- 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° 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 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 judgement
we are not authorised 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 in
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.
I must now
say a few words on what are
called accidental
means, but which more properly
might be called occasional means
of distribution. I shall here
confine myself to plants. In
botanical works, this or that
plant is stated to be ill adapted
for wide dissemination; but for
transport across the sea, the
greater or less facilities may
be said to be almost wholly unknown.
Until I tried, with Mr Berkeley's
aid, a few experiments, it was
not even known how far seeds
could resist the injurious action
of sea-water. To my surprise
I found that out of 87 kinds,
64 germinated after an immersion
of 28 days, and a few survived
an immersion of 137 days. For
convenience sake I chiefly tried
small seeds, without the capsule
or fruit; and as all of these
sank in a few days, they could
not be floated across wide spaces
of the sea, whether or not they
were injured by the salt-water.
Afterwards I tried some larger
fruits, capsules, &c., and
some of these floated for a long
time. It is well known what a
difference there is in the buoyancy
of green and seasoned timber;
and it occurred to me that floods
might wash down plants or branches,
and that these might be dried
on the banks, and then by a fresh
rise in the stream be washed
into the sea. Hence I was led
to dry stems and branches of
94 plants with ripe fruit, and
to place them on sea water. The
majority sank quickly, but some
which whilst green floated for
a very short time, when dried
floated much longer; for instance,
ripe hazel-nuts sank immediately,
but when dried, they floated
for 90 days and afterwards when
planted they germinated; an asparagus
plant with ripe berries floated
for 23 days, when dried it floated
for 85 days, and the seeds afterwards
germinated: the ripe seeds of
Helosciadium sank in two days,
when dried they floated for above
90 days, and afterwards germinated.
Altogether out of the 94 dried
plants, 18 floated for above
28 days, and some of the 18 floated
for a very much longer period.
So that as 64/87 seeds germinated
after an immersion of 28 days;
and as 18/94 plants with ripe
fruit (but not all the same species
as in the foregoing experiment)
floated, after being dried, for
above 28 days, as far as we may
infer anything from these scanty
facts, we may conclude that the
seeds of 14/100 plants of any
country might be floated by sea-currents
during 28 days, and would retain
their power of germination. In
Johnston's physical Atlas, the
average rate of the several Atlantic
currents is 33 miles per diem
(some currents running at the
rate of 60 miles per diem); on
this average, the seeds of 14/100
plants belonging to one country
might be floated across 924 miles
of sea to another country; and
when stranded, if blown to a
favourable spot by an inland
gale, they would germinate.
Subsequently to my experiments,
M. Martens tried similar ones,
but in a much better manner,
for he placed the seeds in a
box in the actual sea, so that
they were alternately wet and
exposed to the air like really
floating plants. He tried 98
seeds, mostly different from
mine; but he chose many large
fruits and likewise seeds from
plants which live near the sea;
and this would have favoured
the average length of their flotation
and of their resistance to the
injurious action of the salt-water.
On the other hand he did not
previously dry the plants or
branches with the fruit; and
this, as we have seen, would
have caused some of them to have
floated much longer. The result
was that 18/98 of his seeds floated
for 42 days, and were then capable
of germination. But I do not
doubt that plants exposed to
the waves would float for a less
time than those protected from
violent movement as in our experiments.
Therefore it would perhaps be
safer to assume that the seeds
of about 10/100 plants of a flora,
after having been dried, could
be floated across a space of
sea 900 miles in width, and would
then germinate. The fact of the
larger fruits often floating
longer than the small, is interesting;
as plants with large seeds or
fruit could hardly be transported
by any other means; and Alph.
de Candolle has shown that such
plants generally have restricted
ranges.
But seeds may be occasionally
transported in another manner.
Drift timber is thrown up on
most islands, even on those in
the midst of the widest oceans;
and the natives of the coral-islands
in the Pacific, procure stones
for their tools, solely from
the roots of drifted trees, these
stones being a valuable royal
tax. I find on examination, that
when irregularly shaped stones
are embedded in the roots of
trees, small parcels of earth
are very frequently enclosed
in their interstices and behind
them, so perfectly that not a
particle could be washed away
in the longest transport: out
of one small portion of earth
thus completely enclosed
by wood in an oak about 50 years
old, three dicotyledonous plants
germinated: I am certain of the
accuracy of this observation.
Again, I can show that the carcasses
of birds, when floating on the
sea, sometimes escape being immediately
devoured; and seeds of many kinds
in the crops of floating birds
long retain their vitality: peas
and vetches, for instance, are
killed by even a few days' immersion
in sea-water; but some taken
out of the crop of a pigeon,
which had floated on artificial
salt-water for 30 days, to my
surprise nearly all germinated.
