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mously 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 be-
longing to the same genera have migrated in radiating lines from a com-
mon centre; and I am inclined to look in the southern, as in the northren
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, Aus-
tralia, and New Zealand may have become slightly tinted by the same
peculiar forms of life.

Sir G. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alterations of climate through-
out the world on geographical distribution. And we have now seen that



GEOGRAPHICAL DISTRIBUTION 285

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 Hnes, 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 low-
lands to a great altitude under the equator. The various beings thus left
stranded may be compared with savage races of man, driven up and sur-
viving 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.



CHAPTER XIII

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 Colonization from
the Nearest Source with Subsequent Modification — Summary of the Last and
Present Chapters.

FRESH-WATER PRODUCTIONS

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 formidable barrier, that they would never have extended to dis-
tant countries. But the case is exactly the reverse. Not only have many
fresh-water species belonging to different classes an enormous range, but
allied species prevail in a remarkable manner throughout the world. When
first collecting in the fresh waters of Brazil, I well remember 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 the wide ranging power of fresh-water productions 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, within their own countries; and liability to wide dis-
persal would follow from this capacity as an almost necessary consequence.
We can here consider only a few cases; of these, some of the most difficult
to explain are presented by fish. It was formerly believed that the same
fresh-water species never existed on two continents distant from each other.
But Dr. GUnther has lately shown that the Galaxias attenuatus inhabits
Tasmania, New Zealand, the Falkland Islands, and the mainland of South
America. This is a wonderful case, and probably indicates dispersal from
an antarctic centre during a former warm period. This case, however, is
rendered in some degree less surprising by the species of this genus having
the power of crossing by some unknown means considerable spaces of open
ocean: thus there is one species common to New Zealand and to the Auck-
land Islands, though separated by a distance of about 230 miles. On the
same continent fresh-water fish often range widely, and as if capriciously;
for in two adjoining river systems some of the species may be the same and
some wholly different.

It is probable that they are occasionally transported by what may be
called accidental means. Thus fishes still alive are not very rarely dropped
at distant points by whirlwinds; and it is known that the ova retain their
vitality for a considerable time after removal from the water. Their dis-
persal may, however, be mainly attributed to changes in the level of the
land within the recent period, causing rivers to flow into each other. In-

286



GEOGRAPHICAL DISTRIBUTION 287

stances, also, could be given of this having occurred during floods, without
any change of level. The wide diff'erences of the fish on the opposite sides
of most mountain-ranges, which are continuous and consequently must,
from an early period, have completely prevented the inosculation of the
river-systems on the two sides, leads to the same conclusion. 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. Moreover, Dr. Giinther has recently been led
by several considerations to infer that with fishes the same forms have
a long endurance. 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 which all the members are confined to fresh water, so that a
marine species belonging to a fresh-water group might travel far along the
shores of the sea, and could, it is probable, become adapted without much
difficulty to the fresh waters of a distant land.

Some species of fresh-water shells have very wide ranges, and allied
species which, on our 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 the ova, as well as the adults, are immediately
killed by sea-water. I could not even understand how some naturalized
species have spread rapidly throughout the same , country. But two facts,
which I have observed — and many others no doubt will be discovered —
throw some light on this subject. When ducks suddenly emerge from a
pond covered with duck-weed, I have twice seen these little plants adhering
to their backs; and it has happened to me, in removing a little duck-weed
from one aquarium to another, that I have unintentionally stocked the one
with fresh-water shells from the other. But another agency is perhaps more
effectual: I suspended the feet of a duck in an aquarium, where many ova
of fresh- water shells were hatching; and I found that numbers of the ex-
tremely 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 mollusks, 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 if blown across the sea to an oceanic island, or to any other distant
point, would be sure to alight on a pool or rivulet. Sir Charles Lyell in-
forms me that a dytiscus 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 been blown
by a favoring 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 illustrated, according



288 THE ORIGIN OF SPECIES

to Alph. de Candolle, in those large groups of terrestrial plants, which have
very few aquatic members; for the latter seem immediately to acquire, as if
in consequence, a wide range. I think favorable means of dispersal explain
this fact. I have before mentioned that earth occasionally 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 wander more than those of any other;
and they are occasionally found on the most remote and barren islands of
the open ocean; they would not be likely to alight on the surface of the.
sea, so that any dirt on their feet would not be washed off; and when
gaining the 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 tablespoonfuls of
mud from three different points, beneath water, on the edge of a little
pond; this mud when dried weighed only six and three-fourths 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 unstocked
ponds and streams, situated at very distant points. 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
many hours afterward in pellets or in the excrement. When I saw the great
size of the seeds of that fine water-lily, the Nelumbium, and remembered
Alph. de Candolle's remarks on the distribution of this plant, I thought
that the means of its dispersal must remain 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.
Now this bird must often have flown with its stomach thus well stocked to
distant ponds, and then, getting a hearty meal of fish, analogy makes me
believe that it would have rejected the seeds in the pellet in a fit state
for germination.

