On the Origin of Species by Means of Natural Selection
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Chapter XI
Geographical Distribution
Present distribution cannot be accounted for by differences in physical
conditions -- Importance of barriers -- Affinity of the productions of the
same continent -- Centres of creation -- Means of dispersal, by changes of
climate and of the level of the land, and by occasional means -- Dispersal
during the Glacial period co-extensive with the world.
In considering the distribution of organic beings over the face of the
globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various regions can
be accounted for by their climatal and other physical conditions. Of late,
almost every author who has studied the subject has come to this
conclusion. The case of America alone would almost suffice to prove its
truth: for if we exclude the northern parts where the circumpolar land is
almost continuous, all authors agree that one of the most fundamental
divisions in geographical distribution is that between the New and Old
Worlds; yet if we travel over the vast American continent, from the central
parts of the United States to its extreme southern point, we meet with the
most diversified conditions; the most humid districts, arid deserts, lofty
mountains, grassy plains, forests, marshes, lakes, and great rivers, under
almost every temperature. There is hardly a climate or condition in the
Old World which cannot be paralleled in the New--at least as closely as the
same species generally require; for it is a most rare case to find a group
of organisms confined to any small spot, having conditions peculiar in only
a slight degree; for instance, small areas in the Old World could be
pointed out hotter than any in the New World, yet these are not inhabited
by a peculiar fauna or flora. Notwithstanding this parallelism in the
conditions of the Old and New Worlds, how widely different are their living
productions!
In the southern hemisphere, if we compare large tracts of land in
Australia, South Africa, and western South America, between latitudes 25
deg and 35 deg, we shall find parts extremely similar in all their
conditions, yet it would not be possible to point out three faunas and
floras more utterly dissimilar. Or again we may compare the productions of
South America south of lat. 35 deg with those north of 25 deg, which
consequently inhabit a considerably different climate, and they will be
found incomparably more closely related to each other, than they are to the
productions of Australia or Africa under nearly the same climate.
Analogous facts could be given with respect to the inhabitants of the sea.
A second great fact which strikes us in our general review is, that
barriers of any kind, or obstacles to free migration, are related in a
close and important manner to the differences between the productions of
various regions. We see this in the great difference of nearly all the
terrestrial productions of the New and Old Worlds, excepting in the
northern parts, where the land almost joins, and where, under a slightly
different climate, there might have been free migration for the northern
temperate forms, as there now is for the strictly arctic productions. We
see the same fact in the great difference between the inhabitants of
Australia, Africa, and South America under the same latitude: for these
countries are almost as much isolated from each other as is possible. On
each continent, also, we see the same fact; for on the opposite sides of
lofty and continuous mountain-ranges, and of great deserts, and sometimes
even of large rivers, we find different productions; though as mountain
chains, deserts, &c., are not as impassable, or likely to have endured so
long as the oceans separating continents, the differences are very inferior
in degree to those characteristic of distinct continents.
Turning to the sea, we find the same law. No two marine faunas are more
distinct, with hardly a fish, shell, or crab in common, than those of the
eastern and western shores of South and Central America; yet these great
faunas are separated only by the narrow, but impassable, isthmus of Panama.
Westward of the shores of America, a wide space of open ocean extends, with
not an island as a halting-place for emigrants; here we have a barrier of
another kind, and as soon as this is passed we meet in the eastern islands
of the Pacific, with another and totally distinct fauna. So that here
three marine faunas range far northward and southward, in parallel lines
not far from each other, under corresponding climates; but from being
separated from each other by impassable barriers, either of land or open
sea, they are wholly distinct. On the other hand, proceeding still further
westward from the eastern islands of the tropical parts of the Pacific, we
encounter no impassable barriers, and we have innumerable islands as
halting-places, until after travelling over a hemisphere we come to the
shores of Africa; and over this vast space we meet with no well-defined and
distinct marine faunas. Although hardly one shell, crab or fish is common
to the above-named three approximate faunas of Eastern and Western America
and the eastern Pacific islands, yet many fish range from the Pacific into
the Indian Ocean, and many shells are common to the eastern islands of the
Pacific and the eastern shores of Africa, on almost exactly opposite
meridians of longitude.
