- Distribution
of fresh-water productions
- On
the inhabitants of oceanic
islands
- Absence
of Batrachians and of terrestrial
Mammals
- On
the relations of the
inhabitants of islands to those
of
the nearest mainland
- On
colonisation from the
nearest source with
subsequent modification
- Summary
of the last and present
chapters
As lakes and
river-systems are separated
from each other
by barriers of land, it might
have been thought that fresh-water
productions would not have ranged
widely within the same country,
and as the sea is apparently
a still more impassable barrier,
that they never would have extended
to distant countries. But the
case is exactly the reverse.
Not only have many fresh-water
species, belonging to quite different
classes, an enormous range, but
allied species prevail in a remarkable
manner throughout the world.
I well remember, when first collecting
in the fresh waters of Brazil,
feeling much surprise at the
similarity of the fresh-water
insects, shells, &c., and
at the dissimilarity of the surrounding
terrestrial beings, compared
with those of Britain.
But this power in fresh-water
productions of ranging widely,
though so unexpected, can, I
think, in most cases be explained
by their having become fitted,
in a manner highly useful to
them, for short and frequent
migrations from pond to pond,
or from stream to stream; and
liability to wide dispersal would
follow from this capacity as
an almost necessary consequence.
We can here consider only a few
cases. In regard to fish, I believe
that the same species never occur
in the fresh waters of distant
continents. But on the same continent
the species often range widely
and almost capriciously; for
two river-systems will have some
fish in common and some different.
A few facts seem to favour the
possibility of their occasional
transport by accidental means;
like that of the live fish not
rarely dropped by whirlwinds
in India, and the vitality of
their ova when removed from the
water. But I am inclined to attribute
the dispersal of fresh-water
fish mainly to slight changes
within the recent period in the
level of the land, having caused
rivers to flow into each other.
Instances, also, could be given
of this having occurred during
floods, without any change of
level. We have evidence in the
loess of the Rhine of considerable
changes of level in the land
within a very recent geological
period, and when the surface
was peopled by existing land
and fresh-water shells. The wide
difference of the fish on opposite
sides of continuous mountain-ranges,
which from an early period must
have parted river-systems and
completely prevented their inosculation,
seems to lead to this same conclusion.
With respect to allied fresh-water
fish occurring at very distant
points of the world, no doubt
there are many cases which cannot
at present be explained: but
some fresh-water fish belong
to very ancient forms, and in
such cases there will have been
ample time for great geographical
changes, and consequently time
and means for much migration.
In the second place, salt-water
fish can with care be slowly
accustomed to live in fresh water;
and, according to Valenciennes,
there is hardly a single group
of fishes confined exclusively
to fresh water, so that we may
imagine that a marine member
of a fresh-water group might
travel far along the shores of
the sea, and subsequently become
modified and adapted to the fresh
waters of a distant land.
Some species of fresh-water
shells have a very wide range,
and allied species, which, on
my theory, are descended from
a common parent and must have
proceeded from a single source,
prevail throughout the world.
Their distribution at first perplexed
me much, as their ova are not
likely to be transported by birds,
and they are immediately killed
by sea water, as are the adults.
I could not even understand how
some naturalised species have
rapidly spread throughout the
same country. But two facts,
which I have observed and no
doubt many others remain to be
observed throw some light on
this subject. When a duck suddenly
emerges from a pond covered with
duck-weed, I have twice seen
these little plants adhering
to its back; and it has happened
to me, in removing a little duck-weed
from one aquarium to another,
that I have quite unintentionally
stocked the one with fresh-water
shells from the other. But another
agency is perhaps more effectual:
I suspended a duck's feet, which
might represent those of a bird
sleeping in a natural pond, in
an aquarium, where many ova of
fresh-water shells were hatching;
and I found that numbers of the
extremely minute and just hatched
shells crawled on the feet, and
clung to them so firmly that
when taken out of the water they
could not be jarred off, though
at a somewhat more advanced age
they would voluntarily drop off.
These just hatched molluscs,
though aquatic in their nature,
survived on the duck's feet,
in damp air, from twelve to twenty
hours; and in this length of
time a duck or heron might fly
at least six or seven hundred
miles, and would be sure to alight
on a pool or rivulet, if blown
across sea to an oceanic island
or to any other distant point.
Sir Charles Lyell also informs
me that a Dyticus has been caught
with an Ancylus (a fresh-water
shell like a limpet) firmly adhering
to it; and a water-beetle of
the same family, a Colymbetes,
once flew on board the `Beagle,'
when forty-five miles distant
from the nearest land: how much
farther it might have flown with
a favouring gale no one can tell.
With respect to plants, it
has long been known what enormous
ranges many fresh-water and even
marsh-species have, both over
continents and to the most remote
oceanic islands. This is strikingly
shown, as remarked by Alph. de
Candolle, in large groups of
terrestrial plants, which have
only a very few aquatic members;
for these latter seem immediately
to acquire, as if in consequence,
a very wide range. I think favourable
means of dispersal explain this
fact. I have before mentioned
that earth occasionally, though
rarely, adheres in some quantity
to the feet and beaks of birds.
Wading birds, which frequent
the muddy edges of ponds, if
suddenly flushed, would be the
most likely to have muddy feet.
