ON THE GEOGRAPHICAL DISTRIBUTION OF ORGANIC BEINGS IN PAST AND PRESENT
TIMES
For convenience sake I shall divide this chapter into three
sections{340}. In the first place I shall endeavour to state the laws of
the distribution of existing beings, as far as our present object is
concerned; in the second, that of extinct; and in the third section I
shall consider how far these laws accord with the theory of allied
species having a common descent.
{340} Chapters XI and XII in the _Origin_, Ed. i., vi. chs. XII and
XIII ("On geographical distribution") show signs of having been
originally one, in the fact that one summary serves for both. The
geological element is not separately treated there, nor is there a
separate section on "how far these laws accord with the theory,
&c."
In the MS. the author has here written in the margin "If same
species appear at two spot at once, fatal to my theory." See
_Origin_, Ed. i. p. 352, vi. p. 499
SECTION FIRST.
_Distribution of the inhabitants in the different continents._
In the following discussion I shall chiefly refer to terrestrial
mammifers, inasmuch as they are better known; their differences in
different countries, strongly marked; and especially as the necessary
means of their transport are more evident, and confusion, from the
accidental conveyance by man of a species from one district to another
district, is less likely to arise. It is known that all mammifers (as
well as all other organisms) are united in one great system; but that
the different species, genera, or families of the same order inhabit
different quarters of the globe. If we divide the land{341} into two
divisions, according to the amount of difference, and disregarding the
numbers of the terrestrial mammifers inhabiting them, we shall have
first Australia including New Guinea; and secondly the rest of the
world: if we make a three-fold division, we shall have Australia, S.
America, and the rest of the world; I must observe that North America is
in some respects neutral land, from possessing some S. American forms,
but I believe it is more closely allied (as it certainly is in its
birds, plants and shells) with Europe. If our division had been
four-fold, we should have had Australia, S. America, Madagascar (though
inhabited by few mammifers) and the remaining land: if five-fold,
Africa, especially the southern eastern parts, would have to be
separated from the remainder of the world. These differences in the
mammiferous inhabitants of the several main divisions of the globe
cannot, it is well known, be explained by corresponding differences in
their conditions{342}; how similar are parts of tropical America and
Africa; and accordingly we find some _analogous_ resemblances,--thus
both have monkeys, both large feline animals, both large Lepidoptera,
and large dung-feeding beetles; both have palms and epiphytes; and yet
the essential difference between their productions is as great as
between those of the arid plains of the Cape of Good Hope and the
grass-covered savannahs of La Plata{343}. Consider the distribution of
the Marsupialia, which are eminently characteristic of Australia, and in
a lesser degree of S. America; when we reflect that animals of this
division, feeding both on animal and vegetable matter, frequent the dry
open or wooded plains and mountains of Australia, the humid impenetrable
forests of New Guinea and Brazil; the dry rocky mountains of Chile, and
the grassy plains of Banda Oriental, we must look to some other cause,
than the nature of the country, for their absence in Africa and other
quarters of the world.
{341} This division of the land into regions does not occur in the
_Origin_, Ed. i.
{342} _Origin_, Ed. i. p. 346, vi. p. 493.
{343} Opposite this passage is written "_not botanically_," in Sir
J. D. Hooker's hand. The word _palms_ is underlined three times and
followed by three exclamation marks. An explanatory note is added
in the margin "singular paucity of palms and epiphytes in Trop.
Africa compared with Trop. America and Ind. Or." <=East Indies>.
Furthermore it may be observed that _all_ the organisms inhabiting any
country are not perfectly adapted to it{344}; I mean by not being
perfectly adapted, only that some few other organisms can generally be
found better adapted to the country than some of the aborigines. We must
admit this when we consider the enormous number of horses and cattle
which have run wild during the three last centuries in the uninhabited
parts of St Domingo, Cuba, and S. America; for these animals must have
supplanted some aboriginal ones. I might also adduce the same fact in
Australia, but perhaps it will be objected that 30 or 40 years has not
been a sufficient period to test this power of struggling <with> and
overcoming the aborigines. We know the European mouse is driving before
it that of New Zealand, like the Norway rat has driven before it the old
English species in England. Scarcely an island can be named, where
casually introduced plants have not supplanted some of the native
species: in La Plata the Cardoon covers square leagues of country on
which some S. American plants must once have grown: the commonest weed
over the whole of India is an introduced Mexican poppy. The geologist
who knows that slow changes are in progress, replacing land and water,
will easily perceive that even if all the organisms of any country had
originally been the best adapted to it, this could hardly continue so
during succeeding ages without either extermination, or changes, first
in the relative proportional numbers of the inhabitants of the country,
and finally in their constitutions and structure.
{344} This partly corresponds to _Origin_, Ed. i. p. 337, vi. p.
483.
Inspection of a map of the world at once shows that the five divisions,
separated according to the greatest amount of difference in the
mammifers inhabiting them, are likewise those most widely separated from
each other by barriers{345} which mammifers cannot pass: thus Australia
is separated from New Guinea and some small adjoining islets only by a
narrow and shallow strait; whereas New Guinea and its adjoining islets
are cut off from the other East Indian islands by deep water. These
latter islands, I may remark, which fall into the great Asiatic group,
are separated from each other and the continent only by shallow water;
and where this is the case we may suppose, from geological oscillations
of level, that generally there has been recent union. South America,
including the southern part of Mexico, is cut off from North America by
the West Indies, and the great table-land of Mexico, except by a mere
fringe of tropical forests along the coast: it is owing, perhaps, to
this fringe that N. America possesses some S. American forms. Madagascar
is entirely isolated. Africa is also to a great extent isolated,
although it approaches, by many promontories and by lines of shallower
sea, to Europe and Asia: southern Africa, which is the most distinct in
its mammiferous inhabitants, is separated from the northern portion by
the Great Sahara Desert and the table-land of Abyssinia. That the
distribution of organisms is related to barriers, stopping their
progress, we clearly see by comparing the distribution of marine and
terrestrial productions. The marine animals being different on the two
sides of land tenanted by the same terrestrial animals, thus the shells
are wholly different on the opposite sides of the temperate parts of
South America{346}, as they are (?) in the Red Sea and the
Mediterranean. We can at once perceive that the destruction of a barrier
would permit two geographical groups of organisms to fuse and blend into
one. But the original cause of groups being different on opposite sides
of a barrier can only be understood on the hypothesis of each organism
having been created or produced on one spot or area, and afterwards
migrating as widely as its means of transport and subsistence permitted
it.
{345} On the general importance of barriers, see _Origin_, Ed. i.
p. 347, vi. p. 494.
{346} _Origin_, Ed. i. p. 348, vi. p. 495.
_Relation of range in genera and species._
It is generally{347} found, that where a genus or group ranges over
nearly the entire world, many of the species composing the group have
wide ranges: on the other hand, where a group is restricted to any one
country, the species composing it generally have restricted ranges in
that country{348}. Thus among mammifers the feline and canine genera are
widely distributed, and many of the individual species have enormous
ranges [the genus Mus I believe, however, is a strong exception to the
rule]. Mr Gould informs me that the rule holds with birds, as in the
owl genus, which is mundane, and many of the species range widely. The
rule holds also with land and fresh-water mollusca, with butterflies and
very generally with plants. As instances of the converse rule, I may
give that division of the monkeys which is confined to S. America, and
amongst plants, the Cacti, confined to the same continent, the species
of both of which have generally narrow ranges. On the ordinary theory of
the separate creation of each species, the cause of these relations is
not obvious; we can see no reason, because many allied species have been
created in the several main divisions of the world, that several of
these species should have wide ranges; and on the other hand, that
species of the same group should have narrow ranges if all have been
created in one main division of the world. As the result of such and
probably many other unknown relations, it is found that, even in the
same great classes of beings, the different divisions of the world are
characterised by either merely different species, or genera, or even
families: thus in cats, mice, foxes, S. America differs from Asia and
Africa only in species; in her pigs, camels and monkeys the difference
is generic or greater. Again, whilst southern Africa and Australia
differ more widely in their mammalia than do Africa and S. America, they
are more closely (though indeed very distantly) allied in their plants.
{347} <Note in original.> The same laws seem to govern distribution
of species and genera, and individuals in time and space. <See
_Origin_, Ed. i. p. 350, vi. p. 497, also a passage in the last
chapter, p. 146.>
{348} _Origin_, Ed. i. p. 404, vi. p. 559.
