On the Origin of Species by Means of Natural Selection
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I must now say a few words on what are called accidental means, but which
more properly might be called occasional means of distribution. I shall
here confine myself to plants. In botanical works, this or that plant is
stated to be ill adapted for wide dissemination; but for transport across
the sea, the greater or less facilities may be said to be almost wholly
unknown. Until I tried, with Mr. Berkeley's aid, a few experiments, it was
not even known how far seeds could resist the injurious action of
sea-water. To my surprise I found that out of 87 kinds, 64 germinated
after an immersion of 28 days, and a few survived an immersion of 137 days.
For convenience sake I chiefly tried small seeds, without the capsule or
fruit; and as all of these sank in a few days, they could not be floated
across wide spaces of the sea, whether or not they were injured by the
salt-water. Afterwards I tried some larger fruits, capsules, &c., and some
of these floated for a long time. It is well known what a difference there
is in the buoyancy of green and seasoned timber; and it occurred to me that
floods might wash down plants or branches, and that these might be dried on
the banks, and then by a fresh rise in the stream be washed into the sea.
Hence I was led to dry stems and branches of 94 plants with ripe fruit, and
to place them on sea water. The majority sank quickly, but some which
whilst green floated for a very short time, when dried floated much longer;
for instance, ripe hazel-nuts sank immediately, but when dried, they
floated for 90 days and afterwards when planted they germinated; an
asparagus plant with ripe berries floated for 23 days, when dried it
floated for 85 days, and the seeds afterwards germinated: the ripe seeds
of Helosciadium sank in two days, when dried they floated for above 90
days, and afterwards germinated. Altogether out of the 94 dried plants, 18
floated for above 28 days, and some of the 18 floated for a very much
longer period. So that as 64/87 seeds germinated after an immersion of 28
days; and as 18/94 plants with ripe fruit (but not all the same species as
in the foregoing experiment) floated, after being dried, for above 28 days,
as far as we may infer anything from these scanty facts, we may conclude
that the seeds of 14/100 plants of any country might be floated by
sea-currents during 28 days, and would retain their power of germination.
In Johnston's Physical Atlas, the average rate of the several Atlantic
currents is 33 miles per diem (some currents running at the rate of 60
miles per diem); on this average, the seeds of 14/100 plants belonging to
one country might be floated across 924 miles of sea to another country;
and when stranded, if blown to a favourable spot by an inland gale, they
would germinate.
Subsequently to my experiments, M. Martens tried similar ones, but in a
much better manner, for he placed the seeds in a box in the actual sea, so
that they were alternately wet and exposed to the air like really floating
plants. He tried 98 seeds, mostly different from mine; but he chose many
large fruits and likewise seeds from plants which live near the sea; and
this would have favoured the average length of their flotation and of their
resistance to the injurious action of the salt-water. On the other hand he
did not previously dry the plants or branches with the fruit; and this, as
we have seen, would have caused some of them to have floated much longer.
The result was that 18/98 of his seeds floated for 42 days, and were then
capable of germination. But I do not doubt that plants exposed to the
waves would float for a less time than those protected from violent
movement as in our experiments. Therefore it would perhaps be safer to
assume that the seeds of about 10/100 plants of a flora, after having been
dried, could be floated across a space of sea 900 miles in width, and would
then germinate. The fact of the larger fruits often floating longer than
the small, is interesting; as plants with large seeds or fruit could hardly
be transported by any other means; and Alph. de Candolle has shown that
such plants generally have restricted ranges.
But seeds may be occasionally transported in another manner. Drift timber
is thrown up on most islands, even on those in the midst of the widest
oceans; and the natives of the coral-islands in the Pacific, procure stones
for their tools, solely from the roots of drifted trees, these stones being
a valuable royal tax. I find on examination, that when irregularly shaped
stones are embedded in the roots of trees, small parcels of earth are very
frequently enclosed in their interstices and behind them,--so perfectly
that not a particle could be washed away in the longest transport: out of
one small portion of earth thus completely enclosed by wood in an oak about
50 years old, three dicotyledonous plants germinated: I am certain of the
accuracy of this observation. Again, I can show that the carcasses of
birds, when floating on the sea, sometimes escape being immediately
devoured; and seeds of many kinds in the crops of floating birds long
retain their vitality: peas and vetches, for instance, are killed by even
a few days' immersion in sea-water; but some taken out of the crop of a
pigeon, which had floated on artificial salt-water for 30 days, to my
surprise nearly all germinated.
Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how frequently
birds of many kinds are blown by gales to vast distances across the ocean.
We may I think safely assume that under such circumstances their rate of
flight would often be 35 miles an hour; and some authors have given a far
higher estimate. I have never seen an instance of nutritious seeds passing
through the intestines of a bird; but hard seeds of fruit will pass
uninjured through even the digestive organs of a turkey. In the course of
two months, I picked up in my garden 12 kinds of seeds, out of the
excrement of small birds, and these seemed perfect, and some of them, which
I tried, germinated. But the following fact is more important: the crops
of birds do not secrete gastric juice, and do not in the least injure, as I
know by trial, the germination of seeds; now after a bird has found and
devoured a large supply of food, it is positively asserted that all the
grains do not pass into the gizzard for 12 or even 18 hours. A bird in
this interval might easily be blown to the distance of 500 miles, and hawks
are known to look out for tired birds, and the contents of their torn crops
might thus readily get scattered. Mr. Brent informs me that a friend of
his had to give up flying carrier-pigeons from France to England, as the
hawks on the English coast destroyed so many on their arrival. Some hawks
and owls bolt their prey whole, and after an interval of from twelve to
twenty hours, disgorge pellets, which, as I know from experiments made in
the Zoological Gardens, include seeds capable of germination. Some seeds
of the oat, wheat, millet, canary, hemp, clover, and beet germinated after
having been from twelve to twenty-one hours in the stomachs of different
birds of prey; and two seeds of beet grew after having been thus retained
for two days and fourteen hours. Freshwater fish, I find, eat seeds of
many land and water plants: fish are frequently devoured by birds, and
thus the seeds might be transported from place to place. I forced many
kinds of seeds into the stomachs of dead fish, and then gave their bodies
to fishing-eagles, storks, and pelicans; these birds after an interval of
many hours, either rejected the seeds in pellets or passed them in their
excrement; and several of these seeds retained their power of germination.
Certain seeds, however, were always killed by this process.
Although the beaks and feet of birds are generally quite clean, I can show
that earth sometimes adheres to them: in one instance I removed twenty-two
grains of dry argillaceous earth from one foot of a partridge, and in this
earth there was a pebble quite as large as the seed of a vetch. Thus seeds
might occasionally be transported to great distances; for many facts could
be given showing that soil almost everywhere is charged with seeds.
Reflect for a moment on the millions of quails which annually cross the
Mediterranean; and can we doubt that the earth adhering to their feet would
sometimes include a few minute seeds? But I shall presently have to recur
to this subject.
As icebergs are known to be sometimes loaded with earth and stones, and
have even carried brushwood, bones, and the nest of a land-bird, I can
hardly doubt that they must occasionally have transported seeds from one
part to another of the arctic and antarctic regions, as suggested by Lyell;
and during the Glacial period from one part of the now temperate regions to
another. In the Azores, from the large number of the species of plants
common to Europe, in comparison with the plants of other oceanic islands
nearer to the mainland, and (as remarked by Mr. H. C. Watson) from the
somewhat northern character of the flora in comparison with the latitude, I
suspected that these islands had been partly stocked by ice-borne seeds,
during the Glacial epoch. At my request Sir C. Lyell wrote to M. Hartung
to inquire whether he had observed erratic boulders on these islands, and
he answered that he had found large fragments of granite and other rocks,
which do not occur in the archipelago. Hence we may safely infer that
icebergs formerly landed their rocky burthens on the shores of these
mid-ocean islands, and it is at least possible that they may have brought
thither the seeds of northern plants.
