Gua Shen - "Meng Xi Bi Tan, volume 1 26 [1031 1095 A.D.]"

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Thomas Belt - "The Naturalist in Nicaragua"

Kate Douglas Wiggin - "Homespun Tales"

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Books: On the Origin of Species

C >> Charles Darwin >> On the Origin of Species




On the southern mountains of Australia, Dr. F. Muller has discovered
several European species; other species, not introduced by man, occur
on the lowlands; and a long list can be given, as I am informed by Dr.
Hooker, of European genera, found in Australia, but not in the
intermediate torrid regions. In the admirable 'Introduction to the
Flora of New Zealand,' by Dr. Hooker, analogous and striking facts are
given in regard to the plants of that large island. Hence we see that
throughout the world, the plants growing on the more lofty mountains,
and on the temperate lowlands of the northern and southern
hemispheres, are sometimes identically the same; but they are much
oftener specifically distinct, though related to each other in a most
remarkable manner.

This brief abstract applies to plants alone: some strictly analogous
facts could be given on the distribution of terrestrial animals. In
marine productions, similar cases occur; as an example, I may quote a
remark by the highest authority, 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.

It should be observed that the northern species and forms found in the
southern parts of the southern hemisphere, and on the mountain-ranges
of the intertropical regions, are not arctic, but belong to the
northern temperate zones. As Mr. H. C. Watson has recently remarked,
"In receding from polar towards equatorial latitudes, the Alpine or
mountain floras really become less and less arctic." Many of the forms
living on the mountains of the warmer regions of the earth and in the
southern hemisphere are of doubtful value, being ranked by some
naturalists as specifically distinct, by others as varieties; but some
are certainly identical, and many, though closely related to northern
forms, must be ranked as distinct species.

Now let us see what light can be thrown on the foregoing facts, on the
belief, supported as it is by a large body of geological evidence,
that the whole world, or a large part of it, was during the Glacial
period simultaneously much colder than at present. The Glacial period,
as measured by years, must have been very long; and when we remember
over what vast spaces some naturalised plants and animals have spread
within a few centuries, this period will have been ample for any
amount of migration. As the cold came slowly on, all the tropical
plants and other productions will have retreated from both sides
towards the equator, followed in the rear by the temperate
productions, and these by the arctic; but with the latter we are not
now concerned. The tropical plants probably suffered much extinction;
how much no one can say; perhaps formerly the tropics supported as
many species as we see at the present day crowded together at the Cape
of Good Hope, and in parts of temperate Australia. As we know that
many tropical plants and animals can withstand a considerable amount
of cold, many might have escaped extermination during a moderate fall
of temperature, more especially by escaping into the warmest spots.
But the great fact to bear in mind is, that all tropical productions
will have suffered to a certain extent. On the other hand, the
temperate productions, after migrating nearer to the equator, though
they will have been placed under somewhat new conditions, will have
suffered less. And it is certain that many temperate plants, if
protected from the inroads of competitors, can withstand a much warmer
climate than their own. Hence, it seems to me possible, bearing in
mind that the tropical productions were in a suffering state and could
not have presented a firm front against intruders, that a certain
number of the more vigorous and dominant temperate forms might have
penetrated the native ranks and have reached or even crossed the
equator. The invasion would, of course, have been greatly favoured by
high land, and perhaps by a dry climate; for Dr. Falconer informs me
that it is the damp with the heat of the tropics which is so
destructive to perennial plants from a temperate climate. On the other
hand, the most humid and hottest districts will have afforded an
asylum to the tropical natives. The mountain-ranges north-west of the
Himalaya, and the long line of the Cordillera, seem to have afforded
two great lines of invasion: and it is a striking fact, lately
communicated to me by Dr. Hooker, that all the flowering plants, about
forty-six in number, common to Tierra del Fuego and to Europe still
exist in North America, which must have lain on the line of march. But
I do not doubt that some temperate productions entered and crossed
even the LOWLANDS of the tropics at the period when the cold was most
intense,--when arctic forms had migrated some twenty-five degrees of
latitude from their native country and covered the land at the foot of
the Pyrenees. At this period of extreme cold, I believe that the
climate under the equator at the level of the sea was about the same
with that now felt there at the height of six or seven thousand feet.
During this the coldest period, I suppose that large spaces of the
tropical lowlands were clothed with a mingled tropical and temperate
vegetation, like that now growing with strange luxuriance at the base
of the Himalaya, as graphically described by Hooker.

