GEOGRAPHICAL DISTRIBUTION.
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 co-extensive with the world.
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 accounted for by their 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 northern parts where the circumpolar land is
almost continuous, 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; the most 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 as closely as the
same species generally require; for it is a most rare case to find a group
of organisms confined to any small spot, having conditions peculiar in only
a slight {347} degree; for instance, small areas in the Old World could be
pointed out hotter than any in the New World, yet these are not inhabited
by a peculiar fauna or flora. Notwithstanding this parallelism in the
conditions of the 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°, 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 lat. 35° with those north of 25°, which consequently inhabit a
considerably different climate, and they will be found 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 of 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
{348} lofty and continuous mountain-ranges, and of great deserts, and
sometimes even of large rivers, we find different productions; though as
mountain-chains, deserts, &c., 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. No two marine faunas are more
distinct, with hardly a fish, shell, or crab in common, than those of the
eastern and western shores of South and Central America; yet these great
faunas are separated only by the narrow, but impassable, isthmus of Panama.
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 here three
marine faunas range far northward and southward, in parallel lines not far
from each other, under corresponding climates; but from being separated
from each other by impassable barriers, either of land or open sea, they
are 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 hardly one shell, crab or fish is
common to the above-named three approximate faunas of Eastern and Western
America and the eastern Pacific islands, yet many fish range from the
Pacific into the Indian Ocean, and many shells are common to the eastern
islands of the Pacific {349} and the eastern shores of Africa, on almost
exactly opposite meridians of longitude.
A third great fact, partly included in the foregoing statements, is the
affinity of the productions of the same continent or 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, yet clearly 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 found in 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 musk-rat, but the coypu and
capybara, rodents of the 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, though they may
be all peculiar species, are essentially American. We may look back to past
ages, as shown in the last chapter, and we find American types then
prevalent on {350} the American continent and in the American seas. We see
in these facts some deep organic bond, prevailing throughout space and
time, over the same areas of land and water, and independent of their
physical conditions. The naturalist must feel little curiosity, who is not
led to inquire what this bond is.
This bond, on my theory, is simply inheritance, that cause which alone, as
far as we positively know, produces organisms quite like, or, as we see in
the case of varieties, nearly like each other. The dissimilarity of the
inhabitants of different regions may be attributed to modification through
natural selection, and in a quite subordinate degree to the direct
influence of different physical conditions. The degree of dissimilarity
will depend on the migration of the more dominant forms of life from one
region into another having been effected with more or less ease, at periods
more or less remote;--on the nature and number of the former
immigrants;--and on their action and reaction, in their mutual struggles
for life;--the relation of organism to organism 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 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, {351} and even
families are confined to the same areas, as is so commonly and notoriously
the case.
I believe, as was remarked in the last chapter, in no 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 the individual in its complex struggle for life, so the degree of
modification in different species will be no uniform quantity. If, for
instance, a number of species, which stand in direct competition with each
other, migrate in a body into a new and afterwards isolated country, they
will be little liable to modification; for neither migration nor isolation
in themselves can do 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 modified.
On 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 have descended from the same
progenitor. In the case of those species, which have undergone during whole
geological periods but little modification, there is not much difficulty in
believing that they may have migrated from the same region; for during the
vast geographical and climatal changes which will 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 {352} 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 explained in the last chapter, it is incredible that individuals
identically the same should ever have been produced through natural
selection from parents specifically distinct.
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 very 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 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
be easily passed over by migration, the fact is given as something
remarkable and exceptional. The capacity of migrating across the sea is
more distinctly limited in terrestrial mammals, than perhaps in any other
organic beings; and, accordingly, we find no inexplicable cases of the same
mammal inhabiting distant points of the world. No geologist will feel any
difficulty in such cases as Great Britain having been formerly united to
Europe, and consequently possessing the same quadrupeds. But if the same
species can be produced at two separate points, why do we not find a single
mammal common to Europe and {353} 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 vast and broken
interspace. The great and striking influence which barriers of every kind
have had on distribution, is intelligible only on the view that the great
majority of species have been produced on one side alone, and have not been
able to migrate to the other side. Some few families, many sub-families,
very many genera, and 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 local, or
confined to one area. What a strange anomaly it would be, if, when coming
one step lower in the series, to the individuals of the same species, a
directly opposite rule prevailed; and species were not local, but had been
produced in two or more distinct areas!