Living birds can hardly fail
to be highly effective agents
in the transportation of seeds.
I could give many facts showing
how frequently birds of many
kinds are blown by gales to vast
distances across the ocean. We
may I think safely assume that
under such circumstances their
rate of flight would often be
35 miles an hour; and some authors
have given a far higher estimate.
I have never seen an instance
of nutritious seeds passing through
the intestines of a bird; but
hard seeds of fruit will pass
uninjured through even the digestive
organs of a turkey. In the course
of two months, I picked up in
my garden 12 kinds of seeds,
out of the excrement of small
birds, and these seemed perfect,
and some of them, which I tried,
germinated. But the following
fact is more important: the crops
of birds do not secrete gastric
juice, and do not in the least
injure, as I know by trial, the
germination of seeds; now after
a bird has found and devoured
a large supply of food, it is
positively asserted that all
the grains do not pass into the
gizzard for 12 or even 18 hours.
A bird in this interval might
easily be blown to the distance
of 500 miles, and hawks are known
to look out for tired birds,
and the contents of their torn
crops might thus readily get
scattered. Mr Brent informs me
that a friend of his had to give
up flying carrier-pigeons from
France to England, as the hawks
on the English coast destroyed
so many on their arrival. Some
hawks and owls bolt their prey
whole, and after an interval
of from twelve to twenty hours,
disgorge pellets, which, as I
know from experiments made in
the Zoological Gardens, include
seeds capable of germination.
Some seeds of the oat, wheat,
millet, canary, hemp, clover,
and beet germinated after having
been from twelve to twenty-one
hours in the stomachs of different
birds of prey; and two seeds
of beet grew after having been
thus retained for two days and
fourteen hours. Freshwater fish,
I find, eat seeds of many land
and water plants: fish are frequently
devoured by birds, and thus the
seeds might be transported from
place to place. I forced many
kinds of seeds into the stomachs
of dead fish, and then gave their
bodies to fishing-eagles, storks,
and pelicans; these birds after
an interval of many hours, either
rejected the seeds in pellets
or passed them in their excrement;
and several of these seeds retained
their power of germination. Certain
seeds, however, were always killed
by this process.
Although the beaks and feet
of birds are generally quite
clean, I can show that earth
sometimes adheres to them: in
one instance I removed twenty-two
grains of dry argillaceous earth
from one foot of a partridge,
and in this earth there was a
pebble quite as large as the
seed of a vetch. Thus seeds might
occasionally be transported to
great distances; for many facts
could be given showing that soil
almost everywhere is charged
with seeds. Reflect for a moment
on the millions of quails which
annually cross the Mediterranean;
and can we doubt that the earth
adhering to their feet would
sometimes include a few minute
seeds? But I shall presently
have to recur to this subject.
As icebergs are known to be
sometimes loaded with earth and
stones, and have even carried
brushwood, bones, and the nest
of a land-bird, I can hardly
doubt that they must occasionally
have transported seeds from one
part to another of the arctic
and antarctic regions, as suggested
by Lyell; and during the Glacial
period from one part of the now
temperate regions to another.
In the Azores, from the large
number of the species of plants
common to Europe, in comparison
with the plants of other oceanic
islands nearer to the mainland,
and (as remarked by Mr H. C.
Watson) from the somewhat northern
character of the flora in comparison
with the latitude, I suspected
that these islands had been partly
stocked by ice-borne seeds, during
the Glacial epoch. At my request
Sir C. Lyell wrote to M. Hartung
to inquire whether he had observed
erratic boulders on these islands,
and he answered that he had found
large fragments of granite and
other rocks, which do not occur
in the archipelago. Hence we
may safely infer that icebergs
formerly landed their rocky burthens
on the shores of these mid-ocean
islands, and it is at least possible
that they may have brought thither
the seeds of northern plants.
Considering that the several
above means of transport, and
that several other means, which
without doubt remain to be discovered,
have been in action year after
year, for centuries and tens
of thousands of years, it would
I think be a marvellous fact
if many plants had not thus become
widely transported. These means
of transport are sometimes called
accidental, but this is not strictly
correct: the currents of the
sea are not accidental, nor is
the direction of prevalent gales
of wind. It should be observed
that scarcely any means of transport
would carry seeds for very great
distances; for seeds do not retain
their vitality when exposed for
a great length of time to the
action of seawater; nor could
they be long carried in the crops
or intestines of birds. These
means, however, would suffice
for occasional transport across
tracts of sea some hundred miles
in breadth, or from island to
island, or from a continent to
a neighbouring island, but not
from one distant continent to
another. The floras of distant
continents would not by such
means become mingled in any great
degree; but would remain as distinct
as we now see them to be. The
currents, from their course,
would never bring seeds from
North America to Britain, though
they might and do bring seeds
from the West Indies to our western
shores, where, if not killed
by so long an immersion in salt-water,
they could not endure our climate.
