another region, whence it has been stocked. 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
plainly related by inheritance to the inhabitants of the continent. Cases
of this nature are common, and are, as we shall hereafter more fully see,
inexplicable on the theory of independent creation. This view of the
relation of species in one region to those in another, does not differ much
(by substituting the word variety for species) from that lately advanced in
an ingenious paper by Mr. Wallace, in which he concludes, that 'every
species has come into existence coincident both in space and time with a
pre-existing closely allied species.' And I now know from correspondence,
that this coincidence he attributes to generation with modification.
The previous remarks on 'single and multiple centres of creation' do not
directly bear on another allied question,--namely whether all the
individuals of the same species have descended from a single pair, or
single hermaphrodite, or whether, as some authors suppose, from many
individuals simultaneously created. With those organic beings which never
intercross (if such exist), the species, on my theory, must have descended
from a succession of improved varieties, which will never have blended with
other individuals or varieties, but will have supplanted each other; so
that, at each successive stage of modification and improvement, all the
individuals of each variety will have descended from a single parent. But
in the majority of cases, namely, with all organisms which habitually unite
for each birth, or which often intercross, I believe that during the slow
process of modification the individuals of the species will have been kept
nearly uniform by intercrossing; so that many individuals will have gone on
simultaneously changing, and the whole amount of modification will not have
been due, at each stage, to descent from a single parent. To illustrate
what I mean: our English racehorses differ slightly 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 selecting and
training many individuals during many generations.
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 when its climate was different may have been a high
road for migration, but now be impassable; 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 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 will
dispute that great mutations of level have occurred within the period of
existing organisms. Edward Forbes insisted that all the islands in the
Atlantic must recently have been connected with Europe or Africa, and
Europe likewise with America. Other authors have thus hypothetically
bridged over every ocean, and have united almost every island to 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 any 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 of level in our continents; but not of such vast changes in
their position and extension, 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, as I believe, by rings of coral or atolls standing
over them. Whenever it is fully admitted, as I believe 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 continents which are now 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,--a certain degree of
relation (as we shall hereafter see) between the distribution of mammals
and the depth of the sea,--these and other such facts seem to me opposed to
the admission of such prodigious geographical revolutions within the recent
period, as are necessitated on the view advanced by Forbes and admitted by
his many followers. The nature and relative proportions of the inhabitants
of oceanic islands likewise seem to me opposed to the belief of their
former continuity with continents. Nor does their almost universally
volcanic composition favour the admission that they are the wrecks of
sunken continents;--if they had originally existed as mountain-ranges on
the land, some at least of the islands would have been formed, like other
mountain-summits, of granite, metamorphic schists, old fossiliferous or
other such 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 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 sev
eral 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