ダーウィン『種の起源』第12・13章、第6版にもとづく
CHAPTER XII. GEOGRAPHICAL DISTRIBUTION. from "THE ORIGIN OF SPECIES BY MEANS OF NATURAL SELECTION or THE PRESERVATION OF FAVOURED RACES IN THE STRUGGLE FOR LIFE." by By Charles Darwin, M.A., F.R.S., Sixth London Edition. source: http://www.gutenberg.org/files/2009/2009-h/2009-h.htm
Present
distribution cannot be accounted for by differences in physical
conditions�\Importance of barriers�\Affinity of the productions of the
same continent�\Centres of creation�\Means of dispersal by changes of
climate and of the level of the land, and by occasional
means�\Dispersal
during the Glacial period�\Alternate Glacial periods in the North and
South.
In considering the distribution of organic beings over the face of the
globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various regions
can be wholly accounted for by climatal and other physical conditions.
Of late, almost every author who has studied the subject has come to
this conclusion. The case of America alone would almost suffice to
prove its truth; for if we exclude the arctic and northern temperate
parts, all authors agree that one of the most fundamental divisions in
geographical distribution is that between the New and Old Worlds; yet
if we travel over the vast American continent, from the central parts
of the United States to its extreme southern point, we meet with the
most diversified conditions; humid districts, arid deserts, lofty
mountains, grassy plains, forests, marshes, lakes and great rivers,
under almost every temperature. There is hardly a climate or condition
in the Old World which cannot be paralleled in the New�\at least so
closely as the same species generally require. No doubt small areas can
be pointed out in the Old World hotter than any in the New World; but
these are not inhabited by a fauna different from that of the
surrounding districts; for it is rare to find a group of organisms
confined to a small area, of which the conditions are peculiar in only
a slight degree. Notwithstanding this general parallelism in the
conditions of Old and New Worlds, how widely different are their living
productions!
In the southern hemisphere, if we compare large tracts of land in
Australia, South Africa, and western South America, between latitudes
25 and 35 degrees, we shall find parts extremely similar in all their
conditions, yet it would not be possible to point out three faunas and
floras more utterly dissimilar. Or, again, we may compare the
productions of South America south of latitude 35 degrees with those
north of 25 degrees, which consequently are separated by a space of ten
degrees of latitude, and are exposed to considerably different
conditions; yet they are incomparably more closely related to each
other than they are to the productions of Australia or Africa under
nearly the same climate. Analogous facts could be given with respect to
the inhabitants of the sea.
A second great fact which strikes us in our general review is, that
barriers of any kind, or obstacles to free migration, are related in a
close and important manner to the differences between the productions
of various regions. We see this in the great difference in nearly all
the terrestrial productions of the New and Old Worlds, excepting in the
northern parts, where the land almost joins, and where, under a
slightly different climate, there might have been free migration for
the northern temperate forms, as there now is for the strictly arctic
productions. We see the same fact in the great difference between the
inhabitants of Australia, Africa, and South America under the same
latitude; for these countries are almost as much isolated from each
other as is possible. On each continent, also, we see the same fact;
for on the opposite sides of lofty and continuous mountain-ranges, and
of great deserts and even of large rivers, we find different
productions; though as mountain chains, deserts, etc., are not as
impassable, or likely to have endured so long, as the oceans separating
continents, the differences are very inferior in degree to those
characteristic of distinct continents.
Turning to the sea, we find the same law. The marine inhabitants of the
eastern and western shores of South America are very distinct, with
extremely few shells, crustacea, or echinodermata in common; but Dr.
Gunther has recently shown that about thirty per cent of the fishes are
the same on the opposite sides of the isthmus of Panama; and this fact
has led naturalists to believe that the isthmus was formerly open.
Westward of the shores of America, a wide space of open ocean extends,
with not an island as a halting-place for emigrants; here we have a
barrier of another kind, and as soon as this is passed we meet in the
eastern islands of the Pacific with another and totally distinct fauna.
So that three marine faunas range northward and southward in parallel
lines not far from each other, under corresponding climate; but from
being separated from each other by impassable barriers, either of land
or open sea, they are almost wholly distinct. On the other hand,
proceeding still 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, or continuous coasts,
until, after travelling over a hemisphere, we come to the shores of
Africa; and over this vast space we meet with no well-defined and
distinct marine faunas. Although so few marine animals are common to
the above-named three approximate faunas of Eastern and Western America
and the eastern Pacific islands, yet many fishes range from the Pacific
into the Indian Ocean, and many shells are common to the eastern
islands of the Pacific and the eastern shores of Africa on almost
exactly opposite meridians of longitude.
A third great fact, partly included in the foregoing statement, is the
affinity of the productions of the same continent or of the same sea,
though the species themselves are distinct at different points and
stations. It is a law of the widest generality, and every continent
offers innumerable instances. Nevertheless, the naturalist, in
travelling, for instance, from north to south, never fails to be struck
by the manner in which successive groups of beings, specifically
distinct, though nearly related, replace each other. He hears from
closely allied, yet distinct kinds of birds, notes nearly similar, and
sees their nests similarly constructed, but not quite alike, with eggs
coloured in nearly the same manner. The plains near the Straits of
Magellan are inhabited by one species of Rhea (American ostrich), and
northward the plains of La Plata by another species of the same genus;
and not by a true ostrich or emu, like those inhabiting Africa and
Australia under the same latitude. On these same plains of La Plata we
see the agouti and bizcacha, animals having nearly the same habits as
our hares and rabbits, and belonging to the same order of Rodents, but
they plainly display an American type of structure. We ascend the lofty
peaks of the Cordillera, and we find an alpine species of bizcacha; we
look to the waters, and we do not find the beaver or muskrat, but the
coypu and capybara, rodents of the South American type. Innumerable
other instances could be given. If we look to the islands off the
American shore, however much they may differ in geological structure,
the inhabitants are essentially American, though they may be all
peculiar species. We may look back to past ages, as shown in the last
chapter, and we find American types then prevailing on the American
continent and in the American seas. We see in these facts some deep
organic bond, throughout space and time, over the same areas of land
and water, independently of physical conditions. The naturalist must be
dull who is not led to inquire what this bond is.
The bond is simply inheritance, that cause which alone, as far as we
positively know, produces organisms quite like each other, or, as we
see in the case of varieties, nearly alike. The dissimilarity of the
inhabitants of different regions may be attributed to modification
through variation and natural selection, and probably in a subordinate
degree to the definite influence of different physical conditions. The
degrees of dissimilarity will depend on the migration of the more
dominant forms of life from one region into another having been more or
less effectually prevented, at periods more or less remote�\on the
nature and number of the former immigrants�\and on the action of the
inhabitants on each other in leading to the preservation of different
modifications; the relation of organism to organism in the struggle for
life being, as I have already often remarked, the most important of all
relations. Thus the high importance of barriers comes into play by
checking migration; as does time for the slow process of modification
through natural selection. Widely-ranging species, abounding in
individuals, which have already triumphed over many competitors in
their own widely-extended homes, will have the best chance of seizing
on new places, when they spread out into new countries. In their new
homes they will be exposed to new conditions, and will frequently
undergo further modification and improvement; and thus they will become
still further victorious, and will produce groups of modified
descendants. On this principle of inheritance with modification we can
understand how it is that sections of genera, whole genera, and even
families, are confined to the same areas, as is so commonly and
notoriously the case.
There is no evidence, as was remarked in the last chapter, of the
existence of any law of necessary development. As the variability of
each species is an independent property, and will be taken advantage of
by natural selection, only so far as it profits each individual in its
complex struggle for life, so the amount of modification in different
species will be no uniform quantity. If a number of species, after
having long competed with each other in their old home, were to migrate
in a body into a new and afterwards isolated country, they would be
little liable to modification; for neither migration nor isolation in
themselves effect anything. These principles come into play only by
bringing organisms into new relations with each other and in a lesser
degree with the surrounding physical conditions. As we have seen in the
last chapter that some forms have retained nearly the same character
from an enormously remote geological period, so certain species have
migrated over vast spaces, and have not become greatly or at all
modified.
According to these views, it is obvious that the several species of the
same genus, though inhabiting the most distant quarters of the world,
must originally have proceeded from the same source, as they are
descended from the same progenitor. In the case of those species which
have undergone, during whole geological periods, little modification,
there is not much difficulty in believing that they have migrated from
the same region; for during the vast geographical and climatical
changes which have supervened since ancient times, almost any amount of
migration is possible. But in many other cases, in which we have reason
to believe that the species of a genus have been produced within
comparatively recent times, there is great difficulty on this head. It
is also obvious that the individuals of the same species, though now
inhabiting distant and isolated regions, must have proceeded from one
spot, where their parents were first produced: for, as has been
explained, it is incredible that individuals identically the same
should have been produced from parents specifically distinct.
