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Charles Darwin: The origin of species
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CHAPTER XII.
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 -- Alternate Glacial periods in the North and
South.
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 wholly accounted for by 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 arctic and northern temperate parts, 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;
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 so closely as the same species generally require. No
doubt small areas can be pointed out in the Old World hotter than any in
the New World; but these are not inhabited by a fauna different from that
of the surrounding districts; for it is rare to find a group of organisms
confined to a small area, of which the conditions are peculiar in only a
slight degree. Notwithstanding this general parallelism in the conditions
of 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 degrees, 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 latitude 35 degrees with those north of 25
degrees, which consequently are separated by a space of ten degrees of
latitude, and are exposed to considerably different conditions; yet they
are 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 in 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 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. The marine inhabitants of the
eastern and western shores of South America are very distinct, with
extremely few shells, crustacea, or echinodermata in common; but Dr.
Gunther has recently shown that about thirty per cent of the fishes are the
same on the opposite sides of the isthmus of Panama; and this fact has led
naturalists to believe that the isthmus was formerly open. 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 three marine faunas range
northward and southward in parallel lines not far from each other, under
corresponding climate; but from being separated from each other by
impassable barriers, either of land or open sea, they are almost 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 so few marine animals are common to
the above-named three approximate faunas of Eastern and Western America and
the eastern Pacific islands, yet many fishes 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 statement, is the
affinity of the productions of the same continent or of the same 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, though nearly
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 inhabiting 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
muskrat, but the coypu and capybara, rodents of the South 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 are essentially American, though they may be all peculiar
species. We may look back to past ages, as shown in the last chapter, and
we find American types then prevailing on the American continent and in the
American seas. We see in these facts some deep organic bond, throughout
space and time, over the same areas of land and water, independently of
physical conditions. The naturalist must be dull who is not led to inquire
what this bond is.
The bond is simply inheritance, that cause which alone, as far as we
positively know, produces organisms quite like each other, or, as we see in
the case of varieties, nearly alike. The dissimilarity of the inhabitants
of different regions may be attributed to modification through variation
and natural selection, and probably in a subordinate degree to the definite
influence of different physical conditions. The degrees of dissimilarity
will depend on the migration of the more dominant forms of life from one
region into another having been more or less effectually prevented, at
periods more or less remote--on the nature and number of the former
immigrants--and on the action of the inhabitants on each other in leading
to the preservation of different modifications; the relation of organism to
organism in the struggle for life 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 out 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.
There is no evidence, as was remarked in the last chapter, of the existence
of any 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 each individual in its complex
struggle for life, so the amount of modification in different species will
be no uniform quantity. If a number of species, after having long competed
with each other in their old home, were to migrate in a body into a new and
afterwards isolated country, they would be little liable to modification;
for neither migration nor isolation in themselves effect 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 or at all modified.
According to 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 are descended from
the same progenitor. In the case of those species which have undergone,
during whole geological periods, little modification, there is not much
difficulty in believing that they have migrated from the same region; for
during the vast geographical and climatical changes which 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
has been explained, it is incredible that individuals identically the same
should have been produced from parents specifically distinct.
SINGLE CENTRES OF SUPPOSED CREATION.
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 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 that 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 have
been easily passed over by migration, the fact is given as something
remarkable and exceptional. The incapacity of migrating across a wide sea
is more clear in the case of terrestrial mammals than perhaps with any
other organic beings; and, accordingly, we find no inexplicable instances
of the same mammals inhabiting distant points of the world. No geologist
feels any difficulty in Great Britain possessing the same quadrupeds with
the rest of Europe, for they were no doubt once united. 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 wide and broken
interspaces. The great and striking influence of barriers of all kinds, is
intelligible only on the view that the great majority of species have been
produced on one side, and have not been able to migrate to the opposite
side. Some few families, many subfamilies, very many genera, 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 confined to the same country, or if they have a wide
range that their range is continuous. What a strange anomaly it would be
if a directly opposite rule were to prevail when we go down one step lower
in the series, namely to the individuals of the same species, and these had
not been, at least at first, confined to some one region!
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 climatical changes which have certainly occurred within
recent geological times, must have 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 instances. 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 fresh
water productions; and thirdly, the occurrence of the same terrestrial
species on islands and on the nearest 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
climatical and geographical changes, and to the various occasional means of
transport, the belief that a single birthplace is the 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 species of a
genus which must on our theory all be descended from a common progenitor,
can have migrated, undergoing modification during their migration from some
one area. If, when most of the species inhabiting one region are different
from those of another region, though closely allied to them, it can be
shown that migration from the one region to the other has probably occurred
at some former period, our general view will be much strengthened; for the
explanation is obvious on the principle of descent with modification. 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 related by inheritance to the inhabitants of that continent.