Birds of this order I can show
are the greatest wanderers, and
are occasionally found on the
most remote and barren islands
in the open ocean; they would
not be likely to alight on the
surface of the sea, so that the
dirt would not be washed off
their feet; when making land,
they would be sure to fly to
their natural fresh-water haunts.
I do not believe that botanists
are aware how charged the mud
of ponds is with seeds: I have
tried several little experiments,
but will here give only the most
striking case: I took in February
three table-spoonfuls of mud
from three different points,
beneath water, on the edge of
a little pond; this mud when
dry weighed only 6 3/4 ounces;
I kept it covered up in my study
for six months, pulling up and
counting each plant as it grew;
the plants were of many kinds,
and were altogether 537 in number;
and yet the viscid mud was all
contained in a breakfast cup!
Considering these facts, I think
it would be an inexplicable circumstance
if water-birds did not transport
the seeds of fresh-water plants
to vast distances, and if consequently
the range of these plants was
not very great. The same agency
may have come into play with
the eggs of some of the smaller
fresh-water animals.
Other and unknown agencies
probably have also played a part.
I have stated that fresh-water
fish eat some kinds of seeds,
though they reject many other
kinds after having swallowed
them; even small fish swallow
seeds of moderate size, as of
the yellow water-lily and Potamogeton.
Herons and other birds, century
after century, have gone on daily
devouring fish; they then take
flight and go to other waters,
or are blown across the sea;
and we have seen that seeds retain
their power of germination, when
rejected in pellets or in excrement,
many hours afterwards. When I
saw the great size of the seeds
of that fine water-lily, the
Nelumbium, and remembered Alph.
de Candolle's remarks on this
plant, I thought that its distribution
must remain quite inexplicable;
but Audubon states that he found
the seeds of the great southern
water-lily (probably, according
to Dr Hooker, the Nelumbium luteum)
in a heron's stomach; although
I do not know the fact, yet analogy
makes me believe that a heron
flying to another pond and getting
a hearty meal of fish, would
probably reject from its stomach
a pellet containing the seeds
of the Nelumbium undigested;
or the seeds might be dropped
by the bird whilst feeding its
young, in the same way as fish
are known sometimes to be dropped.
In considering these several
means of distribution, it should
be remembered that when a pond
or stream is first formed, for
instance, on a rising islet,
it will be unoccupied; and a
single seed or egg will have
a good chance of succeeding.
Although there will always be
a struggle for life between the
individuals of the species, however
few, already occupying any pond,
yet as the number of kinds is
small, compared with those on
the land, the competition will
probably be less severe between
aquatic than between terrestrial
species; consequently an intruder
from the waters of a foreign
country, would have a better
chance of seizing on a place,
than in the case of terrestrial
colonists. We should, also, remember
that some, perhaps many, fresh-water
productions are low in the scale
of nature, and that we have reason
to believe that such low beings
change or become modified less
quickly than the high; and this
will give longer time than the
average for the migration of
the same aquatic species. We
should not forget the probability
of many species having formerly
ranged as continuously as fresh-water
productions ever can range, over
immense areas, and having subsequently
become extinct in intermediate
regions. But the wide distribution
of fresh-water plants and of
the lower animals, whether retaining
the same identical form or in
some degree modified, I believe
mainly depends on the wide dispersal
of their seeds and eggs by animals,
more especially by fresh-water
birds, which have large powers
of flight, and naturally travel
from one to another and often
distant piece of water. Nature,
like a careful gardener, thus
takes her seeds from a bed of
a particular nature, and drops
them in another equally well
fitted for them.
On the Inhabitants of Oceanic
Islands
We now come to the last of
the three classes of facts, which
I have selected as presenting
the greatest amount of difficulty,
on the view that all the individuals
both of the same and of allied
species have descended from a
single parent; and therefore
have all proceeded from a common
birthplace, notwithstanding that
in the course of time they have
come to inhabit distant points
of the globe. I have already
stated that I cannot honestly
admit Forbes's view on continental
extensions, which, if legitimately
followed out, would lead to the
belief that within the recent
period all existing islands have
been nearly or quite joined to
some continent. This view would
remove many difficulties, but
it would not, I think, explain
all the facts in regard to insular
productions. In the following
remarks I shall not confine myself
to the mere question of dispersal;
but shall consider some other
facts, which bear on the truth
of the two theories of independent
creation and of descent with
modification.
The species of all kinds which
inhabit oceanic islands are few
in number compared with those
on equal continental areas: Alph.
de Candolle admits this for plants,
and Wollaston for insects. If
we look to the large size and
varied stations of New Zealand,
extending over 780 miles of latitude,
and compare its flowering plants,
only 750 in number, with those
on an equal area at the Cape
of Good Hope or in Australia,
we must, I think, admit that
something quite independently
of any difference in physical
conditions has caused so great
a difference in number. Even
the uniform county of Cambridge
has 847 plants, and the little
island of Anglesea 764, but a
few ferns and a few introduced
plants are included in these
numbers, and the comparison in
some other respects is not quite
fair. We have evidence that the
barren island of Ascension aboriginally
possessed under half-a-dozen
flowering plants; yet many have
become naturalised on it, as
they have on New Zealand and
on every other oceanic island
which can be named. In St. Helena
there is reason to believe that
the naturalised plants and animals
have nearly or quite exterminated
many native productions. He who
admits the doctrine of the creation
of each separate species, will
have to admit, that a sufficient
number of the best adapted plants
and animals have not been created
on oceanic islands; for man has
unintentionally stocked them
from various sources far more
fully and perfectly than has
nature.