SINGLE CENTRES OF SUPPOSED CREATION.
We are thus brought to the question which has been largely discussed by
naturalists, namely, whether species have been created at one or more
points of the earth's surface. Undoubtedly there are many cases of
extreme difficulty in understanding how the same species could possibly
have migrated from some one point to the several distant and isolated
points, where now found. Nevertheless the simplicity of the view that
each species was first produced within a single region captivates the
mind. He who rejects it, rejects the vera causa of ordinary generation
with subsequent migration, and calls in the agency of a miracle. It is
universally admitted, that in most cases the area inhabited by a
species is continuous; and that when a plant or animal inhabits two
points so distant from each other, or with an interval of such a
nature, that the space could not have been easily passed over by
migration, the fact is given as something remarkable and exceptional.
The incapacity of migrating across a wide sea is more clear in the case
of terrestrial mammals than perhaps with any other organic beings; and,
accordingly, we find no inexplicable instances of the same mammals
inhabiting distant points of the world. No geologist feels any
difficulty in Great Britain possessing the same quadrupeds with the
rest of Europe, for they were no doubt once united. But if the same
species can be produced at two separate points, why do we not find a
single mammal common to Europe and Australia or South America? The
conditions of life are nearly the same, so that a multitude of European
animals and plants have become naturalised in America and Australia;
and some of the aboriginal plants are identically the same at these
distant points of the northern and southern hemispheres? The answer, as
I believe, is, that mammals have not been able to migrate, whereas some
plants, from their varied means of dispersal, have migrated across the
wide and broken interspaces. The great and striking influence of
barriers of all kinds, is intelligible only on the view that the great
majority of species have been produced on one side, and have not been
able to migrate to the opposite side. Some few families, many
subfamilies, very many genera, a still greater number of sections of
genera, are confined to a single region; and it has been observed by
several naturalists that the most natural genera, or those genera in
which the species are most closely related to each other, are generally
confined to the same country, or if they have a wide range that their
range is continuous. What a strange anomaly it would be if a directly
opposite rule were to prevail when we go down one step lower in the
series, namely to the individuals of the same species, and these had
not been, at least at first, confined to some one region!
Hence, it seems to me, as it has to many other naturalists, that the
view of each species having been produced in one area alone, and having
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 climatical changes which have certainly occurred
within recent geological times, must have rendered discontinuous the
formerly continuous range of many species. So that we are reduced to
consider whether the exceptions to continuity of range are so numerous,
and of so grave a nature, that we ought to give up the belief, rendered
probable by general considerations, that each species has been produced
within one area, and has migrated thence as far as it could. It would
be hopelessly tedious to discuss all the exceptional cases of the same
species, now living at distant and separated points; nor do I for a
moment pretend that any explanation could be offered of many instances.
But, after some preliminary remarks, I will discuss a few of the most
striking classes of facts, namely, the existence of the same species on
the summits of distant mountain ranges, and at distant points in the
Arctic and Antarctic regions; and secondly (in the following chapter),
the wide distribution of fresh water productions; and thirdly, the
occurrence of the same terrestrial species on islands and on the
nearest mainland, though separated by hundreds of miles of open sea. If
the existence of the same species at distant and isolated points of the
earth's surface can in many instances be explained on the view of each
species having migrated from a single birthplace; then, considering our
ignorance with respect to former climatical and geographical changes,
and to the various occasional means of transport, the belief that a
single birthplace is the law seems to me incomparably the safest.
In discussing this subject we shall be enabled at the same time to
consider a point equally important for us, namely, whether the several
species of a genus which must on our theory all be descended from a
common progenitor, can have migrated, undergoing modification during
their migration from some one area. If, when most of the species
inhabiting one region are different from those of another region,
though closely allied to them, it can be shown that migration from the
one region to the other has probably occurred at some former period,
our general view will be much strengthened; for the explanation is
obvious on the principle of descent with modification. A volcanic
island, for instance, upheaved and formed at the distance of a few
hundreds of miles from a continent, would probably receive from it in
the course of time a few colonists, and their descendants, though
modified, would still be related by inheritance to the inhabitants of
that continent. Cases of this nature are common, and are, as we shall
hereafter see, inexplicable on the theory of independent creation. This
view of the relation of the species of one region to those of another,
does not differ much from that advanced by Mr. Wallace, who concludes
that "every species has come into existence coincident both in space
and time with a pre-existing closely allied species." And it is now
well known that he attributes this coincidence to descent with
modification.
The question of single or multiple centres of creation differs from
another though allied question, namely, whether all the individuals of
the same species are descended from a single pair, or single
hermaphrodite, or whether, as some authors suppose, from many
individuals simultaneously created. With organic beings which never
intercross, if such exist, each species, must be descended from a
succession of modified varieties, that have supplanted each other, but
have never blended with other individuals or varieties of the same
species, so that, at each successive stage of modification, all the
individuals of the same form will be descended from a single parent.
But in the great majority of cases, namely, with all organisms which
habitually unite for each birth, or which occasionally intercross, the
individuals of the same species inhabiting the same area will be kept
nearly uniform by intercrossing; so that many individuals will go on
simultaneously changing, and the whole amount of modification at each
stage will not be due to descent from a single parent. To illustrate
what I mean: our English race-horses differ from the horses of every
other breed; but they do not owe their difference and superiority to
descent from any single pair, but to continued care in the selecting
and training of many individuals during each generation.
Before discussing the three classes of facts, which I have selected as
presenting the greatest amount of difficulty on the theory of "single
centres of creation," I must say a few words on the means of dispersal.
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 now impassable to certain organisms from the nature of its
climate, might have been a high road for migration, when the climate
was different. I shall, however, 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 together, or may formerly have blended. Where
the sea now extends, land may at a former period have connected islands
or possibly even continents together, and thus have allowed terrestrial
productions to pass from one to the other. No geologist disputes that
great mutations of level have occurred within the period of existing
organisms. Edward Forbes insisted that all the islands in the Atlantic
must have been recently connected with Europe or Africa, and Europe
likewise with America. Other authors have thus hypothetically bridged
over every ocean, and united almost every island with some mainland.
If, indeed, the arguments used by Forbes are to be trusted, it must be
admitted that scarcely a single island exists which has not recently
been united to some continent. This view cuts the Gordian knot of the
dispersal of the same species to the most distant points, and removes
many a difficulty; but to the best of my judgment we are not authorized
in admitting such enormous geographical changes within the period of
existing species. It seems to me that we have abundant evidence of
great oscillations in the level of the land or sea; but not of such
vast changes in the position and extension of our continents, as to
have united them within the recent period to each other and to the
several intervening oceanic islands. I freely admit the former
existence of many islands, now buried beneath the sea, which may have
served as halting places for plants and for many animals during their
migration. In the coral-producing oceans such sunken islands are now
marked by rings of coral or atolls standing over them. Whenever it is
fully admitted, as 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 most of our continents which now stand 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�\the degree
of affinity between the mammals inhabiting islands with those of the
nearest continent, being in part determined (as we shall hereafter see)
by the depth of the intervening ocean�\these and other such facts are
opposed to the admission of such prodigious geographical revolutions
within the recent period, as are necessary on the view advanced by
Forbes and admitted by his followers. The nature and relative
proportions of the inhabitants of oceanic islands are likewise opposed
to the belief of their former continuity of continents. Nor does the
almost universally volcanic composition of such islands favour the
admission that they are the wrecks of sunken continents; if they had
originally existed as continental mountain ranges, some at least of the
islands would have been formed, like other mountain summits, of
granite, metamorphic schists, old fossiliferous and other 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 should be called occasional means of distribution.