Cases of this nature are common, and are, as we shall hereafter see,
inexplicable on the theory of independent creation. This view of the
relation of the species of one region to those of another, does not differ
much from that advanced by Mr. Wallace, who concludes that "every species
has come into existence coincident both in space and time with a
pre-existing closely allied species." And it is now well known that he
attributes this coincidence to descent with modification.
The question of single or multiple centres of creation differs from another
though allied question, namely, whether all the individuals of the same
species are descended from a single pair, or single hermaphrodite, or
whether, as some authors suppose, from many individuals simultaneously
created. With organic beings which never intercross, if such exist, each
species, must be descended from a succession of modified varieties, that
have supplanted each other, but have never blended with other individuals
or varieties of the same species, so that, at each successive stage of
modification, all the individuals of the same form will be descended from a
single parent. But in the great majority of cases, namely, with all
organisms which habitually unite for each birth, or which occasionally
intercross, the individuals of the same species inhabiting the same area
will be kept nearly uniform by intercrossing; so that many individuals will
go on simultaneously changing, and the whole amount of modification at each
stage will not be due to descent from a single parent. To illustrate what
I mean: our English race-horses differ 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 the selecting and training of
many individuals during each generation.
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 now
impassable to certain organisms from the nature of its climate, might have
been a high road for migration, when the climate was different. I shall,
however, presently have to discuss this branch of the subject in some
detail. Changes of level in the land must also have been highly
influential: a narrow isthmus now separates two marine faunas; submerge
it, or let it formerly have been submerged, and the two faunas will now
blend together, or may formerly have blended. Where the sea now extends,
land may at a former period have connected islands or possibly even
continents together, and thus have allowed terrestrial productions to pass
from one to the other. No geologist disputes that great mutations of level
have occurred within the period of existing organisms. Edward Forbes
insisted that all the islands in the Atlantic must have been recently
connected with Europe or Africa, and Europe likewise with America. Other
authors have thus hypothetically bridged over every ocean, and united
almost every island with some mainland. If, indeed, the arguments used by
Forbes are to be trusted, it must be admitted that scarcely a single island
exists which has not recently been united to some continent. This view
cuts the Gordian knot of the dispersal of the same species to the 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 in the level of the land or sea;
but not of such vast changes in the position and extension of our
continents, as to have united them within the recent period to each other
and to the several intervening oceanic islands. I freely admit the former
existence of many islands, now buried beneath the sea, which may have
served as halting places for plants and for many animals during their
migration. In the coral-producing oceans such sunken islands are now
marked by rings of coral or atolls standing over them. Whenever it is
fully admitted, as it will some day be, that each species has proceeded
from a single birthplace, and when in the course of time we know something
definite about the means of distribution, we shall be enabled to speculate
with security on the former extension of the land. But I do not believe
that it will ever be proved that within the recent period most of our
continents which now stand quite separate, have been continuously, or
almost continuously united with each other, and with the many existing
oceanic islands. Several facts in distribution--such as the great
difference in the marine faunas on the opposite sides of almost every
continent--the close relation of the tertiary inhabitants of several lands
and even seas to their present inhabitants--the degree of affinity between
the mammals inhabiting islands with those of the nearest continent, being
in part determined (as we shall hereafter see) by the depth of the
intervening ocean--these and other such facts are opposed to the admission
of such prodigious geographical revolutions within the recent period, as
are necessary on the view advanced by Forbes and admitted by his followers.
The nature and relative proportions of the inhabitants of oceanic islands
are likewise opposed to the belief of their former continuity of
continents. Nor does the almost universally volcanic composition of such
islands favour the admission that they are the wrecks of sunken continents;
if they had originally existed as continental mountain ranges, some at
least of the islands would have been formed, like other mountain summits,
of granite, metamorphic schists, old fossiliferous and other rocks, instead
of consisting of mere piles of volcanic matter.
