On the Origin of Species
by
Charles Darwin
Part 6 out of 9
species being given, but this is unimportant for us. The horizontal
lines may represent successive geological formations, and all the
forms beneath the uppermost line may be considered as extinct. The
three existing genera, a14, q14, p14, will form a small family; b14
and f14 a closely allied family or sub-family; and o14, e14, m14, a
third family. These three families, together with the many extinct
genera on the several lines of descent diverging from the parent-form
A, will form an order; for all will have inherited something in common
from their ancient and common progenitor. On the principle of the
continued tendency to divergence of character, which was formerly
illustrated by this diagram, the more recent any form is, the more it
will generally differ from its ancient progenitor. Hence we can
understand the rule that the most ancient fossils differ most from
existing forms. We must not, however, assume that divergence of
character is a necessary contingency; it depends solely on the
descendants from a species being thus enabled to seize on many and
different places in the economy of nature. Therefore it is quite
possible, as we have seen in the case of some Silurian forms, that a
species might go on being slightly modified in relation to its
slightly altered conditions of life, and yet retain throughout a vast
period the same general characteristics. This is represented in the
diagram by the letter F14.
All the many forms, extinct and recent, descended from A, make, as
before remarked, one order; and this order, from the continued effects
of extinction and divergence of character, has become divided into
several sub-families and families, some of which are supposed to have
perished at different periods, and some to have endured to the present
day.
By looking at the diagram we can see that if many of the extinct
forms, supposed to be embedded in the successive formations, were
discovered at several points low down in the series, the three
existing families on the uppermost line would be rendered less
distinct from each other. If, for instance, the genera a1, a5, a10,
f8, m3, m6, m9 were disinterred, these three families would be so
closely linked together that they probably would have to be united
into one great family, in nearly the same manner as has occurred with
ruminants and pachyderms. Yet he who objected to call the extinct
genera, which thus linked the living genera of three families
together, intermediate in character, would be justified, as they are
intermediate, not directly, but only by a long and circuitous course
through many widely different forms. If many extinct forms were to be
discovered above one of the middle horizontal lines or geological
formations--for instance, above Number VI.--but none from beneath this
line, then only the two families on the left hand (namely, a14, etc.,
and b14, etc.) would have to be united into one family; and the two
other families (namely, a14 to f14 now including five genera, and o14
to m14) would yet remain distinct. These two families, however, would
be less distinct from each other than they were before the discovery
of the fossils. If, for instance, we suppose the existing genera of
the two families to differ from each other by a dozen characters, in
this case the genera, at the early period marked VI., would differ by
a lesser number of characters; for at this early stage of descent they
have not diverged in character from the common progenitor of the
order, nearly so much as they subsequently diverged. Thus it comes
that ancient and extinct genera are often in some slight degree
intermediate in character between their modified descendants, or
between their collateral relations.
In nature the case will be far more complicated than is represented in
the diagram; for the groups will have been more numerous, they will
have endured for extremely unequal lengths of time, and will have been
modified in various degrees. As we possess only the last volume of the
geological record, and that in a very broken condition, we have no
right to expect, except in very rare cases, to fill up wide intervals
in the natural system, and thus unite distinct families or orders. All
that we have a right to expect, is that those groups, which have
within known geological periods undergone much modification, should in
the older formations make some slight approach to each other; so that
the older members should differ less from each other in some of their
characters than do the existing members of the same groups; and this
by the concurrent evidence of our best palaeontologists seems
frequently to be the case.
Thus, on the theory of descent with modification, the main facts with
respect to the mutual affinities of the extinct forms of life to each
other and to living forms, seem to me explained in a satisfactory
manner. And they are wholly inexplicable on any other view.
On this same theory, it is evident that the fauna of any great period
in the earth's history will be intermediate in general character
between that which preceded and that which succeeded it. Thus, the
species which lived at the sixth great stage of descent in the diagram
are the modified offspring of those which lived at the fifth stage,
and are the parents of those which became still more modified at the
seventh stage; hence they could hardly fail to be nearly intermediate
in character between the forms of life above and below. We must,
however, allow for the entire extinction of some preceding forms, and
for the coming in of quite new forms by immigration, and for a large
amount of modification, during the long and blank intervals between
the successive formations. Subject to these allowances, the fauna of
each geological period undoubtedly is intermediate in character,
between the preceding and succeeding faunas. I need give only one
instance, namely, the manner in which the fossils of the Devonian
system, when this system was first discovered, were at once recognised
by palaeontologists as intermediate in character between those of the
overlying carboniferous, and underlying Silurian system. But each
fauna is not necessarily exactly intermediate, as unequal intervals of
time have elapsed between consecutive formations.
It is no real objection to the truth of the statement, that the fauna
of each period as a whole is nearly intermediate in character between
the preceding and succeeding faunas, that certain genera offer
exceptions to the rule. For instance, mastodons and elephants, when
arranged by Dr. Falconer in two series, first according to their
mutual affinities and then according to their periods of existence, do
not accord in arrangement. The species extreme in character are not
the oldest, or the most recent; nor are those which are intermediate
in character, intermediate in age. But supposing for an instant, in
this and other such cases, that the record of the first appearance and
disappearance of the species was perfect, we have no reason to believe
that forms successively produced necessarily endure for corresponding
lengths of time: a very ancient form might occasionally last much
longer than a form elsewhere subsequently produced, especially in the
case of terrestrial productions inhabiting separated districts. To
compare small things with great: if the principal living and extinct
races of the domestic pigeon were arranged as well as they could be in
serial affinity, this arrangement would not closely accord with the
order in time of their production, and still less with the order of
their disappearance; for the parent rock-pigeon now lives; and many
varieties between the rock-pigeon and the carrier have become extinct;
and carriers which are extreme in the important character of length of
beak originated earlier than short-beaked tumblers, which are at the
opposite end of the series in this same respect.
Closely connected with the statement, that the organic remains from an
intermediate formation are in some degree intermediate in character,
is the fact, insisted on by all palaeontologists, that fossils from
two consecutive formations are far more closely related to each other,
than are the fossils from two remote formations. Pictet gives as a
well-known instance, the general resemblance of the organic remains
from the several stages of the chalk formation, though the species are
distinct in each stage. This fact alone, from its generality, seems to
have shaken Professor Pictet in his firm belief in the immutability of
species. He who is acquainted with the distribution of existing
species over the globe, will not attempt to account for the close
resemblance of the distinct species in closely consecutive formations,
by the physical conditions of the ancient areas having remained nearly
the same. Let it be remembered that the forms of life, at least those
inhabiting the sea, have changed almost simultaneously throughout the
world, and therefore under the most different climates and conditions.
