in shallow water than the lower coral-forming types. Among _Medusæ_,
the young are either attached to the bottom, or grow from the depth,
while the perfect free forms of these animals come to the surface.
Among _Echinoderms_, the _Crinoids_ are deep-water forms; free
star-fishes and _Echini_, and, above all _Holothuriæ_, living nearer
the surface. Among _Mollusca_, the _Acephala_, which are lowest, have
their lower types,--the _Brachiopods_, entirely confined to deep
waters; the _Monomyarians_ appear next, and, above them, the
_Dimyarians_; among these latter, the highest family, the _Nayades_,
rises above the level of the ocean into the fresh waters, and extends
even to considerable heights above the sea, in lakes and rivers. A
number of examples of all classes should be mentioned, to shew that
this is the universal case; as, for instance, among _Crustacea_ the
_Macrura_ are, in general species of deeper water than the true crabs,
of which some come even upon dry land. Again, on the slopes of our
mountains, the highest forms among _Mammalia_ which remain numerous
are the _Ruminants_ and _Rodents_. There are no _Carnivora_ living in
high regions. Among birds of prey, we have the vultures, rising above
the highest summits of mountains, while eagles and falcons hover over
the woods and plains, by the water sides, and along the sea-shores.
Among reptiles, salamanders, frogs, and toads occur higher than any
turtles, lizards, &c. But the same adaptation may be traced with
reference to the latitudes under which animals are found. Those of the
higher latitudes, the arctic and antarctic species, resemble both the
animals of high, prominent mountain chains, and those of the deep
sea-waters, which there meet in the most unexpected combinations (and
it is surprising to see how extensively this is the case); while, in
lower latitudes, towards the tropics, we find everywhere the higher
representatives of the same families. For instance, among _Mammalia_
we observe monkeys only in warm latitudes, and they die out in the
warmer parts of the temperate zone. The great development of
_Digitigrades_--lions, tigers, &c., takes place within the tropics,
smaller species, like wolves and foxes, weasels, &c., occurring in the
north, whilst the _Plantigrades_, which come nearer and nearer to the
seal, follow an inverse progression, the largest and most powerful of
them being the arctic ice bear, which meets there his family
relations, the _Pinnipedia_, that are so numerous in the polar
regions. Again, the families of _Ruminants_ and _Pachyderms_ seem to
form an exception, for though belonging to the lower types of
_Mammalia_, they prevail in the tropical zone; but let us remember
that they were among the earlier inhabitants of our globe, and the
fact of their occurring more extensively in warm climates is rather a
reminiscence of the plan of creation in older times, than an
adaptation to the law regulating at present the distribution of
organized beings. The gradation of animals among birds being less
satisfactorily ascertained, we do not venture to say anything
respecting their geographical distribution, in relation to climates.
But among reptiles, we cannot overlook the fact, that the crocodiles,
which are the highest in structure, are altogether tropical, and
the _Batrachians_, which rank lowest, especially the salamandroid
forms, are rather types of the colder temperate zone than of the warm,
&c. From these facts it is plain, that the geographical distribution
of all groups has a direct reference to atmospheric and hydrostatic
pressure on one side, and also to the intensity of light and heat over
the surface of the globe.
The special adaptation of minor groups begins very early in the
history of our globe, and extends at present all over its surface. In
the same manner as animals are adapted to natural limits in their
large primitive groups which we call classes, we find also the minor
divisions more closely adapted to particular circumstances of the
physical condition of all parts of the globe. Among _Mammalia_, the
great type of _Marsupialia_ is placed in New Holland, and extends
little beyond that continent into the adjacent islands. A very few
representatives of that family are found in America. Asia, Africa, the
colder parts of North America, and its southern extremity, are
entirely deprived of this type. The family of _Edentata_, again, has
its centre of development in South America, where the sloth, dasypus,
ant-eaters, &c., form characteristic types, of which a few analogues
occur in Africa, along its southern extremity and western coast. Now
it is a fact upon which we cannot insist too strongly, that the same
districts of New Holland and South America were, during an earlier
geological period comparatively recent, the seat of an equally wide
development of the same animals in the same extensive proportion as at
present. We need only refer to the beautiful investigations of Dr
Lund, upon the fossil mammalia of Brazil, and to those, no less
important, of Professor Owen, upon the fossil remains of mammalia of
New Holland, to leave not a shadow of doubt upon this adaptation,
which indicates distinctly these two regions, at two distinct periods
remote from each other, as the points of development of two distinct
families, which have never spread over other parts of the globe at any
period since the time of their existence, indicating at least two
distinct foci of creation, with the same characters, at two successive
epochs; a fact which, in our opinion, can never be reconciled to the
idea of a unique centre of origin of the animals now living. But
though other families have never been and are not now localized in so
special a manner, we nevertheless find them circumscribed within
certain limits, in particular districts, or, at least, in particular
zones.