I must now say a few words on what are called accidental means, but which
more properly should be called occasional means of distribution. I shall
here confine myself to plants. In botanical works, this or that plant is
often stated to be ill adapted for wide dissemination; but the greater or
less facilities for transport across the sea may be said to be almost
wholly unknown. Until I tried, with Mr. Berkeley's aid, a few experiments,
it was not even known how far seeds could resist the injurious action of
sea-water. To my surprise I found that out of eighty-seven kinds, sixty-
four germinated after an immersion of twenty-eight days, and a few survived
an immersion of 137 days. It deserves notice that certain orders were far
more injured than others: nine Leguminosae were tried, and, with one
exception, they resisted the salt-water badly; seven species of the allied
orders, Hydrophyllaceae and Polemoniaceae, were all killed by a month's
immersion. For convenience sake I chiefly tried small seeds without the
capsules or fruit; and as all of these sank in a few days, they could not
have been floated across wide spaces of the sea, whether or not they were
injured by 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 would often wash into the sea dried plants or
branches with seed-capsules or fruit attached to them. Hence I was led to
dry the stems and branches of ninety-four 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
ninety days, and afterwards when planted germinated; an asparagus plant
with ripe berries floated for twenty-three days, when dried it floated for
eighty-five days, and the seeds afterwards germinated: the ripe seeds of
Helosciadium sank in two days, when dried they floated for above ninety
days, and afterwards germinated. Altogether, out of the ninety-four dried
plants, eighteen floated for above twenty-eight days; and some of the
eighteen floated for a very much longer period. So that as 64/87 kinds of
seeds germinated after an immersion of twenty-eight days; and as 18/94
distinct species with ripe fruit (but not all the same species as in the
foregoing experiment) floated, after being dried, for above twenty-eight
days, we may conclude, as far as anything can be inferred from these scanty
facts, that the seeds of 14/100 kinds of plants of any country might be
floated by sea-currents during twenty-eight days, and would retain their
power of germination. In Johnston's Physical Atlas, the average rate of
the several Atlantic currents is thirty-three miles per diem (some currents
running at the rate of sixty miles per diem); on this average, the seeds of
14/100 plants belonging to one country might be floated across 924 miles of
sea to another country; and when stranded, if blown by an inland gale to a
favourable spot, would germinate.
Subsequently to my experiments, M. Martens tried similar ones, but in a
much better manner, for he placed the seeds in a box in the actual sea, so
that they were alternately wet and exposed to the air like really floating
plants. He tried ninety-eight seeds, mostly different from mine, but he
chose many large fruits, and likewise seeds, from plants which live near
the sea; and this would have favoured both the average length of their
flotation and their resistance to the injurious action of the salt-water.
On the other hand, he did not previously dry the plants or branches with
the fruit; and this, as we have seen, would have caused some of them to
have floated much longer. The result was that 18/98 of his seeds of
different kinds floated for forty-two days, and were then capable of
germination. But I do not doubt that plants exposed to the waves would
float for a less time than those protected from violent movement as in our
experiments. Therefore, it would perhaps be safer to assume that the seeds
of about 10/100 plants of a flora, after having been dried, could be
floated across a space of sea 900 miles in width, and would then germinate.
The fact of the larger fruits often floating longer than the small, is
interesting; as plants with large seeds or fruit which, as Alph. de
Candolle has shown, generally have restricted ranges, could hardly be
transported by any other means.
Seeds may be occasionally transported in another manner. Drift timber is
thrown up on most islands, even on those in the midst of the widest oceans;
and the natives of the coral islands in the Pacific procure stones for
their tools, solely from the roots of drifted trees, these stones being a
valuable royal tax. I find that when irregularly shaped 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 during the longest transport: out of one
small portion of earth thus COMPLETELY enclosed by the roots of an oak
about fifty 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 many kinds of seeds in the crops of floating
birds long retain their vitality: peas and vetches, for instance, are
killed by even a few days' immersion in sea-water; but some taken out of
the crop of a pigeon, which had floated on artificial sea-water for thirty
days, to my surprise nearly all germinated.
Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how frequently
birds of many kinds are blown by gales to vast distances across the ocean.
We may safely assume that under such circumstances their rate of flight
would often be thirty-five 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 twelve kinds of seeds, out of the
excrement of small birds, and these seemed perfect, and some of them, which
were tried, germinated. But the following fact is more important: the
crops of birds do not secrete gastric juice, and do not, as I know by
trial, injure in the least the germination of seeds; now, after a bird has
found and devoured a large supply of food, it is positively asserted that
all the grains do not pass into the gizzard for twelve or even eighteen
hours. A bird in this interval might easily be blown to the distance of
five hundred miles, and hawks are known to look out for tired birds, and
the contents of their torn crops might thus readily get scattered. Some
hawks and owls bolt their prey whole, and after an interval of from twelve
to twenty hours, disgorge pellets, which, as I know from experiments made
in the Zoological Gardens, include seeds capable of germination. Some
seeds of the oat, wheat, millet, canary, hemp, clover, and beet germinated
after having been from twelve to twenty-one hours in the stomachs of
different birds of prey; and two seeds of beet grew after having been thus
retained for two days and fourteen hours. Fresh-water fish, I find, eat
seeds of many land and water plants; fish are frequently devoured by birds,
and thus the seeds might be transported from place to place. I forced many
kinds of seeds into the stomachs of dead fish, and then gave their bodies
to fishing-eagles, storks, and pelicans; these birds, after an interval of
many hours, either rejected the seeds in pellets or passed them in their
excrement; and several of these seeds retained the power of germination.
Certain seeds, however, were always killed by this process.
Locusts are sometimes blown to great distances from the land. I myself
caught one 370 miles from the coast of Africa, and have heard of others
caught at greater distances. The Rev. R.T. Lowe informed Sir C. Lyell that
in November, 1844, swarms of locusts visited the island of Madeira. They
were in countless numbers, as thick as the flakes of snow in the heaviest
snowstorm, and extended upward as far as could be seen with a telescope.