Thus, as I believe, a considerable number of plants, a few terrestrial
animals, and some marine productions, migrated during the Glacial
period from the northern and southern temperate zones into the
intertropical regions, and some even crossed the equator. As the
warmth returned, these temperate forms would naturally ascend the
higher mountains, being exterminated on the lowlands; those which had
not reached the equator, would re-migrate northward or southward
towards their former homes; but the forms, chiefly northern, which had
crossed the equator, would travel still further from their homes into
the more temperate latitudes of the opposite hemisphere. Although we
have reason to believe from geological evidence that the whole body of
arctic shells underwent scarcely any modification during their long
southern migration and re-migration northward, the case may have been
wholly different with those intruding forms which settled themselves
on the intertropical mountains, and in the southern hemisphere. These
being surrounded by strangers will have had to compete with many new
forms of life; and it is probable that selected modifications in their
structure, habits, and constitutions will have profited them. Thus
many of these wanderers, though still plainly related by inheritance
to their brethren of the northern or southern hemispheres, now exist
in their new homes as well-marked varieties or as distinct species.

It is a remarkable fact, strongly insisted on by Hooker in regard to
America, and by Alph. de Candolle in regard to Australia, that many
more identical plants and allied forms have apparently migrated from
the north to the south, than in a reversed direction. We see, however,
a few southern vegetable forms on the mountains of Borneo and
Abyssinia. I suspect that this preponderant migration from north to
south is due to the greater extent of land in the north, and to the
northern forms having existed in their own homes in greater numbers,
and having consequently been advanced through natural selection and
competition to a higher stage of perfection or dominating power, than
the southern forms. And thus, when they became commingled during the
Glacial period, the northern forms were enabled to beat the less
powerful southern forms. Just in the same manner as we see at the
present day, that very many European productions cover the ground in
La Plata, and in a lesser degree in Australia, and have to a certain
extent beaten the natives; whereas extremely few southern forms have
become naturalised in any part of Europe, though hides, wool, and
other objects likely to carry seeds have been largely imported into
Europe during the last two or three centuries from La Plata, and
during the last thirty or forty years from Australia. Something of the
same kind must have occurred on the intertropical mountains: no doubt
before the Glacial period they were stocked with endemic Alpine forms;
but these have almost everywhere largely yielded to the more dominant
forms, generated in the larger areas and more efficient workshops of
the north. In many islands the native productions are nearly equalled
or even outnumbered by the naturalised; and if the natives have not
been actually exterminated, their numbers have been greatly reduced,
and this is the first stage towards extinction. A mountain is an
island on the land; and the intertropical mountains before the Glacial
period must have been completely isolated; and I believe that the
productions of these islands on the land yielded to those produced
within the larger areas of the north, just in the same way as the
productions of real islands have everywhere lately yielded to
continental forms, naturalised by man's agency.

I am far from supposing that all difficulties are removed on the view
here given in regard to the range and affinities of the allied species
which live in the northern and southern temperate zones and on the
mountains of the intertropical regions. Very many difficulties remain
to be solved. I do not pretend to indicate the exact lines and means
of migration, or the reason why certain species and not others have
migrated; why certain species have been modified and have given rise
to new groups of forms, and others have remained unaltered. We cannot
hope to explain such facts, until we can say why one species and not
another becomes naturalised by man's agency in a foreign land; why one
ranges twice or thrice as far, and is twice or thrice as common, as
another species within their own homes.

