GEOGRAPHICAL DISTRIBUTION.
Present distribution cannot be accounted for by differences in physical
conditions—Importance of barriers—Affinity of the productions of the
same continent—Centres of creation—Means of dispersal by changes of
climate and of the level of the land, and by occasional means—Dispersal
during the Glacial period—Alternate Glacial periods in the North and
South.
In considering the distribution of organic beings over the face of the
globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various regions
can be wholly accounted for by climatal and other physical conditions.
Of late, almost every author who has studied the subject has come to
this conclusion. The case of America alone would almost suffice to
prove its truth; for if we exclude the arctic and northern temperate
parts, all authors agree that one of the most fundamental divisions in
geographical distribution is that between the New and Old Worlds; yet
if we travel over the vast American continent, from the central parts
of the United States to its extreme southern point, we meet with the
most diversified conditions; humid districts, arid deserts, lofty
mountains, grassy plains, forests, marshes, lakes and great rivers,
under almost every temperature. There is hardly a climate or condition
in the Old World which cannot be paralleled in the New—at least so
closely as the same species generally require. No doubt small areas can
be pointed out in the Old World hotter than any in the New World; but
these are not inhabited by a fauna different from that of the
surrounding districts; for it is rare to find a group of organisms
confined to a small area, of which the conditions are peculiar in only
a slight degree. Notwithstanding this general parallelism in the
conditions of Old and New Worlds, how widely different are their living
productions!
In the southern hemisphere, if we compare large tracts of land in
Australia, South Africa, and western South America, between latitudes
25° and 35°, we shall find parts extremely similar in all their
conditions, yet it would not be possible to point out three faunas and
floras more utterly dissimilar. Or, again, we may compare the
productions of South America south of latitude 35° with those north of
25°, which consequently are separated by a space of ten degrees of
latitude, and are exposed to considerably different conditions; yet
they are incomparably more closely related to each other than they are
to the productions of Australia or Africa under nearly the same
climate. Analogous facts could be given with respect to the inhabitants
of the sea.
A second great fact which strikes us in our general review is, that
barriers of any kind, or obstacles to free migration, are related in a
close and important manner to the differences between the productions
of various regions. We see this in the great difference in nearly all
the terrestrial productions of the New and Old Worlds, excepting in the
northern parts, where the land almost joins, and where, under a
slightly different climate, there might have been free migration for
the northern temperate forms, as there now is for the strictly arctic
productions. We see the same fact in the great difference between the
inhabitants of Australia, Africa, and South America under the same
latitude; for these countries are almost as much isolated from each
other as is possible. On each continent, also, we see the same fact;
for on the opposite sides of lofty and continuous mountain-ranges, and
of great deserts and even of large rivers, we find different
productions; though as mountain chains, deserts, &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. The marine inhabitants of the
eastern and western shores of South America are very distinct, with
extremely few shells, crustacea, or echinodermata in common; but Dr.
Günther has recently shown that about thirty per cent of the fishes are
the same on the opposite sides of the isthmus of Panama; and this fact
has led naturalists to believe that the isthmus was formerly open.
Westward of the shores of America, a wide space of open ocean extends,
with not an island as a halting-place for emigrants; here we have a
barrier of another kind, and as soon as this is passed we meet in the
eastern islands of the Pacific with another and totally distinct fauna.
So that three marine faunas range northward and southward in parallel
lines not far from each other, under corresponding climate; but from
being separated from each other by impassable barriers, either of land
or open sea, they are almost wholly distinct. On the other hand,
proceeding still farther westward from the eastern islands of the
tropical parts of the Pacific, we encounter no impassable barriers, and
we have innumerable islands as halting-places, or continuous coasts,
until, after travelling over a hemisphere, we come to the shores of
Africa; and over this vast space we meet with no well-defined and
distinct marine faunas. Although so few marine animals are common to
the above-named three approximate faunas of Eastern and Western America
and the eastern Pacific islands, yet many fishes range from the Pacific
into the Indian Ocean, and many shells are common to the eastern
islands of the Pacific and the eastern shores of Africa on almost
exactly opposite meridians of longitude.
A third great fact, partly included in the foregoing statement, is the
affinity of the productions of the same continent or of the same sea,
though the species themselves are distinct at different points and
stations. It is a law of the widest generality, and every continent
offers innumerable instances. Nevertheless, the naturalist, in
travelling, for instance, from north to south, never fails to be struck
by the manner in which successive groups of beings, specifically
distinct, though nearly related, replace each other. He hears from
closely allied, yet distinct kinds of birds, notes nearly similar, and
sees their nests similarly constructed, but not quite alike, with eggs
coloured in nearly the same manner. The plains near the Straits of
Magellan are inhabited by one species of Rhea (American ostrich), and
northward the plains of La Plata by another species of the same genus;
and not by a true ostrich or emu, like those inhabiting Africa and
Australia under the same latitude. On these same plains of La Plata we
see the agouti and bizcacha, animals having nearly the same habits as
our hares and rabbits, and belonging to the same order of Rodents, but
they plainly display an American type of structure. We ascend the lofty
peaks of the Cordillera, and we find an alpine species of bizcacha; we
look to the waters, and we do not find the beaver or muskrat, but the
coypu and capybara, rodents of the South American type. Innumerable
other instances could be given. If we look to the islands off the
American shore, however much they may differ in geological structure,
the inhabitants are essentially American, though they may be all
peculiar species. We may look back to past ages, as shown in the last
chapter, and we find American types then prevailing on the American
continent and in the American seas. We see in these facts some deep
organic bond, throughout space and time, over the same areas of land
and water, independently of physical conditions. The naturalist must be
dull who is not led to inquire what this bond is.
The bond is simply inheritance, that cause which alone, as far as we
positively know, produces organisms quite like each other, or, as we
see in the case of varieties, nearly alike. The dissimilarity of the
inhabitants of different regions may be attributed to modification
through variation and natural selection, and probably in a subordinate
degree to the definite influence of different physical conditions. The
degrees of dissimilarity will depend on the migration of the more
dominant forms of life from one region into another having been more or
less effectually prevented, at periods more or less remote—on the
nature and number of the former immigrants—and on the action of the
inhabitants on each other in leading to the preservation of different
modifications; the relation of organism to organism in the struggle for
life being, as I have already often remarked, the most important of all
relations. Thus the high importance of barriers comes into play by
checking migration; as does time for the slow process of modification
through natural selection. Widely-ranging species, abounding in
individuals, which have already triumphed over many competitors in
their own widely-extended homes, will have the best chance of seizing
on new places, when they spread out into new countries. In their new
homes they will be exposed to new conditions, and will frequently
undergo further modification and improvement; and thus they will become
still further victorious, and will produce groups of modified
descendants. On this principle of inheritance with modification we can
understand how it is that sections of genera, whole genera, and even
families, are confined to the same areas, as is so commonly and
notoriously the case.