During two or three days they slowly careered round and round in an immense
ellipse, at least five or six miles in diameter, and at night alighted on
the taller trees, which were completely coated with them. They then
disappeared over the sea, as suddenly as they had appeared, and have not
since visited the island. Now, in parts of Natal it is believed by some
farmers, though on insufficient evidence, that injurious seeds are
introduced into their grass-land in the dung left by the great flights of
locusts which often visit that country. In consequence of this belief Mr.
Weale sent me in a letter a small packet of the dried pellets, out of which
I extracted under the microscope several seeds, and raised from them seven
grass plants, belonging to two species, of two genera. Hence a swarm of
locusts, such as that which visited Madeira, might readily be the means of
introducing several kinds of plants into an island lying far from the
mainland.
Although the beaks and feet of birds are generally clean, earth sometimes
adheres to them: in one case I removed sixty-one grains, and in another
case twenty-two grains of dry argillaceous earth from the foot of a
partridge, and in the earth there was a pebble as large as the seed of a
vetch. Here is a better case: the leg of a woodcock was sent to me by a
friend, with a little cake of dry earth attached to the shank, weighing
only nine grains; and this contained a seed of the toad-rush (Juncus
bufonius) which germinated and flowered. Mr. Swaysland, of Brighton, who
during the last forty years has paid close attention to our migratory
birds, informs me that he has often shot wagtails (Motacillae), wheatears,
and whinchats (Saxicolae), on their first arrival on our shores, before
they had alighted; and he has several times noticed little cakes of earth
attached to their feet. Many facts could be given showing how generally
soil is charged with seeds. For instance, Professor Newton sent me the leg
of a red-legged partridge (Caccabis rufa) which had been wounded and could
not fly, with a ball of hard earth adhering to it, and weighing six and a
half ounces. The earth had been kept for three years, but when broken,
watered and placed under a bell glass, no less than eighty-two plants
sprung from it: these consisted of twelve monocotyledons, including the
common oat, and at least one kind of grass, and of seventy dicotyledons,
which consisted, judging from the young leaves, of at least three distinct
species. With such facts before us, can we doubt that the many birds which
are annually blown by gales across great spaces of ocean, and which
annually migrate--for instance, the millions of quails across the
Mediterranean--must occasionally transport a few seeds embedded in dirt
adhering to their feet or beaks? But I shall have to recur to this
subject.
As icebergs are known to be sometimes loaded with earth and stones, and
have even carried brushwood, bones, and the nest of a land-bird, it can
hardly be doubted that they must occasionally, as suggested by Lyell, have
transported seeds from one part to another of the arctic and antarctic
regions; and during the Glacial period from one part of the now temperate
regions to another. In the Azores, from the large number of plants common
to Europe, in comparison with the species on the other islands of the
Atlantic, which stand nearer to the mainland, and (as remarked by Mr. H.C.
Watson) from their somewhat northern character, in comparison with the
latitude, I suspected that these islands had been partly stocked by
ice-borne seeds during the Glacial epoch. At my request Sir C. Lyell wrote
to M. Hartung to inquire whether he had observed erratic boulders on these
islands, and he answered that he had found large fragments of granite and
other rocks, which do not occur in the archipelago. Hence we may safely
infer that icebergs formerly landed their rocky burdens 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 these several means of transport, and that other means,
which without doubt remain to be discovered, have been in action year after
year for tens of thousands of years, it would, I think, be a marvellous
fact if many plants had not thus become widely transported. These means of
transport are sometimes called accidental, but this is not strictly
correct: the currents of the sea are not accidental, nor is the direction
of prevalent gales of wind. It should be observed that scarcely any means
of transport would carry seeds for very great distances; for seeds do not
retain their vitality when exposed for a great length of time to the action
of sea water; nor could they be long carried in the crops or intestines of
birds. These means, however, would suffice for occasional transport across
tracts of sea some hundred miles in breadth, or from island to island, or
from a continent to a neighbouring island, but not from one distant
continent to another. The floras of distant continents would not by such
means become mingled; but would remain as distinct as they now are. The
currents, from their course, would never bring seeds from North America to
Britain, though they might and do bring seeds from the West Indies to our
western shores, where, if not killed by their very long immersion in salt
water, they could not endure our climate. Almost every year, one or two
land-birds are blown across the whole Atlantic Ocean, from North America to
the western shores of Ireland and England; but seeds could be transported
by these rare wanderers only by one means, namely, by dirt adhering to
their feet or beaks, which is in itself a rare accident. Even in this
case, how small would be the chance of a seed falling on favourable soil,
and coming to maturity! But it would be a great error to argue that
because a well-stocked island, like Great Britain, has not, as far as is
known (and it would be very difficult to prove this), received within the
last few centuries, through occasional means of transport, immigrants from
Europe or any other continent, that a poorly-stocked island, though
standing more remote from the mainland, would not receive colonists by
similar means. Out of a hundred kinds of seeds or animals transported to
an island, even if far less well-stocked than Britain, perhaps not more
than one would be so well fitted to its new home, as to become naturalised.