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 climatal changes, which have certainly occurred within
recent geological times, must have interrupted or rendered discontinuous
the {354} 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 such cases. 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
freshwater productions; and thirdly, the occurrence of the same terrestrial
species on islands and on the 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 climatal and geographical
changes and various occasional means of transport, the belief that this has
been the universal 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
distinct species of a genus, which on my theory have all descended from a
common progenitor, can have migrated (undergoing modification during some
part of their migration) from the area inhabited by their progenitor. If it
can be shown to be almost invariably the case, that a region, of which
{355} most of its inhabitants are closely related to, or belong to the same
genera with the species of a second region, has probably received at some
former period immigrants from this other region, my theory will be
strengthened; for we can clearly understand, on the principle of
modification, why the inhabitants of a region should be related to those of
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 {356} 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 {357} 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 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 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
{358} 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 {359} 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 {360} 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, {361} 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 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. {362} 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 {363} 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 {364} 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 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 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 {365} (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, if fitted
for the climate, 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 {366} 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 {367} 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 {368} 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, {369} 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.
In illustrating what, as I believe, actually took place during the Glacial
period, I assumed that at its commencement the arctic productions were as
uniform round the polar regions as they are at the present day. But the
foregoing remarks on distribution apply not only to strictly arctic forms,
but also to many sub-arctic and to some few northern temperate forms, for
some of these are the same on the lower mountains and on the plains of
North America and Europe; and it may be reasonably asked how I account for
the necessary degree of uniformity of the sub-arctic and northern temperate
forms round the world, at the commencement of the 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 {370} Atlantic Ocean
and by the extreme northern part of the Pacific. During the Glacial period,
when the inhabitants of the Old and New Worlds lived further southwards
than at present, they must have been still more completely separated by
wider spaces of ocean. I believe the above difficulty may be surmounted by
looking to still earlier changes of climate of an opposite nature. We have
good reason to believe that during the newer Pliocene period, before the
Glacial epoch, and whilst the majority of the inhabitants of the world were
specifically the same as now, the climate was warmer than at the present
day. Hence we may suppose that the organisms now living under the climate
of latitude 60°, during the Pliocene period lived further north under the
Polar Circle, in latitude 66°-67°; and that the strictly arctic productions
then lived on the broken land still nearer to the pole. Now if we look at a
globe, we shall see that under the Polar Circle there is almost continuous
land from western Europe, through Siberia, to eastern America. And to this
continuity of the circumpolar land, and to the consequent freedom for
intermigration under a more favourable climate, I attribute the necessary
amount of uniformity in the sub-arctic and northern 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 large,
but partial 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 {371} 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 most remarkable, considering the distance
of the two areas, and their separation by the 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 must have been completely cut off from each
other. This separation, as far as the more temperate productions are
concerned, took place long ages ago. And as the plants and animals migrated
southward, they will have become mingled in the one great region with the
native American productions, and 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 the two
Worlds. Hence it has come, that when we compare {372} 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 areas completely sundered. Thus, I think, we can understand the
presence of many existing and tertiary representative forms on the eastern
and western shores of temperate North America; and the still more striking
case of many closely allied crustaceans (as described in Dana's admirable
work), of some fish and other marine animals, in the Mediterranean and in
the seas of Japan,--areas now separated by a continent and by nearly a
hemisphere of equatorial ocean.