I shall here confine myself to plants. In botanical works, this or that
plant is often stated to be ill adapted for wide dissemination; but the
greater or less facilities for transport across the sea 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
eighty-seven kinds, sixty-four germinated after an immersion of
twenty-eight days, and a few survived an immersion of 137 days. It
deserves notice that certain orders were far more injured than others:
nine Leguminosae were tried, and, with one exception, they resisted the
salt-water badly; seven species of the allied orders, Hydrophyllaceae
and Polemoniaceae, were all killed by a month's immersion. For
convenience sake I chiefly tried small seeds without the capsules or
fruit; and as all of these sank in a few days, they could not have been
floated across wide spaces of the sea, whether or not they were injured
by salt water. Afterwards I tried some larger fruits, capsules, etc.,
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 would often wash into the sea dried
plants or branches with seed-capsules or fruit attached to them. Hence
I was led to dry the stems and branches of ninety-four 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 ninety days, and afterwards when
planted germinated; an asparagus plant with ripe berries floated for
twenty-three days, when dried it floated for eighty-five days, and the
seeds afterwards germinated: the ripe seeds of Helosciadium sank in two
days, when dried they floated for above ninety days, and afterwards
germinated. Altogether, out of the ninety-four dried plants, eighteen
floated for above twenty-eight days; and some of the eighteen floated
for a very much longer period. So that as 64/87 kinds of seeds
germinated after an immersion of twenty-eight days; and as 18/94
distinct species with ripe fruit (but not all the same species as in
the foregoing experiment) floated, after being dried, for above
twenty-eight days, we may conclude, as far as anything can be inferred
from these scanty facts, that the seeds of 14/100 kinds of plants of
any country might be floated by sea-currents during twenty-eight days,
and would retain their power of germination. In Johnston's Physical
Atlas, the average rate of the several Atlantic currents is
thirty-three miles per diem (some currents running at the rate of sixty
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 by an inland gale to a favourable
spot, 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 ninety-eight 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 both the average
length of their flotation and 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 of different kinds floated for forty-two 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 which, as Alph. de Candolle has shown,
generally have restricted ranges, could hardly be transported by any
other means.
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 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 during the longest
transport: out of one small portion of earth thus COMPLETELY enclosed
by the roots of an oak about fifty 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 many kinds of
seeds 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 sea-water for thirty 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 safely assume that under such circumstances their rate of
flight would often be thirty-five 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 pass uninjured through even the digestive organs of a
turkey. In the course of two months, I picked up in my garden twelve
kinds of seeds, out of the excrement of small birds, and these seemed
perfect, and some of them, which were tried, germinated. But the
following fact is more important: the crops of birds do not secrete
gastric juice, and do not, as I know by trial, injure in the least 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 twelve or even eighteen hours. A bird in this
interval might easily be blown to the distance of five hundred miles,
and hawks are known to look out for tired birds, and the contents of
their torn crops might thus readily get scattered. 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. Fresh-water 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 the power of germination. Certain seeds, however, were
always killed by this process.
Locusts are sometimes blown to great distances from the land. I myself
caught one 370 miles from the coast of Africa, and have heard of others
caught at greater distances. The Rev. R.T. Lowe informed Sir C. Lyell
that in November, 1844, swarms of locusts visited the island of
Madeira. They were in countless numbers, as thick as the flakes of snow
in the heaviest snowstorm, and extended upward as far as could be seen
with a telescope. During two or three days they slowly careered round
and round in an immense ellipse, at least five or six miles in
diameter, and at night alighted on the taller trees, which were
completely coated with them. They then disappeared over the sea, as
suddenly as they had appeared, and have not since visited the island.
Now, in parts of Natal it is believed by some farmers, though on
insufficient evidence, that injurious seeds are introduced into their
grass-land in the dung left by the great flights of locusts which often
visit that country. In consequence of this belief Mr. Weale sent me in
a letter a small packet of the dried pellets, out of which I extracted
under the microscope several seeds, and raised from them seven grass
plants, belonging to two species, of two genera. Hence a swarm of
locusts, such as that which visited Madeira, might readily be the means
of introducing several kinds of plants into an island lying far from
the mainland.
Although the beaks and feet of birds are generally clean, earth
sometimes adheres to them: in one case I removed sixty-one grains, and
in another case twenty-two grains of dry argillaceous earth from the
foot of a partridge, and in the earth there was a pebble as large as
the seed of a vetch. Here is a better case: the leg of a woodcock was
sent to me by a friend, with a little cake of dry earth attached to the
shank, weighing only nine grains; and this contained a seed of the
toad-rush (Juncus bufonius) which germinated and flowered. Mr.
Swaysland, of Brighton, who during the last forty years has paid close
attention to our migratory birds, informs me that he has often shot
wagtails (Motacillae), wheatears, and whinchats (Saxicolae), on their
first arrival on our shores, before they had alighted; and he has
several times noticed little cakes of earth attached to their feet.
Many facts could be given showing how generally soil is charged with
seeds. For instance, Professor Newton sent me the leg of a red-legged
partridge (Caccabis rufa) which had been wounded and could not fly,
with a ball of hard earth adhering to it, and weighing six and a half
ounces. The earth had been kept for three years, but when broken,
watered and placed under a bell glass, no less than eighty-two plants
sprung from it: these consisted of twelve monocotyledons, including the
common oat, and at least one kind of grass, and of seventy
dicotyledons, which consisted, judging from the young leaves, of at
least three distinct species. With such facts before us, can we doubt
that the many birds which are annually blown by gales across great
spaces of ocean, and which annually migrate�\for instance, the millions
of quails across the Mediterranean�\must occasionally transport a few
seeds embedded in dirt adhering to their feet or beaks? But I shall
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, it can
hardly be doubted that they must occasionally, as suggested by Lyell,
have transported seeds from one part to another of the arctic and
antarctic regions; and during the Glacial period from one part of the
now temperate regions to another. In the Azores, from the large number
of plants common to Europe, in comparison with the species on the other
islands of the Atlantic, which stand nearer to the mainland, and (as
remarked by Mr. H.C. Watson) from their somewhat northern character, 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 burdens 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 these several means of transport, and that other
means, which without doubt remain to be discovered, have been in action
year after year for 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 sea water; 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; but would remain as distinct as they now are. 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 their very long
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 rare wanderers only by one
means, namely, by dirt adhering to their feet or beaks, which is in
itself a rare accident. Even in this case, how small would be the
chance 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. Out of a hundred kinds of seeds or animals transported
to an island, even if far less well-stocked than Britain, perhaps not
more than one would be so well fitted to its new home, as to become
naturalised. But this is no valid argument against what would be
effected by occasional means of transport, during the long lapse of
geological time, whilst the island was being upheaved, 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, if fitted for the climate, would 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 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 point to the other. It is
indeed a remarkable fact to see so many plants of the same species
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 many 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 scored rocks plainly reveal a former cold
period.
The former influence of the glacial climate on the distribution of the
inhabitants of Europe, as explained by Edward Forbes, is substantially
as follows. But we shall follow the changes more readily, by supposing
a new glacial period slowly to come on, and then pass away, as formerly
occurred. As the cold came on, and as each more southern zone became
fitted for the inhabitants of the north, these would take the places of
the former inhabitants of the temperate regions. The latter, at the
same time would travel further and further 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 an 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.
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, as the warmth increased and the snow still
further disappeared, higher and higher, whilst their brethren were
pursuing their northern journey. Hence, when the warmth had fully
returned, the same species, which had lately lived together on the
European and North American lowlands, would again be found in the
arctic regions of the Old and New Worlds, and on many isolated
mountain-summits far distant from each other.
Thus we can understand the identity of many plants at points so
immensely remote as the mountains of the United States and those 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 first migration
when the cold came on, and the re-migration on the returning warmth,
would 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 formerly
permitted their migration across the intervening lowlands, now become
too warm for their existence.
As the arctic forms moved first southward and afterwards backward to
the north, in unison with the changing climate, they will not have been
exposed during their long migrations to any great diversity of
temperature; and as they all migrated in a body together, their mutual
relations will not have been much disturbed. Hence, in accordance with
the principles inculcated in this volume, these forms will not have
been liable to much modification. But with the 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 far 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 the coldest period will have been
temporarily driven down to the plains; they will, also, have been
subsequently exposed to somewhat different climatical influences. Their
mutual relations will thus have been in some degree disturbed;
consequently they will have been liable to modification; and they have
been modified; for if we compare the present Alpine plants and animals
of the several great European mountain ranges, one with another, though
many of the species remain identically the same, some exist as
varieties, some as doubtful forms or sub-species and some as distinct
yet closely allied species representing each other on the several
ranges.