But this is no valid argument against what would be effected by occasional
means of transport, during the long lapse of geological time, whilst the
island was being upheaved, and before it had become fully stocked with
inhabitants. On almost bare land, with few or no destructive insects or
birds living there, nearly every seed which chanced to arrive, if fitted
for the climate, would germinate and survive.
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 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 point to the other. It is indeed a
remarkable fact to see so many plants of the same species 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 many 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 scored rocks plainly reveal a former cold period.
The former influence of the glacial climate on the distribution of the
inhabitants of Europe, as explained by Edward Forbes, is substantially as
follows. But we shall follow the changes more readily, by supposing a new
glacial period slowly to come on, and then pass away, as formerly occurred.
As the cold came on, and as each more southern zone became fitted for the
inhabitants of the north, these would take the places of the former
inhabitants of the temperate regions. The latter, at the same time would
travel further and further 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
an 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.
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, as the warmth increased and the snow still further disappeared,
higher and higher, whilst their brethren were pursuing their northern
journey. Hence, when the warmth had fully returned, the same species,
which had lately lived together on the European and North American
lowlands, would again be found in the arctic regions of the Old and New
Worlds, and on many isolated mountain-summits far distant from each other.
Thus we can understand the identity of many plants at points so immensely
remote as the mountains of the United States and those 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 first migration when the cold came on, and the
re-migration on the returning warmth, would 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 formerly permitted
their migration across the intervening lowlands, now become too warm for
their existence.
As the arctic forms moved first southward and afterwards backward to the
north, in unison with the changing climate, they will not have been exposed
during their long migrations to any great diversity of temperature; and as
they all migrated in a body together, their mutual relations will not have
been much disturbed. Hence, in accordance with the principles inculcated
in this volume, these forms will not have been liable to much modification.
But with the 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 far
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 the coldest period will have been
temporarily driven down to the plains; they will, also, have been
subsequently exposed to somewhat different climatical influences. Their
mutual relations will thus have been in some degree disturbed; consequently
they will have been liable to modification; and they have been modified;
for if we compare the present Alpine plants and animals of the several
great European mountain ranges, one with another, though many of the
species remain identically the same, some exist as varieties, some as
doubtful forms or sub-species and some as distinct yet closely allied
species representing each other on the several ranges.
In the foregoing illustration, I have assumed that at the commencement of
our imaginary Glacial period, the arctic productions were as uniform round
the polar regions as they are at the present day. But it is also necessary
to assume that many sub-arctic and some few temperate forms were the same
round the world, for some of the species which now exist on the lower
mountain slopes and on the plains of North America and Europe are the same;
and it may be asked how I account for this degree of uniformity of the
sub-arctic and temperate forms round the world, at the commencement of the
real 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 whole Atlantic Ocean and by the northern part of the Pacific.
During the Glacial period, when the inhabitants of the Old and New Worlds
lived further southwards than they do at present, they must have been still
more completely separated from each other by wider spaces of ocean; so that
it may well be asked how the same species could then or previously have
entered the two continents. The explanation, I believe, lies in the nature
of the climate before the commencement of the Glacial period. At this, the
newer Pliocene period, the majority of the inhabitants of the world were
specifically the same as now, and we have good reason to believe that the
climate was warmer than at the present day. Hence, we may suppose that the
organisms which now live under latitude 60 degrees, lived during the
Pliocene period further north, under the Polar Circle, in latitude 66-67
degrees; and that the present arctic productions then lived on the broken
land still nearer to the pole. Now, if we look at a terrestrial globe, we
see under the Polar Circle that there is almost continuous land from
western Europe through Siberia, to eastern America. And this continuity of
the circumpolar land, with the consequent freedom under a more favourable
climate for intermigration, will account for the supposed uniformity of the
sub-arctic and 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 great
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 highly remarkable, considering the distance of the two areas, and their
separation by the whole 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 will have been completely cut off from each
other. This separation, as far as the more temperate productions are
concerned, must have taken place long ages ago. As the plants and animals
migrated southward, they will have become mingled in the one great region
with the native American productions, and would 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 Europe and North America. 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 marine areas completely sundered. Thus, I think, we can
understand the presence of some closely allied, still existing and extinct
tertiary forms, on the eastern and western shores of temperate North
America; and the still more striking fact of many closely allied
crustaceans (as described in Dana's admirable work), some fish and other
marine animals, inhabiting the Mediterranean and the seas of Japan--these
two areas being now completely separated by the breadth of a whole
continent and by wide spaces of ocean.
