The Origin of Species
Chapter 11: Geographical Distribution
by Charles Darwin
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, &c., are not as impassable,
or likely to have endured so long as the oceans separating continents, the differences are
very inferior in degree to those characteristic of distinct continents.
Turning to the sea, we find the same law. No two marine faunas are more distinct, with
hardly a fish, shell, or crab in common, than those of the eastern and western shores of
South and Central America; yet these great faunas are separated only by the narrow, but
impassable, isthmus of panama. Westward of the shores of America, a wide space of open
ocean extends, with not an island as a halting-place for emigrants; here we have a barrier
of another kind, and as soon as this is passed we meet in the eastern islands of the
Pacific, with another and totally distinct fauna. So that here three marine faunas range
far northward and southward, in parallel lines not far from each other, under
corresponding climates; but from being separated from each other by impassable barriers,
either of land or open sea, they are wholly distinct. On the other hand, proceeding still
further westward from the eastern islands of the tropical parts of the Pacific, we
encounter no impassable barriers, and we have innumerable islands as halting-places, 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 any judgement we are not authorised in admitting such
enormous geographical changes within the period of existing species. It seems to me that
we have abundant evidence of great oscillations 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
in the view advanced by Forbes and admitted by his many followers. The nature and relative
proportions of the inhabitants of oceanic islands likewise seem to me opposed to the
belief of their former continuity with continents. Nor does their almost universally
volcanic composition favour the admission that they are the wrecks of sunken continents;
if they had originally existed as mountain-ranges on the land, some at least of the
islands would have been formed, like other mountain-summits, of granite, metamorphic
schists, old fossiliferous or other such rocks, instead of consisting of mere piles of
volcanic matter.
I must now say a few words on what are called accidental means, but which more properly
might be called occasional means of distribution. I shall here confine myself to plants.
In botanical works, this or that plant is stated to be ill adapted for wide dissemination;
but for transport across the sea, the greater or less facilities may be said to be almost
wholly unknown. Until I tried, with Mr Berkeley's aid, a few experiments, it was not even
known how far seeds could resist the injurious action of sea-water. To my surprise I found
that out of 87 kinds, 64 germinated after an immersion of 28 days, and a few survived an
immersion of 137 days. For convenience sake I chiefly tried small seeds, without the
capsule or fruit; and as all of these sank in a few days, they could not be floated across
wide spaces of the sea, whether or not they were injured by the salt-water. Afterwards I
tried some larger fruits, capsules, &c., and some of these floated for a long time. It
is well known what a difference there is in the buoyancy of green and seasoned timber; and
it occurred to me that floods might wash down plants or branches, and that these might be
dried on the banks, and then by a fresh rise in the stream be washed into the sea. Hence I
was led to dry stems and branches of 94 plants with ripe fruit, and to place them on sea
water. The majority sank quickly, but some which whilst green floated for a very short
time, when dried floated much longer; for instance, ripe hazel-nuts sank immediately, but
when dried, they floated for 90 days and afterwards when planted they germinated; an
asparagus plant with ripe berries floated for 23 days, when dried it floated for 85 days,
and the seeds afterwards germinated: the ripe seeds of Helosciadium sank in two days, when
dried they floated for above 90 days, and afterwards germinated. Altogether out of the 94
dried plants, 18 floated for above 28 days, and some of the 18 floated for a very much
longer period. So that as 64/87 seeds germinated after an immersion of 28 days; and as
18/94 plants with ripe fruit (but not all the same species as in the foregoing experiment)
floated, after being dried, for above 28 days, as far as we may infer anything from these
scanty facts, we may conclude that the seeds of 14/100 plants of any country might be
floated by sea-currents during 28 days, and would retain their power of germination. In
Johnston's physical Atlas, the average rate of the several Atlantic currents is 33 miles
per diem (some currents running at the rate of 60 miles per diem); on this average, the
seeds of 14/100 plants belonging to one country might be floated across 924 miles of sea
to another country; and when stranded, if blown to a favourable spot by an inland gale,
they would germinate.