A third great fact, partly included in the foregoing statements, is the
affinity of the productions of the same continent or sea, though the
species themselves are distinct at different points and stations. It is a
law of the widest generality, and every continent offers innumerable
instances. Nevertheless the naturalist in travelling, for instance, from
north to south never fails to be struck by the manner in which successive
groups of beings, specifically distinct, yet clearly related, replace each
other. He hears from closely allied, yet distinct kinds of birds, notes
nearly similar, and sees their nests similarly constructed, but not quite
alike, with eggs coloured in nearly the same manner. The plains near the
Straits of Magellan are inhabited by one species of Rhea (American
ostrich), and northward the plains of La Plata by another species of the
same genus; and not by a true ostrich or emeu, like those found in Africa
and Australia under the same latitude. On these same plains of La Plata,
we see the agouti and bizcacha, animals having nearly the same habits as
our hares and rabbits and belonging to the same order of Rodents, but they
plainly display an American type of structure. We ascend the lofty peaks
of the Cordillera and we find an alpine species of bizcacha; we look to the
waters, and we do not find the beaver or musk-rat, but the coypu and
capybara, rodents of the American type. Innumerable other instances could
be given. If we look to the islands off the American shore, however much
they may differ in geological structure, the inhabitants, though they may
be all peculiar species, are essentially American. We may look back to
past ages, as shown in the last chapter, and we find American types then
prevalent on the American continent and in the American seas. We see in
these facts some deep organic bond, prevailing throughout space and time,
over the same areas of land and water, and independent of their physical
conditions. The naturalist must feel little curiosity, who is not led to
inquire what this bond is.
This bond, on my theory, is simply inheritance, that cause which alone, as
far as we positively know, produces organisms quite like, or, as we see in
the case of varieties nearly like each other. The dissimilarity of the
inhabitants of different regions may be attributed to modification through
natural selection, and in a quite subordinate degree to the direct
influence of different physical conditions. The degree of dissimilarity
will depend on the migration of the more dominant forms of life from one
region into another having been effected with more or less ease, at periods
more or less remote;--on the nature and number of the former
immigrants;--and on their action and reaction, in their mutual struggles
for life;--the relation of organism to organism being, as I have already
often remarked, the most important of all relations. Thus the high
importance of barriers comes into play by checking migration; as does time
for the slow process of modification through natural selection.
Widely-ranging species, abounding in individuals, which have already
triumphed over many competitors in their own widely-extended homes will
have the best chance of seizing on new places, when they spread into new
countries. In their new homes they will be exposed to new conditions, and
will frequently undergo further modification and improvement; and thus they
will become still further victorious, and will produce groups of modified
descendants. On this principle of inheritance with modification, we can
understand how it is that sections of genera, whole genera, and even
families are confined to the same areas, as is so commonly and notoriously
the case.
I believe, as was remarked in the last chapter, in no law of necessary
development. As the variability of each species is an independent
property, and will be taken advantage of by natural selection, only so far
as it profits the individual in its complex struggle for life, so the
degree of modification in different species will be no uniform quantity.
If, for instance, a number of species, which stand in direct competition
with each other, migrate in a body into a new and afterwards isolated
country, they will be little liable to modification; for neither migration
nor isolation in themselves can do anything. These principles come into
play only by bringing organisms into new relations with each other, and in
a lesser degree with the surrounding physical conditions. As we have seen
in the last chapter that some forms have retained nearly the same character
from an enormously remote geological period, so certain species have
migrated over vast spaces, and have not become greatly modified.
On these views, it is obvious, that the several species of the same genus,
though inhabiting the most distant quarters of the world, must originally
have proceeded from the same source, as they have descended from the same
progenitor. In the case of those species, which have undergone during
whole geological periods but little modification, there is not much
difficulty in believing that they may have migrated from the same region;
for during the vast geographical and climatal changes which will have
supervened since ancient times, almost any amount of migration is possible.
But in many other cases, in which we have reason to believe that the
species of a genus have been produced within comparatively recent times,
there is great difficulty on this head. It is also obvious that the
individuals of the same species, though now inhabiting distant and isolated
regions, must have proceeded from one spot, where their parents were first
produced: for, as explained in the last chapter, it is incredible that
individuals identically the same should ever have been produced through
natural selection from parents specifically distinct.