Consider the prodigious vicissitudes of climate during the pleistocene
period, which includes the whole glacial period, and note how little
the specific forms of the inhabitants of the sea have been affected.
On the theory of descent, the full meaning of the fact of fossil
remains from closely consecutive formations, though ranked as distinct
species, being closely related, is obvious. As the accumulation of
each formation has often been interrupted, and as long blank intervals
have intervened between successive formations, we ought not to expect
to find, as I attempted to show in the last chapter, in any one or two
formations all the intermediate varieties between the species which
appeared at the commencement and close of these periods; but we ought
to find after intervals, very long as measured by years, but only
moderately long as measured geologically, closely allied forms, or, as
they have been called by some authors, representative species; and
these we assuredly do find. We find, in short, such evidence of the
slow and scarcely sensible mutation of specific forms, as we have a
just right to expect to find.
ON THE STATE OF DEVELOPMENT OF ANCIENT FORMS.
There has been much discussion whether recent forms are more highly
developed than ancient. I will not here enter on this subject, for
naturalists have not as yet defined to each other's satisfaction what
is meant by high and low forms. But in one particular sense the more
recent forms must, on my theory, be higher than the more ancient; for
each new species is formed by having had some advantage in the
struggle for life over other and preceding forms. If under a nearly
similar climate, the eocene inhabitants of one quarter of the world
were put into competition with the existing inhabitants of the same or
some other quarter, the eocene fauna or flora would certainly be
beaten and exterminated; as would a secondary fauna by an eocene, and
a palaeozoic fauna by a secondary fauna. I do not doubt that this
process of improvement has affected in a marked and sensible manner
the organisation of the more recent and victorious forms of life, in
comparison with the ancient and beaten forms; but I can see no way of
testing this sort of progress. Crustaceans, for instance, not the
highest in their own class, may have beaten the highest molluscs. From
the extraordinary manner in which European productions have recently
spread over New Zealand, and have seized on places which must have
been previously occupied, we may believe, if all the animals and
plants of Great Britain were set free in New Zealand, that in the
course of time a multitude of British forms would become thoroughly
naturalized there, and would exterminate many of the natives. On the
other hand, from what we see now occurring in New Zealand, and from
hardly a single inhabitant of the southern hemisphere having become
wild in any part of Europe, we may doubt, if all the productions of
New Zealand were set free in Great Britain, whether any considerable
number would be enabled to seize on places now occupied by our native
plants and animals. Under this point of view, the productions of Great
Britain may be said to be higher than those of New Zealand. Yet the
most skilful naturalist from an examination of the species of the two
countries could not have foreseen this result.
Agassiz insists that ancient animals resemble to a certain extent the
embryos of recent animals of the same classes; or that the geological
succession of extinct forms is in some degree parallel to the
embryological development of recent forms. I must follow Pictet and
Huxley in thinking that the truth of this doctrine is very far from
proved. Yet I fully expect to see it hereafter confirmed, at least in
regard to subordinate groups, which have branched off from each other
within comparatively recent times. For this doctrine of Agassiz
accords well with the theory of natural selection. In a future chapter
I shall attempt to show that the adult differs from its embryo, owing
to variations supervening at a not early age, and being inherited at a
corresponding age. This process, whilst it leaves the embryo almost
unaltered, continually adds, in the course of successive generations,
more and more difference to the adult.
Thus the embryo comes to be left as a sort of picture, preserved by
nature, of the ancient and less modified condition of each animal.
This view may be true, and yet it may never be capable of full proof.
Seeing, for instance, that the oldest known mammals, reptiles, and
fish strictly belong to their own proper classes, though some of these
old forms are in a slight degree less distinct from each other than
are the typical members of the same groups at the present day, it
would be vain to look for animals having the common embryological
character of the Vertebrata, until beds far beneath the lowest
Silurian strata are discovered--a discovery of which the chance is
very small.
ON THE SUCCESSION OF THE SAME TYPES WITHIN THE SAME AREAS, DURING THE
LATER TERTIARY PERIODS.
Mr. Clift many years ago showed that the fossil mammals from the
Australian caves were closely allied to the living marsupials of that
continent. In South America, a similar relationship is manifest, even
to an uneducated eye, in the gigantic pieces of armour like those of
the armadillo, found in several parts of La Plata; and Professor Owen
has shown in the most striking manner that most of the fossil mammals,
buried there in such numbers, are related to South American types.
This relationship is even more clearly seen in the wonderful
collection of fossil bones made by MM. Lund and Clausen in the caves
of Brazil. I was so much impressed with these facts that I strongly
insisted, in 1839 and 1845, on this "law of the succession of
types,"--on "this wonderful relationship in the same continent between
the dead and the living." Professor Owen has subsequently extended the
same generalisation to the mammals of the Old World. We see the same
law in this author's restorations of the extinct and gigantic birds of
New Zealand. We see it also in the birds of the caves of Brazil. Mr.
Woodward has shown that the same law holds good with sea-shells, but
from the wide distribution of most genera of molluscs, it is not well
displayed by them. Other cases could be added, as the relation between
the extinct and living land-shells of Madeira; and between the extinct
and living brackish-water shells of the Aralo-Caspian Sea.
Now what does this remarkable law of the succession of the same types
within the same areas mean? He would be a bold man, who after
comparing the present climate of Australia and of parts of South
America under the same latitude, would attempt to account, on the one
hand, by dissimilar physical conditions for the dissimilarity of the
inhabitants of these two continents, and, on the other hand, by
similarity of conditions, for the uniformity of the same types in each
during the later tertiary periods. Nor can it be pretended that it is
an immutable law that marsupials should have been chiefly or solely
produced in Australia; or that Edentata and other American types
should have been solely produced in South America. For we know that
Europe in ancient times was peopled by numerous marsupials; and I have
shown in the publications above alluded to, that in America the law of
distribution of terrestrial mammals was formerly different from what
it now is. North America formerly partook strongly of the present
character of the southern half of the continent; and the southern half
was formerly more closely allied, than it is at present, to the
northern half. In a similar manner we know from Falconer and Cautley's
discoveries, that northern India was formerly more closely related in
its mammals to Africa than it is at the present time. Analogous facts
could be given in relation to the distribution of marine animals.
On the theory of descent with modification, the great law of the long
enduring, but not immutable, succession of the same types within the
same areas, is at once explained; for the inhabitants of each quarter
of the world will obviously tend to leave in that quarter, during the
next succeeding period of time, closely allied though in some degree
modified descendants. If the inhabitants of one continent formerly
differed greatly from those of another continent, so will their
modified descendants still differ in nearly the same manner and
degree. But after very long intervals of time and after great
geographical changes, permitting much inter-migration, the feebler
will yield to the more dominant forms, and there will be nothing
immutable in the laws of past and present distribution.