These cases of relationship, without identity, of the inhabitants of seas
now disjoined, and likewise of the past and present inhabitants of the
temperate lands of North America and Europe, are inexplicable on the theory
of creation. We cannot say that they 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 the
southern continents of the Old World, we see countries closely
corresponding in all their physical conditions, but with their inhabitants
utterly dissimilar. {373}
But we must return to our more immediate subject, the Glacial period. I am
convinced that Forbes's view may be largely extended. In Europe we have the
plainest evidence of the cold period, from the western shores of Britain to
the Oural range, and southward to the Pyrenees. We may infer from the
frozen mammals and nature of the mountain vegetation, that Siberia was
similarly affected. 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 gigantic ancient moraines. South of the equator, we
have some direct evidence of former glacial action in New Zealand; and the
same plants, found on widely separated mountains in that island, tell the
same story. If one account which has been published can be trusted, we have
direct evidence of glacial action in 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 as far south as lat. 36°-37°, and on the
shores of the Pacific, where the climate is now so different, as far south
as lat. 46°; erratic boulders have, also, been noticed on the Rocky
Mountains. In the Cordillera of Equatorial South America, glaciers once
extended far below their present level. In central Chili I was astonished
at the structure of a vast mound of detritus, about 800 feet in height,
crossing a valley of the Andes; and this I now feel convinced was a
gigantic moraine, left far below any existing glacier. Further south on
both sides of the continent, from lat. 41° to the southernmost extremity,
we have the clearest evidence of former glacial action, in huge boulders
transported far from their parent source.
We do not know that the Glacial epoch was strictly simultaneous at these
several far distant points on {374} opposite sides of the world. But we
have good evidence in almost every case, that the epoch was included within
the latest geological period. We have, also, excellent evidence, that it
endured for an enormous time, as measured by years, at each point. The cold
may have come on, or have ceased, earlier at one point of the globe than at
another, but seeing that it endured for long at each, and that it was
contemporaneous in a geological sense, it seems to me probable that it was,
during a part at least of the period, actually simultaneous throughout the
world. Without some distinct evidence to the contrary, we may at least
admit as probable that the glacial action was simultaneous on the eastern
and western sides of North America, in the Cordillera under the equator and
under the warmer temperate zones, and on both sides of the southern
extremity of the continent. If this be admitted, it is difficult to avoid
believing that the temperature of the whole world was at this period
simultaneously cooler. But it would suffice for my purpose, if the
temperature was at the same time lower along certain broad belts of
longitude.
On this view of the whole world, or at least of broad longitudinal belts,
having been simultaneously colder from pole to pole, much light can be
thrown on the present distribution of identical and allied species. In
America, Dr. Hooker has shown that between forty and fifty of the flowering
plants of Tierra del Fuego, forming no inconsiderable part of its scanty
flora, are common to Europe, enormously remote as these two points are; and
there are many closely allied species. On the lofty mountains of equatorial
America a host of peculiar species belonging to European genera occur. On
the highest mountains of Brazil, some few European genera were found by
Gardner, which do not exist in the wide {375} intervening hot countries. So
on the Silla of Caraccas the illustrious Humboldt long ago found species
belonging to genera characteristic of the Cordillera. On the mountains of
Abyssinia, several European forms and some few representatives of the
peculiar flora of the Cape of Good Hope occur. At the Cape of Good Hope a
very few European species, believed not to have been introduced by man, and
on the mountains, some few representative European forms are found, which
have not been discovered in the intertropical parts of Africa. 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 collected on the loftier peaks of Java
raises a picture of a collection made on a hill in Europe! Still more
striking is the fact that southern Australian forms are clearly represented
by 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 as far north as Japan.
On the southern mountains of Australia, Dr. F. Müller 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 throughout the world, the plants
growing on the {376} more lofty mountains, and on the temperate lowlands of
the northern and southern hemispheres, are sometimes identically the same;
but they are much oftener specifically distinct, though related to each
other in a most remarkable manner.
This brief abstract applies to plants alone: some strictly analogous facts
could be given on the distribution of terrestrial animals. In marine
productions, similar cases occur; as an example, I may quote a remark 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, &c., of northern forms of fish. Dr. Hooker informs me that
twenty-five species of Algæ are common to New Zealand and to Europe, but
have not been found in the intermediate tropical seas.