_Distribution of the inhabitants in the same continent._
If we now look at the distribution of the organisms in any one of the
above main divisions of the world, we shall find it split up into many
regions, with all or nearly all their species distinct, but yet
partaking of one common character. This similarity of type in the
subdivisions of a great region is equally well-known with the
dissimilarity of the inhabitants of the several great regions; but it
has been less often insisted on, though more worthy of remark. Thus for
instance, if in Africa or S. America, we go from south to north{349}, or
from lowland to upland, or from a humid to a dryer part, we find wholly
different species of those genera or groups which characterise the
continent over which we are passing. In these subdivisions we may
clearly observe, as in the main divisions of the world, that
sub-barriers divide different groups of species, although the opposite
sides of such sub-barriers may possess nearly the same climate, and may
be in other respects nearly similar: thus it is on the opposite sides of
the Cordillera of Chile, and in a lesser degree on the opposite sides of
the Rocky mountains. Deserts, arms of the sea, and even rivers form the
barriers; mere preoccupied space seems sufficient in several cases: thus
Eastern and Western Australia, in the same latitude, with very similar
climate and soils, have scarcely a plant, and few animals or birds, in
common, although all belong to the peculiar genera characterising
Australia. It is in short impossible to explain the differences in the
inhabitants, either of the main divisions of the world, or of these
sub-divisions, by the differences in their physical conditions, and by
the adaptation of their inhabitants. Some other cause must intervene.
{349} _Origin_, Ed. i. p. 349, vi. p. 496.
We can see that the destruction of sub-barriers would cause (as before
remarked in the case of the main divisions) two sub-divisions to blend
into one; and we can only suppose that the original difference in the
species, on the opposite sides of sub-barriers, is due to the creation
or production of species in distinct areas, from which they have
wandered till arrested by such sub-barriers. Although thus far is pretty
clear, it may be asked, why, when species in the same main division of
the world were produced on opposite sides of a sub-barrier, both when
exposed to similar conditions and when exposed to widely different
influences (as on alpine and lowland tracts, as on arid and humid soils,
as in cold and hot climates), have they invariably been formed on a
similar type, and that type confined to this one division of the world?
Why when an ostrich{350} was produced in the southern parts of America,
was it formed on the American type, instead of on the African or on
Australian types? Why when hare-like and rabbit-like animals were formed
to live on the Savannahs of La Plata, were they produced on the peculiar
Rodent type of S. America, instead of on the true{351} hare-type of
North America, Asia and Africa? Why when borrowing Rodents, and
camel-like animals were formed to tenant the Cordillera, were they
formed on the same type{352} with their representatives on the plains?
Why were the mice, and many birds of different species on the opposite
sides of the Cordillera, but exposed to a very similar climate and soil,
created on the same peculiar S. American type? Why were the plants in
Eastern and Western Australia, though wholly different as species,
formed on the same peculiar Australian types? The generality of the
rule, in so many places and under such different circumstances, makes it
highly remarkable and seems to demand some explanation.
{350} The case of the ostrich (_Rhea_) occurs in the _Origin_, Ed.
i. p. 349, vi. p. 496.
{351} <Note in original.> There is a hare in S. America,--so bad
example.
{352} See _Origin_, Ed. i. p. 349, vi. p. 497.
_Insular Faunas._
If we now look to the character of the inhabitants of small
islands{353}, we shall find that those situated close to other land have
a similar fauna with that land{354}, whilst those at a considerable
distance from other land often possess an almost entirely peculiar
fauna. The Galapagos Archipelago{355} is a remarkable instance of this
latter fact; here almost every bird, its one mammifer, its reptiles,
land and sea shells, and even fish, are almost all peculiar and distinct
species, not found in any other quarter of the world: so are the
majority of its plants. But although situated at the distance of between
500 and 600 miles from the S. American coast, it is impossible to even
glance at a large part of its fauna, especially at the birds, without at
once seeing that they belong to the American type{356}. Hence, in fact,
groups of islands thus circumstanced form merely small but well-defined
sub-divisions of the larger geographical divisions. But the fact is in
such cases far more striking: for taking the Galapagos Archipelago as an
instance; in the first place we must feel convinced, seeing that every
island is wholly volcanic and bristles with craters, that in a
geological sense the whole is of recent origin comparatively with a
continent; and as the species are nearly all peculiar, we must conclude
that they have in the same sense recently been produced on this very
spot; and although in the nature of the soil, and in a lesser degree in
the climate, there is a wide difference with the nearer part of the S.
American coast, we see that the inhabitants have been formed on the same
closely allied type. On the other hand, these islands, as far as their
physical conditions are concerned, resemble closely the Cape de Verde
volcanic group, and yet how wholly unlike are the productions of these
two archipelagoes. The Cape de Verde{357} group, to which may be added
the Canary Islands, are allied in their inhabitants (of which many are
peculiar species) to the coast of Africa and southern Europe, in
precisely the same manner as the Galapagos Archipelago is allied to
America. We here clearly see that mere geographical proximity affects,
more than any relation of adaptation, the character of species. How many
islands in the Pacific exist far more like in their physical conditions
to Juan Fernandez than this island is to the coast of Chile, distant 300
miles; why then, except from mere proximity, should this island alone be
tenanted by two very peculiar species of humming-birds--that form of
birds which is so exclusively American? Innumerable other similar cases
might be adduced.
{353} For the general problem of Oceanic Islands, see _Origin_, Ed.
i. p. 388, vi. p. 541.
{354} This is an illustration of the general theory of barriers
(_Origin_, Ed. i. p. 347, vi. p. 494). At i. p. 391, vi. p. 544 the
question is discussed from the point of view of means of transport.
Between the lines, above the words "with that land," the author
wrote "Cause, formerly joined, no one doubts after Lyell."
{355} _Origin_, Ed. i. p. 390, vi. p. 543.
{356} See _Origin_, Ed. i. p. 397, vi. p. 552.
{357} The Cape de Verde and Galapagos Archipelagoes are compared in
the _Origin_, Ed. i. p. 398, vi. p. 553. See also _Journal of
Researches_, 1860, p. 393.
The Galapagos Archipelago offers another, even more remarkable, example
of the class of facts we are here considering. Most of its genera are,
as we have said, American, many of them are mundane, or found
everywhere, and some are quite or nearly confined to this archipelago.
The islands are of absolutely similar composition, and exposed to the
same climate; most of them are in sight of each other; and yet several
of the islands are inhabited, each by peculiar species (or in some cases
perhaps only varieties) of some of the genera characterising the
archipelago. So that the small group of the Galapagos Islands typifies,
and follows exactly the same laws in the distribution of its
inhabitants, as a great continent. How wonderful it is that two or three
closely similar but distinct species of a mocking-thrush{358} should
have been produced on three neighbouring and absolutely similar islands;
and that these three species of mocking-thrush should be closely related
to the other species inhabiting wholly different climates and different
districts of America, and only in America. No similar case so striking
as this of the Galapagos Archipelago has hitherto been observed; and
this difference of the productions in the different islands may perhaps
be partly explained by the depth of the sea between them (showing that
they could not have been united within recent geological periods), and
by the currents of the sea sweeping _straight_ between them,--and by
storms of wind being rare, through which means seeds and birds could be
blown, or drifted, from one island to another. There are however some
similar facts: it is said that the different, though neighbouring
islands of the East Indian Archipelago are inhabited by some different
species of the same genera; and at the Sandwich group some of the
islands have each their peculiar species of the same genera of plants.
{358} In the _Origin_, Ed. i. p. 390, a strong point is made of
birds which immigrated "with facility and in a body" not having
been modified. Thus the author accounts for the small percentage of
peculiar "marine birds."
Islands standing quite isolated within the intra-tropical oceans have
generally very peculiar floras, related, though feebly (as in the case
of St Helena{359} where almost every species is distinct), with the
nearest continent: Tristan d'Acunha is feebly related, I believe, in its
plants, both to Africa and S. America, not by having species in common,
but by the genera to which they belong{360}. The floras of the numerous
scattered islands of the Pacific are related to each other and to all
the surrounding continents; but it has been said, that they have more of
an Indo-Asiatic than American character{361}. This is somewhat
remarkable, as America is nearer to all the Eastern islands, and lies in
the direction of the trade-wind and prevailing currents; on the other
hand, all the heaviest gales come from the Asiatic side. But even with
the aid of these gales, it is not obvious on the ordinary theory of
creation how the possibility of migration (without we suppose, with
extreme improbability, that each species with an Indo-Asiatic character
has actually travelled from the Asiatic shores, where such species do
not now exist) explains this Asiatic character in the plants of the
Pacific. This is no more obvious than that (as before remarked) there
should exist a relation between the creation of closely allied species
in several regions of the world, and the fact of many such species
having wide ranges; and on the other hand, of allied species confined to
one region of the world having in that region narrow ranges.
{359} "The affinities of the St Helena flora are strongly South
African." Hooker's _Lecture on Insular Floras_ in the _Gardeners'
Chronicle_, Jan. 1867.
{360} It is impossible to make out the precise form which the
author intended to give to this sentence, but the meaning is clear.