We are thus brought to the question which has been largely discussed by
naturalists, namely, whether species have been created at one or more
points of the earth's surface. Undoubtedly there are very many cases of
extreme difficulty, in understanding how the same species could possibly
have migrated from some one point to the several distant and isolated
points, where now found. Nevertheless the simplicity of the view that each
species was first produced within a single region captivates the mind. He
who rejects it, rejects the vera causa of ordinary generation with
subsequent migration, and calls in the agency of a miracle. It is
universally admitted, that in most cases the area inhabited by a species is
continuous; and when a plant or animal inhabits two points so distant from
each other, or with an interval of such a nature, that the space could not
be easily passed over by migration, the fact is given as something
remarkable and exceptional. The capacity of migrating across the sea is
more distinctly limited in terrestrial mammals, than perhaps in any other
organic beings; and, accordingly, we find no inexplicable cases of the same
mammal inhabiting distant points of the world. No geologist will feel any
difficulty in such cases as Great Britain having been formerly united to
Europe, and consequently possessing the same quadrupeds. But if the same
species can be produced at two separate points, why do we not find a single
mammal common to Europe and Australia or South America? The conditions of
life are nearly the same, so that a multitude of European animals and
plants have become naturalised in America and Australia; and some of the
aboriginal plants are identically the same at these distant points of the
northern and southern hemispheres? The answer, as I believe, is, that
mammals have not been able to migrate, whereas some plants, from their
varied means of dispersal, have migrated across the vast and broken
interspace. The great and striking influence which barriers of every kind
have had on distribution, is intelligible only on the view that the great
majority of species have been produced on one side alone, and have not been
able to migrate to the other side. Some few families, many sub-families,
very many genera, and a still greater number of sections of genera are
confined to a single region; and it has been observed by several
naturalists, that the most natural genera, or those genera in which the
species are most closely related to each other, are generally local, or
confined to one area. What a strange anomaly it would be, if, when coming
one step lower in the series, to the individuals of the same species, a
directly opposite rule prevailed; and species were not local, but had been
produced in two or more distinct areas!
Hence it seems to me, as it has to many other naturalists, that the view of
each species having been produced in one area alone, and having
subsequently migrated from that area as far as its powers of migration and
subsistence under past and present conditions permitted, is the most
probable. Undoubtedly many cases occur, in which we cannot explain how the
same species could have passed from one point to the other. But the
geographical and climatal changes, which have certainly occurred within
recent geological times, must have interrupted or rendered discontinuous
the formerly continuous range of many species. So that we are reduced to
consider whether the exceptions to continuity of range are so numerous and
of so grave a nature, that we ought to give up the belief, rendered
probable by general considerations, that each species has been produced
within one area, and has migrated thence as far as it could. It would be
hopelessly tedious to discuss all the exceptional cases of the same
species, now living at distant and separated points; nor do I for a moment
pretend that any explanation could be offered of many such cases. But
after some preliminary remarks, I will discuss a few of the most striking
classes of facts; namely, the existence of the same species on the summits
of distant mountain-ranges, and at distant points in the arctic and
antarctic regions; and secondly (in the following chapter), the wide
distribution of freshwater productions; and thirdly, the occurrence of the
same terrestrial species on islands and on the mainland, though separated
by hundreds of miles of open sea. If the existence of the same species at
distant and isolated points of the earth's surface, can in many instances
be explained on the view of each species having migrated from a single
birthplace; then, considering our ignorance with respect to former climatal
and geographical changes and various occasional means of transport, the
belief that this has been the universal law, seems to me incomparably the
safest.
In discussing this subject, we shall be enabled at the same time to
consider a point equally important for us, namely, whether the several
distinct species of a genus, which on my theory have all descended from a
common progenitor, can have migrated (undergoing modification during some
part of their migration) from the area inhabited by their progenitor. If
it can be shown to be almost invariably the case, that a region, of which
most of its inhabitants are closely related to, or belong to the same
genera with the species of a second region, has probably received at some
former period immigrants from this other region, my theory will be
strengthened; for we can clearly understand, on the principle of
modification, why the inhabitants of a region should be related to those of
another region, whence it has been stocked. A volcanic island, for
instance, upheaved and formed at the distance of a few hundreds of miles
from a continent, would probably receive from it in the course of time a
few colonists, and their descendants, though modified, would still be
plainly related by inheritance to the inhabitants of the continent. Cases
of this nature are common, and are, as we shall hereafter more fully see,
inexplicable on the theory of independent creation. This view of the
relation of species in one region to those in another, does not differ much
(by substituting the word variety for species) from that lately advanced in
an ingenious paper by Mr. Wallace, in which he concludes, that 'every
species has come into existence coincident both in space and time with a
pre-existing closely allied species.' And I now know from correspondence,
that this coincidence he attributes to generation with modification.