It should be observed that the northern species and forms found in the
southern parts of the southern hemisphere, and on the mountain-ranges of
the intertropical regions, are not arctic, but belong to the northern
temperate zones. As Mr. H. C. Watson has recently remarked, "In receding
from polar towards equatorial latitudes, the Alpine or mountain floras
really become less and less arctic." Many of the forms living on the
mountains of the warmer regions of the earth and in the southern hemisphere
are of doubtful value, being ranked by some naturalists as specifically
distinct, by others as varieties; but some are certainly identical, and
many, though closely related to northern forms, must be ranked as distinct
species.
Now let us see what light can be thrown on the foregoing facts, on the
belief, supported as it is by a large {377} body of geological evidence,
that the whole world, or a large part of it, was during the Glacial period
simultaneously much colder than at present. 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 came slowly on, all the tropical plants and other productions will
have retreated from both sides towards the equator, followed in the rear by
the temperate productions, and these by the arctic; but with the latter we
are not now concerned. The tropical plants probably suffered much
extinction; how much no one can say; perhaps formerly the tropics supported
as many species as we see at the present day crowded together at the Cape
of Good Hope, and in parts of temperate Australia. As we know that many
tropical plants and animals can withstand a considerable amount of cold,
many might have escaped extermination during a moderate fall of
temperature, more especially by escaping into the lowest, most protected,
and warmest districts. But the great fact to bear in mind is, that all
tropical productions will have suffered to a certain extent. On the other
hand, the temperate productions, after migrating nearer to the equator,
though they will have been placed under somewhat new conditions, will have
suffered less. And it is certain that many temperate plants, if protected
from the inroads of competitors, can withstand a much warmer climate than
their own. Hence, it seems to me possible, bearing in mind that the
tropical productions were in a suffering state and could not have presented
a firm front against intruders, that a certain number of the more vigorous
and dominant temperate forms might have penetrated the native ranks and
have reached or {378} even crossed the equator. The invasion would, of
course, have been greatly favoured by high land, and perhaps by a dry
climate; for Dr. Falconer informs me that it is the damp with the heat of
the tropics which is so destructive to perennial plants from a temperate
climate. On the other hand, the most humid and hottest districts will have
afforded an asylum to the tropical natives. The mountain-ranges north-west
of the Himalaya, and the long line of the Cordillera, seem to have afforded
two great lines of invasion: and it is a striking fact, lately communicated
to me by Dr. Hooker, that all the flowering plants, about forty-six in
number, common to Tierra del Fuego and to Europe still exist in North
America, which must have lain on the line of march. But I do not doubt that
some temperate productions entered and crossed even the _lowlands_ of the
tropics at the period when the cold was most intense,--when arctic forms
had migrated some twenty-five degrees of latitude from their native country
and covered the land at the foot of the Pyrenees. At this period of extreme
cold, I believe that the climate under the equator at the level of the sea
was about the same with that now felt there at the height of six or seven
thousand feet. During this the coldest period, I suppose that large spaces
of the tropical lowlands were clothed with a mingled tropical and temperate
vegetation, like that now growing with strange luxuriance at the base of
the Himalaya, as graphically described by Hooker.
Thus, as I believe, a considerable number of plants, a few terrestrial
animals, and some marine productions, migrated during the Glacial period
from the northern and southern temperate zones into the intertropical
regions, and some even crossed the equator. As the warmth returned, these
temperate forms would naturally ascend the higher mountains, being
exterminated on the {379} lowlands; those which had not reached the equator
would re-migrate northward or southward towards their former homes; but the
forms, chiefly northern, which had crossed the equator, would travel still
further from their homes into the more temperate latitudes of the opposite
hemisphere. Although we have reason to believe from geological evidence
that the whole body of arctic shells underwent scarcely any modification
during their long southern migration and re-migration northward, the case
may have been wholly different with those intruding forms which settled
themselves on the intertropical mountains, and in the southern hemisphere.