I have said that many difficulties remain to be solved: some of the
most remarkable are stated with admirable clearness by Dr. Hooker in
his botanical works on the antarctic regions. These cannot be here
discussed. I will only say that as far as regards the occurrence of
identical species at points so enormously remote as Kerguelen Land,
New Zealand, and Fuegia, I believe that towards the close of the
Glacial period, icebergs, as suggested by Lyell, have been largely
concerned in their dispersal. But the existence of several quite
distinct species, belonging to genera exclusively confined to the
south, at these and other distant points of the southern hemisphere,
is, on my theory of descent with modification, a far more remarkable
case of difficulty. For some of these species are so distinct, that we
cannot suppose that there has been time since the commencement of the
Glacial period for their migration, and for their subsequent
modification to the necessary degree. The facts seem to me to indicate
that peculiar and very distinct species have migrated in radiating
lines from some common centre; and I am inclined to look in the
southern, as in the northern hemisphere, to a former and warmer
period, before the commencement of the Glacial period, when the
antarctic lands, now covered with ice, supported a highly peculiar and
isolated flora. I suspect that before this flora was exterminated by
the Glacial epoch, a few forms were widely dispersed to various points
of the southern hemisphere by occasional means of transport, and by
the aid, as halting-places, of existing and now sunken islands, and
perhaps at the commencement of the Glacial period, by icebergs. By
these means, as I believe, the southern shores of America, Australia,
New Zealand have become slightly tinted by the same peculiar forms of
vegetable life.

Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate
on geographical distribution. I believe that the world has recently
felt one of his great cycles of change; and that on this view,
combined with modification through natural selection, a multitude of
facts in the present distribution both of the same and of allied forms
of life can be explained. The living waters may be said to have flowed
during one short period from the north and from the south, and to have
crossed at the equator; but to have flowed with greater force from the
north so as to have freely inundated the south. As the tide leaves its
drift in horizontal lines, though rising higher on the shores where
the tide rises highest, so have the living waters left their living
drift on our mountain-summits, in a line gently rising from the arctic
lowlands to a great height under the equator. The various beings thus
left stranded may be compared with savage races of man, driven up and
surviving in the mountain-fastnesses of almost every land, which serve
as a record, full of interest to us, of the former inhabitants of the
surrounding lowlands.


CHAPTER 12. GEOGRAPHICAL DISTRIBUTION--continued.

Distribution of fresh-water productions.
On the inhabitants of oceanic islands.
Absence of Batrachians and of terrestrial Mammals.
On the relation of the inhabitants of islands to those of the nearest
mainland.
On colonisation from the nearest source with subsequent modification.
Summary of the last and present chapters.

As lakes and river-systems are separated from each other by barriers
of land, it might have been thought that fresh-water productions would
not have ranged widely within the same country, and as the sea is
apparently a still more impassable barrier, that they never would have
extended to distant countries. But the case is exactly the reverse.
Not only have many fresh-water species, belonging to quite different
classes, an enormous range, but allied species prevail in a remarkable
manner throughout the world. I well remember, when first collecting in
the fresh waters of Brazil, feeling much surprise at the similarity of
the fresh-water insects, shells, etc., and at the dissimilarity of the
surrounding terrestrial beings, compared with those of Britain.

But this power in fresh-water productions of ranging widely, though so
unexpected, can, I think, in most cases be explained by their having
become fitted, in a manner highly useful to them, for short and
frequent migrations from pond to pond, or from stream to stream; and
liability to wide dispersal would follow from this capacity as an
almost necessary consequence. We can here consider only a few cases.
In regard to fish, I believe that the same species never occur in the
fresh waters of distant continents. But on the same continent the
species often range widely and almost capriciously; for two
river-systems will have some fish in common and some different. A few
facts seem to favour the possibility of their occasional transport by
accidental means; like that of the live fish not rarely dropped by
whirlwinds in India, and the vitality of their ova when removed from
the water. But I am inclined to attribute the dispersal of fresh-water
fish mainly to slight changes within the recent period in the level of
the land, having caused rivers to flow into each other. Instances,
also, could be given of this having occurred during floods, without
any change of level. We have evidence in the loess of the Rhine of
considerable changes of level in the land within a very recent
geological period, and when the surface was peopled by existing land
and fresh-water shells. The wide difference of the fish on opposite
sides of continuous mountain-ranges, which from an early period must
have parted river-systems and completely prevented their inosculation,
seems to lead to this same conclusion. With respect to allied
fresh-water fish occurring at very distant points of the world, no
doubt there are many cases which cannot at present be explained: but
some fresh-water fish belong to very ancient forms, and in such cases
there will have been ample time for great geographical changes, and
consequently time and means for much migration. In the second place,
salt-water fish can with care be slowly accustomed to live in fresh
water; and, according to Valenciennes, there is hardly a single group
of fishes confined exclusively to fresh water, so that we may imagine
that a marine member of a fresh-water group might travel far along the
shores of the sea, and subsequently become modified and adapted to the
fresh waters of a distant land.