Although in oceanic islands
the number of kinds of inhabitants
is scanty, the proportion of
endemic species (i.e. those
found nowhere else in the world)
is often extremely large. If
we compare, for instance, the
number of the endemic land-shells
in Madeira, or of the endemic
birds in the Galapagos Archipelago,
with the number found on any
continent, and then compare the
area of the islands with that
of the continent, we shall see
that this is true. This fact
might have been expected on my
theory for, as already explained,
species occasionally arriving
after long intervals in a new
and isolated district, and having
to compete with new associates,
will be eminently liable to modification,
and will often produce groups
of modified descendants. But
it by no means follows, that,
because in an island nearly all
the species of one class are
peculiar, those of another class,
or of another section of the
same class, are peculiar; and
this difference seems to depend
on the species which do not become
modified having immigrated with
facility and in a body, so that
their mutual relations have not
been much disturbed. Thus in
the Galapagos Islands nearly
every land-bird, but only two
out of the eleven marine birds,
are peculiar; and it is obvious
that marine birds could arrive
at these islands more easily
than land-birds. Bermuda, on
the other hand, which lies at
about the same distance from
North America as the Galapagos
Islands do from South America,
and which has a very peculiar
soil, does not possess one endemic
land bird; and we know from Mr.
J. M. Jones's admirable account
of Bermuda, that very many North
American birds, during their
great annual migrations, visit
either periodically or occasionally
this island. Madeira does not
possess one peculiar bird, and
many European and African birds
are almost every year blown there,
as I am informed by Mr. E. V.
Harcourt. So that these two islands
of Bermuda and Madeira have been
stocked by birds, which for long
ages have struggled together
in their former homes, and have
become mutually adapted to each
other; and when settled in their
new homes, each kind will have
been kept by the others to their
proper places and habits, and
will consequently have been little
liable to modification. Madeira,
again, is inhabited by a wonderful
number of peculiar land-shells,
whereas not one species of sea-shell
is confined to its shores: now,
though we do not know how seashells
are dispersed, yet we can see
that their eggs or larvae, perhaps
attached to seaweed or floating
timber, or to the feet of wading-birds,
might be transported far more
easily than land-shells, across
three or four hundred miles of
open sea. The different orders
of insects in Madeira apparently
present analogous facts.
Oceanic islands are sometimes
deficient in certain classes,
and their places are apparently
occupied by the other inhabitants;
in the Galapagos Islands reptiles,
and in New Zealand gigantic wingless
birds, take the place of mammals.
In the plants of the Galapagos
Islands, Dr. Hooker has shown
that the proportional numbers
of the different orders are very
different from what they are
elsewhere. Such cases are generally
accounted for by the physical
conditions of the islands; but
this explanation seems to me
not a little doubtful. Facility
of immigration, I believe, has
been at least as important as
the nature of the conditions.
Many remarkable little facts
could be given with respect to
the inhabitants of remote islands.
For instance, in certain islands
not tenanted by mammals, some
of the endemic plants have beautifully
hooked seeds; yet few relations
are more striking than the adaptation
of hooked seeds for transportal
by the wool and fur of quadrupeds.
This case presents no difficulty
on my view, for a hooked seed
might be transported to an island
by some other means; and the
plant then becoming slightly
modified, but still retaining
its hooked seeds, would form
an endemic species, having as
useless an appendage as any rudimentary
organ, for instance, as the shrivelled
wings under the soldered elytra
of many insular beetles. Again,
islands often possess trees or
bushes belonging to orders which
elsewhere include only herbaceous
species; now trees, as Alph.
de Candolle has shown, generally
have, whatever the cause may
be, confined ranges. Hence trees
would be little likely to reach
distant oceanic islands; and
an herbaceous plant, though it
would have no chance of successfully
competing in stature with a fully
developed tree, when established
on an island and having to compete
with herbaceous plants alone,
might readily gain an advantage
by growing taller and taller
and overtopping the other plants.
If so, natural selection would
often tend to add to the stature
of herbaceous plants when growing
on an island, to whatever order
they belonged, and thus convert
them first into bushes and ultimately
into trees.
With respect to the absence
of whole orders on oceanic islands,
Bory St. Vincent long ago remarked
that Batrachians (frogs, toads,
newts) have never been found
on any of the many islands with
which the great oceans are studded.
I have taken pains to verify
this assertion, and I have found
it strictly true. I have, however,
been assured that a frog exists
on the mountains of the great
island of New Zealand; but I
suspect that this exception (if
the information be correct) may
be explained through glacial
agency. This general absence
of frogs, toads, and newts on
so many oceanic islands cannot
be accounted for by their physical
conditions; indeed it seems that
islands are peculiarly well fitted
for these animals; for frogs
have been introduced into Madeira,
the Azores, and Mauritius, and
have multiplied so as to become
a nuisance. But as these animals
and their spawn are known to
be immediately killed by sea-water,
on my view we can see that there
would be great difficulty in
their transportal across the
sea, and therefore why they do
not exist on any oceanic island.