{361} This is no doubt true, the flora of the Sandwich group
however has marked American affinities.
_Alpine Floras._
We will now turn to the floras of mountain-summits which are well known
to differ from the floras of the neighbouring lowlands. In certain
characters, such as dwarfness of stature, hairiness, &c., the species
from the most distant mountains frequently resemble each other,--a kind
of analogy like that for instance of the succulency of most desert
plants. Besides this analogy, Alpine plants present some eminently
curious facts in their distribution. In some cases the summits of
mountains, although immensely distant from each other, are clothed by
the same identical species{362} which are likewise the same with those
growing on the likewise very distant Arctic shores. In other cases,
although few or none of the species may be actually identical, they are
closely related; whilst the plants of the lowland districts surrounding
the two mountains in question will be wholly dissimilar. As
mountain-summits, as far as their plants are concerned, are islands
rising out of an ocean of land in which the Alpine species cannot live,
nor across which is there any known means of transport, this fact
appears directly opposed to the conclusion which we have come to from
considering the general distribution of organisms both on continents and
on islands--namely, that the degree of relationship between the
inhabitants of two points depends on the completeness and nature of the
barriers between those points{363}. I believe, however, this anomalous
case admits, as we shall presently see, of some explanation. We might
have expected that the flora of a mountain summit would have presented
the same relation to the flora of the surrounding lowland country, which
any isolated part of a continent does to the whole, or an island does to
the mainland, from which it is separated by a rather wide space of sea.
This in fact is the case with the plants clothing the summits of _some_
mountains, which mountains it may be observed are particularly isolated;
for instance, all the species are peculiar, but they belong to the forms
characteristic of the surrounding continent, on the mountains of
Caraccas, of Van Dieman's Land and of the Cape of Good Hope{364}. On
some other mountains, for instance <in> Tierra del Fuego and in Brazil,
some of the plants though distinct species are S. American forms; whilst
others are allied to or are identical with the Alpine species of Europe.
In islands of which the lowland flora is distinct <from> but allied to
that of the nearest continent, the Alpine plants are sometimes (or
perhaps mostly) eminently peculiar and distinct{365}; this is the case
on Teneriffe, and in a lesser degree even on some of the Mediterranean
islands.
{362} See _Origin_, Ed. i. p. 365, vi. p. 515. The present
discussion was written before the publication of Forbes' celebrated
paper on the same subject; see _Life and Letters_, vol. I. p. 88.
{363} The apparent breakdown of the doctrine of barriers is
slightly touched on in the _Origin_, Ed. i. p. 365, vi. p. 515.
{364} In the _Origin_, Ed. i. p. 375, vi. p. 526, the author points
out that on the mountains at the Cape of Good Hope "some few
representative European forms are found, which have not been
discovered in the inter-tropical parts of Africa."
{365} See Hooker's _Lecture on Insular Floras_ in the _Gardeners'
Chronicle_, Jan. 1867.
If all Alpine floras had been characterised like that of the mountain of
Caraccas, or of Van Dieman's Land, &c., whatever explanation is possible
of the general laws of geographical distribution would have applied to
them. But the apparently anomalous case just given, namely of the
mountains of Europe, of some mountains in the United States (Dr Boott)
and of the summits of the Himalaya (Royle), having many identical
species in common conjointly with the Arctic regions, and many species,
though not identical, closely allied, require a separate explanation.
The fact likewise of several of the species on the mountains of Tierra
del Fuego (and in a lesser degree on the mountains of Brazil) not
belonging to American forms, but to those of Europe, though so immensely
remote, requires also a separate explanation.
_Cause of the similarity in the floras of some distant mountains._
Now we may with confidence affirm, from the number of the then floating
icebergs and low descent of the glaciers, that within a period so near
that species of shells have remained the same, the whole of Central
Europe and of North America (and perhaps of Eastern Asia) possessed a
very cold climate; and therefore it is probable that the floras of these
districts were the same as the present Arctic one,--as is known to have
been to some degree the case with then existing sea-shells, and those
now living on the Arctic shores. At this period the mountains must have
been covered with ice of which we have evidence in the surfaces polished
and scored by glaciers. What then would be the natural and almost
inevitable effects of the gradual change into the present more temperate
climate{366}? The ice and snow would disappear from the mountains, and
as new plants from the more temperate regions of the south migrated
northward, replacing the Arctic plants, these latter would crawl{367} up
the now uncovered mountains, and likewise be driven northward to the
present Arctic shores. If the Arctic flora of that period was a nearly
uniform one, as the present one is, then we should have the same plants
on these mountain-summits and on the present Arctic shores. On this view
the Arctic flora of that period must have been a widely extended one,
more so than even the present one; but considering how similar the
physical conditions must always be of land bordering on perpetual frost,
this does not appear a great difficulty; and may we not venture to
suppose that the almost infinitely numerous icebergs, charged with
great masses of rocks, soil and _brushwood_{368} and often driven high
up on distant beaches, might have been the means of widely distributing
the seeds of the same species?
{366} In the margin the author has written "(Forbes)." This may
have been inserted at a date later than 1844, or it may refer to a
work by Forbes earlier than his Alpine paper.
{367} See _Origin_, Ed. i. p. 367, vi. p. 517.
{368} <Note in original.> Perhaps vitality checked by cold and so
prevented germinating. <On the carriage of seeds by icebergs, see
_Origin_, Ed. i. p. 363, vi. p. 513.>
I will only hazard one other observation, namely that during the change
from an extremely cold climate to a more temperate one the conditions,
both on lowland and mountain, would be singularly favourable for the
diffusion of any existing plants, which could live on land, just freed
from the rigour of eternal winter; for it would possess no inhabitants;
and we cannot doubt that _preoccupation_{369} is the chief bar to the
diffusion of plants. For amongst many other facts, how otherwise can we
explain the circumstance that the plants on the opposite, though
similarly constituted sides of a wide river in Eastern Europe (as I was
informed by Humboldt) should be widely different; across which river
birds, swimming quadrupeds and the wind must often transport seeds; we
can only suppose that plants already occupying the soil and freely
seeding check the germination of occasionally transported seeds.
{369} A note by the author gives "many authors" apparently as
authority for this statement.
At about the same period when icebergs were transporting boulders in N.
America as far as 36 deg. south, where the cotton tree now grows in South
America, in latitude 42 deg. (where the land is now clothed with forests
having an almost tropical aspect with the trees bearing epiphytes and
intertwined with canes), the same ice action was going on; is it not
then in some degree probable that at this period the whole tropical
parts of the two Americas possessed{370} (as Falconer asserts that
India did) a more temperate climate? In this case the Alpine plants of
the long chain of the Cordillera would have descended much lower and
there would have been a broad high-road{371} connecting those parts of
North and South America which were then frigid. As the present climate
supervened, the plants occupying the districts which now are become in
both hemispheres temperate and even semi-tropical must have been driven
to the Arctic and Antarctic{372} regions; and only a few of the loftiest
points of the Cordillera can have retained their former connecting
flora. The transverse chain of Chiquitos might perhaps in a similar
manner during the ice-action period have served as a connecting road
(though a broken one) for Alpine plants to become dispersed from the
Cordillera to the highlands of Brazil. It may be observed that some
(though not strong) reasons can be assigned for believing that at about
this same period the two Americas were not so thoroughly divided as they
now are by the West Indies and tableland of Mexico. I will only further
remark that the present most singularly close similarity in the
vegetation of the lowlands of Kerguelen's Land{373} and of Tierra del
Fuego (Hooker), though so far apart, may perhaps be explained by the
dissemination of seeds during this same cold period, by means of
icebergs, as before alluded to{374}.
{370} Opposite to this passage, in the margin, the author has
written:--"too hypothetical."
{371} The Cordillera is described as supplying a great line of
invasion in the _Origin_, Ed. i. p. 378.
{372} This is an approximation to the author's views on
trans-tropical migration (_Origin_, Ed. i. pp. 376-8). See
Thiselton-Dyer's interesting discussion in _Darwin and Modern
Science_, p. 304.
{373} See Hooker's _Lecture on Insular Floras_ in the _Gardeners'
Chronicle_, Jan. 1867.
{374} <Note by the author.> Similarity of flora of coral islands
easily explained.
Finally, I think we may safely grant from the foregoing facts and
reasoning that the anomalous similarity in the vegetation of certain
very distant mountain-summits is not in truth opposed to the conclusion
of the intimate relation subsisting between proximity in space (in
accordance with the means of transport in each class) and the degree of
affinity of the inhabitants of any two countries. In the case of several
quite isolated mountains, we have seen that the general law holds good.