Considering that the several above means of transport, and that several
other means, which without doubt remain to be discovered, have been in
action year after year, for centuries and tens of thousands of years, it
would I think be a marvellous fact if many plants had not thus become
widely transported. These means of transport are sometimes called
accidental, but this is not strictly correct: the currents of the sea are
not accidental, nor is the direction of prevalent gales of wind. It should
be observed that scarcely any means of transport would carry seeds for very
great distances; for seeds do not retain their vitality when exposed for a
great length of time to the action of seawater; nor could they be long
carried in the crops or intestines of birds. These means, however, would
suffice for occasional transport across tracts of sea some hundred miles in
breadth, or from island to island, or from a continent to a neighbouring
island, but not from one distant continent to another. The floras of
distant continents would not by such means become mingled in any great
degree; but would remain as distinct as we now see them to be. The
currents, from their course, would never bring seeds from North America to
Britain, though they might and do bring seeds from the West Indies to our
western shores, where, if not killed by so long an immersion in salt-water,
they could not endure our climate. Almost every year, one or two
land-birds are blown across the whole Atlantic Ocean, from North America to
the western shores of Ireland and England; but seeds could be transported
by these wanderers only by one means, namely, in dirt sticking to their
feet, which is in itself a rare accident. Even in this case, how small
would the chance be of a seed falling on favourable soil, and coming to
maturity! But it would be a great error to argue that because a
well-stocked island, like Great Britain, has not, as far as is known (and
it would be very difficult to prove this), received within the last few
centuries, through occasional means of transport, immigrants from Europe or
any other continent, that a poorly-stocked island, though standing more
remote from the mainland, would not receive colonists by similar means. I
do not doubt that out of twenty seeds or animals transported to an island,
even if far less well-stocked than Britain, scarcely more than one would be
so well fitted to its new home, as to become naturalised. But this, as it
seems to me, is no valid argument against what would be effected by
occasional means of transport, during the long lapse of geological time,
whilst an island was being upheaved and formed, and before it had become
fully stocked with inhabitants. On almost bare land, with few or no
destructive insects or birds living there, nearly every seed, which chanced
to arrive, would be sure to germinate and survive.
Dispersal during the Glacial period. -- The identity of many plants and
animals, on mountain-summits, separated from each other by hundreds of
miles of lowlands, where the Alpine species could not possibly exist, is
one of the most striking cases known of the same species living at distant
points, without the apparent possibility of their having migrated from one
to the other. It is indeed a remarkable fact to see so many of the same
plants living on the snowy regions of the Alps or Pyrenees, and in the
extreme northern parts of Europe; but it is far more remarkable, that the
plants on the White Mountains, in the United States of America, are all the
same with those of Labrador, and nearly all the same, as we hear from Asa
Gray, with those on the loftiest mountains of Europe. Even as long ago as
1747, such facts led Gmelin to conclude that the same species must have
been independently created at several distinct points; and we might have
remained in this same belief, had not Agassiz and others called vivid
attention to the Glacial period, which, as we shall immediately see,
affords a simple explanation of these facts. We have evidence of almost
every conceivable kind, organic and inorganic, that within a very recent
geological period, central Europe and North America suffered under an
Arctic climate. The ruins of a house burnt by fire do not tell their tale
more plainly, than do the mountains of Scotland and Wales, with their
scored flanks, polished surfaces, and perched boulders, of the icy streams
with which their valleys were lately filled. So greatly has the climate of
Europe changed, that in Northern Italy, gigantic moraines, left by old
glaciers, are now clothed by the vine and maize. Throughout a large part
of the United States, erratic boulders, and rocks scored by drifted
icebergs and coast-ice, plainly reveal a former cold period.