These cases of close relationship in species either now or formerly
inhabiting the seas on the eastern and western shores of North America, the
Mediterranean and Japan, and the temperate lands of North America and
Europe, are inexplicable on the theory of creation. We cannot maintain
that such species 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 parts of South Africa or
Australia, we see countries closely similar in all their physical
conditions, with their inhabitants utterly dissimilar.
ALTERNATE GLACIAL PERIODS IN THE NORTH AND SOUTH.
But we must return to our more immediate subject. I am convinced that
Forbes's view may be largely extended. In Europe we meet with the plainest
evidence of the Glacial period, from the western shores of Britain to the
Ural range, and southward to the Pyrenees. We may infer from the frozen
mammals and nature of the mountain vegetation, that Siberia was similarly
affected. In the Lebanon, according to Dr. Hooker, perpetual snow formerly
covered the central axis, and fed glaciers which rolled 4,000 feet down the
valleys. The same observer has recently found great moraines at a low
level on the Atlas range in North Africa. 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 ancient and gigantic
moraines. Southward of the Asiatic continent, on the opposite side of the
equator, we know, from the excellent researches of Dr. J. Haast and Dr.
Hector, that in New Zealand immense glaciers formerly descended to a low
level; and the same plants, found by Dr. Hooker on widely separated
mountains in this island tell the same story of a former cold period. From
facts communicated to me by the Rev. W.B. Clarke, it appears also that
there are traces of former glacial action on the mountains of 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 of the continent, as far south as
latitude 36 and 37 degrees, and on the shores of the Pacific, where the
climate is now so different, as far south as latitude 46 degrees. Erratic
boulders have, also, been noticed on the Rocky Mountains. In the
Cordillera of South America, nearly under the equator, glaciers once
extended far below their present level. In central Chile I examined a vast
mound of detritus with great boulders, crossing the Portillo valley, which,
there can hardly be a doubt, once formed a huge moraine; and Mr. D. Forbes
informs me that he found in various parts of the Cordillera, from latitude
13 to 30 degrees south, at about the height of 12,000 feet, deeply-furrowed
rocks, resembling those with which he was familiar in Norway, and likewise
great masses of detritus, including grooved pebbles. Along this whole
space of the Cordillera true glaciers do not now exist even at much more
considerable heights. Further south, on both sides of the continent, from
latitude 41 degrees to the southernmost extremity, we have the clearest
evidence of former glacial action, in numerous immense boulders transported
far from their parent source.
>From these several facts, namely, from the glacial action having extended
all round the northern and southern hemispheres--from the period having
been in a geological sense recent in both hemispheres--from its having
lasted in both during a great length of time, as may be inferred from the
amount of work effected--and lastly, from glaciers having recently
descended to a low level along the whole line of the Cordillera, it at one
time appeared to me that we could not avoid the conclusion that the
temperature of the whole world had been simultaneously lowered during the
Glacial period. But now, Mr. Croll, in a series of admirable memoirs, has
attempted to show that a glacial condition of climate is the result of
various physical causes, brought into operation by an increase in the
eccentricity of the earth's orbit. All these causes tend towards the same
end; but the most powerful appears to be the indirect influence of the
eccentricity of the orbit upon oceanic currents. According to Mr. Croll,
cold periods regularly recur every ten or fifteen thousand years; and these
at long intervals are extremely severe, owing to certain contingencies, of
which the most important, as Sir C. Lyell has shown, is the relative
position of the land and water. Mr. Croll believes that the last great
glacial period occurred about 240,000 years ago, and endured, with slight
alterations of climate, for about 160,000 years. With respect to more
ancient glacial periods, several geologists are convinced, from direct
evidence, that such occurred during the miocene and eocene formations, not
to mention still more ancient formations. But the most important result
for us, arrived at by Mr. Croll, is that whenever the northern hemisphere
passes through a cold period the temperature of the southern hemisphere is
actually raised, with the winters rendered much milder, chiefly through
changes in the direction of the ocean currents. So conversely it will be
with the northern hemisphere, while the southern passes through a glacial
period. This conclusion throws so much light on geographical distribution
that I am strongly inclined to trust in it; but I will first give the facts
which demand an explanation.
In South America, Dr. Hooker has shown that besides many closely allied
species, between forty and fifty of the flowering plants of Tierra del
Fuego, forming no inconsiderable part of its scanty flora, are common to
North America and Europe, enormously remote as these areas in opposite
hemispheres are from each other. On the lofty mountains of equatorial
America a host of peculiar species belonging to European genera occur. On
the Organ Mountains of Brazil some few temperate European, some Antarctic
and some Andean genera were found by Gardner which do not exist in the low
intervening hot countries. On the Silla of Caraccas the illustrious
Humboldt long ago found species belonging to genera characteristic of the
Cordillera.
