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allied, but modified, descendants; and these will generally succeed in displac-
ing the groups which are their inferiors in the struggle for existence. Hence,
after long intervals of time, the productions of the world appear to have
changed simultaneously.

We can understand how it is that all the forms of life, ancient and recent,
make together a few grand classes. We can understand, from the continued
tendency to divergence of character, why, the more ancient a form is, the
more it generally differs from those now living; why ancient and extinct
forms often tend to fill up gaps between existing forms, sometimes blending
two groups, previously classed as distinct, into one; but more commonly
bringing them only a little closer together. The more ancient a form is, the
more ofteh it stands in some degree intermediate between groups now dis-
tinct; for the more ancient a form is, the more nearly it will be related to,
and consequently resemble, the common progenitor of groups, since become
widely divergent. Extinct forms are seldom directly intermediate between
existing forms; but are intermediate only by a long and circuitous course
through other extinct and different forms. We can clearly see why the organicj
remains of closely consecutive formations are closely allied; for they arq
closely linked together by generation. We can clearly see why the remains ol
an intermediate formation are intermediate in character.

The inhabitants of the world at each successive period in its history have
beaten their predecessors in the race for life, and are, in so far, higher in the
scale, and their structure has generally become more speciaUzed; and this
may account for the common belief held by so many palaeontologists, that
organization on the whole has progressed. Extinct and ancient animals re-
semble to a certain extent the embryos of the more recent animals belonging
to the same classes, and this wonderful fact receives a simple explanation
according to our views. The succession of the same types of structures within
the same areas during the later geological periods ceases to be mysterious,
and is intelligible on the principle of inheritance.

If, then, the geological record be as imperfect as many believe, and it
may at least be asserted that the record cannot be proved to be much more
perfect, the main objections to the theory of natural selection are greatly
diminished or disappear. On the other hand, all the chief laws of palaeon-
tology plainly proclaim, as it seems to me, that species have been produced by
ordinary generation: old forms having been supplanted by new and im-
proved forms of life, the products of Variation and the Survival of the Fittest.



CHAPTER XII

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 — Alternate Glacial Periods in the North and South.

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 wholly ac-
counted for by 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 arctic and northern temperate parts, all authors agree that one of the
most fundamental divisions in geographical distribution is that between the
New and the Old Worlds; yet if we travel over the vast American con-
tinent, from the central parts of the United States to its extreme southern
point, we meet with the most diversified conditions; humid districts, arid
deserts, lofty mountains, grassy plains, forests, marshes, lakes and great
rivers, under almost every temperature. There is hardly a climate or con-
dition in the Old World which cannot be paralleled in the New — at least
so closely as the same species generally require. No doubt small areas can
be pointed out in the Old World, hotter than any in the New World; but
these are not inhabited by a fauna different from that of the surrounding
districts; for it is rare to find a group of organisms confined to a small area,
of which the conditions are peculiar in only a slight degree. Notwithstand-
ing this general parallelism in the conditions of Old and New Worlds, how
widely different are their living productions!

In the southern hemisphere, if we compare large tracts of land in Aus-
tralia, South Africa, and western South America, between latitudes 25 and
35 degrees, 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 dis-
similar. Or, again, we may compare the productions of South America south
of latitude 35 degrees with those north of 25 degrees, which consequently are
separated by a space of ten degrees of latitude, and are exposed to con-
siderably different conditions; yet they are incomparably more closely re-
lated 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 im-
portant manner to the differences between the productions of various re-
gions. We see this in the great difference in nearly all the terrestrial pro-
ductions of the New and Old Worlds, excepting in the northern parts,

263



264 THE ORIGIN OF SPECIES

%vhere the land almost joins, and where, under a sHghtly different climate,
there might have been free migration for the northern temperate forms, as
there now is for the strictly arctic productions. \Ve see the same fact in the
great difference ber^veen the inhabitants of AustraUa, .\frica. and South
America, under the same latiuide: 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 loft)^ and continuous mountain-
ranges, of great deserts and even of large rivers, we find different produc-
tions: though as mountain-chains, deserts, etc., are not as impassible, or
likely to have endured so long, as the oceans separating continents, the
dififerences are ver\' inferior in degree to those characteristic of distinct
continents.

