Chapter XI Geographical
Distribution
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!
2 In the southern hemisphere, if we compare large
tracts of land in Australia, South Africa, and western South America, between
latitudes 25 deg and 35 deg, 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 deg with those north of 25 deg, 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.
3 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.
4 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.
5 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.
6 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.
7 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.
8 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.
9 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!
10 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.
11 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.
12 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.
13 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.
14 Means of Dispersal. -- Sir C. Lyell and other authors
have ably treated this subject. I can give here only the briefest abstract of
the more important facts. Change of climate must have had a powerful influence
on migration: a region when its climate was different may have been a high
road for migration, but now be impassable; I shall, however, presently have to
discuss this branch of the subject in some detail. Changes of level in the
land must also have been highly influential: a narrow isthmus now separates
two marine faunas; submerge it, or let it formerly have been submerged, and
the two faunas will now blend or may formerly have blended: where the sea now
extends, land may at a former period have connected islands or possibly even
continents together, and thus have allowed terrestrial productions to pass
from one to the other. No geologist will dispute that great mutations of level
have occurred within the period of existing organisms. Edward Forbes insisted
that all the islands in the Atlantic must recently have been connected with
Europe or Africa, and Europe likewise with America. Other authors have thus
hypothetically bridged over every ocean, and have united almost every island
to some mainland. If indeed the arguments used by Forbes are to be trusted, it
must be admitted that scarcely a single island exists which has not recently
been united to some continent. This view cuts the Gordian knot of the
dispersal of the same species to the most distant points, and removes many a
difficulty: but to the best of any judgment we are not authorized in admitting
such enormous geographical changes within the period of existing species. It
seems to me that we have abundant evidence of great oscillations of level in
our continents; but not of such vast changes in their position and extension,
as to have united them within the recent period to each other and to the
several intervening oceanic islands. I freely admit the former existence of
many islands, now buried beneath the sea, which may have served as halting
places for plants and for many animals during their migration. In the
coral-producing oceans such sunken islands are now marked, as I believe, by
rings of coral or atolls standing over them. Whenever it is fully admitted, as
I believe it will some day be, that each species has proceeded from a single
birthplace, and when in the course of time we know something definite about
the means of distribution, we shall be enabled to speculate with security on
the former extension of the land. But I do not believe that it will ever be
proved that within the recent period continents which are now quite separate,
have been continuously, or almost continuously, united with each other, and
with the many existing oceanic islands. Several facts in distribution,--such
as the great difference in the marine faunas on the opposite sides of almost
every continent,--the close relation of the tertiary inhabitants of several
lands and even seas to their present inhabitants,--a certain degree of
relation (as we shall hereafter see) between the distribution of mammals and
the depth of the sea,--these and other such facts seem to me opposed to the
admission of such prodigious geographical revolutions within the recent
period, as are necessitated on the view advanced by Forbes and admitted by his
many followers. The nature and relative proportions of the inhabitants of
oceanic islands likewise seem to me opposed to the belief of their former
continuity with continents. Nor does their almost universally volcanic
composition favour the admission that they are the wrecks of sunken
continents;--if they had originally existed as mountain-ranges on the land,
some at least of the islands would have been formed, like other
mountain-summits, of granite, metamorphic schists, old fossiliferous or other
such rocks, instead of consisting of mere piles of volcanic matter.
15 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.
16 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.
17 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.
18 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.
19 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.
20 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.
21 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.
22 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.
23 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.
24 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.
25 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.
26 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.
27 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.
28 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 deg, during the
Pliocene period lived further north under the Polar Circle, in latitude 66
deg-67 deg; 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.
29 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.
30 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.
31 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.
32 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.
33 But we must return to our more immediate subject, the
Glacial period. I am convinced that Forbes's view may be largely extended. In
Europe we have the plainest evidence of the cold period, from the western
shores of Britain to the Oural range, and southward to the Pyrenees. We may
infer, from the frozen mammals and nature of the mountain vegetation, that
Siberia was similarly affected. Along the Himalaya, at points 900 miles apart,
glaciers have left the marks of their former low descent; and in Sikkim, Dr.
Hooker saw maize growing on gigantic ancient moraines. South of the equator,
we have some direct evidence of former glacial action in New Zealand; and the
same plants, found on widely separated mountains in this island, tell the same
story. If one account which has been published can be trusted, we have direct
evidence of glacial action in the south-eastern corner of Australia.
34 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 deg-37 deg, and on the shores of the Pacific, where the climate is now so
different, as far south as lat. 46 deg; erratic boulders have, also, been
noticed on the Rocky Mountains. In the Cordillera of Equatorial South America,
glaciers once extended far below their present level. In central Chile I was
astonished at the structure of a vast mound of detritus, about 800 feet in
height, crossing a valley of the Andes; and this I now feel convinced was a
gigantic moraine, left far below any existing glacier. Further south on both
sides of the continent, from lat. 41 deg to the southernmost extremity, we
have the clearest evidence of former glacial action, in huge boulders
transported far from their parent source.
35 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.
36 On this view of the whole world, or at least of broad
longitudinal belts, having been simultaneously colder from pole to pole, much
light can be thrown on the present distribution of identical and allied
species. In America, Dr. Hooker has shown that between forty and fifty of the
flowering plants of Tierra del Fuego, forming no inconsiderable part of its
scanty flora, are common to Europe, enormously remote as these two points are;
and there are many closely allied species. On the lofty mountains of
equatorial America a host of peculiar species belonging to European genera
occur. On the highest mountains of Brazil, some few European genera were found
by Gardner, which do not exist in the wide intervening hot countries. So on
the Silla of Caraccas the illustrious Humboldt long ago found species
belonging to genera characteristic of the Cordillera. On the mountains of
Abyssinia, several European forms and some few representatives of the peculiar
flora of the Cape of Good Hope occur. At the Cape of Good Hope a very few
European species, believed not to have been introduced by man, and on the
mountains, some few representative European forms are found, which have not
been discovered in the intertropical parts of Africa. On the Himalaya, and on
the isolated mountain-ranges of the peninsula of India, on the heights of
Ceylon, and on the volcanic cones of Java, many plants occur, either
identically the same or representing each other, and at the same time
representing plants of Europe, not found in the intervening hot lowlands. A
list of the genera collected on the loftier peaks of Java raises a picture of
a collection made on a hill in Europe! Still more striking is the fact that
southern Australian forms are clearly represented by plants growing on the
summits of the mountains of Borneo. Some of these Australian forms, as I hear
from Dr. Hooker, extend along the heights of the peninsula of Malacca, and are
thinly scattered, on the one hand over India and on the other as far north as
Japan.
37 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 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.
38 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.
39 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.
40 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.
41 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.
42 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.
43 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.
44 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.
45 Sir C. Lyell in a striking passage has speculated, in
language almost identical with mine, on the effects of great alternations of
climate on geographical distribution. I believe that the world has recently
felt one of his great cycles of change; and that on this view, combined with
modification through natural selection, a multitude of facts in the present
distribution both of the same and of allied forms of life can be explained.
The living waters may be said to have flowed during one short period from the
north and from the south, and to have crossed at the equator; but to have
flowed with greater force from the north so as to have freely inundated the
south. As the tide leaves its drift in horizontal lines, though rising higher
on the shores where the tide rises highest, so have the living waters left
their living drift on our mountain-summits, in a line gently rising from the
arctic lowlands to a great height under the equator. The various beings thus
left stranded may be compared with savage races of man, driven up and
surviving in the mountain-fastnesses of almost every land, which serve as a
record, full of interest to us, of the former inhabitants of the surrounding
lowlands.