It may be asked in ridicule, whether I suppose that the megatherium
and other allied huge monsters have left behind them in South America
the sloth, armadillo, and anteater, as their degenerate descendants.
This cannot for an instant be admitted. These huge animals have become
wholly extinct, and have left no progeny. But in the caves of Brazil,
there are many extinct species which are closely allied in size and in
other characters to the species still living in South America; and
some of these fossils may be the actual progenitors of living species.
It must not be forgotten that, on my theory, all the species of the
same genus have descended from some one species; so that if six
genera, each having eight species, be found in one geological
formation, and in the next succeeding formation there be six other
allied or representative genera with the same number of species, then
we may conclude that only one species of each of the six older genera
has left modified descendants, constituting the six new genera. The
other seven species of the old genera have all died out and have left
no progeny. Or, which would probably be a far commoner case, two or
three species of two or three alone of the six older genera will have
been the parents of the six new genera; the other old species and the
other whole genera having become utterly extinct. In failing orders,
with the genera and species decreasing in numbers, as apparently is
the case of the Edentata of South America, still fewer genera and
species will have left modified blood-descendants.
SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS.
I have attempted to show that the geological record is extremely
imperfect; that only a small portion of the globe has been
geologically explored with care; that only certain classes of organic
beings have been largely preserved in a fossil state; that the number
both of specimens and of species, preserved in our museums, is
absolutely as nothing compared with the incalculable number of
generations which must have passed away even during a single
formation; that, owing to subsidence being necessary for the
accumulation of fossiliferous deposits thick enough to resist future
degradation, enormous intervals of time have elapsed between the
successive formations; that there has probably been more extinction
during the periods of subsidence, and more variation during the
periods of elevation, and during the latter the record will have been
least perfectly kept; that each single formation has not been
continuously deposited; that the duration of each formation is,
perhaps, short compared with the average duration of specific forms;
that migration has played an important part in the first appearance of
new forms in any one area and formation; that widely ranging species
are those which have varied most, and have oftenest given rise to new
species; and that varieties have at first often been local. All these
causes taken conjointly, must have tended to make the geological
record extremely imperfect, and will to a large extent explain why we
do not find interminable varieties, connecting together all the
extinct and existing forms of life by the finest graduated steps.
He who rejects these views on the nature of the geological record,
will rightly reject my whole theory. For he may ask in vain where are
the numberless transitional links which must formerly have connected
the closely allied or representative species, found in the several
stages of the same great formation. He may disbelieve in the enormous
intervals of time which have elapsed between our consecutive
formations; he may overlook how important a part migration must have
played, when the formations of any one great region alone, as that of
Europe, are considered; he may urge the apparent, but often falsely
apparent, sudden coming in of whole groups of species. He may ask
where are the remains of those infinitely numerous organisms which
must have existed long before the first bed of the Silurian system was
deposited: I can answer this latter question only hypothetically, by
saying that as far as we can see, where our oceans now extend they
have for an enormous period extended, and where our oscillating
continents now stand they have stood ever since the Silurian epoch;
but that long before that period, the world may have presented a
wholly different aspect; and that the older continents, formed of
formations older than any known to us, may now all be in a
metamorphosed condition, or may lie buried under the ocean.
Passing from these difficulties, all the other great leading facts in
palaeontology seem to me simply to follow on the theory of descent
with modification through natural selection. We can thus understand
how it is that new species come in slowly and successively; how
species of different classes do not necessarily change together, or at
the same rate, or in the same degree; yet in the long run that all
undergo modification to some extent. The extinction of old forms is
the almost inevitable consequence of the production of new forms. We
can understand why when a species has once disappeared it never
reappears. Groups of species increase in numbers slowly, and endure
for unequal periods of time; for the process of modification is
necessarily slow, and depends on many complex contingencies. The
dominant species of the larger dominant groups tend to leave many
modified descendants, and thus new sub-groups and groups are formed.
As these are formed, the species of the less vigorous groups, from
their inferiority inherited from a common progenitor, tend to become
extinct together, and to leave no modified offspring on the face of
the earth. But the utter extinction of a whole group of species may
often be a very slow process, from the survival of a few descendants,
lingering in protected and isolated situations. When a group has once
wholly disappeared, it does not reappear; for the link of generation
has been broken.
We can understand how the spreading of the dominant forms of life,
which are those that oftenest vary, will in the long run tend to
people the world with allied, but modified, descendants; and these
will generally succeed in taking the places of those groups of species
which are their inferiors in the struggle for existence. Hence, after
long intervals of time, the productions of the world will appear to
have changed simultaneously.
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. We can understand, from the continued tendency to
divergence of character, why the more ancient a form is, the more it
generally differs from those now living. Why ancient and extinct forms
often tend to fill up gaps between existing forms, sometimes blending
two groups previously classed as distinct into one; but more commonly
only bringing them a little closer together. The more ancient a form
is, the more often, apparently, it displays characters in some degree
intermediate between groups now distinct; for the more ancient a form
is, the more nearly it will be related to, and consequently resemble,
the common progenitor of groups, since become widely divergent.
Extinct forms are seldom directly intermediate between existing forms;
but are intermediate only by a long and circuitous course through many
extinct and very different forms. We can clearly see why the organic
remains of closely consecutive formations are more closely allied to
each other, than are those of remote formations; for the forms are
more closely linked together by generation: we can clearly see why the
remains of an intermediate formation are intermediate in character.
The inhabitants of each successive period in the world's history have
beaten their predecessors in the race for life, and are, in so far,
higher in the scale of nature; and this may account for that vague yet
ill-defined sentiment, felt by many palaeontologists, that
organisation on the whole has progressed. If it should hereafter be
proved that ancient animals resemble to a certain extent the embryos
of more recent animals of the same class, the fact will be
intelligible. The succession of the same types of structure within the
same areas during the later geological periods ceases to be
mysterious, and is simply explained by inheritance.
If then the geological record be as imperfect as I believe it to be,
and it may at least be asserted that the record cannot be proved to be
much more perfect, the main objections to the theory of natural
selection are greatly diminished or disappear. On the other hand, all
the chief laws of palaeontology plainly proclaim, as it seems to me,
that species have been produced by ordinary generation: old forms
having been supplanted by new and improved forms of life, produced by
the laws of variation still acting round us, and preserved by Natural
Selection.