There is no evidence, as was remarked in the last chapter, of the
existence of any law of necessary development. As the variability of
each species is an independent property, and will be taken advantage of
by natural selection, only so far as it profits each individual in its
complex struggle for life, so the amount of modification in different
species will be no uniform quantity. If a number of species, after
having long competed with each other in their old home, were to migrate
in a body into a new and afterwards isolated country, they would be
little liable to modification; for neither migration nor isolation in
themselves effect anything. These principles come into play only by
bringing organisms into new relations with each other and in a lesser
degree with the surrounding physical conditions. As we have seen in the
last chapter that some forms have retained nearly the same character
from an enormously remote geological period, so certain species have
migrated over vast spaces, and have not become greatly or at all
modified.
According to these views, it is obvious that the several species of the
same genus, though inhabiting the most distant quarters of the world,
must originally have proceeded from the same source, as they are
descended from the same progenitor. In the case of those species which
have undergone, during whole geological periods, little modification,
there is not much difficulty in believing that they have migrated from
the same region; for during the vast geographical and climatical
changes which have supervened since ancient times, almost any amount of
migration is possible. But in many other cases, in which we have reason
to believe that the species of a genus have been produced within
comparatively recent times, there is great difficulty on this head. It
is also obvious that the individuals of the same species, though now
inhabiting distant and isolated regions, must have proceeded from one
spot, where their parents were first produced: for, as has been
explained, it is incredible that individuals identically the same
should have been produced from parents specifically distinct.
_Single Centres of supposed Creation._—We are thus brought to the
question which has been largely discussed by naturalists, namely,
whether species have been created at one or more points of the earth’s
surface. Undoubtedly there are many cases of extreme difficulty in
understanding how the same species could possibly have migrated from
some one point to the several distant and isolated points, where now
found. Nevertheless the simplicity of the view that each species was
first produced within a single region captivates the mind. He who
rejects it, rejects the vera causa of ordinary generation with
subsequent migration, and calls in the agency of a miracle. It is
universally admitted, that in most cases the area inhabited by a
species is continuous; and that when a plant or animal inhabits two
points so distant from each other, or with an interval of such a
nature, that the space could not have been easily passed over by
migration, the fact is given as something remarkable and exceptional.
The incapacity of migrating across a wide sea is more clear in the case
of terrestrial mammals than perhaps with any other organic beings; and,
accordingly, we find no inexplicable instances of the same mammals
inhabiting distant points of the world. No geologist feels any
difficulty in Great Britain possessing the same quadrupeds with the
rest of Europe, for they were no doubt once united. But if the same
species can be produced at two separate points, why do we not find a
single mammal common to Europe and Australia or South America? The
conditions of life are nearly the same, so that a multitude of European
animals and plants have become naturalised in America and Australia;
and some of the aboriginal plants are identically the same at these
distant points of the northern and southern hemispheres? The answer, as
I believe, is, that mammals have not been able to migrate, whereas some
plants, from their varied means of dispersal, have migrated across the
wide and broken interspaces. The great and striking influence of
barriers of all kinds, is intelligible only on the view that the great
majority of species have been produced on one side, and have not been
able to migrate to the opposite side. Some few families, many
subfamilies, very many genera, a still greater number of sections of
genera, are confined to a single region; and it has been observed by
several naturalists that the most natural genera, or those genera in
which the species are most closely related to each other, are generally
confined to the same country, or if they have a wide range that their
range is continuous. What a strange anomaly it would be if a directly
opposite rule were to prevail when we go down one step lower in the
series, namely to the individuals of the same species, and these had
not been, at least at first, confined to some one region!
Hence, it seems to me, as it has to many other naturalists, that the
view of each species having been produced in one area alone, and having
subsequently migrated from that area as far as its powers of migration
and subsistence under past and present conditions permitted, is the
most probable. Undoubtedly many cases occur in which we cannot explain
how the same species could have passed from one point to the other. But
the geographical and climatical changes which have certainly occurred
within recent geological times, must have rendered discontinuous the
formerly continuous range of many species. So that we are reduced to
consider whether the exceptions to continuity of range are so numerous,
and of so grave a nature, that we ought to give up the belief, rendered
probable by general considerations, that each species has been produced
within one area, and has migrated thence as far as it could. It would
be hopelessly tedious to discuss all the exceptional cases of the same
species, now living at distant and separated points; nor do I for a
moment pretend that any explanation could be offered of many instances.
But, after some preliminary remarks, I will discuss a few of the most
striking classes of facts, namely, the existence of the same species on
the summits of distant mountain ranges, and at distant points in the
Arctic and Antarctic regions; and secondly (in the following chapter),
the wide distribution of fresh water productions; and thirdly, the
occurrence of the same terrestrial species on islands and on the
nearest mainland, though separated by hundreds of miles of open sea. If
the existence of the same species at distant and isolated points of the
earth’s surface can in many instances be explained on the view of each
species having migrated from a single birthplace; then, considering our
ignorance with respect to former climatical and geographical changes,
and to the various occasional means of transport, the belief that a
single birthplace is the law seems to me incomparably the safest.
In discussing this subject we shall be enabled at the same time to
consider a point equally important for us, namely, whether the several
species of a genus which must on our theory all be descended from a
common progenitor, can have migrated, undergoing modification during
their migration from some one area. If, when most of the species
inhabiting one region are different from those of another region,
though closely allied to them, it can be shown that migration from the
one region to the other has probably occurred at some former period,
our general view will be much strengthened; for the explanation is
obvious on the principle of descent with modification. A volcanic
island, for instance, upheaved and formed at the distance of a few
hundreds of miles from a continent, would probably receive from it in
the course of time a few colonists, and their descendants, though
modified, would still be related by inheritance to the inhabitants of
that continent. Cases of this nature are common, and are, as we shall
hereafter see, inexplicable on the theory of independent creation. This
view of the relation of the species of one region to those of another,
does not differ much from that advanced by Mr. Wallace, who concludes
that “every species has come into existence coincident both in space
and time with a pre-existing closely allied species.” And it is now
well known that he attributes this coincidence to descent with
modification.
The question of single or multiple centres of creation differs from
another though allied question, namely, whether all the individuals of
the same species are descended from a single pair, or single
hermaphrodite, or whether, as some authors suppose, from many
individuals simultaneously created. With organic beings which never
intercross, if such exist, each species, must be descended from a
succession of modified varieties, that have supplanted each other, but
have never blended with other individuals or varieties of the same
species, so that, at each successive stage of modification, all the
individuals of the same form will be descended from a single parent.
But in the great majority of cases, namely, with all organisms which
habitually unite for each birth, or which occasionally intercross, the
individuals of the same species inhabiting the same area will be kept
nearly uniform by intercrossing; so that many individuals will go on
simultaneously changing, and the whole amount of modification at each
stage will not be due to descent from a single parent. To illustrate
what I mean: our English race-horses differ from the horses of every
other breed; but they do not owe their difference and superiority to
descent from any single pair, but to continued care in the selecting
and training of many individuals during each generation.
