CHAPTER XII - GEOGRAPHICAL DISTRIBUTION
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 accounted 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 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; 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. 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. Notwithstanding this general 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 and 35, 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 with those north
of 25, which consequently are separated by a space of ten degrees of
latitude, and are exposed to considerably different conditions; yet
they are 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 in
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, of great deserts and 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. The marine inhabitants
of the eastern and western shores of South America are very
distinct, with extremely few shells, Crustacea, or Echinodermata in
common; but Dr. Gunther has recently shown that about thirty per cent.
of the fishes are the same on the opposite sides of the isthmus of
Panama; and this fact has led naturalists to believe that the
isthmus was formerly open. 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 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 almost
wholly distinct. On the other hand, proceeding still farther
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, or continuous coasts, 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 so few marine animals are common 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 longitude.
A third great fact, partly included in the foregoing statement, is
the affinity 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 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, though nearly 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 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 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 S. 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 are essentially
American, though they may be all peculiar species. We may look back to
past ages, as shown in the last chapter, and we find American types
then prevailing on the American continent and in the American seas. We
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 enquire
what this bond is.
The bond is simply inheritance, that cause which alone, as far as we
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 variation and natural selection, and probably in a subordinate
degree to the definite influence 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 preservation of
different modifications; the relation of organism to organism in the
struggle for life 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.
There is no evidence, as was remarked in the last chapter, of the
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 selection, only so far as it profits each individual
in its complex struggle for life, so the amount of modification in
different species will be no uniform quantity. If a number of species,
after having long competed with each other in their old home, were
to migrate in a body into a new and afterwards isolated country,
they would be little liable to modification; for neither migration nor
isolation in themselves effect 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 or
at all modified.
According to 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 are descended from the same progenitor. In the case of those
species which have undergone during the 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 climatal 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 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 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, accordingly, we find no inexplicable instances of
the same mammals inhabiting distant points of the world. No
geologist feels any difficulty in Great 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
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 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, 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 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 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
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 species having migrated
from a single birthplace; then, considering our ignorance with respect
to former climatal and geographical changes and to the various
occasional means of transport, the belief that a single birthplace
is the 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 species of a genus which must on our theory all be descended
from a common progenitor, can have migrated, undergoing modification
during their migration, 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 hereafter 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
attributes this coincidence to descent with modification.
The question of single or multiple centres of creation differs
from another though allied question,- namely, whether all the
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 descended 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
individuals of the same form will be descended from a single parent.
But in the great majority of cases, namely, with all organisms which
habitually unite for each birth, or which occasionally intercross, the
individuals of the same species inhabiting the same area will be
kept nearly uniform by intercrossing; 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 difference 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.
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