CHAPTER 11. GEOGRAPHICAL DISTRIBUTION.
Present distribution cannot be accounted for by differences in
physical conditions.
Importance of barriers.
Affinity of the productions of the same continent.
Centres of creation.
Means of dispersal, by changes of climate and of the level of the
land, and by occasional means.
Dispersal during the Glacial period co-extensive with the world.
In considering the distribution of organic beings over the face of the
globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various regions
can be accounted for by their climatal and other physical conditions.
Of late, almost every author who has studied the subject has come to
this conclusion. The case of America alone would almost suffice to
prove its truth: for if we exclude the northern parts where the
circumpolar land is almost continuous, all authors agree that one of
the most fundamental divisions in geographical distribution is that
between the New and Old Worlds; yet if we travel over the vast
American continent, from the central parts of the United States to its
extreme southern point, we meet with the most diversified conditions;
the most humid districts, arid deserts, lofty mountains, grassy
plains, forests, marshes, lakes, and great rivers, under almost every
temperature. There is hardly a climate or condition in the Old World
which cannot be paralleled in the New--at least as closely as the same
species generally require; for it is a most rare case to find a group
of organisms confined to any small spot, having conditions peculiar in
only a slight degree; for instance, small areas in the Old World could
be pointed out hotter than any in the New World, yet these are not
inhabited by a peculiar fauna or flora. Notwithstanding this
parallelism in the conditions of the Old and New Worlds, how widely
different are their living productions!
In the southern hemisphere, if we compare large tracts of land in
Australia, South Africa, and western South America, between latitudes
25 deg and 35 deg, we shall find parts extremely similar in all their
conditions, yet it would not be possible to point out three faunas and
floras more utterly dissimilar. Or again we may compare the
productions of South America south of lat. 35 deg with those north of
25 deg, which consequently inhabit a considerably different climate,
and they will be found incomparably more closely related to each
other, than they are to the productions of Australia or Africa under
nearly the same climate. Analogous facts could be given with respect
to the inhabitants of the sea.
A second great fact which strikes us in our general review is, that
barriers of any kind, or obstacles to free migration, are related in a
close and important manner to the differences between the productions
of various regions. We see this in the great difference of nearly all
the terrestrial productions of the New and Old Worlds, excepting in
the northern parts, where the land almost joins, and where, under a
slightly different climate, there might have been free migration for
the northern temperate forms, as there now is for the strictly arctic
productions. We see the same fact in the great difference between the
inhabitants of Australia, Africa, and South America under the same
latitude: for these countries are almost as much isolated from each
other as is possible. On each continent, also, we see the same fact;
for on the opposite sides of lofty and continuous mountain-ranges, and
of great deserts, and sometimes even of large rivers, we find
different productions; though as mountain chains, deserts, etc., are
not as impassable, or likely to have endured so long as the oceans
separating continents, the differences are very inferior in degree to
those characteristic of distinct continents.
Turning to the sea, we find the same law. No two marine faunas are
more distinct, with hardly a fish, shell, or crab in common, than
those of the eastern and western shores of South and Central America;
yet these great faunas are separated only by the narrow, but
impassable, isthmus of Panama. Westward of the shores of America, a
wide space of open ocean extends, with not an island as a
halting-place for emigrants; here we have a barrier of another kind,
and as soon as this is passed we meet in the eastern islands of the
Pacific, with another and totally distinct fauna. So that here three
marine faunas range far northward and southward, in parallel lines not
far from each other, under corresponding climates; but from being
separated from each other by impassable barriers, either of land or
open sea, they are wholly distinct. On the other hand, proceeding
still further westward from the eastern islands of the tropical parts
of the Pacific, we encounter no impassable barriers, and we have
innumerable islands as halting-places, until after travelling over a
hemisphere we come to the shores of Africa; and over this vast space
we meet with no well-defined and distinct marine faunas. Although
hardly one shell, crab or fish is common to the above-named three
approximate faunas of Eastern and Western America and the eastern
Pacific islands, yet many fish range from the Pacific into the Indian
Ocean, and many shells are common to the eastern islands of the
Pacific and the eastern shores of Africa, on almost exactly opposite
meridians of longitude.
A third great fact, partly included in the foregoing statements, is
the affinity of the productions of the same continent or sea, though
the species themselves are distinct at different points and stations.
It is a law of the widest generality, and every continent offers
innumerable instances. Nevertheless the naturalist in travelling, for
instance, from north to south never fails to be struck by the manner
in which successive groups of beings, specifically distinct, yet
clearly related, replace each other. He hears from closely allied, yet
distinct kinds of birds, notes nearly similar, and sees their nests
similarly constructed, but not quite alike, with eggs coloured in
nearly the same manner. The plains near the Straits of Magellan are
inhabited by one species of Rhea (American ostrich), and northward the
plains of La Plata by another species of the same genus; and not by a
true ostrich or emeu, like those found in Africa and Australia under
the same latitude. On these same plains of La Plata, we see the agouti
and bizcacha, animals having nearly the same habits as our hares and
rabbits and belonging to the same order of Rodents, but they plainly
display an American type of structure. We ascend the lofty peaks of
the Cordillera and we find an alpine species of bizcacha; we look to
the waters, and we do not find the beaver or musk-rat, but the coypu
and capybara, rodents of the American type. Innumerable other
instances could be given. If we look to the islands off the American
shore, however much they may differ in geological structure, the
inhabitants, though they may be all peculiar species, are essentially
American. We may look back to past ages, as shown in the last chapter,
and we find American types then prevalent on the American continent
and in the American seas. We see in these facts some deep organic
bond, prevailing throughout space and time, over the same areas of
land and water, and independent of their physical conditions. The
naturalist must feel little curiosity, who is not led to inquire what
this bond is.
This bond, on my theory, is simply inheritance, that cause which
alone, as far as we positively know, produces organisms quite like,
or, as we see in the case of varieties nearly like each other. The
dissimilarity of the inhabitants of different regions may be
attributed to modification through natural selection, and in a quite
subordinate degree to the direct influence of different physical
conditions. The degree of dissimilarity will depend on the migration
of the more dominant forms of life from one region into another having
been effected with more or less ease, at periods more or less
remote;--on the nature and number of the former immigrants;--and on
their action and reaction, in their mutual struggles for life;--the
relation of organism to organism being, as I have already often
remarked, the most important of all relations. Thus the high
importance of barriers comes into play by checking migration; as does
time for the slow process of modification through natural selection.