As already mentioned, the monkeys are entirely tropical. But here,
again, we notice a very intimate adaptation of their types to the
particular continents, as the monkeys of tropical America constitute a
family altogether distinct from the monkeys of the Old World, there
being not one species of any of the genera of _Quadrumana_, so
numerous on this continent, found either in Africa or in Asia. The
monkeys of the Old World, again, constitute a natural family by
themselves, extending equally over Africa and Asia; but the species of
Africa differ from those of Asia; and there is even a close
representative analogy between those of different parts of these two
continents; the orangs of Africa, the chimpanzee and gorilla,
corresponding to the red orang of Sumatra and Borneo, and the smaller
long armed species of continental Asia. And what is not a little
remarkable is the fact, that the black orang occurs upon that
continent which is inhabited by the black human race, whilst the brown
orang inhabits those parts of Asia over which the chocolate-coloured
Malays have been developed. There is again a peculiar family of
_Quadrumana_ confined to the Island of Madagascar--the makis--which
are entirely peculiar to that island, and the eastern coast of Africa
opposite to it, and to one spot on the western shore of Africa. But in
New Holland, and the adjacent islands, there are no monkeys at all,
though the climatic conditions seem not to exclude their existence any
more than those of the large Asiatic islands, upon which such high
types of this order are found. And these facts more than any other,
would indicate that the special adaptation of animals to particular
districts of the surface of our globe is neither accidental, nor
dependent upon physical conditions, but is implied in the primitive
plan of the creation itself. Whatever classes we may take into
consideration, we shall find similar adaptations, and though, perhaps,
the greater uniformity of some families renders the difference of the
types in various parts of the world less striking, they are none the
less real. The _Carnivora_ of tropical Asia are not the same as those
of tropical Africa, or those of tropical America. Their birds and
reptiles present similar differences. The want of an ostrich in Asia,
when we have one, the largest of the family, in Africa, and two
distinct species in Southern America, and two cassowaries, one in New
Holland, and another in the Sunda Islands, shews this constant process
of analogous or representative species repeated over different parts
of the world to be the principle regulating the distribution of
animals, and the fact that these analogous species are different,
again, cannot be reconciled to the idea of a common origin, as each
type is peculiar to the country where it is now found. These
differences are more striking in tropical regions than anywhere else.
The rhinoceros of the Sunda Islands differs from those of Africa, and
there is none in America. The elephant of Asia differs from that of
Africa, and there is none in America. One tapir is found in the Sunda
Islands, there is none in Africa, but we find one in South America,
&c. Everywhere special adaptation, particular forms in each continent,
an omission of some allied type here, when in the next group it occurs
all over the zone.
As we ascend into the temperate zone, we find, however, the similarity
greatly increased. The difference between the species of the same
family in temperate Asia, temperate Europe, and temperate America is
much less than between the corresponding animals of the tropical zone,
and no doubt it is to this great assemblage of more uniform animals,
living originally within the main seat of human civilization, that we
must ascribe the idea of their common origin, which has so long
prevailed and been so serious an obstacle to a real insight into these
natural phenomena. What, indeed, could be more natural for man, when
for the first time reflecting upon nature around him,--when seeing, as
far as he could extend his investigations, all things alike,--than to
imagine that every thing arose from a common centre, and spread with
him over the world, as it has been the fate of the white race, and of
that only, to extend all over the globe, and that, influenced by the
phenomena of the zone in which he lived and wandered, and from which
he extended farther, he took it for granted that all animals followed
the same laws. But now that we know the whole surface of our globe so
satisfactorily, there can no longer be a question about the difference
between animals and plants in the lower latitudes in all continents.
