CHAPTER XII - GEOGRAPHICAL DISTRIBUTION
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 more distant points, and
removes many a difficulty; but to the best of my judgment we are not
authorised in admitting such enormous geographical changes within
the period of existing species. It seems to me that we have abundant
evidence of great oscillations in the level of the land or sea; but
not of such vast change 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 with 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 87 kinds, 64 germinated after an immersion of 28
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 Polemoniacae, were all killed by a month's
immersion. 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 have been 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 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 94 plants with ripe fruit, and to place them on sea-water.
The majority sank rapidly, but some which, whilst green, floated for a
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 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 kinds of seeds germinated
after an immersion of 28 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 28 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 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
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 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 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 18/98ths of his seeds of different kinds
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
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 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 enclosed by the roots of 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 carcases 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 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 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 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 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 500 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 upwards 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), wheat-ears, 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, Prof. 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 82 plants sprung from it: these consisted of 12 monocotyledons,
including the common oat, and at least one kind of grass, and of 70
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
burthens 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, 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.
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