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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
in the sub-arctic and temperate forms round the world, at the commence-
ment 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 farther southward 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 in-
habitants of the world were specifically the same as now, and we have good
reason to believe that the climate was warmer than at the present day.
Hence we may suppose that the organisms which now live under latitude
60 degrees, lived during the Pliocene period farther north, under the Polar
Circle, in latitude 66-67 degrees; and that the present arctic productions
then lived on the broken land still nearer to the pole. Now, if we look at a
terrestrial globe, we see under the Polar Circle that there is almost continu-
ous land from Western Europe through Siberia, to Eastern America. And
this continuity of the circumpolar land, with the consequent freedom under
a more favorable 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 still earlier and still warmer period, such as the
older Pliocene period, a large number of the same plants and animals in-
habited the almost continuous circumpolar land; and that these plants and
animals, both in the Old and New Worlds, began slowly to migrate south-
ward 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 re-
lated 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 inter-migration of their inhabitants.

During the slowly decreasing warmth of the Pliocene period, as soon as



278 THE ORIGIN OF SPECIES

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 con-
cerned, 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 favorable 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 sup-
posed), 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 Amer-
ica; 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 in-
habiting 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 in-
stance, certain parts of South America with parts of South Africa or Aus-
tralia, 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' 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



GEOGRAPHICAL DISTRIBUTION 279

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 com-
municated 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 thirty-six and thirty-seven degrees, and on the shores of the Pacific,
where the climate is now so different, as far south as latitude forty-six
degrees. Erratic bowlders 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 Chili I examined a
vast mound of detritus with great bowlders, 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 thirteen to thirty degrees south, at about the height of 12,000 feet,
deeply-furrowed rocks, resembling those with which he was familiar in
Norway, and likewise great masses of detritus, including grooved pebbles.
Along this whole space of the Cordillera true glaciers do not now exist even
at much more considerable heights. Further south, on both sides of the
continent, from latitude forty-one degrees to the southern-most extremity,
we have the clearest evidence of former glacial action, in numerous im-
mense bowlders 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 de-
scended 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 tempera-
ture of the whole world had been simultaneously lowered during the Glacial
period. But now, Mr. Croll, in a series of admirable memoirs, has at-
tempted 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 toward the same end; but the most
powerful appears to be the indirect influence of the eccentricity of the orbit



280 THE ORIGIN OF SPECIES

upon oceanic currents. According to Mr. Croll, cold periods regularly recur
every ten or fifteen thousand years; and these at long intervals are ex-
tremely severe, owing to certain contingencies, of which the most impor-
tant, 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 thei
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 hem-
ispheres are from each other. On the lofty mountains of equatorial Amer-
ica 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 Caracas the illustrious Hum-
boldt long ago found species belonging to genera characteristic of the
Cordillera.

In Africa, several forms characteristic of Europe, and some few repre-
sentatives 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, be-
lieved not to have been introduced by man, and on the mountains several
representative European forms, are found which have not been discovered
in the inter-tropical 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 neighboring 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 dis-
tribution 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



GEOGRAPHICAL DISTRIBUTION 281

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. Miiller 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 moun-
tain 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 low-
lands.

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. Professor Dana, that "it is certainly a wonderful fact
that New Zealand should have a closer resemblance in its Crustacea to
Great Britain, its antipode, than to any other part of the world." Sir J.
Richardson, also, speaks of the reappearance on the shores of New Zealand,
Tasmania, etc., of northern forms of fish. Dr. Hooker informs me that
twenty-five species of algae are common to New Zealand and to Europe,
but have not been found in the intermediate tropical seas.

From the foregoing facts, namely, the presence of temperate forms on
the highlands across the whole of equatorial Africa, and along the penin-
sula of India, to Ceylon and the Malay Archipelago, and in a less well-
marked manner across the wide expanse of tropical South America, it ap-
pears 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 tem-
perate 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



282 THE ORIGIN OF SPECIES

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 Hima-
laya, 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 naturalized plants and ani-
mals 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 tem-
perature, 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 acclimatization, as shown by
their transmitting to their offspring different constitutional powers of resist-
ing 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



GEOGRAPHICAL DISTRIBUTION 283

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 some-
what different physical conditions: hence, they would be eminently liable
to modification, and would generally now exist as varieties or as representa-
tive 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 afterward to 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 Zea-
land, and to a lesser degree in Australia, and have beaten the natives;
whereas extremely few southern forms have become naturalized 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 naturalized. Before the last
great Glacial period, no doubt the inter-tropical 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 naturalized;
and this is the first stage toward 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
naturalized through man's agency.

The same principles apply to the distribution of terrestrial animals and



284 THE ORIGIN OF SPECIES

of marine productions, in the northern and southern temperate zones, and
on the inter- tropical 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 south-
ward, 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 distribu-
tion 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 naturalized 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 oc-
currence, as shown by Dr. Hooker, of the same plants at points so enor-



Online LibraryCharles DarwinThe origin of species → online text (page 38 of 50)