Widely-ranging species, abounding in individuals, which have already
triumphed over many competitors in their own widely-extended homes
will have the best chance of seizing on new places, when they spread
into new countries. In their new homes they will be exposed to new
conditions, and will frequently undergo further modification and
improvement; and thus they will become still further victorious, and
will produce groups of modified descendants. On this principle of
inheritance with modification, we can understand how it is that
sections of genera, whole genera, and even families are confined to
the same areas, as is so commonly and notoriously the case.
I believe, as was remarked in the last chapter, in no law of necessary
development. As the variability of each species is an independent
property, and will be taken advantage of by natural selection, only so
far as it profits the individual in its complex struggle for life, so
the degree of modification in different species will be no uniform
quantity. If, for instance, a number of species, which stand in direct
competition with each other, migrate in a body into a new and
afterwards isolated country, they will be little liable to
modification; for neither migration nor isolation in themselves can do
anything. These principles come into play only by bringing organisms
into new relations with each other, and in a lesser degree with the
surrounding physical conditions. As we have seen in the last chapter
that some forms have retained nearly the same character from an
enormously remote geological period, so certain species have migrated
over vast spaces, and have not become greatly modified.
On these views, it is obvious, that the several species of the same
genus, though inhabiting the most distant quarters of the world, must
originally have proceeded from the same source, as they have descended
from the same progenitor. In the case of those species, which have
undergone during whole geological periods but little modification,
there is not much difficulty in believing that they may have migrated
from the same region; for during the vast geographical and climatal
changes which will have supervened since ancient times, almost any
amount of migration is possible. But in many other cases, in which we
have reason to believe that the species of a genus have been produced
within comparatively recent times, there is great difficulty on this
head. It is also obvious that the individuals of the same species,
though now inhabiting distant and isolated regions, must have
proceeded from one spot, where their parents were first produced: for,
as explained in the last chapter, it is incredible that individuals
identically the same should ever have been produced through natural
selection from parents specifically distinct.
We are thus brought to the question which has been largely discussed
by naturalists, namely, whether species have been created at one or
more points of the earth's surface. Undoubtedly there are very many
cases of extreme difficulty, in understanding how the same species
could possibly have migrated from some one point to the several
distant and isolated points, where now found. Nevertheless the
simplicity of the view that each species was first produced within a
single region captivates the mind. He who rejects it, rejects the vera
causa of ordinary generation with subsequent migration, and calls in
the agency of a miracle. It is universally admitted, that in most
cases the area inhabited by a species is continuous; and when a plant
or animal inhabits two points so distant from each other, or with an
interval of such a nature, that the space could not be easily passed
over by migration, the fact is given as something remarkable and
exceptional. The capacity of migrating across the sea is more
distinctly limited in terrestrial mammals, than perhaps in any other
organic beings; and, accordingly, we find no inexplicable cases of the
same mammal inhabiting distant points of the world. No geologist will
feel any difficulty in such cases as Great Britain having been
formerly united to Europe, and consequently possessing the same
quadrupeds. But if the same species can be produced at two separate
points, why do we not find a single mammal common to Europe and
Australia or South America? The conditions of life are nearly the
same, so that a multitude of European animals and plants have become
naturalised in America and Australia; and some of the aboriginal
plants are identically the same at these distant points of the
northern and southern hemispheres? The answer, as I believe, is, that
mammals have not been able to migrate, whereas some plants, from their
varied means of dispersal, have migrated across the vast and broken
interspace. The great and striking influence which barriers of every
kind have had on distribution, is intelligible only on the view that
the great majority of species have been produced on one side alone,
and have not been able to migrate to the other side. Some few
families, many sub-families, very many genera, and a still greater
number of sections of genera are confined to a single region; and it
has been observed by several naturalists, that the most natural
genera, or those genera in which the species are most closely related
to each other, are generally local, or confined to one area. What a
strange anomaly it would be, if, when coming one step lower in the
series, to the individuals of the same species, a directly opposite
rule prevailed; and species were not local, but had been produced in
two or more distinct areas!
Hence it seems to me, as it has to many other naturalists, that the
view of each species having been produced in one area alone, and
having subsequently migrated from that area as far as its powers of
migration and subsistence under past and present conditions permitted,
is the most probable. Undoubtedly many cases occur, in which we cannot
explain how the same species could have passed from one point to the
other. But the geographical and climatal changes, which have certainly
occurred within recent geological times, must have interrupted or
rendered discontinuous the formerly continuous range of many species.
So that we are reduced to consider whether the exceptions to
continuity of range are so numerous and of so grave a nature, that we
ought to give up the belief, rendered probable by general
considerations, that each species has been produced within one area,
and has migrated thence as far as it could. It would be hopelessly
tedious to discuss all the exceptional cases of the same species, now
living at distant and separated points; nor do I for a moment pretend
that any explanation could be offered of many such cases. But after
some preliminary remarks, I will discuss a few of the most striking
classes of facts; namely, the existence of the same species on the
summits of distant mountain-ranges, and at distant points in the
arctic and antarctic regions; and secondly (in the following chapter),
the wide distribution of freshwater productions; and thirdly, the
occurrence of the same terrestrial species on islands and on the
mainland, though separated by hundreds of miles of open sea. If the
existence of the same species at distant and isolated points of the
earth's surface, can in many instances be explained on the view of
each species having migrated from a single birthplace; then,
considering our ignorance with respect to former climatal and
geographical changes and various occasional means of transport, the
belief that this has been the universal law, seems to me incomparably
the safest.
In discussing this subject, we shall be enabled at the same time to
consider a point equally important for us, namely, whether the several
distinct species of a genus, which on my theory have all descended
from a common progenitor, can have migrated (undergoing modification
during some part of their migration) from the area inhabited by their
progenitor. If it can be shown to be almost invariably the case, that
a region, of which most of its inhabitants are closely related to, or
belong to the same genera with the species of a second region, has
probably received at some former period immigrants from this other
region, my theory will be strengthened; for we can clearly understand,
on the principle of modification, why the inhabitants of a region
should be related to those of another region, whence it has been
stocked. A volcanic island, for instance, upheaved and formed at the
distance of a few hundreds of miles from a continent, would probably
receive from it in the course of time a few colonists, and their
descendants, though modified, would still be plainly related by
inheritance to the inhabitants of the continent. Cases of this nature
are common, and are, as we shall hereafter more fully see,
inexplicable on the theory of independent creation. This view of the
relation of species in one region to those in another, does not differ
much (by substituting the word variety for species) from that lately
advanced in an ingenious paper by Mr. Wallace, in which he concludes,
that "every species has come into existence coincident both in space
and time with a pre-existing closely allied species." And I now know
from correspondence, that this coincidence he attributes to generation
with modification.