In considering these several means of distribution, it should be remem-
bered 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
inhabitants of the same pond, however few in kind, yet as the number even



GEOGRAPHICAL DISTRIBUTION 289

in a well-stocked pond is small in comparison with the number of species
inhabiting an equal area of land, the competition between them will prob-
ably be less severe than between terrestrial species; consequently an intruder
from the waters of a foreign country would have a better chance of seizing
on a new place, than in the case of terrestrial colonists. We should also re-
member that many fresh-water productions are low in the scale of nature,
and we have reason to believe that such beings become modified more
slowly than the high; and this will give time for the migration of aquatic
species. We should not forget the probability of many fresh-water forms
having formerly ranged continuously over immense areas, and then having
become extinct at intermediate points. But the wide distribution of fresh-
water plants, and of the lower animals, whether retaining the same identical
form, or in some degree modified, apparently depends in main part on the
wide dispersal of their seeds and eggs by animals, more especially by fresh-
water birds, which have great powers of flight, and naturally travel from
one piece of water to another.

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 with respect to dis-
tribution, on the view that not only all the individuals of the same species
have migrated from some one area, but that allied species, although now
inhabiting the most distant points, have proceeded from a single area, the
birthplace of their early progenitors. I have already given my reasons for
disbelieving in continental extensions within the period of existing species
on so enormous a scale that all the many islands of the several oceans were
thus stocked with their present terrestrial inhabitants. This view removes
many difficulties, but it does not accord with all the facts in regard to the
productions of islands. In the following remarks I shall not confine myself
to the mere question of dispersal, but shall consider some other cases bear-
ing 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 WoUaston for insects. New Zealand, for instance, with
its lofty mountains and diversified stations, extending over 780 miles of
latitude, together with the outlying islands of Auckland, Campbell, and
Chatham, contain altogether only 960 kinds of flowering plants; if we com-
pare this moderate number with the species which swarm over equal areas
in Southwestern Australia or at the Cape of Good Hope, we must admit
that some cause, independently of different physical conditions, has given rise
to 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



290 THE ORIGIN OF SPECIES

island of Ascension aboriginally possessed less than half a dozen flowering
plants; yet many species have now become naturalized on it, as they have
in New Zealand and on every other oceanic island which can be named. In
St. Helena there is reason to believe that the naturalized 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 were not created
for oceanic islands; for man has unintentionally stocked them far more
fully and perfectly than did nature.

Although in oceanic islands the species are few in number the propor-
tion of endemic kinds {i.e.y those found nowhere else in the world) is often
extremely large. If we compare, for instance, the number of endemic land-
shells in Madeira, or of endemic birds in the Galapagos Archipelago, with
the number found on any continent, and then compare the area of the
island with that of the continent, we shall see that this is true. This fact
might have been theoretically expected, for, as already explained, species
occasionally arriving, after long intervals of time, in the new and isolated
district, and having to compete with new associates, would be eminently
liable to modification, and would often produce groups of modified de-
scendants. But it by no means follows that, because in an island nearly all
the species of one class are peculiar, those of another class, or of another
section of the same class, are peculiar; and this difference seems to depend
partly on the species which are not modified having immigrated in a body,
so that their mutual relations have not been much disturbed; and partly on
the frequent arrival of unmodified immigrants from the mother-country,
with which the insular forms have intercrossed. It should be borne in mind
that the offspring of such crosses would certainly gain in vigor; so that even
an occasional cross would produce more effect than might have been
anticipated. I will give a few illustrations of the foregoing remarks: in the
Galapagos Islands there are twenty-six land birds; of these, twenty-one (or
perhaps twenty- three) are peculiar, whereas of the eleven marine birds
only two are peculiar; and it is obvious that marine birds could arrive at
those islands much more easily and frequently than land birds. Bermuda,
on the other hand, which lies at about the same distance from North
America as the Galapagos Islands do from South America, and which has
a very peculiar soil, does not possess a single endemic land bird; and we
know from Mr. J. M. Jones' admirable account of Bermuda, that very
many North American birds occasionally or even frequently visit this island.
Almost every year, as I am informed by Mr. E. Harcourt, many European
and African birds are blown to Madeira; this island is inhabited by ninety-
nine kinds, of which one alone is peculiar, though very closely related to a
European form; and three or four other species are confined to this island
and to the Canaries. So that the islands of Bermuda and Madeira have
been stocked from the neighboring continents with birds, which for long
ages have there struggled together, and have become mutually co-adapted.
Hence, when settled in their new homes, each kind will have been kept by



GEOGRAPHICAL DISTRIBUTION 291

the others to its proper place and habits, and will consequently have been
but little liable to modification. Any tendency to modification will also have
been checked by intercrossing with the unmodified immigrants, often arriv-
ing from the mother-country. Madeira again is inhabited by a wonderful
number of peculiar land-shells, whereas not one species of sea-shell is
peculiar to its shores: now, though we do not know how sea-shells are dis-
persed, yet we can see that their eggs or larvae, perhaps attached to sea-weed
or floating timber, or to the feet of wading birds, might be transported
across three or four hundred miles of open sea far more easily than land-
shells. The different orders of insects inhabiting Madeira present nearly
parallel cases.