Besides, we see them equally striking in the southernmost extremities
of the three great continents, so that there can no longer be any
doubt about the primitive adaptation of these various types to the
continents where they live, as we do not find a single one naturally
diffused everywhere over all continents. Notwithstanding, therefore,
the slighter differences we notice between the animals of different
continents in the temperate zone, we are thus led step by step to
ascribe to them also a special origin upon those continents where they
now occur.
But as soon as we rise to the highest latitudes, the uniformity
becomes so close, that there is no longer any marked difference
noticed between the animals about the arctic regions, either in
America, Europe, or Asia; and we are naturally led to restrict the
idea of a common centre of origin, or at least of a narrow circle of
primitive development, to those animals which spread equally over the
icy fields extending around the northern pole upon the three
continents which meet in the north. The phenomena of geographical
distribution which we observe there among the terrestrial animals are
repeated in the same manner among the aquatic ones. The fishes in the
arctic seas do not materially differ on the shores of Europe, Asia,
and America, and through the northern Atlantic and through Behring's
Straits they extend more or less towards the colder temperate zone, or
migrate into it at particular seasons of the year, as do most birds of
the arctic regions also. But in the temperate zone we begin to find
more and more marked differences between the inhabitants of different
continents, and even between those of the opposite shores of the same
ocean; as, for instance, the fishes of Europe (some of the northern
species excepted) are not identical with those of the temperate shores
of North America, notwithstanding the very open field left for their
uniform distribution across the Atlantic. Such is also the case
between the fishes of Western Africa and those of Central America, and
between those of the southern extremities of these continents. The
fishes of the Indian Ocean, and the fishes of the Pacific vary
greatly, and, though some families have a wider range, there are many
which are circumscribed within the narrowest limits. It is one of the
most striking phenomena in the geographical distribution of
aquatic animals, to find entire families of fishes completely
circumscribed within particular groups of islands, such, for instance,
as the _Labyrinthici_, which are peculiar to the Sunda Islands, and
the family of _Goniodonts_, which are found only in the rivers of
South America.
A similar narrow limitation occurs also among the terrestrial animals,
as the family of _Colubris_ is entirely circumscribed within the
boundaries of the warmer parts of the American continent. The
appearance during the warmer season of the year of a few species of
that family in the Northern States, does not make this case less
strong. Examples might be multiplied without end to shew everywhere
special adaptation, narrow circumscription, or representative
adaptation of species in different parts of the world; but those
mentioned will be sufficient to sustain the argument that animals are
naturally antochthones wherever they are found, and have been so at
all geological periods; that in northern regions they are most
uniform; that their diversity goes on increasing through the temperate
zone till it reaches its maximum in the tropics; that this diversity
is again reduced in the aquatic animals towards the antarctic
pole, though the physical difference between the southernmost
extremities of America, Africa, and New Holland, seems to have called
for an increased difference between their terrestrial animals.
We are thus led to distinguish special provinces in the natural
distribution of animals, and we may adopt the following division as
the most natural: _First_, the _arctic province_, with prevailing
uniformity. _Second_, the temperate zone, with at least three distinct
zoological provinces--the _European temperate zone_, west of the Ural
Mountains, the _Asiatic temperate zone_ east of the Ural Mountains,
and the _American temperate zone_, which may be subdivided into two,
the _eastern_ and the _western_--for the animals east and west of the
Rocky Mountains differ sufficiently to constitute two distinct
zoological provinces. Next, the tropical zone, containing the _African
zoological province_, which extends over the main part of the African
continent, including all the country south of the Atlas and north of
the Cape Colonies; the _tropical Asiatic province_, south of the great
Himalayan chain, and including the Sunda Islands, whose _Fauna_ has
quite a continental character, and differs entirely from that of the
Islands of the Pacific, as well as from that of New Holland; the
_American tropical province_, including Central America, the West
Indies, and tropical South America. _New Holland_ constitutes in
itself a special province, notwithstanding the great differences of
its northern and southern climate, the animals of the whole continent
preserving throughout their peculiar typical character. But it were a
mistake to conceive that the _Faunæ_ or natural groups of animals are
to be limited according to the boundaries of the mainland. On the
contrary we may trace their natural limits into the ocean, and refer
to the temperate European _Fauna_ the eastern shores of the Atlantic,
as we refer its western shores to the American temperate _Fauna_.