Almost every year, one or two
land-birds are blown across the
whole Atlantic Ocean, from North
America to the western shores
of Ireland and England; but seeds
could be transported by these
wanderers only by one means,
namely, in dirt sticking to their
feet, which is in itself a rare
accident. Even in this case,
how small would the chance be
of a seed falling on favourable
soil, and coming to maturity!
But it would be a great error
to argue that because a well-stocked
island, like Great Britain, has
not, as far as is known (and
it would be very difficult to
prove this), received within
the last few centuries, through
occasional means of transport,
immigrants from Europe or any
other continent, that a poorly-stocked
island, though standing more
remote from the mainland, would
not receive colonists by similar
means. I do not doubt that out
of twenty seeds or animals transported
to an island, even if far less
well-stocked than Britain, scarcely
more than one would be so well
fitted to its new home, as to
become naturalised. But this,
as it seems to me, is no valid
argument against what would be
effected by occasional means
of transport, during the long
lapse of geological time, whilst
an island was being upheaved
and formed, and before it had
become fully stocked with inhabitants.
On almost bare land, with few
or no destructive insects or
birds living there, nearly every
seed, which chanced to arrive,
would be sure to germinate and
survive.
Dispersal during the Glacial
period
The identity of many plants
and animals, on mountain-summits,
separated from each other by
hundreds of miles of lowlands,
where the Alpine species could
not possibly exist, is one of
the most striking cases known
of the same species living at
distant points, without the apparent
possibility of their having migrated
from one to the other. It is
indeed a remarkable fact to see
so many of the same plants living
on the snowy regions of the Alps
or Pyrenees, and in the extreme
northern parts of Europe; but
it is far more remarkable, that
the plants on the White Mountains,
in the United States of America,
are all the same with those of
Labrador, and nearly all the
same, as we hear from Asa Gray,
with those on the loftiest mountains
of Europe. Even as long ago as
1747, such facts led Gmelin to
conclude that the same species
must have been independently
created at several distinct points;
and we might have remained in
this same belief, had not Agassiz
and others called vivid attention
to the Glacial period, which,
as we shall immediately see,
affords a simple explanation
of these facts. We have evidence
of almost every conceivable kind,
organic and inorganic, that within
a very recent geological period,
central Europe and North America
suffered under an Arctic climate.
The ruins of a house burnt by
fire do not tell their tale more
plainly, than do the mountains
of Scotland and Wales, with their
scored flanks, polished surfaces,
and perched boulders, of the
icy streams with which their
valleys were lately filled. So
greatly has the climate of Europe
changed, that in Northern Italy,
gigantic moraines, left by old
glaciers, are now clothed by
the vine and maize. Throughout
a large part of the United States,
erratic boulders, and rocks scored
by drifted icebergs and coast-ice,
plainly reveal a former cold
period.
The former influence of the
glacial climate on the distribution
of the inhabitants of Europe,
as explained with remarkable
clearness by Edward Forbes, is
substantially as follows. But
we shall follow the changes more
readily, by supposing a new glacial
period to come slowly on, and
then pass away, as formerly occurred.
As the cold came on, and as each
more southern zone became fitted
for arctic beings and ill-fitted
for their former more temperate
inhabitants, the latter would
be supplanted and arctic productions
would take their places. The
inhabitants of the more temperate
regions would at the same time
travel southward, unless they
were stopped by barriers, in
which case they would perish.
The mountains would become covered
with snow and ice, and their
former Alpine inhabitants would
descend to the plains. By the
time that the cold had reached
its maximum, we should have a
uniform arctic fauna and flora,
covering the central parts of
Europe, as far south as the Alps
and Pyrenees, and even stretching
into Spain. The now temperate
regions of the United States
would likewise be covered by
arctic plants and animals, and
these would be nearly the same
with those of Europe; for the
present circumpolar inhabitants,
which we suppose to have everywhere
travelled southward, are remarkably
uniform round the world. We may
suppose that the Glacial period
came on a little earlier or later
in North America than in Europe,
so will the southern migration
there have been a little earlier
or later; but this will make
no difference in the final result.