Some species of fresh-water shells have a very wide range, and allied
species, which, on my theory, are descended from a common parent and
must have proceeded from a single source, prevail throughout the
world. Their distribution at first perplexed me much, as their ova are
not likely to be transported by birds, and they are immediately killed
by sea water, as are the adults. I could not even understand how some
naturalised species have rapidly spread throughout the same country.
But two facts, which I have observed--and no doubt many others remain
to be observed--throw some light on this subject. When a duck suddenly
emerges from a pond covered with duck-weed, I have twice seen these
little plants adhering to its back; and it has happened to me, in
removing a little duck-weed from one aquarium to another, that I have
quite unintentionally stocked the one with fresh-water shells from the
other. But another agency is perhaps more effectual: I suspended a
duck's feet, which might represent those of a bird sleeping in a
natural pond, in an aquarium, where many ova of fresh-water shells
were hatching; and I found that numbers of the extremely minute and
just hatched shells crawled on the feet, and clung to them so firmly
that when taken out of the water they could not be jarred off, though
at a somewhat more advanced age they would voluntarily drop off. These
just hatched molluscs, though aquatic in their nature, survived on the
duck's feet, in damp air, from twelve to twenty hours; and in this
length of time a duck or heron might fly at least six or seven hundred
miles, and would be sure to alight on a pool or rivulet, if blown
across sea to an oceanic island or to any other distant point. Sir
Charles Lyell also informs me that a Dyticus has been caught with an
Ancylus (a fresh-water shell like a limpet) firmly adhering to it; and
a water-beetle of the same family, a Colymbetes, once flew on board
the 'Beagle,' when forty-five miles distant from the nearest land: how
much farther it might have flown with a favouring gale no one can
tell.

With respect to plants, it has long been known what enormous ranges
many fresh-water and even marsh-species have, both over continents and
to the most remote oceanic islands. This is strikingly shown, as
remarked by Alph. de Candolle, in large groups of terrestrial plants,
which have only a very few aquatic members; for these latter seem
immediately to acquire, as if in consequence, a very wide range. I
think favourable means of dispersal explain this fact. I have before
mentioned that earth occasionally, though rarely, adheres in some
quantity to the feet and beaks of birds. Wading birds, which frequent
the muddy edges of ponds, if suddenly flushed, would be the most
likely to have muddy feet. Birds of this order I can show are the
greatest wanderers, and are occasionally found on the most remote and
barren islands in the open ocean; they would not be likely to alight
on the surface of the sea, so that the dirt would not be washed off
their feet; when making land, they would be sure to fly to their
natural fresh-water haunts. I do not believe that botanists are aware
how charged the mud of ponds is with seeds: I have tried several
little experiments, but will here give only the most striking case: I
took in February three table-spoonfuls of mud from three different
points, beneath water, on the edge of a little pond; this mud when dry
weighed only 6 3/4 ounces; I kept it covered up in my study for six
months, pulling up and counting each plant as it grew; the plants were
of many kinds, and were altogether 537 in number; and yet the viscid
mud was all contained in a breakfast cup! Considering these facts, I
think it would be an inexplicable circumstance if water-birds did not
transport the seeds of fresh-water plants to vast distances, and if
consequently the range of these plants was not very great. The same
agency may have come into play with the eggs of some of the smaller
fresh-water animals.