_Whether the same species has been created more than once._
As the fact of the same species of plants having been found on
mountain-summits immensely remote has been one chief cause of the belief
of some species having been contemporaneously produced or created at two
different points{375}, I will here briefly discuss this subject. On the
ordinary theory of creation, we can see no reason why on two similar
mountain-summits two similar species may not have been created; but the
opposite view, independently of its simplicity, has been generally
received from the analogy of the general distribution of all organisms,
in which (as shown in this chapter) we almost always find that great and
continuous barriers separate distinct series; and we are naturally led
to suppose that the two series have been separately created. When taking
a more limited view we see a river, with a quite similar country on both
sides, with one side well stocked with a certain animal and on the other
side not one (as is the case with the Bizcacha{376} on the opposite
sides of the Plata), we are at once led to conclude that the Bizcacha
was produced on some one point or area on the western side of the
river. Considering our ignorance of the many strange chances of
diffusion by birds (which occasionally wander to immense distances) and
quadrupeds swallowing seeds and ova (as in the case of the flying
water-beetle which disgorged the eggs of a fish), and of whirlwinds
carrying seeds and animals into strong upper currents (as in the case of
volcanic ashes and showers of hay, grain and fish{377}), and of the
possibility of species having survived for short periods at intermediate
spots and afterwards becoming extinct there{378}; and considering our
knowledge of the great changes which _have_ taken place from subsidence
and elevation in the surface of the earth, and of our ignorance of the
greater changes which _may have_ taken place, we ought to be very slow
in admitting the probability of double creations. In the case of plants
on mountain-summits, I think I have shown how almost necessarily they
would, under the past conditions of the northern hemisphere, be as
similar as are the plants on the present Arctic shores; and this ought
to teach us a lesson of caution.
{375} On centres of creation see _Origin_, Ed. i. p. 352, vi. p.
499.
{376} In the _Journal of Researches_, Ed. 1860, p. 124, the
distribution of the Bizcacha is described as limited by the river
Uruguay. The case is not I think given in the _Origin_.
{377} In the _Origin_, Ed. i. a special section (p. 356, vi. p.
504) is devoted to _Means of Dispersal_. The much greater
prominence given to this subject in the _Origin_ is partly
accounted for by the author's experiments being of later date,
_i.e._ 1855 (_Life and Letters_, vol. II. p. 53). The carriage of
fish by whirlwinds is given in the _Origin_, Ed. i. p. 384, vi. p.
536.
{378} The case of islands serving as halting places is given in the
_Origin_, Ed. i. p. 357, vi. p. 505. But here the evidence of this
having occurred is supposed to be lost by the subsidence of the
islands, not merely by the extinction of the species.
But the strongest argument against double creations may be drawn from
considering the case of mammifers{379} in which, from their nature and
from the size of their offspring, the means of distribution are more in
view. There are no cases where the same species is found in _very
remote_ localities, except where there is a continuous belt of land:
the Arctic region perhaps offers the strongest exception, and here we
know that animals are transported on icebergs{380}. The cases of lesser
difficulty may all receive a more or less simple explanation; I will
give only one instance; the nutria{381}, I believe, on the eastern coast
of S. America live exclusively in fresh-water rivers, and I was much
surprised how they could have got into rivulets, widely apart, on the
coast of Patagonia; but on the opposite coast I found these quadrupeds
living exclusively in the sea, and hence their migration along the
Patagonian coast is not surprising. There is no case of the same
mammifer being found on an island far from the coast, and on the
mainland, as happens with plants{382}. On the idea of double creations
it would be strange if the same species of several plants should have
been created in Australia and Europe; and no one instance of the same
species of mammifer having been created, or aboriginally existing, in
two as nearly remote and equally isolated points. It is more
philosophical, in such cases, as that of some plants being found in
Australia and Europe, to admit that we are ignorant of the means of
transport. I will allude only to one other case, namely, that of the
Mydas{383}, an Alpine animal, found only on the distant peaks of the
mountains of Java: who will pretend to deny that during the ice period
of the northern and southern hemispheres, and when India is believed to
have been colder, the climate might not have permitted this animal to
haunt a lower country, and thus to have passed along the ridges from
summit to summit? Mr Lyell has further observed that, _as in space, so
in time_, there is no reason to believe that after the extinction of a
species, the self-same form has ever reappeared{384}. I think, then, we
may, notwithstanding the many cases of difficulty, conclude with some
confidence that every species has been created or produced on a single
point or area.
{379} "We find no inexplicable cases of the same mammal inhabiting
distant points of the world." _Origin_, Ed. i. p. 352, vi. p. 500.
See also _Origin_, Ed. i. p. 393, vi. p. 547.
{380} <Note by the author.> Many authors. <See _Origin_, Ed. i. p.
394, vi. p. 547.>
{381} _Nutria_ is the Spanish for otter, and is now a synonym for
_Lutra_. The otter on the Atlantic coast is distinguished by minute
differences from the Pacific species. Both forms are said to take
to the sea. In fact the case presents no especial difficulties.
{382} In _Origin_, Ed. i. p. 394, vi. p. 548, bats are mentioned as
an explicable exception to this statement.
{383} This reference is doubtless to _Mydaus_, a badger-like animal
from the mountains of Java and Sumatra (Wallace, _Geographical
Distribution_, ii. p. 199). The instance does not occur in the
_Origin_ but the author remarks (_Origin_, Ed. i. p. 376, vi. p.
527) that cases, strictly analogous to the distribution of plants,
occur among terrestrial mammals.
{384} See _Origin_, Ed. i. p. 313, vi. p. 454.
_On the number of species, and of the classes to which they belong in
different regions._
The last fact in geographical distribution, which, as far as I can see,
in any way concerns the origin of species, relates to the absolute
number and nature of the organic beings inhabiting different tracts of
land. Although every species is admirably adapted (but not necessarily
better adapted than every other species, as we have seen in the great
increase of introduced species) to the country and station it frequents;
yet it has been shown that the entire difference between the species in
distant countries cannot possibly be explained by the difference of the
physical conditions of these countries. In the same manner, I believe,
neither the number of the species, nor the nature of the great classes
to which they belong, can possibly in all cases be explained by the
conditions of their country. New Zealand{385}, a linear island
stretching over about 700 miles of latitude, with forests, marshes,
plains and mountains reaching to the limits of eternal snow, has far
more diversified habitats than an equal area at the Cape of Good Hope;
and yet, I believe, at the Cape of Good Hope there are, of phanerogamic
plants, from five to ten times the number of species as in all New
Zealand. Why on the theory of absolute creations should this large and
diversified island only have from 400 to 500 (? Dieffenbach)
phanerogamic plants? and why should the Cape of Good Hope, characterised
by the uniformity of its scenery, swarm with more species of plants than
probably any other quarter of the world? Why on the ordinary theory
should the Galapagos Islands abound with terrestrial reptiles? and why
should many equal-sized islands in the Pacific be without a single
one{386} or with only one or two species? Why should the great island of
New Zealand be without one mammiferous quadruped except the mouse, and
that was probably introduced with the aborigines? Why should not one
island (it can be shown, I think, that the mammifers of Mauritius and St
Iago have all been introduced) in the open ocean possess a mammiferous
quadruped? Let it not be said that quadrupeds cannot live in islands,
for we know that cattle, horses and pigs during a long period have run
wild in the West Indian and Falkland Islands; pigs at St Helena; goats
at Tahiti; asses in the Canary Islands; dogs in Cuba; cats at Ascension;
rabbits at Madeira and the Falklands; monkeys at St Iago and the
Mauritius; even elephants during a long time in one of the very small
Sooloo Islands; and European mice on very many of the smallest islands
far from the habitations of man{387}. Nor let it be assumed that
quadrupeds are more slowly created and hence that the oceanic islands,
which generally are of volcanic formation, are of too recent origin to
possess them; for we know (Lyell) that new forms of quadrupeds succeed
each other quicker than Mollusca or Reptilia. Nor let it be assumed
(though such an assumption would be no explanation) that quadrupeds
cannot be created on small islands; for islands not lying in mid-ocean
do possess their peculiar quadrupeds; thus many of the smaller islands
of the East Indian Archipelago possess quadrupeds; as does Fernando Po
on the West Coast of Africa; as the Falkland Islands possess a peculiar
wolf-like fox{388}; so do the Galapagos Islands a peculiar mouse of the
S. American type. These two last are the most remarkable cases with
which I am acquainted; inasmuch as the islands lie further from other
land. It is possible that the Galapagos mouse may have been introduced
in some ship from the S. American coast (though the species is at
present unknown there), for the aboriginal species soon haunts the goods
of man, as I noticed in the roof of a newly erected shed in a desert
country south of the Plata. The Falkland Islands, though between 200 and
300 miles from the S. American coast, may in one sense be considered as
intimately connected with it; for it is certain that formerly many
icebergs loaded with boulders were stranded on its southern coast, and
the old canoes which are occasionally now stranded, show that the
currents still set from Tierra del Fuego. This fact, however, does not
explain the presence of the _Canis antarcticus_ on the Falkland Islands,
unless we suppose that it formerly lived on the mainland and became
extinct there, whilst it survived on these islands, to which it was
borne (as happens with its northern congener, the common wolf) on an
iceberg, but this fact removes the anomaly of an island, in appearance
effectually separated from other land, having its own species of
quadruped, and makes the case like that of Java and Sumatra, each having
their own rhinoceros.