As the warmth returned, the
arctic forms would retreat northward,
closely followed up in their
retreat by the productions of
the more temperate regions. And
as the snow melted from the bases
of the mountains, the arctic
forms would seize on the cleared
and thawed ground, always ascending
higher and higher, as the warmth
increased, whilst their brethren
were pursuing their northern
journey. Hence, when the warmth
had fully returned, the same
arctic species, which had lately
lived in a body together on the
lowlands of the Old and New Worlds,
would be left isolated on distant
mountain-summits (having been
exterminated on all lesser heights)
and in the arctic regions of
both hemispheres.
Thus we can understand the
identity of many plants at points
so immensely remote as on the
mountains of the United States
and of Europe. We can thus also
understand the fact that the
Alpine plants of each mountain-range
are more especially related to
the arctic forms living due north
or nearly due north of them:
for the migration as the cold
came on, and the re-migration
on the returning warmth, will
generally have been due south
and north. The Alpine plants,
for example, of Scotland, as
remarked by Mr H. C. Watson,
and those of the Pyrenees, as
remarked by Ramond, are more
especially allied to the plants
of northern Scandinavia; those
of the United States to Labrador;
those of the mountains of Siberia
to the arctic regions of that
country. These views, grounded
as they are on the perfectly
well-ascertained occurrence of
a former Glacial period, seem
to me to explain in so satisfactory
a manner the present distribution
of the Alpine and Arctic productions
of Europe and America, that when
in other regions we find the
same species on distant mountain-summits,
we may almost conclude without
other evidence, that a colder
climate permitted their former
migration across the low intervening
tracts, since become too warm
for their existence.
If the climate, since the Glacial
period, has ever been in any
degree warmer than at present
(as some geologists in the United
States believe to have been the
case, chiefly from the distribution
of the fossil Gnathodon), then
the arctic and temperate productions
will at a very late period have
marched a little further north,
and subsequently have retreated
to their present homes; but I
have met with no satisfactory
evidence with respect to this
intercalated slightly warmer
period, since the Glacial period.
The arctic forms, during their
long southern migration and re-migration
northward, will have been exposed
to nearly the same climate, and,
as is especially to be noticed,
they will have kept in a body
together; consequently their
mutual relations will not have
been much disturbed, and, in
accordance with the principles
inculcated in this volume, they
will not have been liable to
much modification. But with our
Alpine productions, left isolated
from the moment of the returning
warmth, first at the bases and
ultimately on the summits of
the mountains, the case will
have been somewhat different;
for it is not likely that all
the same arctic species will
have been left on mountain ranges
distant from each other, and
have survived there ever since;
they will, also, in all probability
have become mingled with ancient
Alpine species, which must have
existed on the mountains before
the commencement of the Glacial
epoch, and which during its coldest
period will have been temporarily
driven down to the plains; they
will, also, have been exposed
to somewhat different climatal
influences. Their mutual relations
will thus have been in some degree
disturbed; consequently they
will have been liable to modification;
and this we find has been the
case; for if we compare the present
Alpine plants and animals of
the several great European mountain-ranges,
though very many of the species
are identically the same, some
present varieties, some are ranked
as doubtful forms, and some few
are distinct yet closely allied
or representative species.
In illustrating
what, as I believe, actually
took place
during the Glacial period, I
assumed that at its commencement
the arctic productions were as
uniform round the polar regions
as they are at the present day.
But the foregoing remarks on
distribution apply not only to
strictly arctic forms, but also
to many sub-arctic and to some
few northern temperate forms,
for some of these are the same
on the lower mountains and on
the plains of North America and
Europe; and it may be reasonably
asked how I account for the necessary
degree of uniformity of the sub-arctic
and northern temperate forms
round the world, at the commencement
of the Glacial period. At the
present day, the sub-arctic and
northern temperate productions
of the Old and New Worlds are
separated from each other by
the Atlantic Ocean and by the
extreme northern part of the
Pacific. During the Glacial period,
when the inhabitants of the Old
and New Worlds lived further
southwards than at present, they
must have been still more completely
separated by wider spaces of
ocean. I believe the above difficulty
may be surmounted by looking
to still earlier changes of climate
of an opposite nature. We have
good reason to believe that during
the newer Pliocene period, before
the Glacial epoch, and whilst
the majority of the inhabitants
of the world were specifically
the same as now, the climate
was warmer than at the present
day. Hence we may suppose that
the organisms now living under
the climate of latitude 60°,
during the Pliocene period lived
further north under the Polar
Circle, in latitude 66°-67°;
and that the strictly arctic
productions then lived on the
broken land still nearer to the
pole. Now if we look at a globe,
we shall see that under the Polar
Circle there is almost continuous
land from western Europe, through
Siberia, to eastern America.
