and
rapids in search of their fountainheads. In many cases the ultimate
dividing ridge is not very broad, and we may imagine that at some time
spawn or even young fishes may have been carried across by birds or
other animals, or by man, or more likely by the dash of some summer
whirlwind. Once carried across in favorable circumstances, the species
might survive and spread.
The following is an example of how such transfer of species may be
accomplished, which shows that we need not be left to draw on the
imagination to invent possible means of transit.
=The Suletind.=--There are few watersheds in the world better defined
than the mountain range which forms the "back-bone" of Norway. I
lately climbed a peak in this range, the Suletind. From its summit I
could look down into the valleys of the Lära and the Bägna, flowing in
opposite directions to opposite sides of the peninsula. To the north of
the Suletind is a large double lake called the Sletningenvand. The maps
show this lake to be one of the chief sources of the westward-flowing
river Lära. This lake is in August swollen by the melting of the
snows, and at the time of my visit it was visibly the source of both
these rivers. From its southeastern side flowed a large brook into
the valley of the Bägna, and from its southwestern corner, equally
distinctly, came the waters which fed the Lära. This lake, like
similar mountain ponds in all northern countries, abounds in trout;
and these trout certainly have for part of the year an uninterrupted
line of water communication from the Sognefjord on the west of Norway
to the Christianiafjord on the southeast,--from the North Sea to the
Baltic. Part of the year the lake has probably but a single outlet
through the Lära. A higher temperature would entirely cut off the
flow into the Bägna, and a still higher one might dry up the lake
altogether. This Sletningenvand, with its two outlets on the summit
of a sharp watershed, may serve to show us how other lakes, permanent
or temporary, may elsewhere have acted as agencies for the transfer of
fishes. We can also see how it might be that certain mountain fishes
should be so transferred while the fishes of the upland waters may
be left behind. In some such way as this we may imagine that various
species of fishes have attained their present wide range in the Rocky
Mountain region; and in similar manner perhaps the Eastern brook
trout[117] and some other mountain species[118] may have been carried
across the Alleghanies.
=The Cassiquiare.=--Professor John C. Branner calls my attention to
a marshy upland which separates the valley of the La Plata from that
of the Amazon, and which permits the free movement of fishes from
the Paraguay River to the Tapajos. It is well known that through the
Cassiquiare River the Rio Negro, another branch of the Amazon, is
joined to the Orinoco River. It is thus evident that almost all the
waters of eastern South America form a single basin, so far as the
fishes are concerned.
As to the method of transfer of the trout from the Columbia to the
Missouri, we are not now left in doubt.
=Two-Ocean Pass.=--To this day, as the present writer and later
Evermann and Jenkins[119] have shown, the Yellowstone and Snake Rivers
are connected by two streams crossing the main divide of the Rocky
Mountains from the Yellowstone to the Snake across Two-Ocean Pass.
Prof. Evermann has described the locality as follows:
"Two-Ocean Pass is a high mountain meadow, about 8,200 feet above
the sea and situated just south of the Yellowstone National Park, in
longitude 110° 10' W., latitude 44° 3' N. It is surrounded on all sides
by rather high mountains except where the narrow valleys of Atlantic
and Pacific creeks open out from it. Running back among the mountains
to the northward are two small canyons down which come two small
streams. On the opposite is another canyon down which comes another
small stream. The extreme length of the meadow from east to west is
about a mile, while the width from north to south is not much less. The
larger of the streams coming in from the north is Pacific Creek, which,
after winding along the western side of the meadow, turns abruptly
westward, leaving the meadow through a narrow gorge. Receiving numerous
small affluents, Pacific Creek soon becomes a good-sized stream, which
finally unites with Buffalo Creek a few miles above where the latter
stream flows into Snake River.
"Atlantic Creek was found to have two forks entering the pass. At the
north end of the meadow is a small wooded canyon down which flows the
North Fork. This stream hugs the border of the flat very closely. The
South Fork comes down the canyon on the south side, skirting the brow
of the hill a little less closely than does the North Fork. The two,
coming together near the middle of the eastern border of the meadow,
form Atlantic Creek, which after a course of a few miles flows into the
Upper Yellowstone. But the remarkable phenomena exhibited here remain
to be described.