But why, on the theory of creation,
they should not have been created
there, it would be very difficult
to explain.
Mammals offer
another and similar case. I
have carefully searched
the oldest voyages, but have
not finished my search; as yet
I have not found a single instance,
free from doubt, of a terrestrial
mammal (excluding domesticated
animals kept by the natives)
inhabiting an island situated
above 300 miles from a continent
or great continental island;
and many islands situated at
a much less distance are equally
barren. The Falkland Islands,
which are inhabited by a wolf-like
fox, come nearest to an exception;
but this group cannot be considered
as oceanic, as it lies on a bank
connected with the mainland;
moreover, icebergs formerly brought
boulders to its western shores,
and they may have formerly transported
foxes, as so frequently now happens
in the arctic regions. Yet it
cannot be said that small islands
will not support small mammals,
for they occur in many parts
of the world on very small islands,
if close to a continent; and
hardly an island can be named
on which our smaller quadrupeds
have not become naturalised and
greatly multiplied. It cannot
be said, on the ordinary view
of creation, that there has not
been time for the creation of
mammals; many volcanic islands
are sufficiently ancient, as
shown by the stupendous degradation
which they have suffered and
by their tertiary strata: there
has also been time for the production
of endemic species belonging
to other classes; and on continents
it is thought that mammals appear
and disappear at a quicker rate
than other and lower animals.
Though terrestrial mammals do
not occur on oceanic islands,
aërial mammals do occur
on almost every island. New Zealand
possesses two bats found nowhere
else in the world: Norfolk Island,
the Viti Archipelago, the Bonin
Islands, the Caroline and Marianne
Archipelagoes, and Mauritius,
all possess their peculiar bats.
Why, it may be asked, has the
supposed creative force produced
bats and no other mammals on
remote islands? On my view this
question can easily be answered;
for no terrestrial mammal can
be transported across a wide
space of sea, but bats can fly
across. Bats have been seen wandering
by day far over the Atlantic
Ocean; and two North American
species either regularly or occasionally
visit Bermuda, at the distance
of 600 miles from the mainland.
I hear from Mr. Tomes, who has
specially studied this family,
that many of the same species
have enormous ranges, and are
found on continents and on far
distant islands. Hence we have
only to suppose that such wandering
species have been modified through
natural selection in their new
homes in relation to their new
position, and we can understand
the presence of endemic bats
on islands, with the absence
of all terrestrial mammals.
Besides the absence of terrestrial
mammals in relation to the remoteness
of islands from continents, there
is also a relation, to a certain
extent independent of distance,
between the depth of the sea
separating an island from the
neighbouring mainland, and the
presence in both of the same
mammiferous species or of allied
species in a more or less modified
condition. Mr. Windsor Earl has
made some striking observations
on this head in regard to the
great Malay Archipelago, which
is traversed near Celebes by
a space of deep ocean; and this
space separates two widely distinct
mammalian faunas. On either side
the islands are situated on moderately
deep submarine banks, and they
are inhabited by closely allied
or identical quadrupeds. No doubt
some few anomalies occur in this
great archipelago, and there
is much difficulty in forming
a judgment in some cases owing
to the probable naturalisation
of certain mammals through man's
agency; but we shall soon have
much light thrown on the natural
history of this archipelago by
the admirable zeal and researches
of Mr Wallace. I have not as
yet had time to follow up this
subject in all other quarters
of the world; but as far as I
have gone, the relation generally
holds good. We see Britain separated
by a shallow channel from Europe,
and the mammals are the same
on both sides; we meet with analogous
facts on many islands separated
by similar channels from Australia.
The West Indian Islands stand
on a deeply submerged bank, nearly
1000 fathoms in depth, and here
we find American forms, but the
species and even the genera are
distinct. As the amount of modification
in all cases depends to a certain
degree on the lapse of time,
and as during changes of level
it is obvious that islands separated
by shallow channels are more
likely to have been continuously
united within a recent period
to the mainland than islands
separated by deeper channels,
we can understand the frequent
relation between the depth of
the sea and the degree of affinity
of the mammalian inhabitants
of islands with those of a neighbouring
continent, an explicable relation
on the view of independent acts
of creation.
All the foregoing
remarks on the inhabitants
of oceanic islands,
namely, the scarcity of kinds
-- the richness in endemic forms
in particular classes or sections
of classes, the absence of whole
groups, as of batrachians, and
of terrestrial mammals notwithstanding
the presence of aërial bats,
the singular proportions of certain
orders of plants, herbaceous
forms having been developed into
trees, &c., seem to me to
accord better with the view of
occasional means of transport
having been largely efficient
in the long course of time, than
with the view of all our oceanic
islands having been formerly
connected by continuous land
with the nearest continent; for
on this latter view the migration
would probably have been more
complete; and if modification
be admitted, all the forms of
life would have been more equally
modified, in accordance with
the paramount importance of the
relation of organism to organism.