And to this continuity of the
circumpolar land, and to the
consequent freedom for intermigration
under a more favourable climate,
I attribute the necessary amount
of uniformity in the sub-arctic
and northern temperate productions
of the Old and New Worlds, at
a period anterior to the Glacial
epoch.
Believing, from reasons before
alluded to, that our continents
have long remained in nearly
the same relative position, though
subjected to large, but partial
oscillations of level, I am strongly
inclined to extend the above
view, and to infer that during
some earlier and still warmer
period, such as the older Pliocene
period, a large number of the
same plants and animals inhabited
the almost continuous circumpolar
land; and that these plants and
animals, both in the Old and
New Worlds, began slowly to migrate
southwards as the climate became
less warm, long before the commencement
of the Glacial period. We now
see, as I believe, their descendants,
mostly in a modified condition,
in the central parts of Europe
and the United States. On this
view we can understand the relationship,
with very little identity, between
the productions of North America
and Europe, a relationship which
is most remarkable, considering
the distance of the two areas,
and their separation by the Atlantic
Ocean. We can further understand
the singular fact remarked on
by several observers, that the
productions of Europe and America
during the later tertiary stages
were more closely related to
each other than they are at the
present time; for during these
warmer periods the northern parts
of the Old and New Worlds will
have been almost continuously
united by land, serving as a
bridge, since rendered impassable
by cold, for the inter-migration
of their inhabitants.
During the slowly decreasing
warmth of the Pliocene period,
as soon as the species in common,
which inhabited the New and Old
Worlds, migrated south of the
Polar Circle, they must have
been completely cut off from
each other. This separation,
as far as the more temperate
productions are concerned, took
place long ages ago. And as the
plants and animals migrated southward,
they will have become mingled
in the one great region with
the native American productions,
and have had to compete with
them; and in the other great
region, with those of the Old
World. Consequently we have here
everything favourable for much
modification, for far more modification
than with the Alpine productions,
left isolated, within a much
more recent period, on the several
mountain-ranges and on the arctic
lands of the two Worlds. Hence
it has come, that when we compare
the now living productions of
the temperate regions of the
New and Old Worlds, we find very
few identical species (though
Asa Gray has lately shown that
more plants are identical than
was formerly supposed), but we
find in every great class many
forms, which some naturalists
rank as geographical races, and
others as distinct species; and
a host of closely allied or representative
forms which are ranked by all
naturalists as specifically distinct.
As on the land, so in the waters
of the sea, a slow southern migration
of a marine fauna, which during
the Pliocene or even a somewhat
earlier period, was nearly uniform
along the continuous shores of
the Polar Circle, will account,
on the theory of modification,
for many closely allied forms
now living in areas completely
sundered. Thus, I think, we can
understand the presence of many
existing and tertiary representative
forms on the eastern and western
shores of temperate North America;
and the still more striking case
of many closely allied crustaceans
(as described in Dana's admirable
work), of some fish and other
marine animals, in the Mediterranean
and in the seas of Japan, areas
now separated by a continent
and by nearly a hemisphere of
equatorial ocean.
These cases of relationship,
without identity, of the inhabitants
of seas now disjoined, and likewise
of the past and present inhabitants
of the temperate lands of North
America and Europe, are inexplicable
on the theory of creation. We
cannot say that they have been
created alike, in correspondence
with the nearly similar physical
conditions of the areas; for
if we compare, for instance,
certain parts of South America
with the southern continents
of the Old World, we see countries
closely corresponding in all
their physical conditions, but
with their inhabitants utterly
dissimilar.
But we must return to our more
immediate subject, the Glacial
period. I am convinced that Forbes's
view may be largely extended.
In Europe we have the plainest
evidence of the cold period,
from the western shores of Britain
to the Oural range, and southward
to the Pyrenees. We may infer,
from the frozen mammals and nature
of the mountain vegetation, that
Siberia was similarly affected.
Along the Himalaya, at points
900 miles apart, glaciers have
left the marks of their former
low descent; and in Sikkim, Dr
Hooker saw maize growing on gigantic
ancient moraines. South of the
equator, we have some direct
evidence of former glacial action
in New Zealand; and the same
plants, found on widely separated
mountains in this island, tell
the same story. If one account
which has been published can
be trusted, we have direct evidence
of glacial action in the southeastern
corner of Australia.
Looking to
America; in the northern half,
ice-borne fragments
of rock have been observed on
the eastern side as far south
as lat. 36°-37°, and
on the shores of the Pacific,
where the climate is now so different,
as far south as lat. 46°;
erratic boulders have, also,
been noticed on the Rocky Mountains.