Again, the eastern shores of the Pacific belong to the western
American _Fauna_, as the western Pacific shores belong to the Asiatic
_Fauna_. In the Atlantic Ocean there is no purely oceanic _Fauna_ to
be distinguished, but in the _Pacific_ we have such a _Fauna_,
entirely marine in its main character, though interspread with
innumerable islands extending east of the Sunda Islands and New
Holland to the western shores of tropical America. The islands west of
this continent seem, indeed, to have very slight relations in their
zoological character with the western parts of the mainland. South of
the tropical zone we have the _South American temperate Fauna_, and
that of the _Cape of Good Hope_, as other distinct zoological
provinces. Van Diemen's Land, however, does not constitute a
zoological province in itself, but belongs to the province of New
Holland, by its zoological character. Finally, the antarctic circle
encloses a special zoological province, including the _antarctic
Fauna_, which, in a great measure, corresponds to the arctic _Fauna_
in its uniformity, though it differs from it in having chiefly a
maritime character, while the arctic _Fauna_ has an almost entirely
continental aspect.
The fact that the principal races of man, in their natural
distribution, cover the same extent of ground as the great zoological
provinces, would go far to shew that the differences which we notice
between them are also primitive; but for the present we shall abstain
from further details upon a subject involving so difficult problems as
the question of the unity or plurality of origin of the human family,
satisfied as we are to have shewn that animals, at least, did not
originate from a common centre, nor from single pairs, but according
to the laws which at present still regulate their existence.
_Additional Illustrations of the Geographical Distribution of
Animals._
I.--_Geographical Distribution of Sturgeons._[2]
Footnote 2: Agassiz's Lake Superior, p. 264.
The sturgeons are generally large fishes, which live at the bottom of
the water, feeding with their toothless mouths upon decomposed
organized substances. Their movements are rather sluggish, resembling
somewhat those of the cod-fish.
Their geographical distribution is quite peculiar, and constitutes
one of their prominent peculiarities. Located as they are, in the
colder portions of the temperate zone, they inhabit either the fresh
waters or the seas exclusively, or alternately both these
elements,--remaining during the larger part of the year in the sea,
and ascending the rivers in the spawning season. Although adapted to
the cold regions of the temperate, they do not seem to extend into the
arctic zone, and I am not aware that they have been observed in any of
the waters of the warmer half of the temperate zone. The great basin
of salt-water lakes or seas which extends east of the Mediterranean,
seems to be their principal abode in the Old World, or at least the
region in which the greater number of species occur; and each species
takes a wide range, extending up the Danube and its tributaries, and
all the Russian rivers emptying into the Black Sea. From the Caspian
they ascend the Wolga in immense shoals, and are found further east in
the lakes of Central Asia, even as far as the borders of China. The
great Canadian lakes constitute another centre of distribution of
these fishes in the New World, but here they are not so numerous, nor
do they ever occur in contact with salt water in this basin.
Northwards, there is another great zone of distribution of sturgeons,
which inhabit all the great northern rivers emptying into the Arctic
Sea, in Asia as well as in America. They occur equally in the
intervening seas, being found on the shores of Norway and Sweden, in
the Baltic and North Seas, as well as in the Atlantic Ocean, from
which they ascend the northern rivers of Germany, as well as those of
Holland, France, and Great Britain. Even the Mediterranean and the
Adriatic have their sturgeons, though few in number. There are also
some on the Atlantic shores of North America, along the British
possessions as well as the northern and middle United States. They
seem to be exceedingly numerous in the Northern Pacific, being found
everywhere from Behring's Straits and Japan to the northern shores of
China, and on the north-west coast of America, as far south as the
Columbia River. Again, the so-called western waters of the United
States have their own species, from the Ohio down to the lower portion
of the Mississippi, but it does not appear that these species ascend
the rivers from the Gulf of Mexico. I suppose them to be rather
entirely fluviatile, like those of the great Canadian lakes.