The former influence of the glacial climate on the distribution of the
inhabitants of Europe, as explained with remarkable clearness by Edward
Forbes, is substantially as follows. But we shall follow the changes more
readily, by supposing a new glacial period to come slowly on, and then pass
away, as formerly occurred. As the cold came on, and as each more southern
zone became fitted for arctic beings and ill-fitted for their former more
temperate inhabitants, the latter would be supplanted and arctic
productions would take their places. The inhabitants of the more temperate
regions would at the same time travel southward, unless they were stopped
by barriers, in which case they would perish. The mountains would become
covered with snow and ice, and their former Alpine inhabitants would
descend to the plains. By the time that the cold had reached its maximum,
we should have a uniform arctic fauna and flora, covering the central parts
of Europe, as far south as the Alps and Pyrenees, and even stretching into
Spain. The now temperate regions of the United States would likewise be
covered by arctic plants and animals, and these would be nearly the same
with those of Europe; for the present circumpolar inhabitants, which we
suppose to have everywhere travelled southward, are remarkably uniform
round the world. We may suppose that the Glacial period came on a little
earlier or later in North America than in Europe, so will the southern
migration there have been a little earlier or later; but this will make no
difference in the final result.
As the warmth returned, the arctic forms would retreat northward, closely
followed up in their retreat by the productions of the more temperate
regions. And as the snow melted from the bases of the mountains, the
arctic forms would seize on the cleared and thawed ground, always ascending
higher and higher, as the warmth increased, whilst their brethren were
pursuing their northern journey. Hence, when the warmth had fully
returned, the same arctic species, which had lately lived in a body
together on the lowlands of the Old and New Worlds, would be left isolated
on distant mountain-summits (having been exterminated on all lesser
heights) and in the arctic regions of both hemispheres.
Thus we can understand the identity of many plants at points so immensely
remote as on the mountains of the United States and of Europe. We can thus
also understand the fact that the Alpine plants of each mountain-range are
more especially related to the arctic forms living due north or nearly due
north of them: for the migration as the cold came on, and the re-migration
on the returning warmth, will generally have been due south and north. The
Alpine plants, for example, of Scotland, as remarked by Mr. H. C. Watson,
and those of the Pyrenees, as remarked by Ramond, are more especially
allied to the plants of northern Scandinavia; those of the United States to
Labrador; those of the mountains of Siberia to the arctic regions of that
country. These views, grounded as they are on the perfectly
well-ascertained occurrence of a former Glacial period, seem to me to
explain in so satisfactory a manner the present distribution of the Alpine
and Arctic productions of Europe and America, that when in other regions we
find the same species on distant mountain-summits, we may almost conclude
without other evidence, that a colder climate permitted their former
migration across the low intervening tracts, since become too warm for
their existence.
If the climate, since the Glacial period, has ever been in any degree
warmer than at present (as some geologists in the United States believe to
have been the case, chiefly from the distribution of the fossil Gnathodon),
then the arctic and temperate productions will at a very late period have
marched a little further north, and subsequently have retreated to their
present homes; but I have met with no satisfactory evidence with respect to
this intercalated slightly warmer period, since the Glacial period.
The arctic forms, during their long southern migration and re-migration
northward, will have been exposed to nearly the same climate, and, as is
especially to be noticed, they will have kept in a body together;
consequently their mutual relations will not have been much disturbed, and,
in accordance with the principles inculcated in this volume, they will not
have been liable to much modification. But with our Alpine productions,
left isolated from the moment of the returning warmth, first at the bases
and ultimately on the summits of the mountains, the case will have been
somewhat different; for it is not likely that all the same arctic species
will have been left on mountain ranges distant from each other, and have
survived there ever since; they will, also, in all probability have become
mingled with ancient Alpine species, which must have existed on the
mountains before the commencement of the Glacial epoch, and which during
its coldest period will have been temporarily driven down to the plains;
they will, also, have been exposed to somewhat different climatal
influences. Their mutual relations will thus have been in some degree
disturbed; consequently they will have been liable to modification; and
this we find has been the case; for if we compare the present Alpine plants
and animals of the several great European mountain-ranges, though very many
of the species are identically the same, some present varieties, some are
ranked as doubtful forms, and some few are distinct yet closely allied or
representative species.
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