The previous remarks on "single and multiple centres of creation" do
not directly bear on another allied question,--namely whether all the
individuals of the same species have descended from a single pair, or
single hermaphrodite, or whether, as some authors suppose, from many
individuals simultaneously created. With those organic beings which
never intercross (if such exist), the species, on my theory, must have
descended from a succession of improved varieties, which will never
have blended with other individuals or varieties, but will have
supplanted each other; so that, at each successive stage of
modification and improvement, all the individuals of each variety will
have descended from a single parent. But in the majority of cases,
namely, with all organisms which habitually unite for each birth, or
which often intercross, I believe that during the slow process of
modification the individuals of the species will have been kept nearly
uniform by intercrossing; so that many individuals will have gone on
simultaneously changing, and the whole amount of modification will not
have been due, at each stage, to descent from a single parent. To
illustrate what I mean: our English racehorses differ slightly from
the horses of every other breed; but they do not owe their difference
and superiority to descent from any single pair, but to continued care
in selecting and training many individuals during many generations.
Before discussing the three classes of facts, which I have selected as
presenting the greatest amount of difficulty on the theory of "single
centres of creation," I must say a few words on the means of
dispersal.
MEANS OF DISPERSAL.
Sir C. Lyell and other authors have ably treated this subject. I can
give here only the briefest abstract of the more important facts.
Change of climate must have had a powerful influence on migration: a
region when its climate was different may have been a high road for
migration, but now be impassable; I shall, however, presently have to
discuss this branch of the subject in some detail. Changes of level in
the land must also have been highly influential: a narrow isthmus now
separates two marine faunas; submerge it, or let it formerly have been
submerged, and the two faunas will now blend or may formerly have
blended: where the sea now extends, land may at a former period have
connected islands or possibly even continents together, and thus have
allowed terrestrial productions to pass from one to the other. No
geologist will dispute that great mutations of level have occurred
within the period of existing organisms. Edward Forbes insisted that
all the islands in the Atlantic must recently have been connected with
Europe or Africa, and Europe likewise with America. Other authors have
thus hypothetically bridged over every ocean, and have united almost
every island to some mainland. If indeed the arguments used by Forbes
are to be trusted, it must be admitted that scarcely a single island
exists which has not recently been united to some continent. This view
cuts the Gordian knot of the dispersal of the same species to the most
distant points, and removes many a difficulty: but to the best of my
judgment we are not authorized in admitting such enormous geographical
changes within the period of existing species. It seems to me that we
have abundant evidence of great oscillations of level in our
continents; but not of such vast changes in their position and
extension, as to have united them within the recent period to each
other and to the several intervening oceanic islands. I freely admit
the former existence of many islands, now buried beneath the sea,
which may have served as halting places for plants and for many
animals during their migration. In the coral-producing oceans such
sunken islands are now marked, as I believe, by rings of coral or
atolls standing over them. Whenever it is fully admitted, as I believe
it will some day be, that each species has proceeded from a single
birthplace, and when in the course of time we know something definite
about the means of distribution, we shall be enabled to speculate with
security on the former extension of the land. But I do not believe
that it will ever be proved that within the recent period continents
which are now quite separate, have been continuously, or almost
continuously, united with each other, and with the many existing
oceanic islands. Several facts in distribution,--such as the great
difference in the marine faunas on the opposite sides of almost every
continent,--the close relation of the tertiary inhabitants of several
lands and even seas to their present inhabitants,--a certain degree of
relation (as we shall hereafter see) between the distribution of
mammals and the depth of the sea,--these and other such facts seem to
me opposed to the admission of such prodigious geographical
revolutions within the recent period, as are necessitated on the view
advanced by Forbes and admitted by his many followers. The nature and
relative proportions of the inhabitants of oceanic islands likewise
seem to me opposed to the belief of their former continuity with
continents. Nor does their almost universally volcanic composition
favour the admission that they are the wrecks of sunken
continents;--if they had originally existed as mountain-ranges on the
land, some at least of the islands would have been formed, like other
mountain-summits, of granite, metamorphic schists, old fossiliferous
or other such rocks, instead of consisting of mere piles of volcanic
matter.
I must now say a few words on what are called accidental means, but
which more properly might be called occasional means of distribution.
I shall here confine myself to plants. In botanical works, this or
that plant is stated to be ill adapted for wide dissemination; but for
transport across the sea, the greater or less facilities may be said
to be almost wholly unknown. Until I tried, with Mr. Berkeley's aid, a
few experiments, it was not even known how far seeds could resist the
injurious action of sea-water. To my surprise I found that out of 87
kinds, 64 germinated after an immersion of 28 days, and a few survived
an immersion of 137 days. For convenience sake I chiefly tried small
seeds, without the capsule or fruit; and as all of these sank in a few
days, they could not be floated across wide spaces of the sea, whether
or not they were injured by the salt-water. Afterwards I tried some
larger fruits, capsules, etc., and some of these floated for a long
time. It is well known what a difference there is in the buoyancy of
green and seasoned timber; and it occurred to me that floods might
wash down plants or branches, and that these might be dried on the
banks, and then by a fresh rise in the stream be washed into the sea.
Hence I was led to dry stems and branches of 94 plants with ripe
fruit, and to place them on sea water. The majority sank quickly, but
some which whilst green floated for a very short time, when dried
floated much longer; for instance, ripe hazel-nuts sank immediately,
but when dried, they floated for 90 days and afterwards when planted
they germinated; an asparagus plant with ripe berries floated for 23
days, when dried it floated for 85 days, and the seeds afterwards
germinated: the ripe seeds of Helosciadium sank in two days, when
dried they floated for above 90 days, and afterwards germinated.
Altogether out of the 94 dried plants, 18 floated for above 28 days,
and some of the 18 floated for a very much longer period. So that as
64/87 seeds germinated after an immersion of 28 days; and as 18/94
plants with ripe fruit (but not all the same species as in the
foregoing experiment) floated, after being dried, for above 28 days,
as far as we may infer anything from these scanty facts, we may
conclude that the seeds of 14/100 plants of any country might be
floated by sea-currents during 28 days, and would retain their power
of germination. In Johnston's Physical Atlas, the average rate of the
several Atlantic currents is 33 miles per diem (some currents running
at the rate of 60 miles per diem); on this average, the seeds of
14/100 plants belonging to one country might be floated across 924
miles of sea to another country; and when stranded, if blown to a
favourable spot by an inland gale, they would germinate.