In the Cordillera of Equatorial
South America, glaciers once
extended far below their present
level. In central Chile I was
astonished at the structure of
a vast mound of detritus, about
800 feet in height, crossing
a valley of the Andes; and this
I now feel convinced was a gigantic
moraine, left far below any existing
glacier. Further south on both
sides of the continent, from
lat. 41° to the southernmost
extremity, we have the clearest
evidence of former glacial action,
in huge boulders transported
far from their parent source.
We do not know that the Glacial
epoch was strictly simultaneous
at these several far distant
points on opposite sides of the
world. But we have good evidence
in almost every case, that the
epoch was included within the
latest geological period. We
have, also, excellent evidence,
that it endured for an enormous
time, as measured by years, at
each point. The cold may have
come on, or have ceased, earlier
at one point of the globe than
at another, but seeing that it
endured for long at each, and
that it was contemporaneous in
a geological sense, it seems
to me probable that it was, during
a part at least of the period,
actually simultaneous throughout
the world. Without some distinct
evidence to the contrary, we
may at least admit as probable
that the glacial action was simultaneous
on the eastern and western sides
of North America, in the Cordillera
under the equator and under the
warmer temperate zones, and on
both sides of the southern extremity
of the continent. If this be
admitted, it is difficult to
avoid believing that the temperature
of the whole world was at this
period simultaneously cooler.
But it would suffice for my purpose,
if the temperature was at the
same time lower along certain
broad belts of longitude.
On this view of the whole world,
or at least of broad longitudinal
belts, having been simultaneously
colder from pole to pole, much
light can be thrown on the present
distribution of identical and
allied species. In America, Dr
Hooker has shown that between
forty and fifty of the flowering
plants of Tierra del Fuego, forming
no inconsiderable part of its
scanty flora, are common to Europe,
enormously remote as these two
points are; and there are many
closely allied species. On the
lofty mountains of equatorial
America a host of peculiar species
belonging to European genera
occur. On the highest mountains
of Brazil, some few European
genera were found by Gardner,
which do not exist in the wide
intervening hot countries. So
on the Silla of Caraccas the
illustrious Humboldt long ago
found species belonging to genera
characteristic of the Cordillera.
On the mountains of Abyssinia,
several European forms and some
few representatives of the peculiar
flora of the Cape of Good Hope
occur. At the Cape of Good Hope
a very few European species,
believed not to have been introduced
by man, and on the mountains,
some few representative European
forms are found, which have not
been discovered in the intertropical
parts of Africa. On the Himalaya,
and on the isolated mountain-ranges
of the peninsula of India, on
the heights of Ceylon, and on
the volcanic cones of Java, many
plants occur, either identically
the same or representing each
other, and at the same time representing
plants of Europe, not found in
the intervening hot lowlands.
A list of the genera collected
on the loftier peaks of Java
raises a picture of a collection
made on a hill in Europe! Still
more striking is the fact that
southern Australian forms are
clearly represented by plants
growing on the summits of the
mountains of Borneo. Some of
these Australian forms, as I
hear from Dr. Hooker, extend
along the heights of the peninsula
of Malacca, and are thinly scattered,
on the one hand over India and
on the other as far as Japan.
On the southern
mountains of Australia, Dr.
F. Müller
has discovered several European
species; other species, not introduced
by man, occur on the lowlands;
and a long list can be given,
as I am informed by Dr. Hooker,
of European genera, found in
Australia, but not in the intermediate
torrid regions. In the admirable
`Introduction to the Flora of
New Zealand,' by Dr. Hooker,
analogous and striking facts
are given in regard to the plants
of that large island. Hence we
see that throughout the world,
the plants growing on the more
lofty mountains, and on the temperate
lowlands of the northern and
southern hemispheres, are sometimes
identically the same; but they
are much oftener specifically
distinct, though related to each
other in a most remarkable manner.
This brief
abstract applies to plants
alone: some strictly
analogous facts could be given
on the distribution of terrestrial
animals. In marine productions,
similar cases occur; as an example,
I may quote a remark by the highest
authority, Prof. Dana, that `it
is certainly a wonderful fact
that New Zealand should have
a closer resemblance in its crustacea
to Great Britain, its antipode,
than to any other part of the
world.' Sir J. Richardson, also,
speaks of the reappearance on
the shores of New Zealand, Tasmania, &c.,
of northern forms of fish. Dr
Hooker informs me that twenty-five
species of Algae are common to
New Zealand and to Europe, but
have not been found in the intermediate
tropical seas.
It should be observed that
the northern species and forms
found in the southern parts of
the southern hemisphere, and
on the mountain-ranges of the
intertropical regions, are not
arctic, but belong to the northern
temperate zones. As Mr. H. C.