In the foregoing illustration, I have assumed that at the commencement
of our imaginary Glacial period, the arctic productions were as uniform
round the polar regions as they are at the present day. But it is also
necessary to assume that many sub-arctic and some few temperate forms
were the same round the world, for some of the species which now exist
on the lower mountain slopes and on the plains of North America and
Europe are the same; and it may be asked how I account for this degree
of uniformity of the sub-arctic and temperate forms round the world, at
the commencement of the real 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 whole Atlantic Ocean and by the
northern part of the Pacific. During the Glacial period, when the
inhabitants of the Old and New Worlds lived further southwards than
they do at present, they must have been still more completely separated
from each other by wider spaces of ocean; so that it may well be asked
how the same species could then or previously have entered the two
continents. The explanation, I believe, lies in the nature of the
climate before the commencement of the Glacial period. At this, the
newer Pliocene period, the majority of the inhabitants of the world
were specifically the same as now, and we have good reason to believe
that the climate was warmer than at the present day. Hence, we may
suppose that the organisms which now live under latitude 60 degrees,
lived during the Pliocene period further north, under the Polar Circle,
in latitude 66-67 degrees; and that the present arctic productions then
lived on the broken land still nearer to the pole. Now, if we look at a
terrestrial globe, we see under the Polar Circle that there is almost
continuous land from western Europe through Siberia, to eastern
America. And this continuity of the circumpolar land, with the
consequent freedom under a more favourable climate for intermigration,
will account for the supposed uniformity of the sub-arctic and
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
great 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 highly remarkable,
considering the distance of the two areas, and their separation by the
whole 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 intermigration 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 will have been completely cut off from
each other. This separation, as far as the more temperate productions
are concerned, must have taken place long ages ago. As the plants and
animals migrated southward, they will have become mingled in the one
great region with the native American productions, and would 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 Europe and North
America. 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 marine areas completely sundered.
Thus, I think, we can understand the presence of some closely allied,
still existing and extinct tertiary forms, on the eastern and western
shores of temperate North America; and the still more striking fact of
many closely allied crustaceans (as described in Dana's admirable
work), some fish and other marine animals, inhabiting the Mediterranean
and the seas of Japan�\these two areas being now completely separated
by
the breadth of a whole continent and by wide spaces of ocean.
These cases of close relationship in species either now or formerly
inhabiting the seas on the eastern and western shores of North America,
the Mediterranean and Japan, and the temperate lands of North America
and Europe, are inexplicable on the theory of creation. We cannot
maintain that such species 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 parts of
South Africa or Australia, we see countries closely similar in all
their physical conditions, with their inhabitants utterly dissimilar.
ALTERNATE GLACIAL PERIODS IN THE NORTH AND SOUTH.
But we must return to our more immediate subject. I am convinced that
Forbes's view may be largely extended. In Europe we meet with the
plainest evidence of the Glacial period, from the western shores of
Britain to the Ural range, and southward to the Pyrenees. We may infer
from the frozen mammals and nature of the mountain vegetation, that
Siberia was similarly affected. In the Lebanon, according to Dr.
Hooker, perpetual snow formerly covered the central axis, and fed
glaciers which rolled 4,000 feet down the valleys. The same observer
has recently found great moraines at a low level on the Atlas range in
North Africa. 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 ancient and gigantic moraines. Southward of
the Asiatic continent, on the opposite side of the equator, we know,
from the excellent researches of Dr. J. Haast and Dr. Hector, that in
New Zealand immense glaciers formerly descended to a low level; and the
same plants, found by Dr. Hooker on widely separated mountains in this
island tell the same story of a former cold period. From facts
communicated to me by the Rev. W.B. Clarke, it appears also that there
are traces of former glacial action on the mountains of the
south-eastern corner of Australia.
Looking to America: in the northern half, ice-borne fragments of rock
have been observed on the eastern side of the continent, as far south
as latitude 36 and 37 degrees, and on the shores of the Pacific, where
the climate is now so different, as far south as latitude 46 degrees.
Erratic boulders have, also, been noticed on the Rocky Mountains. In
the Cordillera of South America, nearly under the equator, glaciers
once extended far below their present level. In central Chile I
examined a vast mound of detritus with great boulders, crossing the
Portillo valley, which, there can hardly be a doubt, once formed a huge
moraine; and Mr. D. Forbes informs me that he found in various parts of
the Cordillera, from latitude 13 to 30 degrees south, at about the
height of 12,000 feet, deeply-furrowed rocks, resembling those with
which he was familiar in Norway, and likewise great masses of detritus,
including grooved pebbles. Along this whole space of the Cordillera
true glaciers do not now exist even at much more considerable heights.
Further south, on both sides of the continent, from latitude 41 degrees
to the southernmost extremity, we have the clearest evidence of former
glacial action, in numerous immense boulders transported far from their
parent source.
From these several facts, namely, from the glacial action having
extended all round the northern and southern hemispheres�\from the
period having been in a geological sense recent in both
hemispheres�\from its having lasted in both during a great length of
time, as may be inferred from the amount of work effected�\and lastly,
from glaciers having recently descended to a low level along the whole
line of the Cordillera, it at one time appeared to me that we could not
avoid the conclusion that the temperature of the whole world had been
simultaneously lowered during the Glacial period. But now, Mr. Croll,
in a series of admirable memoirs, has attempted to show that a glacial
condition of climate is the result of various physical causes, brought
into operation by an increase in the eccentricity of the earth's orbit.
All these causes tend towards the same end; but the most powerful
appears to be the indirect influence of the eccentricity of the orbit
upon oceanic currents. According to Mr. Croll, cold periods regularly
recur every ten or fifteen thousand years; and these at long intervals
are extremely severe, owing to certain contingencies, of which the most
important, as Sir C. Lyell has shown, is the relative position of the
land and water. Mr. Croll believes that the last great glacial period
occurred about 240,000 years ago, and endured, with slight alterations
of climate, for about 160,000 years. With respect to more ancient
glacial periods, several geologists are convinced, from direct
evidence, that such occurred during the miocene and eocene formations,
not to mention still more ancient formations. But the most important
result for us, arrived at by Mr. Croll, is that whenever the northern
hemisphere passes through a cold period the temperature of the southern
hemisphere is actually raised, with the winters rendered much milder,
chiefly through changes in the direction of the ocean currents. So
conversely it will be with the northern hemisphere, while the southern
passes through a glacial period. This conclusion throws so much light
on geographical distribution that I am strongly inclined to trust in
it; but I will first give the facts which demand an explanation.
In South America, Dr. Hooker has shown that besides many closely allied
species, between forty and fifty of the flowering plants of Tierra del
Fuego, forming no inconsiderable part of its scanty flora, are common
to North America and Europe, enormously remote as these areas in
opposite hemispheres are from each other. On the lofty mountains of
equatorial America a host of peculiar species belonging to European
genera occur. On the Organ Mountains of Brazil some few temperate
European, some Antarctic and some Andean genera were found by Gardner
which do not exist in the low intervening hot countries. On the Silla
of Caraccas the illustrious Humboldt long ago found species belonging
to genera characteristic of the Cordillera.
In Africa, several forms characteristic of Europe, and some few
representatives of the flora of the Cape of Good Hope, occur on the
mountains of Abyssinia. At the Cape of Good Hope a very few European
species, believed not to have been introduced by man, and on the
mountains several representative European forms are found which have
not been discovered in the intertropical parts of Africa. Dr. Hooker
has also lately shown that several of the plants living on the upper
parts of the lofty island of Fernando Po, and on the neighbouring
Cameroon Mountains, in the Gulf of Guinea, are closely related to those
on the mountains of Abyssinia, and likewise to those of temperate
Europe. It now also appears, as I hear from Dr. Hooker, that some of
these same temperate plants have been discovered by the Rev. R.T. Lowe
on the mountains of the Cape Verde Islands. This extension of the same
temperate forms, almost under the equator, across the whole continent
of Africa and to the mountains of the Cape Verde archipelago, is one of
the most astonishing facts ever recorded in the distribution of plants.
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 of plants collected
on the loftier peaks of Java, raises a picture of a collection made on
a hillock in Europe. Still more striking is the fact that peculiar
Australian forms are represented by certain 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 hand as far north as Japan.
On the southern mountains of Australia, Dr. F. Muller 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
certain plants growing on the more lofty mountains of the tropics in
all parts of the world, and on the temperate plains of the north and
south, are either the same species or varieties of the same species. It
should, however, be observed that these plants are not strictly arctic
forms; for, as Mr. H.C. Watson has remarked, "in receding from polar
toward equatorial latitudes, the Alpine or mountain flora really become
less and less Arctic." Besides these identical and closely allied
forms, many species inhabiting the same widely sundered areas, belong
to genera not now found in the intermediate tropical lowlands.
These brief remarks apply to plants alone; but some few analogous facts
could be given in regard to terrestrial animals. In marine productions,
similar cases likewise occur; as an example, I may quote a statement 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, etc., 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.