"Each fork of Atlantic Creek, just after entering the meadow, divides
as if to flow around an island, but the stream toward the meadow,
instead of returning to the portion from which it had parted, continues
its westerly course across the meadow. Just before reaching the western
border the two streams unite and then pour their combined waters into
Pacific Creek; thus are Atlantic and Pacific creeks united and a
continuous waterway from the Columbia via Two-Ocean Pass to the Gulf of
Mexico is established.
"Pacific Creek is a stream of good size long before it enters the pass,
and its course through the meadow is in a definite channel, but not so
with Atlantic Creek. The west bank of each fork is low and the stream
is liable to break through anywhere and thus send part of its water
across to Pacific Creek. It is probably true that one or two branches
always connect the two creeks under ordinary conditions, and that
following heavy rains or when the snows are melting, a much greater
portion of the water of Atlantic Creek crosses the meadow to the other
side.
[Illustration: FIG. 192.--Silver Surf-fish (viviparous),
_Hypocritichthys analis_ (Agassiz). Monterey.]
"Besides the channels already mentioned, there are several more or
less distinct ones that were dry at the time of our visit. As already
stated, the pass is a nearly level meadow covered with a heavy growth
of grass and many small willows one to three feet high. While it is
somewhat marshy in places it has nothing of the nature of a lake about
it. Of course, during wet weather the small springs at the borders of
the meadow would be stronger, but the important facts are that there
is no lake or even marsh there and that neither Atlantic nor Pacific
Creek has its rise in the meadow. Atlantic Creek, in fact, comes into
the pass as two good-sized streams from opposite directions and leaves
it by at least four channels, thus making an island of a considerable
portion of the meadow. And it is certain that there is, under ordinary
circumstances, a continuous waterway through Two-Ocean Pass of such a
character as to permit fishes to pass easily and readily from Snake
River over to the Yellowstone, or in the opposite direction. Indeed,
it is quite possible, barring certain falls in the Snake River, for a
fish so inclined, to start at the mouth of the Columbia, travel up that
great river to its principal tributary, the Snake, thence on through
the long, tortuous course of that stream, and, under the shadows of the
Grand Teton, enter the cold waters of Pacific Creek, by which it could
journey on up to the very crest of the great continental divide,--to
Two-Ocean Pass; through this pass it may have a choice of two routes
to Atlantic Creek, in which the down-stream journey is begun. Soon it
reaches the Yellowstone, down which it continues to Yellowstone Lake,
then through the lower Yellowstone out into the turbid waters of the
Missouri; for many hundred miles it may continue down this mighty river
before reaching the Father of Waters, which will finally carry it to
the Gulf of Mexico--a wonderful journey of nearly 6,000 miles, by far
the longest possible fresh-water journey in the world.
"We found trout in Pacific Creek at every point where we examined it.
In Two-Ocean Pass we found trout in each of the streams and in such
positions as would have permitted them to pass easily from one side of
the divide to the other. We also found trout in Atlantic Creek below
the pass, and in the upper Yellowstone they were abundant. Thus it is
certain that there is no obstruction, even in dry weather, to prevent
the passage of trout from the Snake River to Yellowstone Lake; it is
quite evident that trout do pass over in this way; and it is almost
certain that Yellowstone Lake was stocked with trout from the west via
Two-Ocean Pass."--EVERMANN.
=Mountain Chains.=--The Sierra Nevada constitutes also a very important
barrier to the diffusion of species. This is, however, broken by the
passage of the Columbia River, and many species thus find their way
across it. That the waters to the west of it are not unfavorable for
the growth of Eastern fishes is shown by the fact of the rapid spread
of the common Eastern catfish,[120] or horned pout, when transported
from the Schuylkill to the Sacramento. The catfish is now one of the
important food fishes of the San Francisco markets, and with the
Chinaman its patron, it has gone from California to Hawaii. The Chinese
catfish, described by Bleeker as _Ameiurus cantonensis_, was doubtless
carried home by some Chinaman returning from San Francisco. In like
fashion the small-mouthed black bass is now frequent in California
streams, as is also the blue-green sunfish, _Apomotis cyanellus_,
introduced as food for the bass.