Turning to the sea, we find the same la\v. The marine inhabitants of the
eastern and \s-estem shores of South America are ver)' distinct, with ex-
tremely few shells, Crustacea, or echinodermata in common: but Dr.
Giinther has recentiy shown that about thirt\- per cent of the fishes are the
same on the opposite sides of the isthmus of Panama: and this fact has led
naturalists to beUeve that the isthmus was formerly open. \Vest\s'ard 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 faima. So that three marine faimas range
northward and southward in parallel lines not far from each other, imder
corresponding climate: but from being separated from each other by im-
passable barriers, either of land or open sea, they are almost wholly distinct.
On the other hand, proceeding still farther west^vard from the eastern
islands of the tropical parts of the Pacific, we encounter no impassable
barriers, and we have innimierable islands as halting-places, or continuous
coasts, until, after travelling over a hemisphere, ^ve come to the shores of
Africa; and over this vast space we meet \dth no well-defined and distinct
marine faimas. Although so fe^v marine animals are coromon to the above-
named three approximate faunas of Eastern and "Western America and the
eastern Pacific islands, yet many fishes 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 lon^tude.

A third great fact, partly included in the foregoing statement, is the
afl&nit\- of the productions of the same continent or of the same sea, though
the species themselves are distinct at different points and stations. It is a
law of the widest generality', and ever\' continent offers innimierable in-
stances. 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, though nearly related, replace each other.
He hears from closely aUied yet distinct kinds of birds, notes nearly similar,
and sees their nests similarly constructed, but not quite alike, with eggs
colored in nearly the same manner. The plains near the Straits of Magellan



GEOGR.\PHIC.\L DISTRIBUTION 265

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 emu, like those inhabiting 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 t\-pe of structure. \Ve ascend the loft)' 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 muskrat, but the co\-pu and cap\i>ara, rodents of
the South American t\pe. Innumerable other instance could be given. If
we look to the islands off the American shore, however much they may
differ in geological structure, the inhabitants are essentially American,
though they may be all peculiar species. ^Ve may look back to past ages, as
shown in the last chapter, and we find American t\-pes then prevailing oh
the American continent and in the American seas. ^Ve see in these facts
some deep organic bond, throughout space and time, over the same areas
of land and water, independently of physical conditions. The naturalist
must be dull who is not led to inquire what this bond is.

The bond is simply inheritance, that cause which alone, as far as wc
positively know^, produces organisms quite like each other, or, as we see in
the case of varieties, nearly alike. The dissimilarity^ of the inhabitants of
different regions may be attributed to modification through \'ariation and
natural selection, and probably in a subordinate degree to the definite in-
fluence of different physical conditions. The degrees of dissimilarity will
depend on the migration of the more dominant forms of life from one
region into another having been more or less effectually prevented, at
periods more or less remote — on the nature and number of the former
immigrants — and on the action of the inhabitants on each other in leading
to the preser\^ation of different modifications: the relation of organism to
organism in the struggle for life beiug, as I have already often remarked,
the most important of aU relations. Thus the high importance of barriers
comes into play by checking migration; as does time for the slow process ctf
modification through natural selection. ^Videly ranging species, ^x)unding
in indi\iduals, which have already triumphed over many competitors in
their own \sidelv extended homes, wdll have the best chance of seizing on
new places, when they spread out into new^ countries. In their new homes
they will be exposed to new conditions, and will frequentiy undergo further
modification and improvement; and thus they will become stiU further
victorious, and \Nill produce groups of modified descendants. On tbi^ prin-
ciple of inheritance with modification we can understand how it is that
sections of genera, whole genera, and e\^n famiHes, are confined to the
same areas, as is so commonly and notoriously the case.

There is no e\-idence, as was remarked in the last chapter, of die existence
of any law of necessary- development. As the variability- of each species is
an independent property', and will be taken advantage of by natural selec-
tion, only so far as it profits each individual in its complex stru^le for life.



266 THE ORIGIN OF SPECIES

so the amount of modification in different species will be no uniform
quantity. If a number of species, after having long competed with eack
other in their old home, were to migrate in a body into a new and after-J
ward isolated country, they would be little liable to modification; forr
neither migration nor isolation in themselves effect any thing. These:
principles come into play only by bringing organisms into new relations:
with each other and in a lesser degree with the surrounding physical condi-
tions. 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 or at all modified.

According to these views, it is obvious that the several species of the
^jne genus, though inhabiting the most distant quarters of the world, must
originally have proceeded from the same source, as they are descended
from the same progenitor. In the case of those species which have under-
gone, during whole geological periods, little modification, there is not much-:
difficulty in believing that they have migrated from the same region; for
during the vast geographical and climatical changes which 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
has been explained, it is incredible that individuals identically the same
should have been produced from parents specifically distinct.

