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tion. 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 birth-
place, 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 distribu-
tion — 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



270 THE ORIGIN OF SPECIES

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 behef of their former continuity
of continents. Nor does the almost universally volcanic composition of such
islands favor 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 sunk 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. Afterward I tried
some larger fruits, capsules, etc., and some of these floated for a long time.
It is well known what a diff'erence 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 sunk quickly, but
some which, while green, floated, for a very short time, when dried, floated
much longer; for instance, ripe hazel-nuts sunk immediately, but when dried
they floated for ninety days, and afterward 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 afterward germinated; the ripe
seeds of Helosciadium sunk in two days, when dried they floated for above
ninety days, and afterward 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 ^%7 kinds of
seeds germinated after an immersion of twenty-eight days; and as ■^%4:
distinct species with ripe fruit (but not all the same species as in the fore-
going experiment) floated, after being dried, for above twenty-eight days,



GEOGRAPHICAL DISTRIBUTION 271

we may conclude, as far as anything can be inferred from these scanty
facts, that the seeds of ^%oo 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 ^%oo
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
favorable 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 favored 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 ^%s oi 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 ^%oo 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 facts 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 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 inclosed by the roots of an oak about fifty years old, three dicot-
yledonous plants germinated: I am certain of the accuracy of this observa-
tion. 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: pease 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 transporta-
tion of seeds. I could give many facts showing how frequently birds of many



272 THE ORIGIN OF SPECIES

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, il
I have never seen an instance of nutritious seeds passing through the in|'
testines 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 ii
my garden twelve kinds of seeds, out of the excrement of small birds, an(
these seemed perfect, and some of them, which were tried, germinated. Bui
the following fact is more important : the crops of birds do not secrete gastric i:
juice, and do not, as I know by trial, injure in the least the germination off
seeds; now, after a bird has found and devoured a large supply of food, it it
is positively asserted that all the grains do not pass into the gizzard for r
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 scat-
tered. 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
experiment 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 excre-
ment; 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 im-
mense 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 dis-
appeared 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



GEOGRAPHICAL DISTRIBUTION 273

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 some-
times 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 whin-
chats (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 occasion-
ally 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 Atlan-
tic, which, stand nearer to the mainland and (as remarked by Mr. H. G.
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 G. Lyell wrote to
M. Hartung to inquire whether he had observed erratic bowlders 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 some few seeds of Northern plants.

Considering that these several means of transport, and that other means,



274 THE ORIGIN OF SPECIES

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 r
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 thei/"
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 con-
tinent to a neighboring island, but not from one distant continent to an-
other. 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 favorable 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 naturalized. But this is no valid argument against what would
be efTected by occasional means of transport, during the long lapse of
geological time, while 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, sepa-
rated 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 re-
markable fact to see so many plants of the same species living on the snowy



GEOGRAPHICAL DISTRIBUTION 275

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 Moun-
tains, 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 1 747, 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
burned 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
bowlders, 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 bowlders 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 in-
habitants of the temperate regions. The latter, at the same time, would
travel farther and farther 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, while 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.






276 THE ORIGIN OF SPECIES

Thus we can understand the identity of many plants at points so im-
mensely 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 moun-
tain range are more especially related to the arctic forms living due north I
or nearly due north of them: for the first migration when the cold camei
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 afterward backward to the
north, in unison with the changing climate, they will not have been ex-
posed 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 in-
culcated in this volume, these forms will not have been liable to much
modification. But with the alpine productions, left isolated from the mo-
ment of the returning warmth, first at the bases and ultimately on the sum-
mits 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 commence-
ment of the Glacial epoch, and which during the coldest period will have
been temporarily driven down to the plains; they will, also, have been sub-
sequently 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
of 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 neces-
sary 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



GEOGRAPHICAL DISTRIBUTION 277



Online LibraryCharles DarwinThe origin of species → online text (page 37 of 50)