The mountain mass of Mount Shasta is, as already stated, a considerable
barrier to the range of fishes, though a number of species find their
way around it through the sea. The lower and irregular ridges of the
Coast Range are of small importance in this regard, as the streams of
their east slope reach the sea on the west through San Francisco Bay.
Yet the San Joaquin contains a few species not yet recorded from the
smaller rivers of southwestern California.
The main chain of the Alleghanies forms a barrier of importance
separating the rich fish fauna of the Tennessee and Ohio basins from
the scantier faunæ of the Atlantic streams. Yet this barrier is crossed
by many more species than is the case with either the Rocky Mountains
or the Sierra Nevada. It is lower, narrower, and much more broken,--as
in New York, in Pennsylvania, and in Georgia there are several streams
which pass through it or around it. The much greater age of the
Alleghany chain, as compared with the Rocky Mountains, seems not to
be an element of any importance in this connection. Of the fish which
cross this chain, the most prominent is the brook trout,[121] which
is found in all suitable waters from Hudson's Bay to the head of the
Chattahoochee.
=Upland Fishes.=--A few other species are locally found in the head
waters of certain streams on opposite sides of the range. An example
of this is the little red "fallfish,"[122] found only in the mountain
tributaries of the Savannah and the Tennessee. We may suppose the same
agencies to have assisted these species that we have imagined in the
case of the Rocky Mountain trout, and such agencies were doubtless more
operative in the times immediately following the glacial epoch than
they are now. Prof. Cope calls attention also to the numerous caverns
existing in these mountains as a sufficient medium for the transfer of
many species. I doubt whether the main chains of the Blue Ridge or the
Great Smoky can be crossed in that way, though such channels are not
rare in the subcarboniferous limestones of the Cumberland range. In the
brooks at the head waters of the Roanoke River about Alleghany Springs
in Virginia, fishes of the Tennessee Basin are found, instead of those
characteristic of the lower Roanoke. In this case it is likely that we
have to consider the results of local erosion. Probably the divide has
been so shifted that some small stream with its fishes has been cut off
from the Holston and transferred to the Roanoke.
The passage of species from stream to stream along the Atlantic slope
deserves a moment's notice. It is under present conditions impossible
for any mountain or upland fish, as the trout or the miller's
thumb,[123] to cross from the Potomac River to the James, or from
the Neuse to the Santee, by descending to the lower courses of the
rivers, and thence passing along either through the swamps or by way
of the sea. The lower courses of these streams, warm and muddy, are
uninhabitable by such fishes. Such transfers are, however, possible
farther north. From the rivers of Canada and from many rivers of New
England the trout does descend to the sea and into the sea, and farther
north the whitefish does this also. Thus these fishes readily pass
from one river basin to another. As this is the case now everywhere in
the North, it may have been the case farther south in the time of the
glacial cold. We may, I think, imagine a condition of things in which
the snow-fields of the Alleghany chain might have played some part in
aiding the diffusion of cold-loving fishes. A permanent snow-field on
the Blue Ridge in western North Carolina might render almost any stream
in the Carolinas suitable for trout, from its source to its mouth. An
increased volume of colder water might carry the trout of the head
streams of the Catawba and the Savannah as far down as the sea. We can
even imagine that the trout reached these streams in the first place
through such agencies, though of this there is no positive evidence.
For the presence of trout in the upper Chattahoochee we must account in
some other way.
It is noteworthy that the upland fishes are nearly the same in
all these streams until we reach the southern limit of possible
glacial influence. South of western North Carolina the faunæ of the
different river basins appear to be more distinct from one another.
Certain ripple-loving types are represented by closely related but
unquestionably different species in each river basin, and it would
appear that a thorough mingling of the upland species in these rivers
has never taken place.