Other and unknown agencies probably have also played a part. I have
stated that fresh-water fish eat some kinds of seeds, though they
reject many other kinds after having swallowed them; even small fish
swallow seeds of moderate size, as of the yellow water-lily and
Potamogeton. Herons and other birds, century after century, have gone
on daily devouring fish; they then take flight and go to other waters,
or are blown across the sea; and we have seen that seeds retain their
power of germination, when rejected in pellets or in excrement, many
hours afterwards. When I saw the great size of the seeds of that fine
water-lily, the Nelumbium, and remembered Alph. de Candolle's remarks
on this plant, I thought that its distribution must remain quite
inexplicable; but Audubon states that he found the seeds of the great
southern water-lily (probably, according to Dr. Hooker, the Nelumbium
luteum) in a heron's stomach; although I do not know the fact, yet
analogy makes me believe that a heron flying to another pond and
getting a hearty meal of fish, would probably reject from its stomach
a pellet containing the seeds of the Nelumbium undigested; or the
seeds might be dropped by the bird whilst feeding its young, in the
same way as fish are known sometimes to be dropped.

In considering these several means of distribution, it should be
remembered that when a pond or stream is first formed, for instance,
on a rising islet, it will be unoccupied; and a single seed or egg
will have a good chance of succeeding. Although there will always be a
struggle for life between the individuals of the species, however few,
already occupying any pond, yet as the number of kinds is small,
compared with those on the land, the competition will probably be less
severe between aquatic than between terrestrial species; consequently
an intruder from the waters of a foreign country, would have a better
chance of seizing on a place, than in the case of terrestrial
colonists. We should, also, remember that some, perhaps many,
fresh-water productions are low in the scale of nature, and that we
have reason to believe that such low beings change or become modified
less quickly than the high; and this will give longer time than the
average for the migration of the same aquatic species. We should not
forget the probability of many species having formerly ranged as
continuously as fresh-water productions ever can range, over immense
areas, and having subsequently become extinct in intermediate regions.
But the wide distribution of fresh-water plants and of the lower
animals, whether retaining the same identical form or in some degree
modified, I believe mainly depends on the wide dispersal of their
seeds and eggs by animals, more especially by fresh-water birds, which
have large powers of flight, and naturally travel from one to another
and often distant piece of water. Nature, like a careful gardener,
thus takes her seeds from a bed of a particular nature, and drops them
in another equally well fitted for them.

ON THE INHABITANTS OF OCEANIC ISLANDS.

We now come to the last of the three classes of facts, which I have
selected as presenting the greatest amount of difficulty, on the view
that all the individuals both of the same and of allied species have
descended from a single parent; and therefore have all proceeded from
a common birthplace, notwithstanding that in the course of time they
have come to inhabit distant points of the globe. I have already
stated that I cannot honestly admit Forbes's view on continental
extensions, which, if legitimately followed out, would lead to the
belief that within the recent period all existing islands have been
nearly or quite joined to some continent. This view would remove many
difficulties, but it would not, I think, explain all the facts in
regard to insular productions. In the following remarks I shall not
confine myself to the mere question of dispersal; but shall consider
some other facts, which bear on the truth of the two theories of
independent creation and of descent with modification.

The species of all kinds which inhabit oceanic islands are few in
number compared with those on equal continental areas: Alph. de
Candolle admits this for plants, and Wollaston for insects. If we look
to the large size and varied stations of New Zealand, extending over
780 miles of latitude, and compare its flowering plants, only 750 in
number, with those on an equal area at the Cape of Good Hope or in
Australia, we must, I think, admit that something quite independently
of any difference in physical conditions has caused so great a
difference in number. Even the uniform county of Cambridge has 847
plants, and the little island of Anglesea 764, but a few ferns and a
few introduced plants are included in these numbers, and the
comparison in some other respects is not quite fair. We have evidence
that the barren island of Ascension aboriginally possessed under
half-a-dozen flowering plants; yet many have become naturalised on it,
as they have on New Zealand and on every other oceanic island which
can be named. In St. Helena there is reason to believe that the
naturalised plants and animals have nearly or quite exterminated many
native productions. He who admits the doctrine of the creation of each
separate species, will have to admit, that a sufficient number of the
best adapted plants and animals have not been created on oceanic
islands; for man has unintentionally stocked them from various sources
far more fully and perfectly than has nature.