Before discussing the three classes of facts, which I have selected as
presenting the greatest amount of difficulty on the theory of “single
centres of creation,” I must say a few words on the means of dispersal.
_Means of Dispersal._
Sir C. Lyell and other authors have ably treated this subject. I can
give here only the briefest abstract of the more important facts.
Change of climate must have had a powerful influence on migration. A
region now impassable to certain organisms from the nature of its
climate, might have been a high road for migration, when the climate
was different. I shall, however, presently have to discuss this branch
of the subject in some detail. Changes of level in the land must also
have been highly influential: a narrow isthmus now separates two marine
faunas; submerge it, or let it formerly have been submerged, and the
two faunas will now blend together, or may formerly have blended. Where
the sea now extends, land may at a former period have connected islands
or possibly even continents together, and thus have allowed terrestrial
productions to pass from one to the other. No geologist disputes that
great mutations of level have occurred within the period of existing
organisms. Edward Forbes insisted that all the islands in the Atlantic
must have been recently connected with Europe or Africa, and Europe
likewise with America. Other authors have thus hypothetically bridged
over every ocean, and united almost every island with some mainland.
If, indeed, the arguments used by Forbes are to be trusted, it must be
admitted that scarcely a single island exists which has not recently
been united to some continent. This view cuts the Gordian knot of the
dispersal of the same species to the most distant points, and removes
many a difficulty; but to the best of my judgment we are not authorized
in admitting such enormous geographical changes within the period of
existing species. It seems to me that we have abundant evidence of
great oscillations in the level of the land or sea; but not of such
vast changes in the position and extension of our continents, as to
have united them within the recent period to each other and to the
several intervening oceanic islands. I freely admit the former
existence of many islands, now buried beneath the sea, which may have
served as halting-places for plants and for many animals during their
migration. In the coral-producing oceans such sunken islands are now
marked by rings of coral or atolls standing over them. Whenever it is
fully admitted, as it will some day be, that each species has proceeded
from a single birthplace, and when in the course of time we know
something definite about the means of distribution, we shall be enabled
to speculate with security on the former extension of the land. But I
do not believe that it will ever be proved that within the recent
period most of our continents which now stand quite separate, have been
continuously, or almost continuously united with each other, and with
the many existing oceanic islands. Several facts in distribution—such
as the great difference in the marine faunas on the opposite sides of
almost every continent—the close relation of the tertiary inhabitants
of several lands and even seas to their present inhabitants—the degree
of affinity between the mammals inhabiting islands with those of the
nearest continent, being in part determined (as we shall hereafter see)
by the depth of the intervening ocean—these and other such facts are
opposed to the admission of such prodigious geographical revolutions
within the recent period, as are necessary on the view advanced by
Forbes and admitted by his followers. The nature and relative
proportions of the inhabitants of oceanic islands are likewise opposed
to the belief of their former continuity of continents. Nor does the
almost universally volcanic composition of such islands favour the
admission that they are the wrecks of sunken continents; if they had
originally existed as continental mountain ranges, some at least of the
islands would have been formed, like other mountain summits, of
granite, metamorphic schists, old fossiliferous and other rocks,
instead of consisting of mere piles of volcanic matter.
I must now say a few words on what are called accidental means, but
which more properly should be called occasional means of distribution.
I shall here confine myself to plants. In botanical works, this or that
plant is often stated to be ill adapted for wide dissemination; but the
greater or less facilities for transport across the sea may be said to
be almost wholly unknown. Until I tried, with Mr. Berkeley’s aid, a few
experiments, it was not even known how far seeds could resist the
injurious action of sea-water. To my surprise I found that out of
eighty-seven kinds, sixty-four germinated after an immersion of
twenty-eight days, and a few survived an immersion of 137 days. It
deserves notice that certain orders were far more injured than others:
nine Leguminosæ were tried, and, with one exception, they resisted the
salt-water badly; seven species of the allied orders, Hydrophyllaceæ
and Polemoniaceæ, were all killed by a month’s immersion. For
convenience’ sake I chiefly tried small seeds without the capsules or
fruit; and as all of these sank in a few days, they could not have been
floated across wide spaces of the sea, whether or not they were injured
by salt water. Afterwards I tried some larger fruits, capsules, &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 would often wash into the sea dried
plants or branches with seed-capsules or fruit attached to them. Hence
I was led to dry the stems and branches of ninety-four plants with ripe
fruit, and to place them on sea-water. The majority sank quickly, but
some which, whilst green, floated for a very short time, when dried
floated much longer; for instance, ripe hazel-nuts sank immediately,
but when dried they floated for ninety days, and afterwards when
planted germinated; an asparagus plant with ripe berries floated for
twenty-three days, when dried it floated for eighty-five days, and the
seeds afterwards germinated: the ripe seeds of Helosciadium sank in two
days, when dried they floated for above ninety days, and afterwards
germinated. Altogether, out of the ninety-four dried plants, eighteen
floated for above twenty-eight days; and some of the eighteen floated
for a very much longer period. So that as 64/87 kinds of seeds
germinated after an immersion of twenty-eight days; and as 18/94
distinct species with ripe fruit (but not all the same species as in
the foregoing experiment) floated, after being dried, for above
twenty-eight days, we may conclude, as far as anything can be inferred
from these scanty facts, that the seeds of 14/100 kinds of plants of
any country might be floated by sea-currents during twenty-eight days,
and would retain their power of germination. In Johnston’s Physical
Atlas, the average rate of the several Atlantic currents is
thirty-three miles per diem (some currents running at the rate of sixty
miles per diem); on this average, the seeds of 14/100 plants belonging
to one country might be floated across 924 miles of sea to another
country; and when stranded, if blown by an inland gale to a favourable
spot, would germinate.
Subsequently to my experiments, M. Martens tried similar ones, but in a
much better manner, for he placed the seeds in a box in the actual sea,
so that they were alternately wet and exposed to the air like really
floating plants. He tried ninety-eight seeds, mostly different from
mine, but he chose many large fruits, and likewise seeds, from plants
which live near the sea; and this would have favoured both the average
length of their flotation and their resistance to the injurious action
of the salt-water. On the other hand, he did not previously dry the
plants or branches with the fruit; and this, as we have seen, would
have caused some of them to have floated much longer. The result was
that 18/98 of his seeds of different kinds floated for forty-two days,
and were then capable of germination. But I do not doubt that plants
exposed to the waves would float for a less time than those protected
from violent movement as in our experiments. Therefore, it would
perhaps be safer to assume that the seeds of about 10/100 plants of a
flora, after having been dried, could be floated across a space of sea
900 miles in width, and would then germinate. The fact of the larger
fruits often floating longer than the small, is interesting; as plants
with large seeds or fruit which, as Alph. de Candolle has shown,
generally have restricted ranges, could hardly be transported by any
other means.