The best examples of this are the following: In the Santee basin
are found _Notropis pyrrhomelas_, _Notropis niveus_, and _Notropis
chloristius_; in the Altamaha, _Notropis xænurus_ and _Notropis
callisemus_; in the Chattahoochee, _Notropis hypselopterus_ and
_Notropis eurystomus_; in the Alabama, _Notropis coeruleus_, _Notropis
trichroistius_, and _Notropis callistius_. In the Alabama, Escambia,
Pearl, and numerous other rivers is found _Notropis cercostigma_. This
species descends to the sea in the cool streams of the pine woods. Its
range is wider than that of the others, and in the rivers of Texas
it reappears in the form of a scarcely distinct variety, _Notropis
venustus_. In the Tennessee and Cumberland, and in the rivers of the
Ozark range, is _Notropis galacturus_; and in the upper Arkansas
_Notropis camurus_,--all distinct species of the same general type.
Northward, in all the streams from the Potomac to the Oswego, and
westward to the Des Moines and the Arkansas, occurs a single species
of this type, _Notropis whipplei_, varying eastward into _Notropis
analostanus_. But this species is not known from any of the streams
inhabited by any of the other species mentioned, although very likely
it is the parent stock of them all.
=Lowland Fishes.=--With the lowland species of the Southern rivers
it is different. Few of these are confined within narrow limits. The
streams of the whole South Atlantic and Gulf Coast flow into shallow
bays, mostly bounded by sand-spits or sand-bars which the rivers
themselves have brought down. In these bays the waters are often
neither fresh nor salt; or, rather, they are alternately fresh and
salt, the former condition being that of the winter and spring. Many
species descend into these bays, thus finding every facility for
transfer from river to river. There is a continuous inland passage in
fresh or brackish waters, traversable by such fishes, from Chesapeake
Bay nearly to Cape Fear; and similar conditions exist on the coasts
of Louisiana, Texas, and much of Florida. In Perdido Bay I have found
fresh-water minnows[124] and silversides[125] living together with
marine gobies[126] and salt-water eels.[127] Fresh-water alligator
gars[128] and marine sharks compete for the garbage thrown over from
the Pensacola wharves. In Lake Pontchartrain the fauna is a remarkable
mixture of fresh-water fishes from the Mississippi and marine fishes
from the Gulf. Channel-cats, sharks, sea-crabs, sunfishes, and mullets
can all be found there together. It is therefore to be expected that
the lowland fauna of all the rivers of the Gulf States would closely
resemble that of the lower Mississippi; and this, in fact, is the case.
The streams of southern Florida and those of southwestern Texas offer
some peculiarities connected with their warmer climate. The Florida
streams contain a few peculiar fishes;[129] while the rivers of Texas,
with the same general fauna as those farther north, have also a few
distinctly tropical types,[130] immigrants from the lowlands of Mexico.
=Cuban Fishes.=--The fresh waters of Cuba are inhabited by fishes
unlike those found in the United States. Some of these are evidently
indigenous, derived in the waters they now inhabit directly from marine
forms. Two of these are eyeless species,[131] inhabiting streams in
the caverns. They have no relatives in the fresh waters of any other
region, the blind fishes[132] of our caves being of a wholly different
type. Some of the Cuban fishes are common to the fresh waters of the
other West Indies. Of Northern types, only one, the alligator gar,[133]
is found in Cuba, and this is evidently a filibuster immigrant from the
coasts of Florida.
=Swampy Watersheds.=--The low and irregular watershed which separates
the tributaries of Lake Michigan and Lake Erie from those of the Ohio
is of little importance in determining the range of species. Many of
the distinctively Northern fishes are found in the headwaters of the
Wabash and the Scioto. The considerable difference in the general fauna
of the Ohio Valley as compared with that of the streams of Michigan is
due to the higher temperature of the former region, rather than to any
existing barriers between the river and the Great Lakes. In northern
Indiana the watershed is often swampy, and in many places large ponds
exist in the early spring.