I do not deny that there are
many and grave difficulties in
understanding how several of
the inhabitants of the more remote
islands, whether still retaining
the same specific form or modified
since their arrival, could have
reached their present homes.
But the probability of many islands
having existed as halting-places,
of which not a wreck now remains,
must not be overlooked. I will
here give a single instance of
one of the cases of difficulty.
Almost all oceanic islands, even
the most isolated and smallest,
are inhabited by land-shells,
generally by endemic species,
but sometimes by species found
elsewhere. Dr. Aug. A. Gould
has given several interesting
cases in regard to the land-shells
of the islands of the Pacific.
Now it is notorious that land-shells
are very easily killed by salt;
their eggs, at least such as
I have tried, sink in sea-water
and are killed by it. Yet there
must be, on my view, some unknown,
but highly efficient means for
their transportal. Would the
just-hatched young occasionally
crawl on and adhere to the feet
of birds roosting on the ground,
and thus get transported? It
occurred to me that land-shells,
when hybernating and having a
membranous diaphragm over the
mouth of the shell, might be
floated in chinks of drifted
timber across moderately wide
arms of the sea. And I found
that several species did in this
state withstand uninjured an
immersion in sea-water during
seven days: one of these shells
was the Helix pomatia, and after
it had again hybernated I put
it in sea-water for twenty days,
and it perfectly recovered. As
this species has a thick calcareous
operculum, I removed it, and
when it had formed a new membranous
one, I immersed it for fourteen
days in sea-water, and it recovered
and crawled away: but more experiments
are wanted on this head.
The most striking and important
fact for us in regard to the
inhabitants of islands, is their
affinity to those of the nearest
mainland, without being actually
the same species. Numerous instances
could be given of this fact.
I will give only one, that of
the Galapagos Archipelago, situated
under the equator, between 500
and 600 miles from the shores
of South America. Here almost
every product of the land and
water bears the unmistakeable
stamp of the American continent.
There are twenty-six land birds,
and twenty-five of those are
ranked by Mr Gould as distinct
species, supposed to have been
created here; yet the close affinity
of most of these birds to American
species in every character, in
their habits, gestures, and tones
of voice, was manifest. So it
is with the other animals, and
with nearly all the plants, as
shown by Dr. Hooker in his admirable
memoir on the Flora of this archipelago.
The naturalist, looking at the
inhabitants of these volcanic
islands in the Pacific, distant
several hundred miles from the
continent, yet feels that he
is standing on American land.
Why should this be so? why should
the species which are supposed
to have been created in the Galapagos
Archipelago, and nowhere else,
bear so plain a stamp of affinity
to those created in America?
There is nothing in the conditions
of life, in the geological nature
of the islands, in their height
or climate, or in the proportions
in which the several classes
are associated together, which
resembles closely the conditions
of the South American coast:
in fact there is a considerable
dissimilarity in all these respects.
On the other hand, there is a
considerable degree of resemblance
in the volcanic nature of the
soil, in climate, height, and
size of the islands, between
the Galapagos and Cape de Verde
Archipelagos: but what an entire
and absolute difference in their
inhabitants! The inhabitants
of the Cape de Verde Islands
are related to those of Africa,
like those of the Galapagos to
America. I believe this grand
fact can receive no sort of explanation
on the ordinary view of independent
creation; whereas on the view
here maintained, it is obvious
that the Galapagos Islands would
be likely to receive colonists,
whether by occasional means of
transport or by formerly continuous
land, from America; and the Cape
de Verde Islands from Africa;
and that such colonists would
be liable to modifications; the
principle of inheritance still
betraying their original birthplace.
Many analogous facts could
be given: indeed it is an almost
universal rule that the endemic
productions of islands are related
to those of the nearest continent,
or of other near islands. The
exceptions are few, and most
of them can be explained. Thus
the plants of Kerguelen Land,
though standing nearer to Africa
than to America, are related,
and that very closely, as we
know from Dr. Hooker's account,
to those of America: but on the
view that this island has been
mainly stocked by seeds brought
with earth and stones on icebergs,
drifted by the prevailing currents,
this anomaly disappears. New
Zealand in its endemic plants
is much more closely related
to Australia, the nearest mainland,
than to any other region: and
this is what might have been
expected; but it is also plainly
related to South America, which,
although the next nearest continent,
is so enormously remote, that
the fact becomes an anomaly.
But this difficulty almost disappears
on the view that both New Zealand,
South America, and other southern
lands were long ago partially
stocked from a nearly intermediate
though distant point, namely
from the antarctic islands, when
they were clothed with vegetation,
before the commencement of the
Glacial period. The affinity,
which, though feeble, I am assured
by Dr. Hooker is real, between
the flora of the south-western
corner of Australia and of the
Cape of Good Hope, is a far more
remarkable case, and is at present
inexplicable: but this affinity
is confined to the plants, and
will, I do not doubt, be some
day explained.
The law which
causes the inhabitants of an
archipelago, though specifically
distinct, to be closely allied
to those of the nearest continent,
we sometimes see displayed on
a small scale, yet in a most
interesting manner, within the
limits of the same archipelago.