Seeds may be occasionally transported in another manner. Drift timber
is thrown up on most islands, even on those in the midst of the widest
oceans; and the natives of the coral islands in the Pacific procure
stones for their tools, solely from the roots of drifted trees, these
stones being a valuable royal tax. I find that when irregularly shaped
stones are embedded in the roots of trees, small parcels of earth are
very frequently enclosed in their interstices and behind them, so
perfectly that not a particle could be washed away during the longest
transport: out of one small portion of earth thus _completely_ enclosed
by the roots of an oak about fifty years old, three dicotyledonous
plants germinated: I am certain of the accuracy of this observation.
Again, I can show that the carcasses of birds, when floating on the
sea, sometimes escape being immediately devoured; and many kinds of
seeds in the crops of floating birds long retain their vitality: peas
and vetches, for instance, are killed by even a few days’ immersion in
sea-water; but some taken out of the crop of a pigeon, which had
floated on artificial sea-water for thirty days, to my surprise nearly
all germinated.
Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how frequently
birds of many kinds are blown by gales to vast distances across the
ocean. We may safely assume that under such circumstances their rate of
flight would often be thirty-five miles an hour; and some authors have
given a far higher estimate. I have never seen an instance of
nutritious seeds passing through the intestines of a bird; but hard
seeds of fruit pass uninjured through even the digestive organs of a
turkey. In the course of two months, I picked up in my garden twelve
kinds of seeds, out of the excrement of small birds, and these seemed
perfect, and some of them, which were tried, germinated. But the
following fact is more important: the crops of birds do not secrete
gastric juice, and do not, as I know by trial, injure in the least the
germination of seeds; now, after a bird has found and devoured a large
supply of food, it is positively asserted that all the grains do not
pass into the gizzard for twelve or even eighteen hours. A bird in this
interval might easily be blown to the distance of five hundred miles,
and hawks are known to look out for tired birds, and the contents of
their torn crops might thus readily get scattered. Some hawks and owls
bolt their prey whole, and after an interval of from twelve to twenty
hours, disgorge pellets, which, as I know from experiments made in the
Zoological Gardens, include seeds capable of germination. Some seeds of
the oat, wheat, millet, canary, hemp, clover, and beet germinated after
having been from twelve to twenty-one hours in the stomachs of
different birds of prey; and two seeds of beet grew after having been
thus retained for two days and fourteen hours. Fresh-water fish, I
find, eat seeds of many land and water plants; fish are frequently
devoured by birds, and thus the seeds might be transported from place
to place. I forced many kinds of seeds into the stomachs of dead fish,
and then gave their bodies to fishing-eagles, storks, and pelicans;
these birds, after an interval of many hours, either rejected the seeds
in pellets or passed them in their excrement; and several of these
seeds retained the power of germination. Certain seeds, however, were
always killed by this process.
Locusts are sometimes blown to great distances from the land. I myself
caught one 370 miles from the coast of Africa, and have heard of others
caught at greater distances. The Rev. R.T. Lowe informed Sir C. Lyell
that in November, 1844, swarms of locusts visited the island of
Madeira. They were in countless numbers, as thick as the flakes of snow
in the heaviest snowstorm, and extended upward as far as could be seen
with a telescope. During two or three days they slowly careered round
and round in an immense ellipse, at least five or six miles in
diameter, and at night alighted on the taller trees, which were
completely coated with them. They then disappeared over the sea, as
suddenly as they had appeared, and have not since visited the island.
Now, in parts of Natal it is believed by some farmers, though on
insufficient evidence, that injurious seeds are introduced into their
grass-land in the dung left by the great flights of locusts which often
visit that country. In consequence of this belief Mr. Weale sent me in
a letter a small packet of the dried pellets, out of which I extracted
under the microscope several seeds, and raised from them seven grass
plants, belonging to two species, of two genera. Hence a swarm of
locusts, such as that which visited Madeira, might readily be the means
of introducing several kinds of plants into an island lying far from
the mainland.
Although the beaks and feet of birds are generally clean, earth
sometimes adheres to them: in one case I removed sixty-one grains, and
in another case twenty-two grains of dry argillaceous earth from the
foot of a partridge, and in the earth there was a pebble as large as
the seed of a vetch. Here is a better case: the leg of a woodcock was
sent to me by a friend, with a little cake of dry earth attached to the
shank, weighing only nine grains; and this contained a seed of the
toad-rush (Juncus bufonius) which germinated and flowered. Mr.
Swaysland, of Brighton, who during the last forty years has paid close
attention to our migratory birds, informs me that he has often shot
wagtails (Motacillæ), wheatears, and whinchats (Saxicolæ), on their
first arrival on our shores, before they had alighted; and he has
several times noticed little cakes of earth attached to their feet.
Many facts could be given showing how generally soil is charged with
seeds. For instance, Professor Newton sent me the leg of a red-legged
partridge (Caccabis rufa) which had been wounded and could not fly,
with a ball of hard earth adhering to it, and weighing six and a half
ounces. The earth had been kept for three years, but when broken,
watered and placed under a bell glass, no less than eighty-two plants
sprung from it: these consisted of twelve monocotyledons, including the
common oat, and at least one kind of grass, and of seventy
dicotyledons, which consisted, judging from the young leaves, of at
least three distinct species. With such facts before us, can we doubt
that the many birds which are annually blown by gales across great
spaces of ocean, and which annually migrate—for instance, the millions
of quails across the Mediterranean—must occasionally transport a few
seeds embedded in dirt adhering to their feet or beaks? But I shall
have to recur to this subject.
As icebergs are known to be sometimes loaded with earth and stones, and
have even carried brushwood, bones, and the nest of a land-bird, it can
hardly be doubted that they must occasionally, as suggested by Lyell,
have transported seeds from one part to another of the arctic and
antarctic regions; and during the Glacial period from one part of the
now temperate regions to another. In the Azores, from the large number
of plants common to Europe, in comparison with the species on the other
islands of the Atlantic, which stand nearer to the mainland, and (as
remarked by Mr. H.C. Watson) from their somewhat northern character, in
comparison with the latitude, I suspected that these islands had been
partly stocked by ice-borne seeds during the Glacial epoch. At my
request Sir C. Lyell wrote to M. Hartung to inquire whether he had
observed erratic boulders on these islands, and he answered that he had
found large fragments of granite and other rocks, which do not occur in
the archipelago. Hence we may safely infer that icebergs formerly
landed their rocky burdens on the shores of these mid-ocean islands,
and it is at least possible that they may have brought thither the
seeds of northern plants.
Considering that these several means of transport, and that other
means, which without doubt remain to be discovered, have been in action
year after year for tens of thousands of years, it would, I think, be a
marvellous fact if many plants had not thus become widely transported.