{385} The comparison between New Zealand and the Cape is given in
the _Origin_, Ed. i. p. 389, vi. p. 542.
{386} In a corresponding discussion in the _Origin_, Ed. i. p. 393,
vi. p. 546, stress is laid on the distribution of Batrachians not
of reptiles.
{387} The whole argument is given--more briefly than here--in the
_Origin_, Ed. i. p. 394, vi. p. 547.
{388} See _Origin_, Ed i. p. 393, vi. p. 547. The discussion is
much fuller in the present Essay.
Before summing up all the facts given in this section on the present
condition of organic beings, and endeavouring to see how far they admit
of explanation, it will be convenient to state all such facts in the
past geographical distribution of extinct beings as seem anyway to
concern the theory of descent.
SECTION SECOND.
_Geographical distribution of extinct organisms._
I have stated that if the land of the entire world be divided into (we
will say) three sections, according to the amount of difference of the
terrestrial mammifers inhabiting them, we shall have three unequal
divisions of (1st) Australia and its dependent islands, (2nd) South
America, (3rd) Europe, Asia and Africa. If we now look to the mammifers
which inhabited these three divisions during the later Tertiary periods,
we shall find them almost as distinct as at the present day, and
intimately related in each division to the existing forms in that
division{389}. This is wonderfully the case with the several fossil
Marsupial genera in the caverns of New South Wales and even more
wonderfully so in South America, where we have the same peculiar group
of monkeys, of a guanaco-like animal, of many rodents, of the Marsupial
Didelphys, of Armadilloes and other Edentata. This last family is at
present very characteristic of S. America, and in a late Tertiary epoch
it was even more so, as is shown by the numerous enormous animals of the
Megatheroid family, some of which were protected by an osseous armour
like that, but on a gigantic scale, of the recent Armadillo. Lastly,
over Europe the remains of the several deer, oxen, bears, foxes,
beavers, field-mice, show a relation to the present inhabitants of this
region; and the contemporaneous remains of the elephant, rhinoceros,
hippopotamus, hyaena, show a relation with the grand Africo-Asiatic
division of the world. In Asia the fossil mammifers of the Himalaya
(though mingled with forms long extinct in Europe) are equally related
to the existing forms of the Africo-Asiatic division; but especially to
those of India itself. As the gigantic and now extinct quadrupeds of
Europe have naturally excited more attention than the other and smaller
remains, the relation between the past and the present mammiferous
inhabitants of Europe has not been sufficiently attended to. But in fact
the mammifers of Europe are at present nearly as much Africo-Asiatic as
they were formerly when Europe had its elephants and rhinoceroses, etc.;
Europe neither now nor then possessed peculiar groups as does Australia
and S. America. The extinction of certain peculiar forms in one quarter
does not make the remaining mammifers of that quarter less related to
its own great division of the world: though Tierra del Fuego possesses
only a fox, three rodents, and the guanaco, no one (as these all belong
to S. American types, but not to the most characteristic forms) would
doubt for one minute <as to> classifying this district with S. America;
and if fossil Edentata, Marsupials and monkeys were to be found in
Tierra del Fuego, it would not make this district more truly S. American
than it now is. So it is with Europe{390}, and so far as is known with
Asia, for the lately past and present mammifers all belong to the
Africo-Asiatic division of the world. In every case, I may add, the
forms which a country has is of more importance in geographical
arrangement than what it has not.
{389} See _Origin_, Ed. i. p. 339, vi. p. 485.
{390} In the _Origin_, Ed. i. p. 339, vi. p. 485, which corresponds
to this part of the present Essay, the author does not make a
separate section for such cases as the occurrence of fossil
Marsupials in Europe (_Origin_, Ed. i. p. 340, vi. p. 486) as he
does in the present Essay; see the section on _Changes in
geographical distribution_, p. 177.
We find some evidence of the same general fact in a relation between the
recent and the Tertiary sea-shells, in the different main divisions of
the marine world.
This general and most remarkable relation between the lately past and
present mammiferous inhabitants of the three main divisions of the world
is precisely the same kind of fact as the relation between the different
species of the several sub-regions of any one of the main divisions. As
we usually associate great physical changes with the total extinction of
one series of beings, and its succession by another series, this
identity of relation between the past and the present races of beings in
the same quarters of the globe is more striking than the same relation
between existing beings in different sub-regions: but in truth we have
no reason for supposing that a change in the conditions has in any of
these cases supervened, greater than that now existing between the
temperate and tropical, or between the highlands and lowlands of the
same main divisions, now tenanted by related beings. Finally, then, we
clearly see that in each main division of the world the same relation
holds good between its inhabitants in time as over space{391}.
{391} "We can understand how it is that all the forms of life,
ancient and recent, make together one grand system; for all are
connected by generation." _Origin_, Ed. i. p. 344, vi. p. 491.
_Changes in geographical distribution._
If, however, we look closer, we shall find that even Australia, in
possessing a terrestrial Pachyderm, was so far less distinct from the
rest of the world than it now is; so was S. America in possessing the
Mastodon, horse, [hyaena,]{392} and antelope. N. America, as I have
remarked, is now, in its mammifers, in some respects neutral ground
between S. America and the great Africo-Asiatic division; formerly, in
possessing the horse, Mastodon and three Megatheroid animals, it was
more nearly related to S. America; but in the horse and Mastodon, and
likewise in having the elephant, oxen, sheep, and pigs, it was as much,
if not more, related to the Africo-Asiatic division. Again, northern
India was much more closely related (in having the giraffe,
hippopotamus, and certain musk-deer) to southern Africa than it now is;
for southern and eastern Africa deserve, if we divide the world into
five parts, to make one division by itself. Turning to the dawn of the
Tertiary period, we must, from our ignorance of other portions of the
world, confine ourselves to Europe; and at that period, in the presence
of Marsupials{393} and Edentata, we behold an _entire_ blending of those
mammiferous forms which now eminently characterise Australia and S.
America{394}.
{392} The word _hyaena_ is erased. There appear to be no fossil
Hyaenidae in S. America.
{393} See note 1{390}, p. 175, also _Origin_, Ed. i. p. 340, vi. p. 486.
{394} <Note by the author.> And see Eocene European mammals in
N. America.
If we now look at the distribution of sea-shells, we find the same
changes in distribution. The Red Sea and the Mediterranean were more
nearly related in these shells than they now are. In different parts of
Europe, on the other hand, during the Miocene period, the sea-shells
seem to have been more different than at present. In{395} the Tertiary
period, according to Lyell, the shells of N. America and Europe were
less related than at present, and during the Cretaceous still less like;
whereas, during this same Cretaceous period, the shells of India and
Europe were more like than at present. But going further back to the
Carbonaceous period, in N. America and Europe, the productions were much
more like than they now are{396}. These facts harmonise with the
conclusions drawn from the present distribution of organic beings, for
we have seen, that from species being created in different points or
areas, the formation of a barrier would cause or make two distinct
geographical areas; and the destruction of a barrier would permit their
diffusion{397}. And as long-continued geological changes must both
destroy and make barriers, we might expect, the further we looked
backwards, the more changed should we find the present distribution.
This conclusion is worthy of attention; because, finding in widely
different parts of the same main division of the world, and in volcanic
islands near them, groups of distinct, but related, species;--and
finding that a singularly analogous relation holds good with respect to
the beings of past times, when none of the present species were living,
a person might be tempted to believe in some mystical relation between
certain areas of the world, and the production of certain organic forms;
but we now see that such an assumption would have to be complicated by
the admission that such a relation, though holding good for long
revolutions of years, is not truly persistent.
{395} <Note by the author.> All this requires much verification.
{396} This point seems to be less insisted on in the _Origin_.
{397} _Origin_, Ed. i. p. 356, vi. p. 504.
I will only add one more observation to this section. Geologists
finding in the most remote period with which we are acquainted, namely
in the Silurian period, that the shells and other marine
productions{398} in North and South America, in Europe, Southern Africa,
and Western Asia, are much more similar than they now are at these
distant points, appear to have imagined that in these ancient times the
laws of geographical distribution were quite different than what they
now are: but we have only to suppose that great continents were extended
east and west, and thus did not divide the inhabitants of the temperate
and tropical seas, as the continents now do; and it would then become
probable that the inhabitants of the seas would be much more similar
than they now are. In the immense space of ocean extending from the east
coast of Africa to the eastern islands of the Pacific, which space is
connected either by lines of tropical coast or by islands not very
distant from each other, we know (Cuming) that many shells, perhaps even
as many as 200, are common to the Zanzibar coast, the Philippines, and
the eastern islands of the Low or Dangerous Archipelago in the Pacific.