Watson has recently remarked,
`In receding from polar towards
equatorial latitudes, the Alpine
or mountain floras really become
less and less arctic.' Many of
the forms living on the mountains
of the warmer regions of the
earth and in the southern hemisphere
are of doubtful value, being
ranked by some naturalists as
specifically distinct, by others
as varieties; but some are certainly
identical, and many, though closely
related to northern forms, must
be ranked as distinct species.
Now let us see what light can
be thrown on the foregoing facts,
on the belief, supported as it
is by a large body of geological
evidence, that the whole world,
or a large part of it, was during
the Glacial period simultaneously
much colder than at present.
The Glacial period, as measured
by years, must have been very
long; and when we remember over
what vast spaces some naturalised
plants and animals have spread
within a few centuries, this
period will have been ample for
any amount of migration. As the
cold came slowly on, all the
tropical plants and other productions
will have retreated from both
sides towards the equator, followed
in the rear by the temperate
productions, and these by the
arctic; but with the latter we
are not now concerned. The tropical
plants probably suffered much
extinction; how much no one can
say; perhaps formerly the tropics
supported as many species as
we see at the present day crowded
together at the Cape of Good
Hope, and in parts of temperate
Australia. As we know that many
tropical plants and animals can
withstand a considerable amount
of cold, many might have escaped
extermination during a moderate
fall of temperature, more especially
by escaping into the warmest
spots. But the great fact to
bear in mind is, that all tropical
productions will have suffered
to a certain extent. On the other
hand, the temperate productions,
after migrating nearer to the
equator, though they will have
been placed under somewhat new
conditions, will have suffered
less. And it is certain that
many temperate plants, if protected
from the inroads of competitors,
can withstand a much warmer climate
than their own. Hence, it seems
to me possible, bearing in mind
that the tropical productions
were in a suffering state and
could not have presented a firm
front against intruders, that
a certain number of the more
vigorous and dominant temperate
forms might have penetrated the
native ranks and have reached
or even crossed the equator.
The invasion would, of course,
have been greatly favoured by
high land, and perhaps by a dry
climate; for Dr. Falconer informs
me that it is the damp with the
heat of the tropics which is
so destructive to perennial plants
from a temperate climate. On
the other hand, the most humid
and hottest districts will have
afforded an asylum to the tropical
natives. The mountain-ranges
north-west of the Himalaya, and
the long line of the Cordillera,
seem to have afforded two great
lines of invasion: and it is
a striking fact, lately communicated
to me by Dr. Hooker, that all
the flowering plants, about forty-six
in number, common to Tierra del
Fuego and to Europe still exist
in North America, which must
have lain on the line of march.
But I do not doubt that some
temperate productions entered
and crossed even the lowlands of
the tropics at the period when
the cold was most intense, when
arctic forms had migrated some
twenty-five degrees of latitude
from their native country and
covered the land at the foot
of the Pyrenees. At this period
of extreme cold, I believe that
the climate under the equator
at the level of the sea was about
the same with that now felt there
at the height of six or seven
thousand feet. During this the
coldest period, I suppose that
large spaces of the tropical
lowlands were clothed with a
mingled tropical and temperate
vegetation, like that now growing
with strange luxuriance at the
base of the Himalaya, as graphically
described by Hooker.
Thus, as I believe, a considerable
number of plants, a few terrestrial
animals, and some marine productions,
migrated during the Glacial period
from the northern and southern
temperate zones into the intertropical
regions, and some even crossed
the equator. As the warmth returned,
these temperate forms would naturally
ascend the higher mountains,
being exterminated on the lowlands;
those which had not reached the
equator, would re-migrate northward
or southward towards their former
homes; but the forms, chiefly
northern, which had crossed the
equator, would travel still further
from their homes into the more
temperate latitudes of the opposite
hemisphere. Although we have
reason to believe from geological
evidence that the whole body
of arctic shells underwent scarcely
any modification during their
long southern migration and re-migration
northward, the case may have
been wholly different with those
intruding forms which settled
themselves on the intertropical
mountains, and in the southern
hemisphere. These being surrounded
by strangers will have had to
compete with many new forms of
life; and it is probable that
selected modifications in their
structure, habits, and constitutions
will have profited them. Thus
many of these wanderers, though
still plainly related by inheritance
to their brethren of the northern
or southern hemispheres, now
exist in their new homes as well-marked
varieties or as distinct species.
It is a remarkable fact, strongly
insisted on by Hooker in regard
to America, and by Alph. de Candolle
in regard to Australia, that
many more identical plants and
allied forms have apparently
migrated from the north to the
south, than in a reversed direction.