These means of transport are sometimes called accidental, but this is
not strictly correct: the currents of the sea are not accidental, nor
is the direction of prevalent gales of wind. It should be observed that
scarcely any means of transport would carry seeds for very great
distances; for seeds do not retain their vitality when exposed for a
great length of time to the action of sea water; nor could they be long
carried in the crops or intestines of birds. These means, however,
would suffice for occasional transport across tracts of sea some
hundred miles in breadth, or from island to island, or from a continent
to a neighbouring island, but not from one distant continent to
another. The floras of distant continents would not by such means
become mingled; but would remain as distinct as they now are. The
currents, from their course, would never bring seeds from North America
to Britain, though they might and do bring seeds from the West Indies
to our western shores, where, if not killed by their very long
immersion in salt water, they could not endure our climate. Almost
every year, one or two land-birds are blown across the whole Atlantic
Ocean, from North America to the western shores of Ireland and England;
but seeds could be transported by these rare wanderers only by one
means, namely, by dirt adhering to their feet or beaks, which is in
itself a rare accident. Even in this case, how small would be the
chance of a seed falling on favourable soil, and coming to maturity!
But it would be a great error to argue that because a well-stocked
island, like Great Britain, has not, as far as is known (and it would
be very difficult to prove this), received within the last few
centuries, through occasional means of transport, immigrants from
Europe or any other continent, that a poorly-stocked island, though
standing more remote from the mainland, would not receive colonists by
similar means. Out of a hundred kinds of seeds or animals transported
to an island, even if far less well-stocked than Britain, perhaps not
more than one would be so well fitted to its new home, as to become
naturalised. But this is no valid argument against what would be
effected by occasional means of transport, during the long lapse of
geological time, whilst the island was being upheaved, and before it
had become fully stocked with inhabitants. On almost bare land, with
few or no destructive insects or birds living there, nearly every seed
which chanced to arrive, if fitted for the climate, would germinate and
survive.
_Dispersal during the Glacial Period._
The identity of many plants and animals, on mountain-summits, separated
from each other by hundreds of miles of lowlands, where Alpine species
could not possibly exist, is one of the most striking cases known of
the same species living at distant points, without the apparent
possibility of their having migrated from one point to the other. It is
indeed a remarkable fact to see so many plants of the same species
living on the snowy regions of the Alps or Pyrenees, and in the extreme
northern parts of Europe; but it is far more remarkable, that the
plants on the White Mountains, in the United States of America, are all
the same with those of Labrador, and nearly all the same, as we hear
from Asa Gray, with those on the loftiest mountains of Europe. Even as
long ago as 1747, such facts led Gmelin to conclude that the same
species must have been independently created at many distinct points;
and we might have remained in this same belief, had not Agassiz and
others called vivid attention to the Glacial period, which, as we shall
immediately see, affords a simple explanation of these facts. We have
evidence of almost every conceivable kind, organic and inorganic, that,
within a very recent geological period, central Europe and North
America suffered under an Arctic climate. The ruins of a house burnt by
fire do not tell their tale more plainly than do the mountains of
Scotland and Wales, with their scored flanks, polished surfaces, and
perched boulders, of the icy streams with which their valleys were
lately filled. So greatly has the climate of Europe changed, that in
Northern Italy, gigantic moraines, left by old glaciers, are now
clothed by the vine and maize. Throughout a large part of the United
States, erratic boulders and scored rocks plainly reveal a former cold
period.
The former influence of the glacial climate on the distribution of the
inhabitants of Europe, as explained by Edward Forbes, is substantially
as follows. But we shall follow the changes more readily, by supposing
a new glacial period slowly to come on, and then pass away, as formerly
occurred. As the cold came on, and as each more southern zone became
fitted for the inhabitants of the north, these would take the places of
the former inhabitants of the temperate regions. The latter, at the
same time would travel further and further southward, unless they were
stopped by barriers, in which case they would perish. The mountains
would become covered with snow and ice, and their former Alpine
inhabitants would descend to the plains. By the time that the cold had
reached its maximum, we should have an arctic fauna and flora, covering
the central parts of Europe, as far south as the Alps and Pyrenees, and
even stretching into Spain. The now temperate regions of the United
States would likewise be covered by arctic plants and animals and these
would be nearly the same with those of Europe; for the present
circumpolar inhabitants, which we suppose to have everywhere travelled
southward, are remarkably uniform round the world.
As the warmth returned, the arctic forms would retreat northward,
closely followed up in their retreat by the productions of the more
temperate regions. And as the snow melted from the bases of the
mountains, the arctic forms would seize on the cleared and thawed
ground, always ascending, as the warmth increased and the snow still
further disappeared, higher and higher, whilst their brethren were
pursuing their northern journey. Hence, when the warmth had fully
returned, the same species, which had lately lived together on the
European and North American lowlands, would again be found in the
arctic regions of the Old and New Worlds, and on many isolated
mountain-summits far distant from each other.
Thus we can understand the identity of many plants at points so
immensely remote as the mountains of the United States and those of
Europe. We can thus also understand the fact that the Alpine plants of
each mountain-range are more especially related to the arctic forms
living due north or nearly due north of them: for the first migration
when the cold came on, and the re-migration on the returning warmth,
would generally have been due south and north. The Alpine plants, for
example, of Scotland, as remarked by Mr. H.C. Watson, and those of the
Pyrenees, as remarked by Ramond, are more especially allied to the
plants of northern Scandinavia; those of the United States to Labrador;
those of the mountains of Siberia to the arctic regions of that
country. These views, grounded as they are on the perfectly
well-ascertained occurrence of a former Glacial period, seem to me to
explain in so satisfactory a manner the present distribution of the
Alpine and Arctic productions of Europe and America, that when in other
regions we find the same species on distant mountain-summits, we may
almost conclude, without other evidence, that a colder climate formerly
permitted their migration across the intervening lowlands, now become
too warm for their existence.
As the arctic forms moved first southward and afterwards backward to
the north, in unison with the changing climate, they will not have been
exposed during their long migrations to any great diversity of
temperature; and as they all migrated in a body together, their mutual
relations will not have been much disturbed. Hence, in accordance with
the principles inculcated in this volume, these forms will not have
been liable to much modification. But with the Alpine productions, left
isolated from the moment of the returning warmth, first at the bases
and ultimately on the summits of the mountains, the case will have been
somewhat different; for it is not likely that all the same arctic
species will have been left on mountain ranges far distant from each
other, and have survived there ever since; they will also, in all
probability, have become mingled with ancient Alpine species, which
must have existed on the mountains before the commencement of the
Glacial epoch, and which during the coldest period will have been
temporarily driven down to the plains; they will, also, have been
subsequently exposed to somewhat different climatical influences. Their
mutual relations will thus have been in some degree disturbed;
consequently they will have been liable to modification; and they have
been modified; for if we compare the present Alpine plants and animals
of the several great European mountain ranges, one with another, though
many of the species remain identically the same, some exist as
varieties, some as doubtful forms or sub-species and some as distinct
yet closely allied species representing each other on the several
ranges.