At times of heavy rains many species will move through considerable
distances by means of temporary ponds and brooks. Fishes that have
thus emigrated often reach places ordinarily inaccessible, and people
finding them in such localities often imagine that they have "rained
down." Once, near Indianapolis, after a heavy shower, I found in a
furrow in a corn-field a small pike,[134] some half a mile from the
creek in which he should belong. The fish was swimming along in a
temporary brook, apparently wholly unconscious that he was not in his
native stream. Migratory fishes, which ascend small streams to spawn,
are especially likely to be transferred in this way. By some such means
any of the watersheds in Ohio, Indiana, or Illinois may be passed.
[Illustration: FIG. 193.--Creekfish or Chub-sucker, _Erimyzon sucetta_
(Lacépède). Nipisink Lake, Illinois. Family _Catostomidæ_.]
It is certain that the limits of Lake Erie and Lake Michigan were once
more extended than now. It is reasonably probable that some of the
territory now drained by the Wabash and the Illinois was once covered
by the waters of Lake Michigan. The cisco[135] of Lake Tippecanoe,
Lake Geneva, and the lakes of the Oconomowoc chain is evidently a
modified descendant of the so-called lake herring.[136] Its origin most
likely dates from the time when these small deep lakes of Indiana and
Wisconsin were connected with Lake Michigan. The changes in habits
which the cisco has undergone are considerable. The changes in external
characters are but trifling. The presence of the cisco in these lakes
and its periodical disappearance--that is, retreat into deep water
when not in the breeding season--have given rise to much nonsensical
discussion as to whether any or all of these lakes are still joined to
Lake Michigan by subterranean channels. Several of the larger fishes,
properly characteristic of the Great Lake region,[137] are occasionally
taken in the Ohio River, where they are usually recognized as rare
stragglers. The difference in physical conditions is probably the sole
cause of their scarcity in the Ohio basin.
=The Great Basin of Utah.=--The similarity of the fishes in the
different streams and lakes of the Great Basin is doubtless to be
attributed to the general mingling of their waters which took place
during and after the Glacial Epoch. Since that period the climate in
that region has grown hotter and drier, until the overflow of the
various lakes into the Columbia basin through the Snake River has long
since ceased. These lakes have become isolated from each other, and
many of them have become salt or alkaline and therefore uninhabitable.
In some of these lakes certain species may now have become extinct
which still remain in others. In some cases, perhaps, the differences
in surroundings may have caused divergence into distinct species of
what was once one parent stock. The suckers in Lake Tahoe[138] and
those in Utah Lake are certainly now different from each other and
from those in the Columbia. The trout[139] in the same waters can be
regarded as more or less tangible species, while the whitefishes[140]
show no differences at all. The differences in the present faunas of
Lake Tahoe and Utah Lake must be chiefly due to influences which have
acted since the Glacial Epoch, when the whole Utah Basin was part of
the drainage of the Columbia.
=Arctic Species in Lakes.=--Connected perhaps with changes due to
glacial influences is the presence in the deep waters of the Great
Lakes of certain marine types,[141] as shown by the explorations of
Professor Sidney I. Smith and others. One of these is a genus of
fishes,[142] of which the nearest allies now inhabit the Arctic Seas.
In his review of the fish fauna of Finland,[143] Professor A. J.
Malmgren finds a number of Arctic species in the waters of Finland
which are not found either in the North Sea or in the southern portions
of the Baltic. These fishes are said to "agree with their 'forefathers'
in the Glacial Ocean in every point, but remain comparatively smaller,
leaner, almost starved." Professor Lovén[144] also has shown that
numerous small animals of marine origin are found in the deep lakes of
Sweden and Finland as well as in the Gulf of Bothnia. These anomalies
of distribution are explained by Lovén and Malmgren on the supposition
of the former continuity of the Baltic through the Gulf of Bothnia
with the Glacial Ocean. During the second half of the Glacial Period,
according to Lovén, "the greater part of Finland and of the middle
of Sweden was submerged, and the Baltic was a great gulf of the
Glacial Ocean, and not connected with the German Ocean. By the gradual
elevation of the Scandinavian Continent, the Baltic became disconnected
from the Glacial Ocean and the Great Lakes separated from the Baltic.