Subsequently to my experiments, M. Martens tried similar ones, but in a much better
manner, for he placed the seeds in a box in the actual sea, so that they were alternately
wet and exposed to the air like really floating plants. He tried 98 seeds, mostly
different from mine; but he chose many large fruits and likewise seeds from plants which
live near the sea; and this would have favoured the average length of their flotation and
of their resistance to the injurious action of the salt-water. On the other hand he did
not previously dry the plants or branches with the fruit; and this, as we have seen, would
have caused some of them to have floated much longer. The result was that 18/98 of his
seeds floated for 42 days, and were then capable of germination. But I do not doubt that
plants exposed to the waves would float for a less time than those protected from violent
movement as in our experiments. Therefore it would perhaps be safer to assume that the
seeds of about 10/100 plants of a flora, after having been dried, could be floated across
a space of sea 900 miles in width, and would then germinate. The fact of the larger fruits
often floating longer than the small, is interesting; as plants with large seeds or fruit
could hardly be transported by any other means; and Alph. de Candolle has shown that such
plants generally have restricted ranges.
But seeds may be occasionally transported in another manner. Drift timber is thrown up
on most islands, even on those in the midst of the widest oceans; and the natives of the
coral-islands in the Pacific, procure stones for their tools, solely from the roots of
drifted trees, these stones being a valuable royal tax. I find on examination, that when
irregularly shaped stones are embedded in the roots of trees, small parcels of earth are
very frequently enclosed in their interstices and behind them, so perfectly that not a
particle could be washed away in the longest transport: out of one small portion of earth
thus completely enclosed by wood in an oak about 50 years old, three dicotyledonous
plants germinated: I am certain of the accuracy of this observation. Again, I can show
that the carcasses of birds, when floating on the sea, sometimes escape being immediately
devoured; and seeds of many kinds in the crops of floating birds long retain their
vitality: peas and vetches, for instance, are killed by even a few days' immersion in
sea-water; but some taken out of the crop of a pigeon, which had floated on artificial
salt-water for 30 days, to my surprise nearly all germinated.
Living birds can hardly fail to be highly effective agents in the transportation of
seeds. I could give many facts showing how frequently birds of many kinds are blown by
gales to vast distances across the ocean. We may I think safely assume that under such
circumstances their rate of flight would often be 35 miles an hour; and some authors have
given a far higher estimate. I have never seen an instance of nutritious seeds passing
through the intestines of a bird; but hard seeds of fruit will pass uninjured through even
the digestive organs of a turkey. In the course of two months, I picked up in my garden 12
kinds of seeds, out of the excrement of small birds, and these seemed perfect, and some of
them, which I tried, germinated. But the following fact is more important: the crops of
birds do not secrete gastric juice, and do not in the least injure, as I know by trial,
the germination of seeds; now after a bird has found and devoured a large supply of food,
it is positively asserted that all the grains do not pass into the gizzard for 12 or even
18 hours. A bird in this interval might easily be blown to the distance of 500 miles, and
hawks are known to look out for tired birds, and the contents of their torn crops might
thus readily get scattered. Mr Brent informs me that a friend of his had to give up flying
carrier-pigeons from France to England, as the hawks on the English coast destroyed so
many on their arrival. Some hawks and owls bolt their prey whole, and after an interval of
from twelve to twenty hours, disgorge pellets, which, as I know from experiments made in
the Zoological Gardens, include seeds capable of germination. Some seeds of the oat,
wheat, millet, canary, hemp, clover, and beet germinated after having been from twelve to
twenty-one hours in the stomachs of different birds of prey; and two seeds of beet grew
after having been thus retained for two days and fourteen hours. Freshwater fish, I find,
eat seeds of many land and water plants: fish are frequently devoured by birds, and thus
the seeds might be transported from place to place. I forced many kinds of seeds into the
stomachs of dead fish, and then gave their bodies to fishing-eagles, storks, and pelicans;
these birds after an interval of many hours, either rejected the seeds in pellets or
passed them in their excrement; and 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 southeastern
corner of Australia.
Looking to America; in the northern half, ice-borne fragments of rock have been
observed on the eastern side as far south as lat. 36°-37°, and on the shores of the
Pacific, where the climate is now so different, as far south as lat. 46°; erratic
boulders have, also, been noticed on the Rocky Mountains. In the Cordillera of Equatorial
South America, glaciers once extended far below their present level. In central 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 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, Prof. Dana, that `it
is certainly a wonderful fact that New Zealand should have a closer resemblance in its
crustacea to Great Britain, its antipode, than to any other part of the world.' Sir J.
Richardson, also, speaks of the reappearance on the shores of New Zealand, Tasmania,
&c., of northern forms of fish. Dr Hooker informs me that twenty-five species of Algae
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 alterations 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.