In the foregoing illustration, I have assumed that at the commencement
of our imaginary Glacial period, the arctic productions were as uniform
round the polar regions as they are at the present day. But it is also
necessary to assume that many sub-arctic and some few temperate forms
were the same round the world, for some of the species which now exist
on the lower mountain slopes and on the plains of North America and
Europe are the same; and it may be asked how I account for this degree
of uniformity of the sub-arctic and temperate forms round the world, at
the commencement of the real Glacial period. At the present day, the
sub-arctic and northern temperate productions of the Old and New Worlds
are separated from each other by the whole Atlantic Ocean and by the
northern part of the Pacific. During the Glacial period, when the
inhabitants of the Old and New Worlds lived further southwards than
they do at present, they must have been still more completely separated
from each other by wider spaces of ocean; so that it may well be asked
how the same species could then or previously have entered the two
continents. The explanation, I believe, lies in the nature of the
climate before the commencement of the Glacial period. At this, the
newer Pliocene period, the majority of the inhabitants of the world
were specifically the same as now, and we have good reason to believe
that the climate was warmer than at the present day. Hence, we may
suppose that the organisms which now live under latitude 60°, lived
during the Pliocene period further north, under the Polar Circle, in
latitude 66°–67°; and that the present arctic productions then lived on
the broken land still nearer to the pole. Now, if we look at a
terrestrial globe, we see under the Polar Circle that there is almost
continuous land from western Europe through Siberia, to eastern
America. And this continuity of the circumpolar land, with the
consequent freedom under a more favourable climate for intermigration,
will account for the supposed uniformity of the sub-arctic and
temperate productions of the Old and New Worlds, at a period anterior
to the Glacial epoch.
Believing, from reasons before alluded to, that our continents have
long remained in nearly the same relative position, though subjected to
great oscillations of level, I am strongly inclined to extend the above
view, and to infer that during some earlier and still warmer period,
such as the older Pliocene period, a large number of the same plants
and animals inhabited the almost continuous circumpolar land; and that
these plants and animals, both in the Old and New Worlds, began slowly
to migrate southwards as the climate became less warm, long before the
commencement of the Glacial period. We now see, as I believe, their
descendants, mostly in a modified condition, in the central parts of
Europe and the United States. On this view we can understand the
relationship with very little identity, between the productions of
North America and Europe—a relationship which is highly remarkable,
considering the distance of the two areas, and their separation by the
whole Atlantic Ocean. We can further understand the singular fact
remarked on by several observers that the productions of Europe and
America during the later tertiary stages were more closely related to
each other than they are at the present time; for during these warmer
periods the northern parts of the Old and New Worlds will have been
almost continuously united by land, serving as a bridge, since rendered
impassable by cold, for the intermigration of their inhabitants.
During the slowly decreasing warmth of the Pliocene period, as soon as
the species in common, which inhabited the New and Old Worlds, migrated
south of the Polar Circle, they will have been completely cut off from
each other. This separation, as far as the more temperate productions
are concerned, must have taken place long ages ago. As the plants and
animals migrated southward, they will have become mingled in the one
great region with the native American productions, and would have had
to compete with them; and in the other great region, with those of the
Old World. Consequently we have here everything favourable for much
modification—for far more modification than with the Alpine
productions, left isolated, within a much more recent period, on the
several mountain ranges and on the arctic lands of Europe and North
America. Hence, it has come, that when we compare the now living
productions of the temperate regions of the New and Old Worlds, we find
very few identical species (though Asa Gray has lately shown that more
plants are identical than was formerly supposed), but we find in every
great class many forms, which some naturalists rank as geographical
races, and others as distinct species; and a host of closely allied or
representative forms which are ranked by all naturalists as
specifically distinct.
As on the land, so in the waters of the sea, a slow southern migration
of a marine fauna, which, during the Pliocene or even a somewhat
earlier period, was nearly uniform along the continuous shores of the
Polar Circle, will account, on the theory of modification, for many
closely allied forms now living in marine areas completely sundered.
Thus, I think, we can understand the presence of some closely allied,
still existing and extinct tertiary forms, on the eastern and western
shores of temperate North America; and the still more striking fact of
many closely allied crustaceans (as described in Dana’s admirable
work), some fish and other marine animals, inhabiting the Mediterranean
and the seas of Japan—these two areas being now completely separated by
the breadth of a whole continent and by wide spaces of ocean.
These cases of close relationship in species either now or formerly
inhabiting the seas on the eastern and western shores of North America,
the Mediterranean and Japan, and the temperate lands of North America
and Europe, are inexplicable on the theory of creation. We cannot
maintain that such species have been created alike, in correspondence
with the nearly similar physical conditions of the areas; for if we
compare, for instance, certain parts of South America with parts of
South Africa or Australia, we see countries closely similar in all
their physical conditions, with their inhabitants utterly dissimilar.
_Alternate Glacial Periods in the North and South._
But we must return to our more immediate subject. I am convinced that
Forbes’s view may be largely extended. In Europe we meet with the
plainest evidence of the Glacial period, from the western shores of
Britain to the Ural range, and southward to the Pyrenees. We may infer
from the frozen mammals and nature of the mountain vegetation, that
Siberia was similarly affected. In the Lebanon, according to Dr.
Hooker, perpetual snow formerly covered the central axis, and fed
glaciers which rolled 4,000 feet down the valleys. The same observer
has recently found great moraines at a low level on the Atlas range in
North Africa. Along the Himalaya, at points 900 miles apart, glaciers
have left the marks of their former low descent; and in Sikkim, Dr.
Hooker saw maize growing on ancient and gigantic moraines. Southward of
the Asiatic continent, on the opposite side of the equator, we know,
from the excellent researches of Dr. J. Haast and Dr. Hector, that in
New Zealand immense glaciers formerly descended to a low level; and the
same plants, found by Dr. Hooker on widely separated mountains in this
island tell the same story of a former cold period. From facts
communicated to me by the Rev. W.B. Clarke, it appears also that there
are traces of former glacial action on the mountains of the
south-eastern corner of Australia.
Looking to America: in the northern half, ice-borne fragments of rock
have been observed on the eastern side of the continent, as far south
as latitude 36° and 37°, and on the shores of the Pacific, where the
climate is now so different, as far south as latitude 46°. Erratic
boulders have, also, been noticed on the Rocky Mountains. In the
Cordillera of South America, nearly under the equator, glaciers once
extended far below their present level. In central Chile I examined a
vast mound of detritus with great boulders, crossing the Portillo
valley, which, there can hardly be a doubt, once formed a huge moraine;
and Mr. D. Forbes informs me that he found in various parts of the
Cordillera, from latitude 13° to 30° south, at about the height of
12,000 feet, deeply-furrowed rocks, resembling those with which he was
familiar in Norway, and likewise great masses of detritus, including
grooved pebbles. Along this whole space of the Cordillera true glaciers
do not now exist even at much more considerable heights. Further south,
on both sides of the continent, from latitude 41° to the southernmost
extremity, we have the clearest evidence of former glacial action, in
numerous immense boulders transported far from their parent source.