In consequence of the gradual change of the salt water into fresh,
the marine fauna became gradually extinct, with the exception of the
glacial forms mentioned above."
It is possible that the presence of marine types in our Great Lakes
is to be regarded as due to some depression of the land which would
connect their waters with those of the Gulf of St. Lawrence. On this
point, however, our data are still incomplete.
To certain species of upland or mountain fishes the depression of the
Mississippi basin itself forms a barrier which cannot be passed. The
black-spotted trout,[145] very closely related species of which abound
in all waters of northern Asia, Europe, and western North America,
has nowhere crossed the basin of the Mississippi, although one of its
species finds no difficulty in passing Bering Strait. The trout and
whitefish of the Rocky Mountain region are all species different from
those of the Great Lakes or the streams of the Alleghany system. To
the grayling, the trout, the whitefish, the pike, and to arctic and
subarctic species generally, Bering Strait has evidently proved no
serious obstacle to diffusion; and it is not unlikely that much of the
close resemblance of the fresh-water faunæ of northern Europe, Asia,
and North America is due to this fact. To attempt to decide from which
side the first migration came in regard to each group of fishes might
be interesting; but without a wider range of facts than is now in our
possession, most such attempts, based on guesswork, would have little
value. The interlocking of the fish faunas of Asia and North America
presents, however, a number of interesting problems, for migrations in
both directions have doubtless taken place.
=Causes of Dispersion Still in Operation.=--One might go on
indefinitely with the discussion of special cases, each more or less
interesting or suggestive in itself, but the general conclusion is in
all cases the same. The present distribution of fishes is the result
of the long-continued action of forces still in operation. The species
have entered our waters in many invasions from the Old World or from
the sea. Each species has been subjected to the various influences
implied in the term "natural selection," and under varying conditions
its representatives have undergone many different modifications. Each
of the six hundred fresh-water species we now know in the United States
may be conceived as making every year inroads on territory occupied
by other species. If these colonies are able to hold their own in the
struggle for possession, they will multiply in the new conditions,
and the range of the species becomes widened. If the surroundings are
different, new species or varieties may be formed with time; and these
new forms may again invade the territory of the parent species. Again,
colony after colony of species after species may be destroyed by other
species or by uncongenial surroundings.
The ultimate result of centuries on centuries of the restlessness of
individuals is seen in the facts of geographical distribution. Only
in the most general way can the history of any species be traced; but
could we know it all, it would be as long and as eventful a story as
the history of the colonization and settlement of North America by
immigrants from Europe. But by the fishes each river in America has
been a hundred times discovered, its colonization a hundred times
attempted. In these efforts there is no co-operation. Every individual
is for himself, every struggle a struggle of life and death; for each
fish is a cannibal, and to each species each member of every other
species is an alien and a savage.
FOOTNOTES:
[100] Except possibly the Sacramento.
[101] Unless the fauna of certain cave streams in the United States and
Cuba be regarded as forming an exception.
[102] For example, Elk River, Duck River, etc.
[103] There are three species of darters (_Cottogaster copelandi_
Jordan, _Hadropterus evides_ Jordan and Copeland, _Hadropterus
scierus_ Swain) which are now known only from the Ozark region or
beyond and from the uplands of Indiana, not yet having been found
at any point between Indiana and Missouri. These constitute perhaps
isolated colonies, now separated from the parent stock in Arkansas by
the prairie districts of Illinois, a region at present uninhabitable
for these fishes. But the non-occurrence of these species over the
intervening areas needs confirmation, as do most similar cases of
anomalous distribution.
[104] Thus, _Dorosoma cepedianum_ Le Sueur and _Pomolobus
chrysochloris_ Rafinesque have found their way into Lake Michigan
through canals.
[105] _Oncorhynchus tschawytscha_ Walbaum.
[106] _Oncorhynchus nerka_ Walbaum.