In Africa, several forms characteristic of Europe, and some few
representatives of the flora of the Cape of Good Hope, occur on the
mountains of Abyssinia. At the Cape of Good Hope a very few European
species, believed not to have been introduced by man, and on the mountains
several representative European forms are found which have not been
discovered in the intertropical parts of Africa. Dr. Hooker has also
lately shown that several of the plants living on the upper parts of the
lofty island of Fernando Po, and on the neighbouring Cameroon Mountains, in
the Gulf of Guinea, are closely related to those on the mountains of
Abyssinia, and likewise to those of temperate Europe. It now also appears,
as I hear from Dr. Hooker, that some of these same temperate plants have
been discovered by the Rev. R.T. Lowe on the mountains of the Cape Verde
Islands. This extension of the same temperate forms, almost under the
equator, across the whole continent of Africa and to the mountains of the
Cape Verde archipelago, is one of the most astonishing facts ever recorded
in the distribution of plants.
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 of plants collected on the loftier
peaks of Java, raises a picture of a collection made on a hillock in
Europe. Still more striking is the fact that peculiar Australian forms are
represented by certain 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 hand as far north as Japan.
On the southern mountains of Australia, Dr. F. Muller 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 certain plants growing on the
more lofty mountains of the tropics in all parts of the world, and on the
temperate plains of the north and south, are either the same species or
varieties of the same species. It should, however, be observed that these
plants are not strictly arctic forms; for, as Mr. H.C. Watson has remarked,
"in receding from polar toward equatorial latitudes, the Alpine or mountain
flora really become less and less Arctic." Besides these identical and
closely allied forms, many species inhabiting the same widely sundered
areas, belong to genera not now found in the intermediate tropical
lowlands.
These brief remarks apply to plants alone; but some few analogous facts
could be given in regard to terrestrial animals. In marine productions,
similar cases likewise occur; as an example, I may quote a statement 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, etc., of northern forms of fish. Dr. Hooker informs me that
twenty-five species of Algae are common to New Zealand and to Europe, but
have not been found in the intermediate tropical seas.
From the foregoing facts, namely, the presence of temperate forms on the
highlands across the whole of equatorial Africa, and along the Peninsula of
India, to Ceylon and the Malay Archipelago, and in a less well-marked
manner across the wide expanse of tropical South America, it appears almost
certain that at some former period, no doubt during the most severe part of
a Glacial period, the lowlands of these great continents were everywhere
tenanted under the equator by a considerable number of temperate forms. At
this period the equatorial climate at the level of the sea was probably
about the same with that now experienced at the height of from five to six
thousand feet under the same latitude, or perhaps even rather cooler.
During this, the coldest period, the lowlands under the equator must have
been clothed with a mingled tropical and temperate vegetation, like that
described by Hooker as growing luxuriantly at the height of from four to
five thousand feet on the lower slopes of the Himalaya, but with perhaps a
still greater preponderance of temperate forms. So again in the
mountainous island of Fernando Po, in the Gulf of Guinea, Mr. Mann found
temperate European forms beginning to appear at the height of about five
thousand feet. On the mountains of Panama, at the height of only two
thousand feet, Dr. Seemann found the vegetation like that of Mexico, "with
forms of the torrid zone harmoniously blended with those of the temperate."
Now let us see whether Mr. Croll's conclusion that when the northern
hemisphere suffered from the extreme cold of the great Glacial period, the
southern hemisphere was actually warmer, throws any clear light on the
present apparently inexplicable distribution of various organisms in the
temperate parts of both hemispheres, and on the mountains of the tropics.
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 became more and more intense, we know
that Arctic forms invaded the temperate regions; and from the facts just
given, there can hardly be a doubt that some of the more vigorous, dominant
and widest-spreading temperate forms invaded the equatorial lowlands. The
inhabitants of these hot lowlands would at the same time have migrated to
the tropical and subtropical regions of the south, for the southern
hemisphere was at this period warmer. On the decline of the Glacial
period, as both hemispheres gradually recovered their former temperature,
the northern temperate forms living on the lowlands under the equator,
would have been driven to their former homes or have been destroyed, being
replaced by the equatorial forms returning from the south. Some, however,
of the northern temperate forms would almost certainly have ascended any
adjoining high land, where, if sufficiently lofty, they would have long
survived like the Arctic forms on the mountains of Europe. They might have
survived, even if the climate was not perfectly fitted for them, for the
change of temperature must have been very slow, and plants undoubtedly
possess a certain capacity for acclimatisation, as shown by their
transmitting to their offspring different constitutional powers of
resisting heat and cold.