From these several facts, namely, from the glacial action having
extended all round the northern and southern hemispheres—from the
period having been in a geological sense recent in both
hemispheres—from its having lasted in both during a great length of
time, as may be inferred from the amount of work effected—and lastly,
from glaciers having recently descended to a low level along the whole
line of the Cordillera, it at one time appeared to me that we could not
avoid the conclusion that the temperature of the whole world had been
simultaneously lowered during the Glacial period. But now, Mr. Croll,
in a series of admirable memoirs, has attempted to show that a glacial
condition of climate is the result of various physical causes, brought
into operation by an increase in the eccentricity of the earth’s orbit.
All these causes tend towards the same end; but the most powerful
appears to be the indirect influence of the eccentricity of the orbit
upon oceanic currents. According to Mr. Croll, cold periods regularly
recur every ten or fifteen thousand years; and these at long intervals
are extremely severe, owing to certain contingencies, of which the most
important, as Sir C. Lyell has shown, is the relative position of the
land and water. Mr. Croll believes that the last great glacial period
occurred about 240,000 years ago, and endured, with slight alterations
of climate, for about 160,000 years. With respect to more ancient
glacial periods, several geologists are convinced, from direct
evidence, that such occurred during the miocene and eocene formations,
not to mention still more ancient formations. But the most important
result for us, arrived at by Mr. Croll, is that whenever the northern
hemisphere passes through a cold period the temperature of the southern
hemisphere is actually raised, with the winters rendered much milder,
chiefly through changes in the direction of the ocean currents. So
conversely it will be with the northern hemisphere, while the southern
passes through a glacial period. This conclusion throws so much light
on geographical distribution that I am strongly inclined to trust in
it; but I will first give the facts which demand an explanation.
In South America, Dr. Hooker has shown that besides many closely allied
species, between forty and fifty of the flowering plants of Tierra del
Fuego, forming no inconsiderable part of its scanty flora, are common
to North America and Europe, enormously remote as these areas in
opposite hemispheres are from each other. On the lofty mountains of
equatorial America a host of peculiar species belonging to European
genera occur. On the Organ Mountains of Brazil some few temperate
European, some Antarctic and some Andean genera were found by Gardner
which do not exist in the low intervening hot countries. On the Silla
of Caraccas the illustrious Humboldt long ago found species belonging
to genera characteristic of the Cordillera.
In Africa, several forms characteristic of Europe, and some few
representatives of the flora of the Cape of Good Hope, occur on the
mountains of Abyssinia. At the Cape of Good Hope a very few European
species, believed not to have been introduced by man, and on the
mountains several representative European forms are found which have
not been discovered in the intertropical parts of Africa. Dr. Hooker
has also lately shown that several of the plants living on the upper
parts of the lofty island of Fernando Po, and on the neighbouring
Cameroon Mountains, in the Gulf of Guinea, are closely related to those
on the mountains of Abyssinia, and likewise to those of temperate
Europe. It now also appears, as I hear from Dr. Hooker, that some of
these same temperate plants have been discovered by the Rev. R.T. Lowe
on the mountains of the Cape Verde Islands. This extension of the same
temperate forms, almost under the equator, across the whole continent
of Africa and to the mountains of the Cape Verde archipelago, is one of
the most astonishing facts ever recorded in the distribution of plants.
On the Himalaya, and on the isolated mountain ranges of the peninsula
of India, on the heights of Ceylon, and on the volcanic cones of Java,
many plants occur either identically the same or representing each
other, and at the same time representing plants of Europe not found in
the intervening hot lowlands. A list of the genera of plants collected
on the loftier peaks of Java, raises a picture of a collection made on
a hillock in Europe. Still more striking is the fact that peculiar
Australian forms are represented by certain plants growing on the
summits of the mountains of Borneo. Some of these Australian forms, as
I hear from Dr. Hooker, extend along the heights of the peninsula of
Malacca, and are thinly scattered on the one hand over India, and on
the other hand as far north as Japan.
On the southern mountains of Australia, Dr. F. 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
certain plants growing on the more lofty mountains of the tropics in
all parts of the world, and on the temperate plains of the north and
south, are either the same species or varieties of the same species. It
should, however, be observed that these plants are not strictly arctic
forms; for, as Mr. H.C. Watson has remarked, “in receding from polar
toward equatorial latitudes, the Alpine or mountain flora really become
less and less Arctic.” Besides these identical and closely allied
forms, many species inhabiting the same widely sundered areas, belong
to genera not now found in the intermediate tropical lowlands.
These brief remarks apply to plants alone; but some few analogous facts
could be given in regard to terrestrial animals. In marine productions,
similar cases likewise occur; as an example, I may quote a statement by
the highest authority, Prof. Dana, that “it is certainly a wonderful
fact that New Zealand should have a closer resemblance in its crustacea
to Great Britain, its antipode, than to any other part of the world.”
Sir J. Richardson, also, speaks of the reappearance on the shores of
New Zealand, Tasmania, &c., of northern forms of fish. Dr. Hooker
informs me that twenty-five species of Algæ are common to New Zealand
and to Europe, but have not been found in the intermediate tropical
seas.
From the foregoing facts, namely, the presence of temperate forms on
the highlands across the whole of equatorial Africa, and along the
Peninsula of India, to Ceylon and the Malay Archipelago, and in a less
well-marked manner across the wide expanse of tropical South America,
it appears almost certain that at some former period, no doubt during
the most severe part of a Glacial period, the lowlands of these great
continents were everywhere tenanted under the equator by a considerable
number of temperate forms. At this period the equatorial climate at the
level of the sea was probably about the same with that now experienced
at the height of from five to six thousand feet under the same
latitude, or perhaps even rather cooler. During this, the coldest
period, the lowlands under the equator must have been clothed with a
mingled tropical and temperate vegetation, like that described by
Hooker as growing luxuriantly at the height of from four to five
thousand feet on the lower slopes of the Himalaya, but with perhaps a
still greater preponderance of temperate forms. So again in the
mountainous island of Fernando Po, in the Gulf of Guinea, Mr. Mann
found temperate European forms beginning to appear at the height of
about five thousand feet. On the mountains of Panama, at the height of
only two thousand feet, Dr. Seemann found the vegetation like that of
Mexico, “with forms of the torrid zone harmoniously blended with those
of the temperate.”
Now let us see whether Mr. Croll’s conclusion that when the northern
hemisphere suffered from the extreme cold of the great Glacial period,
the southern hemisphere was actually warmer, throws any clear light on
the present apparently inexplicable distribution of various organisms
in the temperate parts of both hemispheres, and on the mountains of the
tropics. The Glacial period, as measured by years, must have been very
long; and when we remember over what vast spaces some naturalised
plants and animals have spread within a few centuries, this period will
have been ample for any amount of migration. As the cold became more
and more intense, we know that Arctic forms invaded the temperate
regions; and from the facts just given, there can hardly be a doubt
that some of the more vigorous, dominant and widest-spreading temperate
forms invaded the equatorial lowlands. The inhabitants of these hot
lowlands would at the same time have migrated to the tropical and
subtropical regions of the south, for the southern hemisphere was at
this period warmer. On the decline of the Glacial period, as both
hemispheres gradually recovered their former temperature, the northern
temperate forms living on the lowlands under the equator, would have
been driven to their former homes or have been destroyed, being
replaced by the equatorial forms returning from the south. Some,
however, of the northern temperate forms would almost certainly have
ascended any adjoining high land, where, if sufficiently lofty, they
would have long survived like the Arctic forms on the mountains of
Europe. They might have survived, even if the climate was not perfectly
fitted for them, for the change of temperature must have been very
slow, and plants undoubtedly possess a certain capacity for
acclimatisation, as shown by their transmitting to their offspring
different constitutional powers of resisting heat and cold.