SINGLE CENTRES OF SUPPOSED CREATION

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 many cases of extreme
difficulty in understanding how the same species could possibly have mi-
grated 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
i% 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 that 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 have been easily passed over by
migration, the fact is given as something remarkable and exceptional. The
incapacity of migrating across a wide sea is more clear in the case of
terrestrial mammals than perhaps with any other organic beings; and, ac-
cordingly, we find no inexplicable instances of the same mammals inhabit-
ing distant points of the world. No geologist feels any difficulty in Great



GEOGRAPHICAL DISTRIBUTION 267

Britain possessing the same quadrupeds with the rest of Europe, for they
were no doubt once united. 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
naturalized 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 wide and broken interspaces. The great and striking
influence of barriers of all kinds, is intelligible only on the view that the
great majority of species have been produced on one side, and have not
been able to migrate to the opposite side. Some few families, many sub-
families, very many genera, 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 confined to the same country, or
if they have a wide range that their range is continuous. What a strange
anomaly it would be if a directly opposite rule were to prevail when we
go down one step lower in the series, namely, to the individuals of the same
species, and these had not been, at least at first, confined to some one
region !

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 sub-
sequently migrated from that area as far as its powers of migration and
subsistence under past and present conditions permitted, is the most prob-
able. Undoubtedly many cases occur in which we cannot explain how the
same species could have passed from one point to the other. But the geo-
graphical and climatical changes which have certainly occurred within
recent geological times, must have 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 instances. 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 fresh-water productions;
and thirdly, the occurrence of the same terrestrial species on islands and
on the nearest 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



268 THE ORIGIN OF SPECIES

species having migrated from a single birthplace, then, considering ouri
ignorance with respect to former climatical and geographical changes, and;
to the various occasional means of transport, the belief that a single birth4
place is the law seems to me incomparably the safest.

In discussing this subject we shall be enabled at the same time to con-
sider a point equally important for us, namely, whether the several species
of a genus which must on our theory all be descended from a common
progenitor^, can have migrated, undergoing modification during their migra-
tion from some one area. If, when most of the species inhabiting one
region are different from those of another region, though closely allied to
them, it can be shown that migration from the one region to the other has
probably occurred at some former period, our general view will be much
strengthened; for the explanation is obvious on the principle of descent
with modification. 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 related by inheritance to the inhabitants of
that continent. Cases of this nature are common, and are, as we shall here-
after see, inexplicable on the theory of independent creation. This view of
the relation of the species of one region to those of another, does not differ
much from that advanced by Mr. Wallace, who concludes that "every
species has come into existence coincident both in space and time with a
pre-existing closely allied species." And it is now well known that he at-
tributes this coincidence to descent with modification.

The question of single or multiple centres of creation differs from another
though allied question; namely, whether all individuals of the same species
are descended from a single pair, or single hermaphrodite, or whether, as
some authors suppose, from many individuals simultaneously created. With
organic beings which never intercross, if such exist, each species must be de-
scended from a succession of modified varieties, that have supplanted each
other, but have never blended with other individuals or varieties of the
same species; so that, at each successive stage of modification, all the in-
dividuals of the same form will be descended from a single parent. But in
the great majority of cases, namely, with all organisms which habitually
mnite for each birth, or which occasionally intercross, the individuals of the
same species inhabiting the same area will be kept nearly uniform by inter-
crossing; so that many individuals will go on simultaneously changing, and
the whole amount of modification at each stage will not be due to descent
from a single parent. To illustrate what I mean: our English race-horses
differ from the horses of every other breed ; but they do not owe their differ-
ence and superiority to descent from any single pair, but to continued care
in the selecting and training of many individuals during each generation.

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.



GEOGRAPHICAL DISTRIBUTION 269



MEANS OF DISPERSAL

Sir C. LyeU and other authors have ably treated this subject. I can give
here only the briefest abstract of the more important facts. Change of cHmate
must have had a powerful influence on migration. A region now impassable
to certain organisms from the nature of its climate, might have been a high
road for migration, when the climate was different. I shall, however, pres-
ently 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 together, 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 al-
lowed terrestrial productions to pass from one to the other. No geologist
disputes that great mutations of level have occurred within the period of
existing organisms. Edward Forbes insisted that all the islands in the Atlantic
must have been recently connected with Europe or Africa, and Europe like-
wise with America. Other authors have thus hypothetically bridged over
every ocean, and united almost every island with 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 my 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 in the level of the land
or sea; but not of such vast changes in the position and extension of our
continents, 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 migra-



Online LibraryCharles DarwinThe origin of species → online text (page 36 of 50)