Subsequently to my experiments, M. Martens tried similar ones, but in
a much better manner, for he placed the seeds in a box in the actual
sea, so that they were alternately wet and exposed to the air like
really floating plants. He tried 98 seeds, mostly different from mine;
but he chose many large fruits and likewise seeds from plants which
live near the sea; and this would have favoured the average length of
their flotation and of their resistance to the injurious action of the
salt-water. On the other hand he did not previously dry the plants or
branches with the fruit; and this, as we have seen, would have caused
some of them to have floated much longer. The result was that 18/98 of
his seeds floated for 42 days, and were then capable of germination.
But I do not doubt that plants exposed to the waves would float for a
less time than those protected from violent movement as in our
experiments. Therefore it would perhaps be safer to assume that the
seeds of about 10/100 plants of a flora, after having been dried,
could be floated across a space of sea 900 miles in width, and would
then germinate. The fact of the larger fruits often floating longer
than the small, is interesting; as plants with large seeds or fruit
could hardly be transported by any other means; and Alph. de Candolle
has shown that such plants generally have restricted ranges.
But seeds may be occasionally transported in another manner. Drift
timber is thrown up on most islands, even on those in the midst of the
widest oceans; and the natives of the coral-islands in the Pacific,
procure stones for their tools, solely from the roots of drifted
trees, these stones being a valuable royal tax. I find on examination,
that when irregularly shaped stones are embedded in the roots of
trees, small parcels of earth are very frequently enclosed in their
interstices and behind them,--so perfectly that not a particle could
be washed away in the longest transport: out of one small portion of
earth thus COMPLETELY enclosed by wood in an oak about 50 years old,
three dicotyledonous plants germinated: I am certain of the accuracy
of this observation. Again, I can show that the carcasses of birds,
when floating on the sea, sometimes escape being immediately devoured;
and seeds of many kinds in the crops of floating birds long retain
their vitality: peas and vetches, for instance, are killed by even a
few days' immersion in sea-water; but some taken out of the crop of a
pigeon, which had floated on artificial salt-water for 30 days, to my
surprise nearly all germinated.
Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how
frequently birds of many kinds are blown by gales to vast distances
across the ocean. We may I think safely assume that under such
circumstances their rate of flight would often be 35 miles an hour;
and some authors have given a far higher estimate. I have never seen
an instance of nutritious seeds passing through the intestines of a
bird; but hard seeds of fruit will pass uninjured through even the
digestive organs of a turkey. In the course of two months, I picked up
in my garden 12 kinds of seeds, out of the excrement of small birds,
and these seemed perfect, and some of them, which I tried, germinated.
But the following fact is more important: the crops of birds do not
secrete gastric juice, and do not in the least injure, as I know by
trial, the germination of seeds; now after a bird has found and
devoured a large supply of food, it is positively asserted that all
the grains do not pass into the gizzard for 12 or even 18 hours. A
bird in this interval might easily be blown to the distance of 500
miles, and hawks are known to look out for tired birds, and the
contents of their torn crops might thus readily get scattered. Mr.
Brent informs me that a friend of his had to give up flying
carrier-pigeons from France to England, as the hawks on the English
coast destroyed so many on their arrival. Some hawks and owls bolt
their prey whole, and after an interval of from twelve to twenty
hours, disgorge pellets, which, as I know from experiments made in the
Zoological Gardens, include seeds capable of germination. Some seeds
of the oat, wheat, millet, canary, hemp, clover, and beet germinated
after having been from twelve to twenty-one hours in the stomachs of
different birds of prey; and two seeds of beet grew after having been
thus retained for two days and fourteen hours. Freshwater fish, I
find, eat seeds of many land and water plants: fish are frequently
devoured by birds, and thus the seeds might be transported from place
to place. I forced many kinds of seeds into the stomachs of dead fish,
and then gave their bodies to fishing-eagles, storks, and pelicans;
these birds after an interval of many hours, either rejected the seeds
in pellets or passed them in their excrement; and several of these
seeds retained their power of germination. Certain seeds, however,
were always killed by this process.
Although the beaks and feet of birds are generally quite clean, I can
show that earth sometimes adheres to them: in one instance I removed
twenty-two grains of dry argillaceous earth from one foot of a
partridge, and in this earth there was a pebble quite as large as the
seed of a vetch. Thus seeds might occasionally be transported to great
distances; for many facts could be given showing that soil almost
everywhere is charged with seeds. Reflect for a moment on the millions
of quails which annually cross the Mediterranean; and can we doubt
that the earth adhering to their feet would sometimes include a few
minute seeds? But I shall presently have to recur to this subject.
As icebergs are known to be sometimes loaded with earth and stones,
and have even carried brushwood, bones, and the nest of a land-bird, I
can hardly doubt that they must occasionally have transported seeds
from one part to another of the arctic and antarctic regions, as
suggested by Lyell; and during the Glacial period from one part of the
now temperate regions to another. In the Azores, from the large number
of the species of plants common to Europe, in comparison with the
plants of other oceanic islands nearer to the mainland, and (as
remarked by Mr. H. C. Watson) from the somewhat northern character of
the flora in comparison with the latitude, I suspected that these
islands had been partly stocked by ice-borne seeds, during the Glacial
epoch. At my request Sir C. Lyell wrote to M. Hartung to inquire
whether he had observed erratic boulders on these islands, and he
answered that he had found large fragments of granite and other rocks,
which do not occur in the archipelago. Hence we may safely infer that
icebergs formerly landed their rocky burthens on the shores of these
mid-ocean islands, and it is at least possible that they may have
brought thither the seeds of northern plants.
Considering that the several above means of transport, and that
several other means, which without doubt remain to be discovered, have
been in action year after year, for centuries and tens of thousands of
years, it would I think be a marvellous fact if many plants had not
thus become widely transported. These means of transport are sometimes
called accidental, but this is not strictly correct: the currents of
the sea are not accidental, nor is the direction of prevalent gales of
wind. It should be observed that scarcely any means of transport would
carry seeds for very great distances; for seeds do not retain their
vitality when exposed for a great length of time to the action of
seawater; nor could they be long carried in the crops or intestines of
birds. These means, however, would suffice for occasional transport
across tracts of sea some hundred miles in breadth, or from island to
island, or from a continent to a neighbouring island, but not from one
distant continent to another. The floras of distant continents would
not by such means become mingled in any great degree; but would remain
as distinct as we now see them to be. The currents, from their course,
would never bring seeds from North America to Britain, though they
might and do bring seeds from the West Indies to our western shores,
where, if not killed by so long an immersion in salt-water, they could
not endure our climate. Almost every year, one or two land-birds are
blown across the whole Atlantic Ocean, from North America to the
western shores of Ireland and England; but seeds could be transported
by these wanderers only by one means, namely, in dirt sticking to
their feet, which is in itself a rare accident. Even in this case, how
small would the chance be of a seed falling on favourable soil, and
coming to maturity! But it would be a great error to argue that
because a well-stocked island, like Great Britain, has not, as far as
is known (and it would be very difficult to prove this), received
within the last few centuries, through occasional means of transport,
immigrants from Europe or any other continent, that a poorly-stocked
island, though standing more remote from the mainland, would not
receive colonists by similar means. I do not doubt that out of twenty
seeds or animals transported to an island, even if far less
well-stocked than Britain, scarcely more than one would be so well
fitted to its new home, as to become naturalised. But this, as it
seems to me, is no valid argument against what would be effected by
occasional means of transport, during the long lapse of geological
time, whilst an island was being upheaved and formed, and before it
had become fully stocked with inhabitants. On almost bare land, with
few or no destructive insects or birds living there, nearly every
seed, which chanced to arrive, would be sure to germinate and survive.