From the foregoing facts, namely, the presence of temperate forms on
the highlands across the whole of equatorial Africa, and along the
Peninsula of India, to Ceylon and the Malay Archipelago, and in a less
well-marked manner across the wide expanse of tropical South America,
it appears almost certain that at some former period, no doubt during
the most severe part of a Glacial period, the lowlands of these great
continents were everywhere tenanted under the equator by a considerable
number of temperate forms. At this period the equatorial climate at the
level of the sea was probably about the same with that now experienced
at the height of from five to six thousand feet under the same
latitude, or perhaps even rather cooler. During this, the coldest
period, the lowlands under the equator must have been clothed with a
mingled tropical and temperate vegetation, like that described by
Hooker as growing luxuriantly at the height of from four to five
thousand feet on the lower slopes of the Himalaya, but with perhaps a
still greater preponderance of temperate forms. So again in the
mountainous island of Fernando Po, in the Gulf of Guinea, Mr. Mann
found temperate European forms beginning to appear at the height of
about five thousand feet. On the mountains of Panama, at the height of
only two thousand feet, Dr. Seemann found the vegetation like that of
Mexico, "with forms of the torrid zone harmoniously blended with those
of the temperate."
Now let us see whether Mr. Croll's conclusion that when the northern
hemisphere suffered from the extreme cold of the great Glacial period,
the southern hemisphere was actually warmer, throws any clear light on
the present apparently inexplicable distribution of various organisms
in the temperate parts of both hemispheres, and on the mountains of the
tropics. 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 became more
and more intense, we know that Arctic forms invaded the temperate
regions; and from the facts just given, there can hardly be a doubt
that some of the more vigorous, dominant and widest-spreading temperate
forms invaded the equatorial lowlands. The inhabitants of these hot
lowlands would at the same time have migrated to the tropical and
subtropical regions of the south, for the southern hemisphere was at
this period warmer. On the decline of the Glacial period, as both
hemispheres gradually recovered their former temperature, the northern
temperate forms living on the lowlands under the equator, would have
been driven to their former homes or have been destroyed, being
replaced by the equatorial forms returning from the south. Some,
however, of the northern temperate forms would almost certainly have
ascended any adjoining high land, where, if sufficiently lofty, they
would have long survived like the Arctic forms on the mountains of
Europe. They might have survived, even if the climate was not perfectly
fitted for them, for the change of temperature must have been very
slow, and plants undoubtedly possess a certain capacity for
acclimatisation, as shown by their transmitting to their offspring
different constitutional powers of resisting heat and cold.
In the regular course of events the southern hemisphere would in its
turn be subjected to a severe Glacial period, with the northern
hemisphere rendered warmer; and then the southern temperate forms would
invade the equatorial lowlands. The northern forms which had before
been left on the mountains would now descend and mingle with the
southern forms. These latter, when the warmth returned, would return to
their former homes, leaving some few species on the mountains, and
carrying southward with them some of the northern temperate forms which
had descended from their mountain fastnesses. Thus, we should have some
few species identically the same in the northern and southern temperate
zones and on the mountains of the intermediate tropical regions. But
the species left during a long time on these mountains, or in opposite
hemispheres, would have to compete with many new forms and would be
exposed to somewhat different physical conditions; hence, they would be
eminently liable to modification, and would generally now exist as
varieties or as representative species; and this is the case. We must,
also, bear in mind the occurrence in both hemispheres of former Glacial
periods; for these will account, in accordance with the same
principles, for the many quite distinct species inhabiting the same
widely separated areas, and belonging to genera not now found in the
intermediate torrid zones.
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 or slightly modified species have migrated from the
north to the south, than in a reversed direction. We see, however, a
few southern forms on the mountains of Borneo and Abyssinia. I suspect
that this preponderant migration from the north to the 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 the two sets became commingled in the equatorial
regions, during the alternations of the Glacial periods, the northern
forms were the more powerful and were able to hold their places on the
mountains, and afterwards migrate southward with the southern forms;
but not so the southern in regard to the northern forms. In the same
manner, at the present day, we see that very many European productions
cover the ground in La Plata, New Zealand, and to a lesser degree in
Australia, and have beaten the natives; whereas extremely few southern
forms have become naturalised in any part of the northern hemisphere,
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 forty or fifty years from Australia.
The Neilgherrie Mountains in India, however, offer a partial exception;
for here, as I hear from Dr. Hooker, Australian forms are rapidly
sowing themselves and becoming naturalised. Before the last great
Glacial period, no doubt the intertropical mountains were stocked with
endemic Alpine forms; but these have almost everywhere 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 those which have become
naturalised; and this is the first stage towards their extinction.
Mountains are islands on the land; and their inhabitants have yielded
to those produced within the larger areas of the north, just in the
same way as the inhabitants of real islands have everywhere yielded and
are still yielding to continental forms naturalised through man's
agency.
The same principles apply to the distribution of terrestrial animals
and of marine productions, in the northern and southern temperate
zones, and on the intertropical mountains. When, during the height of
the Glacial period, the ocean-currents were widely different to what
they now are, some of the inhabitants of the temperate seas might have
reached the equator; of these a few would perhaps at once be able to
migrate southwards, by keeping to the cooler currents, while others
might remain and survive in the colder depths until the southern
hemisphere was in its turn subjected to a glacial climate and permitted
their further progress; in nearly the same manner as, according to
Forbes, isolated spaces inhabited by Arctic productions exist to the
present day in the deeper parts of the northern temperate seas.
I am far from supposing that all the difficulties in regard to the
distribution and affinities of the identical and allied species, which
now live so widely separated in the north and south, and sometimes on
the intermediate mountain ranges, are removed on the views above given.
The exact lines of migration cannot be indicated. We cannot say why
certain species and not others have migrated; why certain species have
been modified and have given rise to new forms, while 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 species ranges twice or thrice as far, and
is twice or thrice as common, as another species within their own homes.
Various special difficulties also remain to be solved; for instance,
the occurrence, as shown by Dr. Hooker, of the same plants at points so
enormously remote as Kerguelen Land, New Zealand, and Fuegia; but
icebergs, as suggested by Lyell, may have been concerned in their
dispersal. The existence at these and other distant points of the
southern hemisphere, of species, which, though distinct, belong to
genera exclusively confined to the south, is a more remarkable case.
Some of these species are so distinct, that we cannot suppose that
there has been time since the commencement of the last Glacial period
for their migration and subsequent modification to the necessary
degree. The facts seem to indicate that distinct species belonging to
the same genera have migrated in radiating lines from a 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 last Glacial period, when the Antarctic lands, now covered with
ice, supported a highly peculiar and isolated flora. It may be
suspected that before this flora was exterminated during the last
Glacial epoch, a few forms had been already widely dispersed to various
points of the southern hemisphere by occasional means of transport, and
by the aid, as halting-places, of now sunken islands. Thus the southern
shores of America, Australia, and New Zealand may have become slightly
tinted by the same peculiar forms of life.
Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate
throughout the world on geographical distribution. And we have now seen
that Mr. Croll's conclusion that successive Glacial periods in the one
hemisphere coincide with warmer periods in the opposite hemisphere,
together with the admission of the slow modification of species,
explains a multitude of facts in the distribution of the same and of
the allied forms of life in all parts of the globe. The living waters
have flowed during one period from the north and during another from
the south, and in both cases have reached the equator; but the stream
of life has flowed with greater force from the north than in the
opposite direction, and has consequently more freely inundated the
south. As the tide leaves its drift in horizontal lines, 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 latitude 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 serves as a record, full of interest to us, of the
former inhabitants of the surrounding lowlands.
+++
CHAPTER XIII. GEOGRAPHICAL
DISTRIBUTION�\continued.
Distribution of fresh-water productions�\On the inhabitants of oceanic
islands�\Absence of Batrachians and of terrestrial Mammals�\On the
relation 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.
FRESH-WATER PRODUCTIONS.
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 formidable barrier, that they would never have
extended to distant countries. But the case is exactly the reverse. Not
only have many fresh-water species, belonging to different classes, an
enormous range, but allied species prevail in a remarkable manner
throughout the world. When first collecting in the fresh waters of
Brazil, I well remember feeling much surprise at the similarity of the
fresh-water insects, shells, etc., and at the dissimilarity of the
surrounding terrestrial beings, compared with those of Britain.
But the wide ranging power of fresh-water productions 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, within their own countries; and
liability to wide dispersal would follow from this capacity as an
almost necessary consequence. We can here consider only a few cases; of
these, some of the most difficult to explain are presented by fish. It
was formerly believed that the same fresh-water species never existed
on two continents distant from each other. But Dr. Gunther has lately
shown that the Galaxias attenuatus inhabits Tasmania, New Zealand, the
Falkland Islands and the mainland of South America. This is a wonderful
case, and probably indicates dispersal from an Antarctic centre during
a former warm period. This case, however, is rendered in some degree
less surprising by the species of this genus having the power of
crossing by some unknown means considerable spaces of open ocean: thus
there is one species common to New Zealand and to the Auckland Islands,
though separated by a distance of about 230 miles. On the same
continent fresh-water fish often range widely, and as if capriciously;
for in two adjoining river systems some of the species may be the same
and some wholly different.