This space equals that from the Arctic to the Antarctic pole! Pass over
the space of quite open ocean, from the Dangerous Archipelago to the
west coast of S. America, and every shell is different: pass over the
narrow space of S. America, to its eastern shores, and again every shell
is different! Many fish, I may add, are also common to the Pacific and
Indian Oceans.
{398} <Note by the author.> D'Orbigny shows that this is not so.
_Summary on the distribution of living and extinct organic beings._
Let us sum up the several facts now given with respect to the past and
present geographical distribution of organic beings. In a previous
chapter it was shown that species are not exterminated by universal
catastrophes, and that they are slowly produced: we have also seen that
each species is probably only once produced, on one point or area once
in time; and that each diffuses itself, as far as barriers and its
conditions of life permit. If we look at any one main division of the
land, we find in the different parts, whether exposed to different
conditions or to the same conditions, many groups of species wholly or
nearly distinct as species, nevertheless intimately related. We find the
inhabitants of islands, though distinct as species, similarly related to
the inhabitants of the nearest continent; we find in some cases, that
even the different islands of one such group are inhabited by species
distinct, though intimately related to one another and to those of the
nearest continent:--thus typifying the distribution of organic beings
over the whole world. We find the floras of distant mountain-summits
either very similar (which seems to admit, as shown, of a simple
explanation) or very distinct but related to the floras of the
surrounding region; and hence, in this latter case, the floras of two
mountain-summits, although exposed to closely similar conditions, will
be very different. On the mountain-summits of islands, characterised by
peculiar faunas and floras, the plants are often eminently peculiar. The
dissimilarity of the organic beings inhabiting nearly similar countries
is best seen by comparing the main divisions of the world; in each of
which some districts may be found very similarly exposed, yet the
inhabitants are wholly unlike;--far more unlike than those in very
dissimilar districts in the same main division. We see this strikingly
in comparing two volcanic archipelagoes, with nearly the same climate,
but situated not very far from two different continents; in which case
their inhabitants are totally unlike. In the different main divisions of
the world, the amount of difference between the organisms, even in the
same class, is widely different, each main division having only the
species distinct in some families, in other families having the genera
distinct. The distribution of aquatic organisms is very different from
that of the terrestrial organisms; and necessarily so, from the barriers
to their progress being quite unlike. The nature of the conditions in an
isolated district will not explain the number of species inhabiting it;
nor the absence of one class or the presence of another class. We find
that terrestrial mammifers are not present on islands far removed from
other land. We see in two regions, that the species though distinct are
more or less related, according to the greater or less _possibility_ of
the transportal in past and present times of species from one to the
other region; although we can hardly admit that all the species in such
cases have been transported from the first to the second region, and
since have become extinct in the first: we see this law in the presence
of the fox on the Falkland Islands; in the European character of some of
the plants of Tierra del Fuego; in the Indo-Asiatic character of the
plants of the Pacific; and in the circumstance of those genera which
range widest having many species with wide ranges; and those genera with
restricted ranges having species with restricted ranges. Finally, we
find in each of the main divisions of the land, and probably of the sea,
that the existing organisms are related to those lately extinct.
Looking further backwards we see that the past geographical distribution
of organic beings was different from the present; and indeed,
considering that geology shows that all our land was once under water,
and that where water now extends land is forming, the reverse could
hardly have been possible.
Now these several facts, though evidently all more or less connected
together, must by the creationist (though the geologist may explain some
of the anomalies) be considered as so many ultimate facts. He can only
say, that it so pleased the Creator that the organic beings of the
plains, deserts, mountains, tropical and temperature forests, of S.
America, should all have some affinity together; that the inhabitants of
the Galapagos Archipelago should be related to those of Chile; and that
some of the species on the similarly constituted islands of this
archipelago, though most closely related, should be distinct; that all
its inhabitants should be totally unlike those of the similarly volcanic
and arid Cape de Verde and Canary Islands; that the plants on the summit
of Teneriffe should be eminently peculiar; that the diversified island
of New Zealand should have not many plants, and not one, or only one,
mammifer; that the mammifers of S. America, Australia and Europe should
be clearly related to their ancient and exterminated prototypes; and so
on with other facts. But it is absolutely opposed to every analogy,
drawn from the laws imposed by the Creator on inorganic matter, that
facts, when connected, should be considered as ultimate and not the
direct consequences of more general laws.
SECTION THIRD.
_An attempt to explain the foregoing laws of geographical distribution,
on the theory of allied species having a common descent._
First let us recall the circumstances most favourable for variation
under domestication, as given in the first chapter--viz. 1st, a change,
or repeated changes, in the conditions to which the organism has been
exposed, continued through several seminal (_i.e._ not by buds or
divisions) generations: 2nd, steady selection of the slight varieties
thus generated with a fixed end in view: 3rd, isolation as perfect as
possible of such selected varieties; that is, the preventing their
crossing with other forms; this latter condition applies to all
terrestrial animals, to most if not all plants and perhaps even to most
(or all) aquatic organisms. It will be convenient here to show the
advantage of isolation in the formation of a new breed, by comparing the
progress of two persons (to neither of whom let time be of any
consequence) endeavouring to select and form some very peculiar new
breed. Let one of these persons work on the vast herds of cattle in the
plains of La Plata{399}, and the other on a small stock of 20 or 30
animals in an island. The latter might have to wait centuries (by the
hypothesis of no importance){400} before he obtained a "sport"
approaching to what he wanted; but when he did and saved the greater
number of its offspring and their offspring again, he might hope that
his whole little stock would be in some degree affected, so that by
continued selection he might gain his end. But on the Pampas, though
the man might get his first approach to his desired form sooner, how
hopeless would it be to attempt, by saving its offspring amongst so many
of the common kind, to affect the whole herd: the effect of this one
peculiar "sport{401}" would be quite lost before he could obtain a
second original sport of the same kind. If, however, he could separate a
small number of cattle, including the offspring of the desirable
"sport," he might hope, like the man on the island, to effect his end.
If there be organic beings of which two individuals _never_ unite, then
simple selection whether on a continent or island would be equally
serviceable to make a new and desirable breed; and this new breed might
be made in surprisingly few years from the great and geometrical powers
of propagation to beat out the old breed; as has happened
(notwithstanding crossing) where good breeds of dogs and pigs have been
introduced into a limited country,--for instance, into the islands of
the Pacific.
{399} This instance occurs in the Essay of 1842, p. 32, but not in
the _Origin_; though the importance of isolation is discussed
(_Origin_, Ed. i. p. 104, vi. p. 127).
{400} The meaning of the words within parenthesis is obscure.
{401} It is unusual to find the author speaking of the selection of
_sports_ rather than small variations.
Let us now take the simplest natural case of an islet upheaved by the
volcanic or subterranean forces in a deep sea, at such a distance from
other land that only a few organic beings at rare intervals were
transported to it, whether borne by the sea{402} (like the seeds of
plants to coral-reefs), or by hurricanes, or by floods, or on rafts, or
in roots of large trees, or the germs of one plant or animal attached to
or in the stomach of some other animal, or by the intervention (in most
cases the most probable means) of other islands since sunk or destroyed.
It may be remarked that when one part of the earth's crust is raised it
is probably the general rule that another part sinks. Let this island
go on slowly, century after century, rising foot by foot; and in the
course of time we shall have instead <of> a small mass of rock{403},
lowland and highland, moist woods and dry sandy spots, various soils,
marshes, streams and pools: under water on the sea shore, instead of a
rocky steeply shelving coast, we shall have in some parts bays with mud,
sandy beaches and rocky shoals. The formation of the island by itself
must often slightly affect the surrounding climate. It is impossible
that the first few transported organisms could be perfectly adapted to
all these stations; and it will be a chance if those successively
transported will be so adapted. The greater number would probably come
from the lowlands of the nearest country; and not even all these would
be perfectly adapted to the new islet whilst it continued low and
exposed to coast influences. Moreover, as it is certain that all
organisms are nearly as much adapted in their structure to the other
inhabitants of their country as they are to its physical conditions, so
the mere fact that a _few_ beings (and these taken in great degree by
chance) were in the first case transported to the islet, would in itself
greatly modify their conditions{404}. As the island continued rising we
might also expect an occasional new visitant; and I repeat that even one
new being must often affect beyond our calculation by occupying the room
and taking part of the subsistence of another (and this again from
another and so on), several or many other organisms. Now as the first
transported and any occasional successive visitants spread or tended to
spread over the growing island, they would undoubtedly be exposed
through several generations to new and varying conditions: it might also
easily happen that some of the species _on an average_ might obtain an
increase of food, or food of a more nourishing quality{405}. According
then to every analogy with what we have seen takes place in every
country, with nearly every organic being under domestication, we might
expect that some of the inhabitants of the island would "sport," or have
their organization rendered in some degree plastic. As the number of the
inhabitants are supposed to be few and as all these cannot be so well
adapted to their new and varying conditions as they were in their native
country and habitat, we cannot believe that every place or office in the
economy of the island would be as well filled as on a continent where
the number of aboriginal species is far greater and where they
consequently hold a more strictly limited place. We might therefore
expect on our island that although very many slight variations were of
no use to the plastic individuals, yet that occasionally in the course
of a century an individual might be born{406} of which the structure or
constitution in some slight degree would allow it better to fill up some
office in the insular economy and to struggle against other species. If
such were the case the individual and its offspring would have a better
_chance_ of surviving and of beating out its parent form; and if (as is
probable) it and its offspring crossed with the unvaried parent form,
yet the number of the individuals being not very great, there would be a
chance of the new and more serviceable form being nevertheless in some
slight degree preserved. The struggle for existence would go on annually
selecting such individuals until a new race or species was formed.