In the regular course of events the southern hemisphere would in its
turn be subjected to a severe Glacial period, with the northern
hemisphere rendered warmer; and then the southern temperate forms would
invade the equatorial lowlands. The northern forms which had before
been left on the mountains would now descend and mingle with the
southern forms. These latter, when the warmth returned, would return to
their former homes, leaving some few species on the mountains, and
carrying southward with them some of the northern temperate forms which
had descended from their mountain fastnesses. Thus, we should have some
few species identically the same in the northern and southern temperate
zones and on the mountains of the intermediate tropical regions. But
the species left during a long time on these mountains, or in opposite
hemispheres, would have to compete with many new forms and would be
exposed to somewhat different physical conditions; hence, they would be
eminently liable to modification, and would generally now exist as
varieties or as representative species; and this is the case. We must,
also, bear in mind the occurrence in both hemispheres of former Glacial
periods; for these will account, in accordance with the same
principles, for the many quite distinct species inhabiting the same
widely separated areas, and belonging to genera not now found in the
intermediate torrid zones.
It is a remarkable fact, strongly insisted on by Hooker in regard to
America, and by Alph. de Candolle in regard to Australia, that many
more identical or slightly modified species have migrated from the
north to the south, than in a reversed direction. We see, however, a
few southern forms on the mountains of Borneo and Abyssinia. I suspect
that this preponderant migration from the north to the south is due to
the greater extent of land in the north, and to the northern forms
having existed in their own homes in greater numbers, and having
consequently been advanced through natural selection and competition to
a higher stage of perfection, or dominating power, than the southern
forms. And thus, when the two sets became commingled in the equatorial
regions, during the alternations of the Glacial periods, the northern
forms were the more powerful and were able to hold their places on the
mountains, and afterwards migrate southward with the southern forms;
but not so the southern in regard to the northern forms. In the same
manner, at the present day, we see that very many European productions
cover the ground in La Plata, New Zealand, and to a lesser degree in
Australia, and have beaten the natives; whereas extremely few southern
forms have become naturalised in any part of the northern hemisphere,
though hides, wool, and other objects likely to carry seeds have been
largely imported into Europe during the last two or three centuries
from La Plata and during the last forty or fifty years from Australia.
The Neilgherrie Mountains in India, however, offer a partial exception;
for here, as I hear from Dr. Hooker, Australian forms are rapidly
sowing themselves and becoming naturalised. Before the last great
Glacial period, no doubt the intertropical mountains were stocked with
endemic Alpine forms; but these have almost everywhere yielded to the
more dominant forms generated in the larger areas and more efficient
workshops of the north. In many islands the native productions are
nearly equalled, or even outnumbered, by those which have become
naturalised; and this is the first stage towards their extinction.
Mountains are islands on the land; and their inhabitants have yielded
to those produced within the larger areas of the north, just in the
same way as the inhabitants of real islands have everywhere yielded and
are still yielding to continental forms naturalised through man’s
agency.
The same principles apply to the distribution of terrestrial animals
and of marine productions, in the northern and southern temperate
zones, and on the intertropical mountains. When, during the height of
the Glacial period, the ocean-currents were widely different to what
they now are, some of the inhabitants of the temperate seas might have
reached the equator; of these a few would perhaps at once be able to
migrate southwards, by keeping to the cooler currents, while others
might remain and survive in the colder depths until the southern
hemisphere was in its turn subjected to a glacial climate and permitted
their further progress; in nearly the same manner as, according to
Forbes, isolated spaces inhabited by Arctic productions exist to the
present day in the deeper parts of the northern temperate seas.
I am far from supposing that all the difficulties in regard to the
distribution and affinities of the identical and allied species, which
now live so widely separated in the north and south, and sometimes on
the intermediate mountain ranges, are removed on the views above given.
The exact lines of migration cannot be indicated. We cannot say why
certain species and not others have migrated; why certain species have
been modified and have given rise to new forms, while others have
remained unaltered. We cannot hope to explain such facts, until we can
say why one species and not another becomes naturalised by man’s agency
in a foreign land; why one species ranges twice or thrice as far, and
is twice or thrice as common, as another species within their own
homes.
Various special difficulties also remain to be solved; for instance,
the occurrence, as shown by Dr. Hooker, of the same plants at points so
enormously remote as Kerguelen Land, New Zealand, and Fuegia; but
icebergs, as suggested by Lyell, may have been concerned in their
dispersal. The existence at these and other distant points of the
southern hemisphere, of species, which, though distinct, belong to
genera exclusively confined to the south, is a more remarkable case.
Some of these species are so distinct, that we cannot suppose that
there has been time since the commencement of the last Glacial period
for their migration and subsequent modification to the necessary
degree. The facts seem to indicate that distinct species belonging to
the same genera have migrated in radiating lines from a common centre;
and I am inclined to look in the southern, as in the northern
hemisphere, to a former and warmer period, before the commencement of
the last Glacial period, when the Antarctic lands, now covered with
ice, supported a highly peculiar and isolated flora. It may be
suspected that before this flora was exterminated during the last
Glacial epoch, a few forms had been already widely dispersed to various
points of the southern hemisphere by occasional means of transport, and
by the aid, as halting-places, of now sunken islands. Thus the southern
shores of America, Australia, and New Zealand may have become slightly
tinted by the same peculiar forms of life.
Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate
throughout the world on geographical distribution. And we have now seen
that Mr. Croll’s conclusion that successive Glacial periods in the one
hemisphere coincide with warmer periods in the opposite hemisphere,
together with the admission of the slow modification of species,
explains a multitude of facts in the distribution of the same and of
the allied forms of life in all parts of the globe. The living waters
have flowed during one period from the north and during another from
the south, and in both cases have reached the equator; but the stream
of life has flowed with greater force from the north than in the
opposite direction, and has consequently more freely inundated the
south. As the tide leaves its drift in horizontal lines, rising higher
on the shores where the tide rises highest, so have the living waters
left their living drift on our mountain summits, in a line gently
rising from the Arctic lowlands to a great latitude under the equator.
The various beings thus left stranded may be compared with savage races
of man, driven up and surviving in the mountain fastnesses of almost
every land, which serves as a record, full of interest to us, of the
former inhabitants of the surrounding lowlands.