The previous remarks on 'single and multiple centres of creation' do not
directly bear on another allied question,--namely whether all the
individuals of the same species have descended from a single pair, or
single hermaphrodite, or whether, as some authors suppose, from many
individuals simultaneously created. With those organic beings which never
intercross (if such exist), the species, on my theory, must have descended
from a succession of improved varieties, which will never have blended with
other individuals or varieties, but will have supplanted each other; so
that, at each successive stage of modification and improvement, all the
individuals of each variety will have descended from a single parent. But
in the majority of cases, namely, with all organisms which habitually unite
for each birth, or which often intercross, I believe that during the slow
process of modification the individuals of the species will have been kept
nearly uniform by intercrossing; so that many individuals will have gone on
simultaneously changing, and the whole amount of modification will not have
been due, at each stage, to descent from a single parent. To illustrate
what I mean: our English racehorses differ slightly from the horses of
every other breed; but they do not owe their difference and superiority to
descent from any single pair, but to continued care in selecting and
training many individuals during many generations.
Before discussing the three classes of facts, which I have selected as
presenting the greatest amount of difficulty on the theory of 'single
centres of creation,' I must say a few words on the means of dispersal.
Means of Dispersal. -- Sir C. Lyell and other authors have ably treated
this subject. I can give here only the briefest abstract of the more
important facts. Change of climate must have had a powerful influence on
migration: a region when its climate was different may have been a high
road for migration, but now be impassable; I shall, however, presently have
to discuss this branch of the subject in some detail. Changes of level in
the land must also have been highly influential: a narrow isthmus now
separates two marine faunas; submerge it, or let it formerly have been
submerged, and the two faunas will now blend or may formerly have blended:
where the sea now extends, land may at a former period have connected
islands or possibly even continents together, and thus have allowed
terrestrial productions to pass from one to the other. No geologist will
dispute that great mutations of level have occurred within the period of
existing organisms. Edward Forbes insisted that all the islands in the
Atlantic must recently have been connected with Europe or Africa, and
Europe likewise with America. Other authors have thus hypothetically
bridged over every ocean, and have united almost every island to some
mainland. If indeed the arguments used by Forbes are to be trusted, it
must be admitted that scarcely a single island exists which has not
recently been united to some continent. This view cuts the Gordian knot of
the dispersal of the same species to the most distant points, and removes
many a difficulty: but to the best of any judgment we are not authorized
in admitting such enormous geographical changes within the period of
existing species. It seems to me that we have abundant evidence of great
oscillations of level in our continents; but not of such vast changes in
their position and extension, as to have united them within the recent
period to each other and to the several intervening oceanic islands. I
freely admit the former existence of many islands, now buried beneath the
sea, which may have served as halting places for plants and for many
animals during their migration. In the coral-producing oceans such sunken
islands are now marked, as I believe, by rings of coral or atolls standing
over them. Whenever it is fully admitted, as I believe it will some day
be, that each species has proceeded from a single birthplace, and when in
the course of time we know something definite about the means of
distribution, we shall be enabled to speculate with security on the former
extension of the land. But I do not believe that it will ever be proved
that within the recent period continents which are now quite separate, have
been continuously, or almost continuously, united with each other, and with
the many existing oceanic islands. Several facts in distribution,--such as
the great difference in the marine faunas on the opposite sides of almost
every continent,--the close relation of the tertiary inhabitants of several
lands and even seas to their present inhabitants,--a certain degree of
relation (as we shall hereafter see) between the distribution of mammals
and the depth of the sea,--these and other such facts seem to me opposed to
the admission of such prodigious geographical revolutions within the recent
period, as are necessitated on the view advanced by Forbes and admitted by
his many followers. The nature and relative proportions of the inhabitants
of oceanic islands likewise seem to me opposed to the belief of their
former continuity with continents. Nor does their almost universally
volcanic composition favour the admission that they are the wrecks of
sunken continents;--if they had originally existed as mountain-ranges on
the land, some at least of the islands would have been formed, like other
mountain-summits, of granite, metamorphic schists, old fossiliferous or
other such rocks, instead of consisting of mere piles of volcanic matter.
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