Thus the several islands of the
Galapagos Archipelago are tenanted,
as I have elsewhere shown, in
a quite marvellous manner, by
very closely related species;
so that the inhabitants of each
separate island, though mostly
distinct, are related in an incomparably
closer degree to each other than
to the inhabitants of any other
part of the world. And this is
just what might have been expected
on my view, for the islands are
situated so near each other that
they would almost certainly receive
immigrants from the same original
source, or from each other. But
this dissimilarity between the
endemic inhabitants of the islands
may be used as an argument against
my views; for it may be asked,
how has it happened in the several
islands situated within sight
of each other, having the same
geological nature, the same height,
climate, &c., that many of
the immigrants should have been
differently modified, though
only in a small degree. This
long appeared to me a great difficulty:
but it arises in chief part from
the deeply-seated error of considering
the physical conditions of a
country as the most important
for its inhabitants; whereas
it cannot, I think, be disputed
that the nature of the other
inhabitants, with which each
has to compete, is at least as
important, and generally a far
more important element of success.
Now if we look to those inhabitants
of the Galapagos Archipelago
which are found in other parts
of the world (having on one side
for the moment the endemic species,
which cannot be here fairly included,
as we are considering how they
have come to be modified since
their arrival), we find a considerable
amount of difference in the several
islands. This difference might
indeed have been expected on
the view of the islands having
been stocked by occasional means
of transport a seed, for instance,
of one plant having been brought
to one island, and that of another
plant to another island. Hence
when in former times an immigrant
settled on any one or more of
the islands, or when it subsequently
spread from one island to another,
it would undoubtedly be exposed
to different conditions of life
in the different islands, for
it would have to compete with
different sets of organisms:
a plant, for instance, would
find the best-fitted ground more
perfectly occupied by distinct
plants in one island than in
another, and it would be exposed
to the attacks of somewhat different
enemies. If then it varied, natural
selection would probably favour
different varieties in the different
islands. Some species, however,
might spread and yet retain the
same character throughout the
group, just as we see on continents
some species' spreading widely
and remaining the same.
The really surprising fact
in this case of the Galapagos
Archipelago, and in a lesser
degree in some analogous instances,
is that the new species formed
in the separate islands have
not quickly spread to the other
islands. But the islands, though
in sight of each other, are separated
by deep arms of the sea, in most
cases wider than the British
Channel, and there is no reason
to suppose that they have at
any former period been continuously
united. The currents of the sea
are rapid and sweep across the
archipelago, and gales of wind
are extraordinarily rare; so
that the islands are far more
effectually separated from each
other than they appear to be
on a map. Nevertheless a good
many species, both those found
in other parts of the world and
those confined to the archipelago,
are common to the several islands,
and we may infer from certain
facts that these have probably
spread from some one island to
the others. But we often take,
I think, an erroneous view of
the probability of closely allied
species invading each other's
territory, when put into free
intercommunication. Undoubtedly
if one species has any advantage
whatever over another, it will
in a very brief time wholly or
in part supplant it; but if both
are equally well fitted for their
own places in nature, both probably
will hold their own places and
keep separate for almost any
length of time. Being familiar
with the fact that many species,
naturalised through man's agency,
have spread with astonishing
rapidity over new countries,
we are apt to infer that most
species would thus spread; but
we should remember that the forms
which become naturalised in new
countries are not generally closely
allied to the aboriginal inhabitants,
but are very distinct species,
belonging in a large proportion
of cases, as shown by Alph. de
Candolle, to distinct genera.
In the Galapagos Archipelago,
many even of the birds, though
so well adapted for flying from
island to island, are distinct
on each; thus there are three
closely-allied species of mocking-thrush,
each confined to its own island.
Now let us suppose the mocking-thrush
of Chatham Island to be blown
to Charles Island, which has
its own mocking-thrush: why should
it succeed in establishing itself
there? We may safely infer that
Charles Island is well stocked
with its own species, for annually
more eggs are laid there than
can possibly be reared; and we
may infer that the mocking-thrush
peculiar to Charles Island is
at least as well fitted for its
home as is the species peculiar
to Chatham Island. Sir C. Lyell
and Mr. Wollaston have communicated
to me a remarkable fact bearing
on this subject; namely, that
Madeira and the adjoining islet
of Porto Santo possess many distinct
but representative land-shells,
some of which live in crevices
of stone; and although large
quantities of stone are annually
transported from Porto Santo
to Madeira, yet this latter island
has not become colonised by the
Porto Santo species: nevertheless
both islands have been colonised
by some European land-shells,
which no doubt had some advantage
over the indigenous species.
From these considerations I think
we need not greatly marvel at
the endemic and representative
species, which inhabit the several
islands of the Galapagos Archipelago,
not having universally spread
from island to island. In many
other instances, as in the several
districts of the same continent,
pre-occupation has probably played
an important part in checking
the commingling of species under
the same conditions of life.
Thus, the south-east and south-west
corners of Australia have nearly
the same physical conditions,
and are united by continuous
land, yet they are inhabited
by a vast number of distinct
mammals, birds, and plants.