These being surrounded by strangers will have had to compete with many new
forms of life; and it is probable that selected modifications in their
structure, habits, and constitutions will have profited them. Thus many of
these wanderers, though still plainly related by inheritance to their
brethren of the northern or southern hemispheres, now exist in their new
homes as well-marked varieties or as distinct species.
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 plants and allied forms have apparently migrated from the north
to the south, than in a reversed direction. We see, however, a few southern
vegetable forms on the mountains of Borneo and Abyssinia. I suspect that
this preponderant migration from north to 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 they became
commingled during the Glacial period, the northern forms {380} were enabled
to beat the less powerful southern forms. Just in the same manner as we see
at the present day, that very many European productions cover the ground in
La Plata, and in a lesser degree in Australia, and have to a certain extent
beaten the natives; whereas extremely few southern forms have become
naturalised in any part of Europe, 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 thirty or
forty years from Australia. Something of the same kind must have occurred
on the intertropical mountains: no doubt before the Glacial period they
were stocked with endemic Alpine forms; but these have almost everywhere
largely 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 the naturalised; and
if the natives have not been actually exterminated, their numbers have been
greatly reduced, and this is the first stage towards extinction. A mountain
is an island on the land; and the intertropical mountains before the
Glacial period must have been completely isolated; and I believe that the
productions of these islands on the land yielded to those produced within
the larger areas of the north, just in the same way as the productions of
real islands have everywhere lately yielded to continental forms,
naturalised by man's agency.
I am far from supposing that all difficulties are removed on the view here
given in regard to the range and affinities of the allied species which
live in the northern and southern temperate zones and on the mountains of
the intertropical regions. Very many difficulties remain to be solved. I do
not pretend to {381} indicate the exact lines and means of migration, or
the reason why certain species and not others have migrated; why certain
species have been modified and have given rise to new groups of forms, and
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 ranges twice or thrice as far, and is
twice or thrice as common, as another species within their own homes.
I have said that many difficulties remain to be solved: some of the most
remarkable are stated with admirable clearness by Dr. Hooker in his
botanical works on the antarctic regions. These cannot be here discussed. I
will only say that as far as regards the occurrence of identical species at
points so enormously remote as Kerguelen Land, New Zealand, and Fuegia, I
believe that towards the close of the Glacial period, icebergs, as
suggested by Lyell, have been largely concerned in their dispersal. But the
existence of several quite distinct species, belonging to genera
exclusively confined to the south, at these and other distant points of the
southern hemisphere, is, on my theory of descent with modification, a far
more remarkable case of difficulty. For some of these species are so
distinct, that we cannot suppose that there has been time since the
commencement of the Glacial period for their migration, and for their
subsequent modification to the necessary degree. The facts seem to me to
indicate that peculiar and very distinct species have migrated in radiating
lines from some 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 Glacial period, when the antarctic lands, now covered
with ice, supported a highly peculiar {382} and isolated flora. I suspect
that before this flora was exterminated by the Glacial epoch, a few forms
were widely dispersed to various points of the southern hemisphere by
occasional means of transport, and by the aid, as halting-places, of
existing and now sunken islands: By these means, as I believe, the southern
shores of America, Australia, New Zealand, have become slightly tinted by
the same peculiar forms of vegetable life.
Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate on
geographical distribution. I believe that the world has recently felt one
of his great cycles of change; and that on this view, combined with
modification through natural selection, a multitude of facts in the present
distribution both of the same and of allied forms of life can be explained.
The living waters may be said to have flowed during one short period from
the north and from the south, and to have crossed at the equator; but to
have flowed with greater force from the north so as to have freely
inundated the south. As the tide leaves its drift in horizontal lines,
though 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 height 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 serve as a record, full of interest to us, of the former
inhabitants of the surrounding lowlands.
* * * * *
{383}