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