Either few or all the first visitants to the island might become
modified, according as the physical conditions of the island and those
resulting from the kind and number of other transported species were
different from those of the parent country--according to the
difficulties offered to fresh immigration--and according to the length
of time since the first inhabitants were introduced. It is obvious that
whatever was the country, generally the nearest from which the first
tenants were transported, they would show an affinity, even if all had
become modified, to the natives of that country and even if the
inhabitants of the same source (?) had been modified. On this view we
can at once understand the cause and meaning of the affinity of the
fauna and flora of the Galapagos Islands with that of the coast of S.
America; and consequently why the inhabitants of these islands show not
the smallest affinity with those inhabiting other volcanic islands, with
a very similar climate and soil, near the coast of Africa{407}.
{402} This brief discussion is represented in the _Origin_, Ed. i.
by a much fuller one (pp. 356, 383, vi. pp. 504, 535). See,
however, the section in the present Essay, p. 168.
{403} On the formation of new stations, see _Origin_, Ed. i. p.
292, vi. p. 429.
{404} _Origin_, Ed. i. pp. 390, 400, vi. pp. 543, 554.
{405} In the MS. _some of the species ... nourishing quality_ is
doubtfully erased. It seems clear that he doubted whether such a
problematical supply of food would be likely to cause variation.
{406} At this time the author clearly put more faith in the
importance of sport-like variation than in later years.
{407} _Origin_, Ed. i. p. 398, vi. p. 553.
To return once again to our island, if by the continued action of the
subterranean forces other neighbouring islands were formed, these would
generally be stocked by the inhabitants of the first island, or by a few
immigrants from the neighbouring mainland; but if considerable obstacles
were interposed to any communication between the terrestrial productions
of these islands, and their conditions were different (perhaps only by
the number of different species on each island), a form transported from
one island to another might become altered in the same manner as one
from the continent; and we should have several of the islands tenanted
by representative races or species, as is so wonderfully the case with
the different islands of the Galapagos Archipelago. As the islands
become mountainous, if mountain-species were not introduced, as could
rarely happen, a greater amount of variation and selection would be
requisite to adapt the species, which originally came from the lowlands
of the nearest continent, to the mountain-summits than to the lower
districts of our islands. For the lowland species from the continent
would have first to struggle against other species and other conditions
on the coast-land of the island, and so probably become modified by the
selection of its best fitted varieties, then to undergo the same process
when the land had attained a moderate elevation; and then lastly when it
had become Alpine. Hence we can understand why the faunas of insular
mountain-summits are, as in the case of Teneriffe, eminently peculiar.
Putting on one side the case of a widely extended flora being driven up
the mountain-summits, during a change of climate from cold to temperate,
we can see why in other cases the floras of mountain-summits (or as I
have called them islands in a sea of land) should be tenanted by
peculiar species, but related to those of the surrounding lowlands, as
are the inhabitants of a real island in the sea to those of the nearest
continent{408}.
{408} See _Origin_, Ed. i. p. 403, vi. p. 558, where the author
speaks of Alpine humming birds, rodents, plants, &c. in S. America,
all of strictly American forms. In the MS. the author has added
between the lines "As world has been getting hotter, there has been
radiation from high-lands,--old view?--curious; I presume Diluvian
in origin."
Let us now consider the effect of a change of climate or of other
conditions on the inhabitants of a continent and of an isolated island
without any great change of level. On a continent the chief effects
would be changes in the numerical proportion of the individuals of the
different species; for whether the climate became warmer or colder,
drier or damper, more uniform or extreme, some species are at present
adapted to its diversified districts; if for instance it became cooler,
species would migrate from its more temperate parts and from its higher
land; if damper, from its damper regions, &c. On a small and isolated
island, however, with few species, and these not adapted to much
diversified conditions, such changes instead of merely increasing the
number of certain species already adapted to such conditions, and
decreasing the number of other species, would be apt to affect the
constitutions of some of the insular species: thus if the island became
damper it might well happen that there were no species living in any
part of it adapted to the consequences resulting from more moisture. In
this case therefore, and still more (as we have seen) during the
production of new stations from the elevation of the land, an island
would be a far more fertile source, as far as we can judge, of new
specific forms than a continent. The new forms thus generated on an
island, we might expect, would occasionally be transported by accident,
or through long-continued geographical changes be enabled to emigrate
and thus become slowly diffused.
But if we look to the origin of a continent; almost every geologist will
admit that in most cases it will have first existed as separate islands
which gradually increased in size{409}; and therefore all that which has
been said concerning the probable changes of the forms tenanting a small
archipelago is applicable to a continent in its early state.
Furthermore, a geologist who reflects on the geological history of
Europe (the only region well known) will admit that it has been many
times depressed, raised and left stationary. During the sinking of a
continent and the probable generally accompanying changes of climate the
effect would be little, _except_ on the numerical proportions and in the
extinction (from the lessening of rivers, the drying of marshes and the
conversion of high-lands into low &c.) of some or of many of the
species. As soon however as the continent became divided into many
isolated portions or islands, preventing free immigration from one part
to another, the effect of climatic and other changes on the species
would be greater. But let the now broken continent, forming isolated
islands, begin to rise and new stations thus to be formed, exactly as in
the first case of the upheaved volcanic islet, and we shall have equally
favourable conditions for the modification of old forms, that is the
formation of new races or species. Let the islands become reunited into
a continent; and then the new and old forms would all spread, as far as
barriers, the means of transportal, and the preoccupation of the land by
other species, would permit. Some of the new species or races would
probably become extinct, and some perhaps would cross and blend
together. We should thus have a multitude of forms, adapted to all kinds
of slightly different stations, and to diverse groups of either
antagonist or food-serving species. The oftener these oscillations of
level had taken place (and therefore generally the older the land) the
greater the number of species <which> would tend to be formed. The
inhabitants of a continent being thus derived in the first stage from
the same original parents, and subsequently from the inhabitants of one
wide area, since often broken up and reunited, all would be obviously
related together and the inhabitants of the most _dissimilar_ stations
on the same continent would be more closely allied than the inhabitants
of two very _similar_ stations on two of the main divisions of the
world{410}.
{409} See the comparison between the Malay Archipelago and the
probable former state of Europe, _Origin_, Ed. i. p. 299, vi. p.
438, also _Origin_, Ed. i. p. 292, vi. p. 429.
{410} _Origin_, Ed. i. p. 349, vi. p. 496. The arrangement of the
argument in the present Essay leads to repetition of statements
made in the earlier part of the book: in the _Origin_ this is
avoided.
I need hardly point out that we now can obviously see why the number of
species in two districts, independently of the number of stations in
such districts, should be in some cases as widely different as in New
Zealand and the Cape of Good Hope{411}. We can see, knowing the
difficulty in the transport of terrestrial mammals, why islands far from
mainlands do not possess them{412}; we see the general reason, namely
accidental transport (though not the precise reason), why certain
islands should, and others should not, possess members of the class of
reptiles. We can see why an ancient channel of communication between two
distant points, as the Cordillera probably was between southern Chile
and the United States during the former cold periods; and icebergs
between the Falkland Islands and Tierra del Fuego; and gales, at a
former or present time, between the Asiatic shores of the Pacific and
eastern islands in this ocean; is connected with (or we may now say
causes) an affinity between the species, though distinct, in two such
districts. We can see how the better chance of diffusion, from several
of the species of any genus having wide ranges in their own countries,
explains the presence of other species of the same genus in other
countries{413}; and on the other hand, of species of restricted powers
of ranging, forming genera with restricted ranges.
{411} _Origin_, Ed. i. p. 389, vi. p. 542.