Oceanic islands are sometimes deficient in animals of certain whole
classes, and their places are occupied by other classes ; thus in the Galapagos
Islands reptiles, and in New Zealand gigantic wingless birds, take, or re-
cently took, the place of mammals. Although New Zealand is here spoken
of as an oceanic island, it is in some degree doubtful whether it should be
so ranked; it is of large size, and is not separated from Australia by a
profoundly deep sea; from its geological character and the direction of its
mountain ranges, the Rev. W. B. Clarke has lately maintained that this
island, as well as New Caledonia, should be considered as appurtenances of
Australia. Turning to plants. Dr. Hooker has shown that in the Galapagos
Islands the proportional numbers of the different orders are very different
from what they are elsewhere. All such differences in number, and the
absence of certain whole groups of animals and plants, are generally ac-
counted for by supposed differences in the physical conditions of the islands;
but this explanation is not a little doubtful. Facility of immigration seems
to have been fully as important as the nature of the conditions.

Many remarkable little facts could be given with respect to the in-
habitants of oceanic islands. For instance, in certain islands not tenanted by
a single mammal, some of the endemic plants have beautifully hooked
seeds; yet few relations are more manifest than that hooks serve for the
transportal. of seeds in the wool or fur of quadrupeds. But a hooked seed
might be carried to an island by other means; and the plant then becoming
modified would form an endemic species, still retaining its hooks, which
would form a useless appendage, like the shrivelled wings under the soldered
wing-covers of many insular beetles. Again, islands often possess trees or
bushes belonging to orders which elsewhere include only herbaceous species;
now trees, as Alph. de Candolle has shown, generally have, whatever the
cause may be, confined ranges. Hence trees would be little likely to reach
distant oceanic islands; and an herbaceous plant, which had no chance of
successfully competing with the many fully developed trees growing on a
continent, might, when established on an island, gain an advantage over
other herbaceous plants by growing taller and taller and over-topping them.
In this case, natural selection would tend to add to the stature of the plant,
to whatever order it belonged, and thus first convert it into a bush and then
into a tree.



292 THE ORIGIN OF SPECIES

ABSENCE OF BATRACHIANS AND TERRESTRIAL MAMMALS ON

OCEANIC ISLANDS

With respect to the absence of whole orders of animals on oceanic islands,
Bory St. Vincent long ago remarked that Batrachians (frogs, toads, newts)
are never found on any of the many islands with which the great oceans are
studded. I have taken pains to verify this assertion, and have found it true,
with the exception of New Zealand, New Caledonia, the Andaman Islands,
and perhaps the Solomon Islands and the Seychelles. But I have already
remarked that it is doubtful whether New Zealand and New Caledonia
ought to be classed as oceanic islands; and this is still more doubtful with
respect to the Andaman and Solomon groups and the Seychelles. This gen-
eral absence of frogs, toads, and newts on so many true oceanic islands
cannot be accounted for, by their physical conditions : indeed, it seems that
islands are peculiarly fitted for these animals; for frogs have been intro-
duced into Madeira, the Azores, and Mauritius, and have multiplied so as
to become a nuisance. But as these animals and their spawn are immediately
killed (with the exception, as far as known, of one Indian species) by sea-
water, there would be great difficulty in their transportal across the sea, and
therefore we can see why they do not exist on strictly oceanic islands. But
why, on the theory of creation, they should not have been created there, it
would be very difficult to explain.

Mammals offer another and similar case. I have carefully searched the
oldest voyages, and have not found a single instance, free from doubt, of
a terrestrial mammal (excluding domesticated animals kept by the natives)
inhabiting an island situated about 300 miles from a continent or great
continental island; and many islands situated at a much less distance are
equally barren. The Falkland Islands, which are inhabited by a wolf-like
fox, come nearest to an exception; but this group cannot be considered as
oceanic, as it lies on a bank in connection with the mainland at a distance
of about 280 miles; moreover, icebergs formerly brought bowlders to its
western shores, and they may have formerly transported foxes, as now
frequently happens in the arctic regions. Yet it cannot be said that small
islands will not support at least small mammals, for they occur in many
parts of the world on very small islands, when lying close to a continent;



Online LibraryCharles DarwinThe origin of species → online text (page 39 of 50)