Beyond the above limits southwards there are nowhere sturgeons to be
found, not even in the Nile, though emptying into a sea in which they
occur; and as for the great rivers of Southern Asia and of tropical
Africa, not only the sturgeons, but another family is wanting
there,--I mean the family of _Goniodonts_, which in Central and
Southern America takes the place of the sturgeons of the north. Again,
all the species in different parts of the world are different.
It is a most extraordinary fact, which will hereafter throw much light
upon the laws of geographical distribution of animals and their mode
of association, viz., that certain families are entirely circumscribed
within comparatively narrow limits, and that their special location
has an unquestionable reference to the location of other animals; or,
in other words, that natural families, apparently little related to
each other, are confined to different parts of the world, but are
linked together by some intermediate form, which itself is located in
the intermediate track between the two extremes. In the case now
before us, we have the sturgeons extending all around the world in the
northern temperate hemisphere, in its seas as well as in its fresh
waters, all closely related to each other. Neither in Asia nor in
Africa is there an aberrant form of that type, or any representative
type in the warmer zones; but in North America we have the genus
_Scaphirhynhus_, which occurs in the Ohio and Mississippi, and which
forms a most natural link with the family of _Goniodonts_, all the
species of which are confined exclusively to the fresh waters of
Central and South America. The closeness of this connection will be at
once perceived by attempting to compare the species of true
_Sonicariæ_ with the _Scaphirhynhus_. I know very well, that the
affinities of _Goniodonts_ and _Siluroids_ with sturgeons are denied,
but I still strongly insist upon their close relationship, which I
hope to establish satisfactorily in a special paper, as I continued to
insist upon the relation between sturgeons and gar-pikes, at one time
positively contradicted and even ridiculed. I trust then to be able to
shew, that the remarkable form of the brains of _Siluridæ_ comes
nearer to that of sturgeons and _Lepidostei_ than to that of any other
family of fishes. This being the case, it is obvious that there must
be in the physical condition of the continent of America some
inducement not yet understood, for adaptations so special and so
different from what we observe in the Old World. Indeed, such
analogies between the organized beings almost from one pole to
another, occur from man down to the plants in America only, among its
native products; while, in the Old World, plants as well as animals
have more circumscribed homes, and more closely characterized
features, in the various continents, at different latitudes.
As for the species of sturgeons which occur in the Canadian lakes, I
know only three from personal examination, one of which was obtained
in Lake Superior, at Michipicotin, another at the Pic, and the third
at the Sault; though I know that they occur in all other Canadian
lakes, yet it remains to be ascertained how the species said to be so
common in Lake Huron, compared with those of Lake Superior, and with
those in the other great lakes and the St Lawrence itself. As for the
Atlantic species, ascending the rivers of the United States west and
south of Cape Cod, I know them to differ from those of the lakes, at
least from those which I possess from Lake Superior. The number of
species of this interesting family which occur in the United States
is, at all events, far greater than would be supposed from an
examination of the published records. Upon close comparison of the
specimens in my collection from different parts of the country,
and in different museums, as those of the Natural History Society of
Boston, of Salem, of the Lyceum of New York, my assistant, Mr Charles
Giran, and myself, have discovered several species not described. For
this comparison I was the better prepared, as I had an opportunity in
former years of studying almost all the European species in a fresh
condition, during a prolonged visit in Vienna.
II.--_Fishes of Lake Superior compared with those of the other
great Canadian Lakes._
Besides the interest there is everywhere in studying the living
animals of a new country, there is a particular interest to a
naturalist in ascertaining their peculiar geographical distribution,
and their true affinities with those of other countries. It is only by
following such a course, that we can hope to arrive at any exact
results as to their origin. In this respect the fresh-water animals
have a peculiar interest, as from the element they inhabit, they are
placed under exceptional circumstances.