It is probable that they are occasionally transported by what may be
called accidental means. Thus fishes still alive are not very rarely
dropped at distant points by whirlwinds; and it is known that the ova
retain their vitality for a considerable time after removal from the
water. Their dispersal may, however, be mainly attributed to changes in
the level of the land within the recent period, causing rivers to flow
into each other. Instances, also, could be given of this having
occurred during floods, without any change of level. The wide
differences of the fish on the opposite sides of most mountain-ranges,
which are continuous and consequently must, from an early period, have
completely prevented the inosculation of the river systems on the two
sides, leads to the same conclusion. 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. Moreover, Dr. Gunther has recently been led by several
considerations to infer that with fishes the same forms have a long
endurance. 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 which all the members are confined to fresh water, so
that a marine species belonging to a fresh-water group might travel far
along the shores of the sea, and could, it is probable, become adapted
without much difficulty to the fresh waters of a distant land.
Some species of fresh-water shells have very wide ranges, and allied
species which, on our 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 the ova, as well as the adults,
are immediately killed by sea-water. I could not even understand how
some naturalised species have spread rapidly throughout the same
country. But two facts, which I have observed�\and many others no doubt
will be discovered�\throw some light on this subject. When ducks
suddenly emerge from a pond covered with duck-weed, I have twice seen
these little plants adhering to their backs; and it has happened to me,
in removing a little duck-weed from one aquarium to another, that I
have unintentionally stocked the one with fresh-water shells from the
other. But another agency is perhaps more effectual: I suspended the
feet of a duck 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 if blown across the sea to an oceanic island, or to any
other distant point, would be sure to alight on a pool or rivulet. Sir
Charles Lyell 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 been blown by 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 illustrated,
according to Alph. de Candolle, in those large groups of terrestrial
plants, which have very few aquatic members; for the latter seem
immediately to acquire, as if in consequence, a wide range. I think
favourable means of dispersal explain this fact. I have before
mentioned that earth occasionally 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 wander more than those of any other; and are
occasionally found on the most remote and barren islands of the open
ocean; they would not be likely to alight on the surface of the sea, so
that any dirt on their feet would not be washed off; and when gaining
the 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
tablespoonfuls of mud from three different points, beneath water, on
the edge of a little pond; this mud when dry weighed only 6 and 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 unstocked ponds and streams, situated at very
distant points. 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 many hours afterwards in pellets or
in the excrement. When I saw the great size of the seeds of that fine
water-lily, the Nelumbium, and remembered Alph. de Candolle's remarks
on the distribution of this plant, I thought that the means of its
dispersal must remain 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. Now this bird must
often have flown with its stomach thus well stocked to distant ponds,
and, then getting a hearty meal of fish, analogy makes me believe that
it would have rejected the seeds in the pellet in a fit state for
germination.
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 inhabitants of the same pond, however few
in kind, yet as the number even in a well-stocked pond is small in
comparison with the number of species inhabiting an equal area of land,
the competition between them will probably be less severe than between
terrestrial species; consequently an intruder from the waters of a
foreign country would have a better chance of seizing on a new place,
than in the case of terrestrial colonists. We should also remember that
many fresh-water productions are low in the scale of nature, and we
have reason to believe that such beings become modified more slowly
than the high; and this will give time for the migration of aquatic
species. We should not forget the probability of many fresh-water forms
having formerly ranged continuously over immense areas, and then having
become extinct at intermediate points. But the wide distribution of
fresh-water plants, and of the lower animals, whether retaining the
same identical form, or in some degree modified, apparently depends in
main part on the wide dispersal of their seeds and eggs by animals,
more especially by fresh-water birds, which have great powers of
flight, and naturally travel from one piece of water to another.
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 with respect
to distribution, on the view that not only all the individuals of the
same species have migrated from some one area, but that allied species,
although now inhabiting the most distant points, have proceeded from a
single area, the birthplace of their early progenitors. I have already
given my reasons for disbelieving in continental extensions within the
period of existing species on so enormous a scale that all the many
islands of the several oceans were thus stocked with their present
terrestrial inhabitants. This view removes many difficulties, but it
does not accord with all the facts in regard to the productions of
islands. In the following remarks I shall not confine myself to the
mere question of dispersal, but shall consider some other cases bearing
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. New
Zealand, for instance, with its lofty mountains and diversified
stations, extending over 780 miles of latitude, together with the
outlying islands of Auckland, Campbell and Chatham, contain altogether
only 960 kinds of flowering plants; if we compare this moderate number
with the species which swarm over equal areas in Southwestern Australia
or at the Cape of Good Hope, we must admit that some cause,
independently of different physical conditions, has given rise to 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 less than
half-a-dozen flowering plants; yet many species have now become
naturalised on it, as they have in 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 were not created for
oceanic islands; for man has unintentionally stocked them far more
fully and perfectly than did nature.
Although in oceanic islands the species are few in number, the
proportion of endemic kinds (i.e. those found nowhere else in the
world) is often extremely large. If we compare, for instance, the
number of endemic land-shells in Madeira, or of endemic birds in the
Galapagos Archipelago, with the number found on any continent, and then
compare the area of the island with that of the continent, we shall see
that this is true. This fact might have been theoretically expected,
for, as already explained, species occasionally arriving, after long
intervals of time in the new and isolated district, and having to
compete with new associates, would be eminently liable to modification,
and would 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
partly on the species which are not modified having immigrated in a
body, so that their mutual relations have not been much disturbed; and
partly on the frequent arrival of unmodified immigrants from the
mother-country, with which the insular forms have intercrossed. It
should be borne in mind that the offspring of such crosses would
certainly gain in vigour; so that even an occasional cross would
produce more effect than might have been anticipated. I will give a few
illustrations of the foregoing remarks: in the Galapagos Islands there
are twenty-six land birds; of these twenty-one (or perhaps
twenty-three) are peculiar; whereas of the eleven marine birds only two
are peculiar; and it is obvious that marine birds could arrive at these
islands much more easily and frequently 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 a single endemic land bird; and we
know from Mr. J.M. Jones's admirable account of Bermuda, that very many
North American birds occasionally or even frequently visit this island.
Almost every year, as I am informed by Mr. E.V. Harcourt, many European
and African birds are blown to Madeira; this island is inhabited by
ninety-nine kinds, of which one alone is peculiar, though very closely
related to a European form; and three or four other species are
confined to this island and to the Canaries. So that the islands of
Bermuda and Madeira have been stocked from the neighbouring continents
with birds, which for long ages have there struggled together, and have
become mutually co-adapted. Hence, when settled in their new homes,
each kind will have been kept by the others to its proper place and
habits, and will consequently have been but little liable to
modification. Any tendency to modification will also have been checked
by intercrossing with the unmodified immigrants, often arriving from
the mother-country. Madeira again is inhabited by a wonderful number of
peculiar land-shells, whereas not one species of sea-shell is peculiar
to its shores: now, though we do not know how sea-shells 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 across three or four hundred miles of open sea far more
easily than land-shells. The different orders of insects inhabiting
Madeira present nearly parallel cases.
Oceanic islands are sometimes deficient in animals of certain whole
classes, and their places are occupied by other classes; thus in the
Galapagos Islands reptiles, and in New Zealand gigantic wingless birds,
take, or recently took, the place of mammals. Although New Zealand is
here spoken of as an oceanic island, it is in some degree doubtful
whether it should be so ranked; it is of large size, and is not
separated from Australia by a profoundly deep sea; from its geological
character and the direction of its mountain ranges, the Rev. W.B.
Clarke has lately maintained that this island, as well as New
Caledonia, should be considered as appurtenances of Australia. Turning
to plants, Dr. Hooker has shown that in the Galapagos Islands the
proportional numbers of the different orders are very different from
what they are elsewhere. All such differences in number, and the
absence of certain whole groups of animals and plants, are generally
accounted for by supposed differences in the physical conditions of the
islands; but this explanation is not a little doubtful. Facility of
immigration seems to have been fully as important as the nature of the
conditions.