We see, however, a few southern
vegetable forms on the mountains
of Borneo and Abyssinia. I suspect
that this preponderant migration
from north to south is due to
the greater extent of land in
the north, and to the northern
forms having existed in their
own homes in greater numbers,
and having consequently been
advanced through natural selection
and competition to a higher stage
of perfection or dominating power,
than the southern forms. And
thus, when they became commingled
during the Glacial period, the
northern forms were enabled to
beat the less powerful southern
forms. Just in the same manner
as we see at the present day,
that very many European productions
cover the ground in La Plata,
and in a lesser degree in Australia,
and have to a certain extent
beaten the natives; whereas extremely
few southern forms have become
naturalised in any part of Europe,
though hides, wool, and other
objects likely to carry seeds
have been largely imported into
Europe during the last two or
three centuries from La Plata,
and during the last thirty or
forty years from Australia. Something
of the same kind must have occurred
on the intertropical mountains:
no doubt before the Glacial period
they were stocked with endemic
Alpine forms; but these have
almost everywhere largely yielded
to the more dominant forms, generated
in the larger areas and more
efficient workshops of the north.
In many islands the native productions
are nearly equalled or even outnumbered
by the naturalised; and if the
natives have not been actually
exterminated, their numbers have
been greatly reduced, and this
is the first stage towards extinction.
A mountain is an island on the
land; and the intertropical mountains
before the Glacial period must
have been completely isolated;
and I believe that the productions
of these islands on the land
yielded to those produced within
the larger areas of the north,
just in the same way as the productions
of real islands have everywhere
lately yielded to continental
forms, naturalised by man's agency.
I am far from supposing that
all difficulties are removed
on the view here given in regard
to the range and affinities of
the allied species which live
in the northern and southern
temperate zones and on the mountains
of the intertropical regions.
Very many difficulties remain
to be solved. I do not pretend
to indicate the exact lines and
means of migration, or the reason
why certain species and not others
have migrated; why certain species
have been modified and have given
rise to new groups of forms,
and others have remained unaltered.
We cannot hope to explain such
facts, until we can say why one
species and not another becomes
naturalised by man's agency in
a foreign land; why one ranges
twice or thrice as far, and is
twice or thrice as common, as
another species within their
own homes.
I have said that many difficulties
remain to be solved: some of
the most remarkable are stated
with admirable clearness by Dr.
Hooker in his botanical works
on the antarctic regions. These
cannot be here discussed. I will
only say that as far as regards
the occurrence of identical species
at points so enormously remote
as Kerguelen Land, New Zealand,
and Fuegia, I believe that towards
the close of the Glacial period,
icebergs, as suggested by Lyell,
have been largely concerned in
their dispersal. But the existence
of several quite distinct species,
belonging to genera exclusively
confined to the south, at these
and other distant points of the
southern hemisphere, is, on my
theory of descent with modification,
a far more remarkable case of
difficulty. For some of these
species are so distinct, that
we cannot suppose that there
has been time since the commencement
of the Glacial period for their
migration, and for their subsequent
modification to the necessary
degree. The facts seem to me
to indicate that peculiar and
very distinct species have migrated
in radiating lines from some
common centre; and I am inclined
to look in the southern, as in
the northern hemisphere, to a
former and warmer period, before
the commencement of the Glacial
period, when the antarctic lands,
now covered with ice, supported
a highly peculiar and isolated
flora. I suspect that before
this flora was exterminated by
the Glacial epoch, a few forms
were widely dispersed to various
points of the southern hemisphere
by occasional means of transport,
and by the aid, as halting-places,
of existing and now sunken islands,
and perhaps at the commencement
of the Glacial period, by icebergs.
By these means, as I believe,
the southern shores of America,
Australia, New Zealand have become
slightly tinted by the same peculiar
forms of vegetable life.
Sir C. Lyell in a striking
passage has speculated, in language
almost identical with mine, on
the effects of great alterations
of climate on geographical distribution.
I believe that the world has
recently felt one of his great
cycles of change; and that on
this view, combined with modification
through natural selection, a
multitude of facts in the present
distribution both of the same
and of allied forms of life can
be explained. The living waters
may be said to have flowed during
one short period from the north
and from the south, and to have
crossed at the equator; but to
have flowed with greater force
from the north so as to have
freely inundated the south. As
the tide leaves its drift in
horizontal lines, though rising
higher on the shores where the
tide rises highest, so have the
living waters left their living
drift on our mountain-summits,
in a line gently rising from
the arctic lowlands to a great
height under the equator. The
various beings thus left stranded
may be compared with savage races
of man, driven up and surviving
in the mountain-fastnesses of
almost every land, which serve
as a record, full of interest
to us, of the former inhabitants
of the surrounding lowlands. |