The principle
which determines the general
character of the
fauna and flora of oceanic islands,
namely, that the inhabitants,
when not identically the same,
yet are plainly related to the
inhabitants of that region whence
colonists could most readily
have been derived, the colonists
having been subsequently modified
and better fitted to their new
homes, is of the widest application
throughout nature. We see this
on every mountain, in every lake
and marsh. For Alpine species,
excepting in so far as the same
forms, chiefly of plants, have
spread widely throughout the
world during the recent Glacial
epoch, are related to those of
the surrounding lowlands; thus
we have in South America, Alpine
humming-birds, Alpine rodents,
Alpine plants, &c., all of
strictly American forms, and
it is obvious that a mountain,
as it became slowly upheaved,
would naturally be colonised
from the surrounding lowlands.
So it is with the inhabitants
of lakes and marshes, excepting
in so far as great facility of
transport has given the same
general forms to the whole world.
We see this same principle in
the blind animals inhabiting
the caves of America and of Europe.
Other analogous facts could be
given. And it will, I believe,
be universally found to be true,
that wherever in two regions,
let them be ever so distant,
many closely allied or representative
species occur, there will likewise
be found some identical species,
showing, in accordance with the
foregoing view, that at some
former period there has been
intercommunication or migration
between the two regions. And
wherever many closely-allied
species occur, there will be
found many forms which some naturalists
rank as distinct species, and
some as varieties; these doubtful
forms showing us the steps in
the process of modification.
This relation between the power
and extent of migration of a
species, either at the present
time or at some former period
under different physical conditions,
and the existence at remote points
of the world of other species
allied to it, is shown in another
and more general way. Mr. Gould
remarked to me long ago, that
in those genera of birds which
range over the world, many of
the species have very wide ranges.
I can hardly doubt that this
rule is generally true, though
it would be difficult to prove
it. Amongst mammals, we see it
strikingly displayed in Bats,
and in a lesser degree in the
Felidae and Canidae. We see it,
if we compare the distribution
of butterflies and beetles. So
it is with most fresh-water productions,
in which so many genera range
over the world, and many individual
species have enormous ranges.
It is not meant that in world-ranging
genera all the species have a
wide range, or even that they
have on an average a wide range;
but only that some of the species
range very widely; for the facility
with which widely-ranging species
vary and give rise to new forms
will largely determine their
average range. For instance,
two varieties of the same species
inhabit America and Europe, and
the species thus has an immense
range; but, if the variation
had been a little greater, the
two varieties would have been
ranked as distinct species, and
the common range would have been
greatly reduced. Still less is
it meant, that a species which
apparently has the capacity of
crossing barriers and ranging
widely, as in the case of certain
powerfully-winged birds, will
necessarily range widely; for
we should never forget that to
range widely implies not only
the power of crossing barriers,
but the more important power
of being victorious in distant
lands in the struggle for life
with foreign associates. But
on the view of all the species
of a genus having descended from
a single parent, though now distributed
to the most remote points of
the world, we ought to find,
and I believe as a general rule
we do find, that some at least
of the species range very widely;
for it is necessary that the
unmodified parent should range
widely, undergoing modification
during its diffusion, and should
place itself under diverse conditions
favourable for the conversion
of its offspring, firstly into
new varieties and ultimately
into new species.
In considering the wide distribution
of certain genera, we should
bear in mind that some are extremely
ancient, and must have branched
off from a common parent at a
remote epoch; so that in such
cases there will have been ample
time for great climatal and geographical
changes and for accidents of
transport; and consequently for
the migration of some of the
species into all quarters of
the world, where they may have
become slightly modified in relation
to their new conditions. There
is, also, some reason to believe
from geological evidence that
organisms low in the scale within
each great class, generally change
at a slower rate than the higher
forms; and consequently the lower
forms will have had a better
chance of ranging widely and
of still retaining the same specific
character. This fact, together
with the seeds and eggs of many
low forms being very minute and
better fitted for distant transportation,
probably accounts for a law which
has long been observed, and which
has lately been admirably discussed
by Alph. de Candolle in regard
to plants, namely, that the lower
any group of organisms is, the
more widely it is apt to range.
The relations just discussed,
namely, low and slowly-changing
organisms ranging more widely
than the high, some of the species
of widely-ranging genera themselves
ranging widely, such facts, as
alpine, lacustrine, and marsh
productions being related (with
the exceptions before specified)
to those on the surrounding low
lands and dry lands, though these
stations are so different the
very close relation of the distinct
species which inhabit the islets
of the same archipelago, and
especially the striking relation
of the inhabitants of each whole
archipelago or island to those
of the nearest mainland, are,
I think, utterly inexplicable
on the ordinary view of the independent
creation of each species, but
are explicable on the view of
colonisation from the nearest
and readiest source, together
with the subsequent modification
and better adaptation of the
colonists to their new homes.
Summary of last and present
Chapters
In these chapters I have endeavoured
to show, that if we make due
allowance for our ignorance of
the full effects of all the changes
of climate and of the level of
the land, which have certainly
occurred within the recent period,
and of other similar changes
which may have occurred within
the same period; if we remember
how profoundly ignorant we are
with respect to the many and
curious means of occasional transport,
a subject which has hardly ever
been properly experimentised
on; if we bear in mind how often
a species may have ranged continuously
over a wide area, and then have
become extinct in the intermediate
tracts, I think the difficulties
in believing that all the individuals
of the same species, wherever
located, have descended from
the same parents, are not insuperable.