Many remarkable little facts could be given with respect to the
inhabitants of oceanic islands. For instance, in certain islands not
tenanted by a single mammal, some of the endemic plants have
beautifully hooked seeds; yet few relations are more manifest than that
hooks serve for the transportal of seeds in the wool or fur of
quadrupeds. But a hooked seed might be carried to an island by other
means; and the plant then becoming modified would form an endemic
species, still retaining its hooks, which would form a useless
appendage, like the shrivelled wings under the soldered wing-covers 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, which had no
chance of successfully competing with the many fully developed trees
growing on a continent, might, when established on an island, gain an
advantage over other herbaceous plants by growing taller and taller and
overtopping them. In this case, natural selection would tend to add to
the stature of the plant, to whatever order it belonged, and thus first
convert it into a bush and then into a tree.
ABSENCE OF BATRACHIANS AND TERRESTRIAL MAMMALS ON OCEANIC ISLANDS.
With respect to the absence of whole orders of animals on oceanic
islands, Bory St. Vincent long ago remarked that Batrachians (frogs,
toads, newts) are never found on any of the many islands with which the
great oceans are studded. I have taken pains to verify this assertion,
and have found it true, with the exception of New Zealand, New
Caledonia, the Andaman Islands, and perhaps the Solomon Islands and the
Seychelles. But I have already remarked that it is doubtful whether New
Zealand and New Caledonia ought to be classed as oceanic islands; and
this is still more doubtful with respect to the Andaman and Solomon
groups and the Seychelles. This general absence of frogs, toads and
newts on so many true oceanic islands cannot be accounted for by their
physical conditions; indeed it seems that islands are peculiarly 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 immediately killed (with the
exception, as far as known, of one Indian species) by sea-water, there
would be great difficulty in their transportal across the sea, and
therefore we can see why they do not exist on strictly oceanic islands.
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, and 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 in connection with the
mainland at a distance of about 280 miles; moreover, icebergs formerly
brought boulders to its western shores, and they may have formerly
transported foxes, as now frequently happens in the arctic regions. Yet
it cannot be said that small islands will not support at least small
mammals, for they occur in many parts of the world on very small
islands, when lying 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 known that new
species of mammals appear and disappear at a quicker rate than other
and lower animals. Although terrestrial mammals do not occur on oceanic
islands, aerial 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 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 in their new homes in
relation to their new position, and we can understand the presence of
endemic bats on oceanic islands, with the absence of all other
terrestrial mammals.
Another interesting relation exists, namely, between the depth of the
sea separating islands from each other, or from the nearest continent,
and the degree of affinity of their mammalian inhabitants. Mr. Windsor
Earl has made some striking observations on this head, since greatly
extended by Mr. Wallace's admirable researches, in regard to the great
Malay Archipelago, which is traversed near Celebes by a space of deep
ocean, and this separates two widely distinct mammalian faunas. On
either side, the islands stand on a moderately shallow submarine bank,
and these islands are inhabited by the same or by closely allied
quadrupeds. I have not as yet had time to follow up this subject in all
quarters of the world; but as far as I have gone, the relation holds
good. For instance, Britain is separated by a shallow channel from
Europe, and the mammals are the same on both sides; and so it is with
all the islands near the shores of Australia. The West Indian Islands,
on the other hand, stand on a deeply submerged bank, nearly one
thousand fathoms in depth, and here we find American forms, but the
species and even the genera are quite distinct. As the amount of
modification which animals of all kinds undergo partly depends on the
lapse of time, and as the islands which are separated from each other,
or from the mainland, by shallow channels, are more likely to have been
continuously united within a recent period than the islands separated
by deeper channels, we can understand how it is that a relation exists
between the depth of the sea separating two mammalian faunas, and the
degree of their affinity, a relation which is quite inexplicable on the
theory of independent acts of creation.
The foregoing statements in regard to the inhabitants of oceanic
islands, namely, the fewness of the species, with a large proportion
consisting of endemic forms�\the members of certain groups, but not
those of other groups in the same class, having been modified�\the
absence of certain whole orders, as of batrachians and of terrestrial
mammals, notwithstanding the presence of aerial bats, the singular
proportions of certain orders of plants, herbaceous forms having been
developed into trees, etc., seem to me to accord better with the belief
in the efficiency of occasional means of transport, carried on during a
long course of time, than with the belief in the former connection of
all oceanic islands with the nearest continent; for on this latter view
it is probable that the various classes would have immigrated more
uniformly, and from the species having entered in a body, their mutual
relations would not have been much disturbed, and consequently, they
would either have not been modified, or all the species in a more
equable manner.
I do not deny that there are many and serious difficulties in
understanding how many of the inhabitants of the more remote islands,
whether still retaining the same specific form or subsequently
modified, have reached their present homes. But the probability of
other islands having once existed as halting-places, of which not a
wreck now remains, must not be overlooked. I will specify one difficult
case. 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 striking instances of which have
been given by Dr. A.A. Gould in relation to the Pacific. Now it is
notorious that land-shells are easily killed by sea-water; their eggs,
at least such as I have tried, sink in it and are killed. Yet there
must be some unknown, but occasionally efficient means for their
transportal. Would the just-hatched young sometimes 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 find that
several species in this state withstand uninjured an immersion in
sea-water during seven days. One shell, the Helix pomatia, after having
been thus treated, and again hybernating, was put into sea-water for
twenty days and perfectly recovered. During this length of time the
shell might have been carried by a marine country of average swiftness
to a distance of 660 geographical miles. As this Helix has a thick
calcareous operculum I removed it, and when it had formed a new
membranous one, I again immersed it for fourteen days in sea-water, and
again it recovered and crawled away. Baron Aucapitaine has since tried
similar experiments. He placed 100 land-shells, belonging to ten
species, in a box pierced with holes, and immersed it for a fortnight
in the sea. Out of the hundred shells twenty-seven recovered. The
presence of an operculum seems to have been of importance, as out of
twelve specimens of Cyclostoma elegans, which is thus furnished, eleven
revived. It is remarkable, seeing how well the Helix pomatia resisted
with me the salt-water, that not one of fifty-four specimens belonging
to four other species of Helix tried by Aucapitaine recovered. It is,
however, not at all probable that land-shells have often been thus
transported; the feet of birds offer a more probable method.
ON THE RELATIONS OF THE INHABITANTS OF ISLANDS TO THOSE OF THE NEAREST
MAINLAND.
The most striking and important fact for us is the affinity of the
species which inhabit islands to those of the nearest mainland, without
being actually the same. Numerous instances could be given. The
Galapagos Archipelago, situated under the equator, lies at a distance
of between 500 and 600 miles from the shores of South America. Here
almost every product of the land and of the water bears the
unmistakable stamp of the American continent. There are twenty-six land
birds. Of these twenty-one, or perhaps twenty-three, are ranked as
distinct species, and would commonly be assumed to have been here
created; yet the close affinity of most of these birds to American
species is manifest in every character in their habits, gestures, and
tones of voice. So it is with the other animals, and with a large
proportion of the plants, as shown by Dr. Hooker in his admirable Flora
of this archipelago. The naturalist, looking at the inhabitants of
these volcanic islands in the Pacific, distant several hundred miles
from the continent, 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
plainly the 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 closely resembles 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 the climate, height, and size of the islands, between the
Galapagos and Cape Verde Archipelagos: but what an entire and absolute
difference in their inhabitants! The inhabitants of the Cape Verde
Islands are related to those of Africa, like those of the Galapagos to
America. Facts, such as these, admit of 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 from America, whether by occasional means of
transport or (though I do not believe in this doctrine) by formerly
continuous land, and the Cape Verde Islands from Africa; such colonists
would be liable to modification�\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 the nearest large island. The exceptions
are few, and most of them can be explained. Thus, although Kerguelen
Land stands nearer to Africa than to America, the plants 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 partially disappears on
the view that New Zealand, South America, and the other southern lands,
have been stocked in part from a nearly intermediate though distant
point, namely, from the antarctic islands, when they were clothed with
vegetation, during a warmer tertiary period, before the commencement of
the last 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; but this affinity is confined to the plants, and will,
no doubt, some day be explained.
The same law which has determined the relationship between the
inhabitants of islands and the nearest mainland, is sometimes displayed
on a small scale, but in a most interesting manner, within the limits
of the same archipelago. Thus each separate island of the Galapagos
Archipelago is tenanted, and the fact is a marvellous one, by many
distinct species; but these species are related to each other in a very
much closer manner than to the inhabitants of the American continent,
or of any other quarter of the world. This is what might have been
expected, for islands situated so near to each other would almost
necessarily receive immigrants from the same original source, and from
each other. But how is it that many of the immigrants have been
differently modified, though only in a small degree, in islands
situated within sight of each other, having the same geological nature,
the same height, climate, etc? 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;
whereas it cannot be disputed that the nature of the other species 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 the species
which inhabit the Galapagos Archipelago, and are likewise found in
other parts of the world, we find that they differ considerably in the
several islands. This difference might indeed have been expected if the
islands have 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, though all proceeding from the same
general source. Hence, when in former times an immigrant first settled
on one of the islands, or when it subsequently spread from one to
another, it would undoubtedly be exposed to different conditions in the
different islands, for it would have to compete with a different set of
organisms; a plant, for instance, would find the ground best-fitted for
it occupied by somewhat different species in the different islands, and
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
some species spreading widely throughout a continent and remaining the
same.