In the regular course of events the southern hemisphere would in its turn
be subjected to a severe Glacial period, with the northern hemisphere
rendered warmer; and then the southern temperate forms would invade the
equatorial lowlands. The northern forms which had before been left on the
mountains would now descend and mingle with the southern forms. These
latter, when the warmth returned, would return to their former homes,
leaving some few species on the mountains, and carrying southward with them
some of the northern temperate forms which had descended from their
mountain fastnesses. Thus, we should have some few species identically the
same in the northern and southern temperate zones and on the mountains of
the intermediate tropical regions. But the species left during a long time
on these mountains, or in opposite hemispheres, would have to compete with
many new forms and would be exposed to somewhat different physical
conditions; hence, they would be eminently liable to modification, and
would generally now exist as varieties or as representative species; and
this is the case. We must, also, bear in mind the occurrence in both
hemispheres of former Glacial periods; for these will account, in
accordance with the same principles, for the many quite distinct species
inhabiting the same widely separated areas, and belonging to genera not now
found in the intermediate torrid zones.
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 or slightly modified species have migrated from the north to the
south, than in a reversed direction. We see, however, a few southern forms
on the mountains of Borneo and Abyssinia. I suspect that this preponderant
migration from the north to the 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 the two sets became
commingled in the equatorial regions, during the alternations of the
Glacial periods, the northern forms were the more powerful and were able to
hold their places on the mountains, and afterwards migrate southward with
the southern forms; but not so the southern in regard to the northern
forms. In the same manner, at the present day, we see that very many
European productions cover the ground in La Plata, New Zealand, and to a
lesser degree in Australia, and have beaten the natives; whereas extremely
few southern forms have become naturalised in any part of the northern
hemisphere, 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 forty or fifty years from
Australia. The Neilgherrie Mountains in India, however, offer a partial
exception; for here, as I hear from Dr. Hooker, Australian forms are
rapidly sowing themselves and becoming naturalised. Before the last great
Glacial period, no doubt the intertropical mountains were stocked with
endemic Alpine forms; but these have almost everywhere 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 those which have become naturalised; and this is
the first stage towards their extinction. Mountains are islands on the
land; and their inhabitants have yielded to those produced within the
larger areas of the north, just in the same way as the inhabitants of real
islands have everywhere yielded and are still yielding to continental forms
naturalised through man's agency.
The same principles apply to the distribution of terrestrial animals and of
marine productions, in the northern and southern temperate zones, and on
the intertropical mountains. When, during the height of the Glacial
period, the ocean-currents were widely different to what they now are, some
of the inhabitants of the temperate seas might have reached the equator; of
these a few would perhaps at once be able to migrate southwards, by keeping
to the cooler currents, while others might remain and survive in the colder
depths until the southern hemisphere was in its turn subjected to a glacial
climate and permitted their further progress; in nearly the same manner as,
according to Forbes, isolated spaces inhabited by Arctic productions exist
to the present day in the deeper parts of the northern temperate seas.
I am far from supposing that all the difficulties in regard to the
distribution and affinities of the identical and allied species, which now
live so widely separated in the north and south, and sometimes on the
intermediate mountain ranges, are removed on the views above given. The
exact lines of migration cannot be indicated. We cannot say why certain
species and not others have migrated; why certain species have been
modified and have given rise to new forms, while 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 species ranges twice or thrice as far, and is twice or thrice
as common, as another species within their own homes.
Various special difficulties also remain to be solved; for instance, the
occurrence, as shown by Dr. Hooker, of the same plants at points so
enormously remote as Kerguelen Land, New Zealand, and Fuegia; but icebergs,
as suggested by Lyell, may have been concerned in their dispersal. The
existence at these and other distant points of the southern hemisphere, of
species, which, though distinct, belong to genera exclusively confined to
the south, is a more remarkable case. Some of these species are so
distinct, that we cannot suppose that there has been time since the
commencement of the last Glacial period for their migration and subsequent
modification to the necessary degree. The facts seem to indicate that
distinct species belonging to the same genera have migrated in radiating
lines from a 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 last Glacial period, when the Antarctic lands, now
covered with ice, supported a highly peculiar and isolated flora. It may
be suspected that before this flora was exterminated during the last
Glacial epoch, a few forms had been already widely dispersed to various
points of the southern hemisphere by occasional means of transport, and by
the aid, as halting-places, of now sunken islands. Thus the southern
shores of America, Australia, and New Zealand may have become slightly
tinted by the same peculiar forms of life.
Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate
throughout the world on geographical distribution. And we have now seen
that Mr. Croll's conclusion that successive Glacial periods in the one
hemisphere coincide with warmer periods in the opposite hemisphere,
together with the admission of the slow modification of species, explains a
multitude of facts in the distribution of the same and of the allied forms
of life in all parts of the globe. The living waters have flowed during
one period from the north and during another from the south, and in both
cases have reached the equator; but the stream of life has flowed with
greater force from the north than in the opposite direction, and has
consequently more freely inundated the south. As the tide leaves its drift
in horizontal lines, 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
latitude 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 serves as a record, full of interest
to us, of the former inhabitants of the surrounding lowlands.
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