And we are led to this conclusion,
which has been arrived at by
many naturalists under the designation
of single centres of creation,
by some general considerations,
more especially from the importance
of barriers and from the analogical
distribution of sub-genera, genera,
and families.
With respect to the distinct
species of the same genus, which
on my theory must have spread
from one parent-source; if we
make the same allowances as before
for our ignorance, and remember
that some forms of life change
most slowly, enormous periods
of time being thus granted for
their migration, I do not think
that the difficulties are insuperable;
though they often are in this
case, and in that of the individuals
of the same species, extremely
grave.
As exemplifying the effects
of climatal changes on distribution,
I have attempted to show how
important has been the influence
of the modern Glacial period,
which I am fully convinced simultaneously
affected the whole world, or
at least great meridional belts.
As showing how diversified are
the means of occasional transport,
I have discussed at some little
length the means of dispersal
of fresh-water productions.
If the difficulties be not
insuperable in admitting that
in the long course of time the
individuals of the same species,
and likewise of allied species,
have proceeded from some one
source; then I think all the
grand leading facts of geographical
distribution are explicable on
the theory of migration (generally
of the more dominant forms of
life), together with subsequent
modification and the multiplication
of new forms. We can thus understand
the high importance of barriers,
whether of land or water, which
separate our several zoological
and botanical provinces. We can
thus understand the localisation
of sub-genera, genera, and families;
and how it is that under different
latitudes, for instance in South
America, the inhabitants of the
plains and mountains, of the
forests, marshes, and deserts,
are in so mysterious a manner
linked together by affinity,
and are likewise linked to the
extinct beings which formerly
inhabited the same continent.
Bearing in mind that the mutual
relations of organism to organism
are of the highest importance,
we can see why two areas having
nearly the same physical conditions
should often be inhabited by
very different forms of life;
for according to the length of
time which has elapsed since
new inhabitants entered one region;
according to the nature of the
communication which allowed certain
forms and not others to enter,
either in greater or lesser numbers;
according or not, as those which
entered happened to come in more
or less direct competition with
each other and with the aborigines;
and according as the immigrants
were capable of varying more
or less rapidly, there would
ensue in different regions, independently
of their physical conditions,
infinitely diversified conditions
of life, there would be an almost
endless amount of organic action
and reaction, and we should find,
as we do find, some groups of
beings greatly, and some only
slightly modified, some developed
in great force, some existing
in scanty numbers in the different
great geographical provinces
of the world.
On these same
principles, we can understand,
as I have endeavoured
to show, why oceanic islands
should have few inhabitants,
but of these a great number should
be endemic or peculiar; and why,
in relation to the means of migration,
one group of beings, even within
the same class, should have all
its species endemic, and another
group should have all its species
common to other quarters of the
world. We can see why whole groups
of organisms, as batrachians
and terrestrial mammals, should
be absent from oceanic islands,
whilst the most isolated islands
possess their own peculiar species
of aërial mammals or bats.
We can see why there should be
some relation between the presence
of mammals, in a more or less
modified condition, and the depth
of the sea between an island
and the mainland. We can clearly
see why all the inhabitants of
an archipelago, though specifically
distinct on the several islets,
should be closely related to
each other, and likewise be related,
but less closely, to those of
the nearest continent or other
source whence immigrants were
probably derived. We can see
why in two areas, however distant
from each other, there should
be a correlation, in the presence
of identical species, of varieties,
of doubtful species, and of distinct
but representative species.
As the late Edward Forbes often
insisted, there is a striking
parallelism in the laws of life
throughout time and space: the
laws governing the succession
of forms in past times being
nearly the same with those governing
at the present time the differences
in different areas. We see this
in many facts. The endurance
of each species and group of
species is continuous in time;
for the exceptions to the rule
are so few, that they may fairly
be attributed to our not having
as yet discovered in an intermediate
deposit the forms which are therein
absent, but which occur above
and below: so in space, it certainly
is the general rule that the
area inhabited by a single species,
or by a group of species, is
continuous; and the exceptions,
which are not rare, may, as I
have attempted to show, be accounted
for by migration at some former
period under different conditions
or by occasional means of transport,
and by the species having become
extinct in the intermediate tracts.
Both in time and space, species
and groups of species have their
points of maximum development.
Groups of species, belonging
either to a certain period of
time, or to a certain area, are
often characterised by trifling
characters in common, as of sculpture
or colour. In looking to the
long succession of ages, as in
now looking to distant provinces
throughout the world, we find
that some organisms differ little,
whilst others belonging to a
different class, or to a different
order, or even only to a different
family of the same order, differ
greatly. In both time and space
the lower members of each class
generally change less than the
higher; but there are in both
cases marked exceptions to the
rule. On my theory these several
relations throughout time and
space are intelligible; for whether
we look to the forms of life
which have changed during successive
ages within the same quarter
of the world, or to those which
have changed after having migrated
into distant quarters, in both
cases the forms within each class
have been connected by the same
bond of ordinary generation;
and the more nearly any two forms
are related in blood, the nearer
they will generally stand to
each other in time and space;
in both cases the laws of variation
have been the same, and modifications
have been accumulated by the
same power of natural selection. |