[107] "In this isolated region of North America, in this zoological
island of New England, as we may call it, we find neither Lepidosteus,
nor Amia, nor Polyodon, nor Amblodon (_Aplodinotus_), nor Grystes
(_Micropterus_), nor Centrarchus, nor Pomoxis, nor Ambloplites, nor
Calliurus (_Chænobryttus_), nor Carpiodes, nor Hyodon, nor indeed
any of the characteristic forms of North American fishes so common
everywhere else, with the exception of two Pomotis (_Lepomis_), one
Boleosoma, and a few Catostomus."--AGASSIZ, _Amer. Journ. Sci. Arts_,
1854.
[108] _Catostomus_, _Pantosteus_, _Chasmistes_.
[109] _Gila_, _Ptychocheilus_, etc.
[110] _Salmo clarkii_ and its varieties.
[111] Genera _Notropis_, _Chrosomus_, etc.
[112] As the fresh-water surf-fish (_Hysterocarpus traski_) and the
species of salmon.
[113] _Salmo clarki_ Richardson.
[114] _Coregonus williamsoni_ Girard.
[115] _Salvelinus malma_ (Walbaum).
[116] _Thymallus tricolor_ Cope.
[117] _Salvelinus fontinalis_ Mitchill.
[118] _Notropis rubricroceus_ Cope, _Rhinichthys atronasus_ Mitchill,
etc.
[119] Evermann, A Reconnoissance of the Streams and Lakes of Western
Montana and Northwestern Wyoming, in Bull. U. S. Fish. Comm., XI, 1891,
24-28, pls. I and II; Jordan, The Story of a Strange Land, in Pop. Sci.
Monthly, Feb., 1892, 447-458; Evermann, Two-Ocean Pass, in Proc. Ind.
Ac. Sci., 1892, 29-34, pl. I; Evermann, Two-Ocean Pass, in Pop. Sci.
Monthly, June, 1895, with plate.
[120] _Ameiurus nebulosus_ Le Sueur: _Ameiurus catus_ Linnæus.
[121] _Salvelinus fontinalis._
[122] _Notropis rubricroceus_ Cope.
[123] _Cottus ictalops_ Rafinesque.
[124] _Notropis cercostigma_, _Notropis xænocephalus_.
[125] _Labidesthes sicculus._
[126] _Gobiosoma molestum._
[127] _Myrophis punctatus._
[128] _Lepisosteus tristoechus._
[129] _Jordanella_, _Rivulus_, _Heterandria_, etc.
[130] _Heros_, _Tetragonopterus_.
[131] _Lucifuga_ and _Stygicola_, fishes allied to the cusk, and
belonging to the family of _Brotulidæ_.
[132] _Amblyopsis_, _Typhlichthys_.
[133] _Lepisosteus tristoechus._
[134] _Esox vermiculatus_ Le Sueur.
[135] _Argyrosomus sisco_ Jordan.
[136] _Argyrosomus artedi_ Le Sueur.
[137] As _Lota maculosa_; _Percopsis guttata_; _Esox masquinongy_.
[138] _Catostomus tahoensis_, in Lake Tahoe; _Catostomus macrocheilus_
and _discobolus_, in the Columbia; _Catostomus fecundus_; _Catostomus
ardens_; _Chasmistes liorus_ and _Pantosteus generosus_, in Utah Lake.
[139] _Salmo henshawi_ and _virginalis_.
[140] _Coregonus williamsoni._
[141] Species of _Mysis_ and other genera of Crustaceans, similar to
species described by Sars and others, in lakes of Sweden and Finland.
[142] _Triglopsis thompsoni_ Girard, a near ally of the marine species
_Oncocottus quadricornis_ L.
[143] Kritisk Öfversigt of Finlands Fisk-Fauna, Helsingfors, 1863.
[144] See Günther, Zoological Record for 1864, p. 137.
[145] _Salmo fario_ L., in Europe; _Salmo labrax_ Pallas, etc., in
Asia; _Salmo gairdneri_ Richardson, in streams of the Pacific Coast;
_Salmo perryi_, in Japan; _Salmo clarki_ Richardson, throughout the
Rocky Mountain range to the Mexican boundary and the headwaters of the
Kansas, Platte, and Missouri.