{412} _Origin_, Ed. i. p. 393, vi. p. 547.
{413} _Origin_, Ed. i. pp. 350, 404, vi. pp. 498, 559.
As every one would be surprised if two exactly similar but peculiar
varieties{414} of any species were raised by man by long continued
selection, in two different countries, or at two very different periods,
so we ought not to expect that an exactly similar form would be produced
from the modification of an old one in two distinct countries or at two
distinct periods. For in such places and times they would probably be
exposed to somewhat different climates and almost certainly to different
associates. Hence we can see why each species appears to have been
produced singly, in space and in time. I need hardly remark that,
according to this theory of descent, there is no necessity of
modification in a species, when it reaches a new and isolated country.
If it be able to survive and if slight variations better adapted to the
new conditions are not selected, it might retain (as far as we can see)
its old form for an indefinite time. As we see that some sub-varieties
produced under domestication are more variable than others, so in
nature, perhaps, some species and genera are more variable than others.
The same precise form, however, would probably be seldom preserved
through successive geological periods, or in widely and differently
conditioned countries{415}.
{414} _Origin_, Ed. i. p. 352, vi. p. 500.
{415} _Origin_, Ed. i. p. 313, vi. p. 454.
Finally, during the long periods of time and probably of oscillations of
level, necessary for the formation of a continent, we may conclude (as
above explained) that many forms would become extinct. These extinct
forms, and those surviving (whether or not modified and changed in
structure), will all be related in each continent in the same manner and
degree, as are the inhabitants of any two different sub-regions in that
same continent. I do not mean to say that, for instance, the present
Marsupials of Australia or Edentata and rodents of S. America have
descended from any one of the few fossils of the same orders which have
been discovered in these countries. It is possible that, in a very few
instances, this may be the case; but generally they must be considered
as merely codescendants of common stocks{416}. I believe in this, from
the improbability, considering the vast number of species, which (as
explained in the last chapter) must by our theory have existed, that
the _comparatively_ few fossils which have been found should chance to
be the immediate and linear progenitors of those now existing. Recent as
the yet discovered fossil mammifers of S. America are, who will pretend
to say that very many intermediate forms may not have existed? Moreover,
we shall see in the ensuing chapter that the very existence of genera
and species can be explained only by a few species of each epoch leaving
modified successors or new species to a future period; and the more
distant that future period, the fewer will be the _linear_ heirs of the
former epoch. As by our theory, all mammifers must have descended from
the same parent stock, so is it necessary that each land now possessing
terrestrial mammifers shall at some time have been so far united to
other land as to permit the passage of mammifers{417}; and it accords
with this necessity, that in looking far back into the earth's history
we find, first changes in the geographical distribution, and secondly a
period when the mammiferous forms most distinctive of two of the present
main divisions of the world were living together{418}.
{416} _Origin_, Ed. i. p. 341, vi. p. 487.
{417} _Origin_, Ed. i. p. 396, vi. p. 549.
{418} _Origin_, Ed. i. p. 340, vi. p. 486.
I think then I am justified in asserting that most of the above
enumerated and often trivial points in the geographical distribution of
past and present organisms (which points must be viewed by the
creationists as so many ultimate facts) follow as a simple consequence
of specific forms being mutable and of their being adapted by natural
selection to diverse ends, conjoined with their powers of dispersal, and
the geologico-geographical changes now in slow progress and which
undoubtedly have taken place. This large class of facts being thus
explained, far more than counterbalances many separate difficulties and
apparent objections in convincing my mind of the truth of this theory of
common descent.
_Improbability of finding fossil forms intermediate between existing
species._
There is one observation of considerable importance that may be here
introduced, with regard to the improbability of the chief transitional
forms between any two species being found fossil. With respect to the
finer shades of transition, I have before remarked that no one has any
cause to expect to trace them in a fossil state, without he be bold
enough to imagine that geologists at a future epoch will be able to
trace from fossil bones the gradations between the Short-Horns,
Herefordshire, and Alderney breeds of cattle{419}. I have attempted to
show that rising islands, in process of formation, must be the best
nurseries of new specific forms, and these points are the least
favourable for the embedment of fossils{420}: I appeal, as evidence, to
the state of the _numerous_ scattered islands in the several great
oceans: how rarely do any sedimentary deposits occur on them; and when
present they are mere narrow fringes of no great antiquity, which the
sea is generally wearing away and destroying. The cause of this lies in
isolated islands being generally volcanic and rising points; and the
effects of subterranean elevation is to bring up the surrounding
newly-deposited strata within the destroying action of the coast-waves:
the strata, deposited at greater distances, and therefore in the depths
of the ocean, will be almost barren of organic remains. These remarks
may be generalised:--periods of subsidence will always be most
favourable to an accumulation of great thicknesses of strata, and
consequently to their long preservation; for without one formation be
protected by successive strata, it will seldom be preserved to a distant
age, owing to the enormous amount of denudation, which seems to be a
general contingent of time{421}. I may refer, as evidence of this
remark, to the vast amount of subsidence evident in the great pile of
the European formations, from the Silurian epoch to the end of the
Secondary, and perhaps to even a later period. Periods of elevation on
the other hand cannot be favourable to the accumulation of strata and
their preservation to distant ages, from the circumstance just alluded
to, viz. of elevation tending to bring to the surface the
circum-littoral strata (always abounding most in fossils) and destroying
them. The bottom of tracts of deep water (little favourable, however, to
life) must be excepted from this unfavourable influence of elevation. In
the quite open ocean, probably no sediment{422} is accumulating, or at a
rate so slow as not to preserve fossil remains, which will always be
subject to disintegration. Caverns, no doubt, will be equally likely to
preserve terrestrial fossils in periods of elevation and of subsidence;
but whether it be owing to the enormous amount of denudation, which all
land seems to have undergone, no cavern with fossil bones has been found
belonging to the Secondary period{423}.
{419} _Origin_, Ed. i. p. 299, vi. p. 437.
{420} "Nature may almost be said to have guarded against the
frequent discovery of her transitional or linking forms," _Origin_,
Ed. i. p. 292. A similar but not identical passage occurs in
_Origin_, Ed. vi. p. 428.
{421} _Origin_, Ed. i. p. 291, vi. p. 426.
{422} _Origin_, Ed. i. p. 288, vi. p. 422.
{423} _Origin_, Ed. i. p. 289, vi. p. 423.
Hence many more remains will be preserved to a distant age, in any
region of the world, during periods of its subsidence{424}, than of its
elevation.
{424} _Origin_, Ed. i. p. 300, vi. p. 439.
But during the subsidence of a tract of land, its inhabitants (as before
shown) will from the decrease of space and of the diversity of its
stations, and from the land being fully preoccupied by species fitted to
diversified means of subsistence, be little liable to modification from
selection, although many may, or rather must, become extinct. With
respect to its circum-marine inhabitants, although during a change from
a continent to a _great_ archipelago, the number of stations fitted for
marine beings will be increased, their means of diffusion (an important
check to change of form) will be greatly improved; for a continent
stretching north and south, or a quite open space of ocean, seems to be
to them the only barrier. On the other hand, during the elevation of a
small archipelago and its conversion into a continent, we have, whilst
the number of stations are increasing, both for aquatic and terrestrial
productions, and whilst these stations are not fully preoccupied by
perfectly adapted species, the most favourable conditions for the
selection of new specific forms; but few of them in their early
transitional states will be preserved to a distant epoch. We must wait
during an enormous lapse of time, until long-continued subsidence shall
have taken the place in this quarter of the world of the elevatory
process, for the best conditions of the embedment and the preservation
of its inhabitants. Generally the great mass of the strata in every
country, from having been chiefly accumulated during subsidence, will be
the tomb, not of transitional forms, but of those either becoming
extinct or remaining unmodified.
The state of our knowledge, and the slowness of the changes of level, do
not permit us to test the truth of these remarks, by observing whether
there are more transitional or "fine" (as naturalists would term them)
species, on a rising and enlarging tract of land, than on an area of
subsidence. Nor do I know whether there are more "fine" species on
isolated volcanic islands in process of formation, than on a continent;
but I may remark, that at the Galapagos Archipelago the number of forms,
which according to some naturalists are true species, and according to
others are mere races, is considerable: this particularly applies to the
different species or races of the same genera inhabiting the different
islands of this archipelago. Furthermore it may be added (as bearing on
the great facts discussed in this chapter) that when naturalists confine
their attention to any one country, they have comparatively little
difficulty in determining what forms to call species and what to call
varieties; that is, those which can or cannot be traced or shown to be
probably descendants of some other form: but the difficulty increases,
as species are brought from many stations, countries and islands. It was
this increasing (but I believe in few cases insuperable) difficulty
which seems chiefly to have urged Lamarck to the conclusion that species
are mutable.