The really surprising fact in this case of the Galapagos Archipelago,
and in a lesser degree in some analogous cases, is that each new
species after being formed in any one island, did not spread quickly 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
deep between the islands, and gales of wind are extraordinarily rare;
so that the islands are far more effectually separated from each other
than they appear on a map. Nevertheless, some of the species, both of
those found in other parts of the world and of those confined to the
archipelago, are common to the several islands; and we may infer from
the present manner of distribution that they have spread from 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 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, both will probably hold their separate places 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 wide areas, we are apt to infer that most species would
thus spread; but we should remember that the species which become
naturalised in new countries are not generally closely allied to the
aboriginal inhabitants, but are very distinct forms, 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, differ on the different
islands; 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 and young birds hatched 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 species of 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 species which inhabit the several islands of the
Galapagos Archipelago not having all spread from island to island. On
the same continent, also, pre-occupation has probably played an
important part in checking the commingling of the species which inhabit
different districts with nearly the same physical conditions. 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; so
it is, according to Mr. Bates, with the butterflies and other animals
inhabiting the great, open, and continuous valley of the Amazons.
The same principle which governs the general character of the
inhabitants of oceanic islands, namely, the relation to the source
whence colonists could have been most easily derived, together with
their subsequent modification, is of the widest application throughout
nature. We see this on every mountain-summit, in every lake and marsh.
For Alpine species, excepting in as far as the same species have become
widely spread during the Glacial epoch, are related to those of the
surrounding lowlands; thus we have in South America, Alpine
humming-birds, Alpine rodents, Alpine plants, etc., all strictly
belonging to American forms; and it is obvious that a mountain, as it
became slowly upheaved, would 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 allowed the same forms to
prevail throughout large portions of the world. We see the same
principle in the character of most of the blind animals inhabiting the
caves of America and of Europe. Other analogous facts could be given.
It will, I believe, be found universally 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; and wherever many closely-allied species occur,
there will be found many forms which some naturalists rank as distinct
species, and others as mere varieties; these doubtful forms showing us
the steps in the process of modification.
The relation between the power and extent of migration in certain
species, either at the present or at some former period, and the
existence at remote points of the world of closely allied species, 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 difficult of proof. Among mammals, we see it
strikingly displayed in Bats, and in a lesser degree in the Felidae and
Canidae. We see the same rule in the distribution of butterflies and
beetles. So it is with most of the inhabitants of fresh water, for many
of the genera in the most distinct classes range over the world, and
many of the species have enormous ranges. It is not meant that all, but
that some of the species have very wide ranges in the genera which
range very widely. Nor is it meant that the species in such genera
have, on an average, a very wide range; for this will largely depend on
how far the process of modification has gone; for instance, two
varieties of the same species inhabit America and Europe, and thus the
species has an immense range; but, if variation were to be carried a
little further, the two varieties would be ranked as distinct species,
and their range would be greatly reduced. Still less is it meant, that
species which have 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 according to the view that all
the species of a genus, though distributed to the most remote points of
the world, are descended from a single progenitor, we ought to find,
and I believe as a general rule we do find, that some at least of the
species range very widely.
We should bear in mind that many genera in all classes are of ancient
origin, and the species in this case will have had ample time for
dispersal and subsequent modification. There is also reason to believe,
from geological evidence, that within each great class the lower
organisms change at a slower rate than the higher; consequently they
will have had a better chance of ranging widely and of still retaining
the same specific character. This fact, together with that of the seeds
and eggs of most lowly organised forms being very minute and better
fitted for distant transportal, probably accounts for a law which has
long been observed, and which has lately been discussed by Alph. de
Candolle in regard to plants, namely, that the lower any group of
organisms stands the more widely it ranges.
The relations just discussed�\namely, lower organisms ranging more
widely than the higher�\some of the species of widely-ranging genera
themselves ranging widely�\such facts, as alpine, lacustrine, and marsh
productions being generally related to those which live on the
surrounding low lands and dry lands�\the striking relationship between
the inhabitants of islands and those of the nearest mainland�\the still
closer relationship of the distinct inhabitants of the islands of the
same archipelago�\are inexplicable on the ordinary view of the
independent creation of each species, but are explicable if we admit
colonisation from the nearest or readiest source, together with the
subsequent adaptation of the colonists to their new homes.
SUMMARY OF THE 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 changes of climate
and of the level of the land, which have certainly occurred within the
recent period, and of other changes which have probably occurred�\if we
remember how ignorant we are with respect to the many curious means of
occasional transport�\if we bear in mind, and this is a very important
consideration, how often a species may have ranged continuously over a
wide area, and then have become extinct in the intermediate tracts�\the
difficulty is not insuperable in believing that all the individuals of
the same species, wherever found, are descended from common parents.
And we are led to this conclusion, which has been arrived at by many
naturalists under the designation of single centres of creation, by
various general considerations, more especially from the importance of
barriers of all kinds, and from the analogical distribution of
subgenera, genera, and families.
With respect to distinct species belonging to the same genus, which on
our theory have spread from one parent-source; if we make the same
allowances as before for our ignorance, and remember that some forms of
life have changed very slowly, enormous periods of time having been
thus granted for their migration, the difficulties are far from
insuperable; though in this case, as in that of the individuals of the
same species, they are often great.
As exemplifying the effects of climatical changes on distribution, I
have attempted to show how important a part the last Glacial period has
played, which affected even the equatorial regions, and which, during
the alternations of the cold in the north and the south, allowed the
productions of opposite hemispheres to mingle, and left some of them
stranded on the mountain-summits in all parts of the world. 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 all the individuals of the same species, and likewise of
the several species belonging to the same genus, have proceeded from
some one source; then all the grand leading facts of geographical
distribution are explicable on the theory of migration, together with
subsequent modification and the multiplication of new forms. We can
thus understand the high importance of barriers, whether of land or
water, in not only separating but in apparently forming the several
zoological and botanical provinces. We can thus understand the
concentration of related species within the same areas; 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 linked together in so mysterious a manner, and are
likewise linked to the extinct beings which formerly inhabited the same
continent. Bearing in mind that the mutual relation of organism to
organism is 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 the colonists entered one of the regions, or both;
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 into 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 the to or more 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 some groups of beings greatly, and some only
slightly modified; some developed in great force, some existing in
scanty numbers�\and this we do find in the several 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 that of
these, a large proportion should be endemic or peculiar; and why, in
relation to the means of migration, one group of beings should have all
its species peculiar, and another group, even within the same class,
should have all its species the same with those in an adjoining quarter
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 should possess their own peculiar species of
aerial mammals or bats. We can see why, in islands, there should be
some relation between the presence of mammals, in a more or less
modified condition, and the depth of the sea between such islands 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 should likewise be related, but
less closely, to those of the nearest continent, or other source whence
immigrants might have been derived. We can see why, if there exist very
closely allied or representative species in two areas, however distant
from each other, some identical species will almost always there be
found.
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 apparent exceptions to
the rule are so few that they may fairly be attributed to our not
having as yet discovered in an intermediate deposit certain forms which
are absent in it, 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 former migrations under different circumstances, or through
occasional means of transport, or 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,
living during the same period of time, or living within the same area,
are often characterised by trifling features in common, as of sculpture
or colour. In looking to the long succession of past ages, as in
looking to distant provinces throughout the world, we find that species
in certain classes differ little from each other, whilst those in
another class, or only in a different section of the same order, differ
greatly from each other. In both time and space the lowly organised
members of each class generally change less than the highly organised;
but there are in both cases marked exceptions to the rule. According to
our theory, these several relations throughout time and space are
intelligible; for whether we look to the allied forms of life which
have changed during successive ages, or to those which have changed
after having migrated into distant quarters, in both cases they are
connected by the same bond of ordinary generation; in both cases the
laws of variation have been the same, and modifications have been
accumulated by the same means of natural selection.
文献
- "THE ORIGIN OF SPECIES BY MEANS OF NATURAL SELECTION or THE PRESERVATION OF FAVOURED RACES IN THE STRUGGLE FOR LIFE." by By Charles Darwin, M.A., F.R.S., Sixth London Edition