DISPERSAL DURING THE GLACIAL PERIOD.
The identity of many plants and animals, on mountain-summits,
separated from each other by hundreds of miles of lowlands, where the
Alpine species could not possibly exist, is one of the most striking
cases known of the same species living at distant points, without the
apparent possibility of their having migrated from one to the other.
It is indeed a remarkable fact to see so many of the same plants
living on the snowy regions of the Alps or Pyrenees, and in the
extreme northern parts of Europe; but it is far more remarkable, that
the plants on the White Mountains, in the United States of America,
are all the same with those of Labrador, and nearly all the same, as
we hear from Asa Gray, with those on the loftiest mountains of Europe.
Even as long ago as 1747, such facts led Gmelin to conclude that the
same species must have been independently created at several distinct
points; and we might have remained in this same belief, had not
Agassiz and others called vivid attention to the Glacial period,
which, as we shall immediately see, affords a simple explanation of
these facts. We have evidence of almost every conceivable kind,
organic and inorganic, that within a very recent geological period,
central Europe and North America suffered under an Arctic climate. The
ruins of a house burnt by fire do not tell their tale more plainly,
than do the mountains of Scotland and Wales, with their scored flanks,
polished surfaces, and perched boulders, of the icy streams with which
their valleys were lately filled. So greatly has the climate of Europe
changed, that in Northern Italy, gigantic moraines, left by old
glaciers, are now clothed by the vine and maize. Throughout a large
part of the United States, erratic boulders, and rocks scored by
drifted icebergs and coast-ice, plainly reveal a former cold period.
The former influence of the glacial climate on the distribution of the
inhabitants of Europe, as explained with remarkable clearness by
Edward Forbes, is substantially as follows. But we shall follow the
changes more readily, by supposing a new glacial period to come slowly
on, and then pass away, as formerly occurred. As the cold came on, and
as each more southern zone became fitted for arctic beings and
ill-fitted for their former more temperate inhabitants, the latter
would be supplanted and arctic productions would take their places.
The inhabitants of the more temperate regions would at the same time
travel southward, unless they were stopped by barriers, in which case
they would perish. The mountains would become covered with snow and
ice, and their former Alpine inhabitants would descend to the plains.
By the time that the cold had reached its maximum, we should have a
uniform arctic fauna and flora, covering the central parts of Europe,
as far south as the Alps and Pyrenees, and even stretching into Spain.
The now temperate regions of the United States would likewise be
covered by arctic plants and animals, and these would be nearly the
same with those of Europe; for the present circumpolar inhabitants,
which we suppose to have everywhere travelled southward, are
remarkably uniform round the world. We may suppose that the Glacial
period came on a little earlier or later in North America than in
Europe, so will the southern migration there have been a little
earlier or later; but this will make no difference in the final
result.
As the warmth returned, the arctic forms would retreat northward,
closely followed up in their retreat by the productions of the more
temperate regions. And as the snow melted from the bases of the
mountains, the arctic forms would seize on the cleared and thawed
ground, always ascending higher and higher, as the warmth increased,
whilst their brethren were pursuing their northern journey. Hence,
when the warmth had fully returned, the same arctic species, which had
lately lived in a body together on the lowlands of the Old and New
Worlds, would be left isolated on distant mountain-summits (having
been exterminated on all lesser heights) and in the arctic regions of
both hemispheres.
Thus we can understand the identity of many plants at points so
immensely remote as on the mountains of the United States and of
Europe. We can thus also understand the fact that the Alpine plants of
each mountain-range are more especially related to the arctic forms
living due north or nearly due north of them: for the migration as the
cold came on, and the re-migration on the returning warmth, will
generally have been due south and north. The Alpine plants, for
example, of Scotland, as remarked by Mr. H. C. Watson, and those of
the Pyrenees, as remarked by Ramond, are more especially allied to the
plants of northern Scandinavia; those of the United States to
Labrador; those of the mountains of Siberia to the arctic regions of
that country. These views, grounded as they are on the perfectly
well-ascertained occurrence of a former Glacial period, seem to me to
explain in so satisfactory a manner the present distribution of the
Alpine and Arctic productions of Europe and America, that when in
other regions we find the same species on distant mountain-summits, we
may almost conclude without other evidence, that a colder climate
permitted their former migration across the low intervening tracts,
since become too warm for their existence.
If the climate, since the Glacial period, has ever been in any degree
warmer than at present (as some geologists in the United States
believe to have been the case, chiefly from the distribution of the
fossil Gnathodon), then the arctic and temperate productions will at a
very late period have marched a little further north, and subsequently
have retreated to their present homes; but I have met with no
satisfactory evidence with respect to this intercalated slightly
warmer period, since the Glacial period.
The arctic forms, during their long southern migration and
re-migration northward, will have been exposed to nearly the same
climate, and, as is especially to be noticed, they will have kept in a
body together; consequently their mutual relations will not have been
much disturbed, and, in accordance with the principles inculcated in
this volume, they will not have been liable to much modification. But
with our Alpine productions, left isolated from the moment of the
returning warmth, first at the bases and ultimately on the summits of
the mountains, the case will have been somewhat different; for it is
not likely that all the same arctic species will have been left on
mountain ranges distant from each other, and have survived there ever
since; they will, also, in all probability have become mingled with
ancient Alpine species, which must have existed on the mountains
before the commencement of the Glacial epoch, and which during its
coldest period will have been temporarily driven down to the plains;
they will, also, have been exposed to somewhat different climatal
influences. Their mutual relations will thus have been in some degree
disturbed; consequently they will have been liable to modification;
and this we find has been the case; for if we compare the present
Alpine plants and animals of the several great European
mountain-ranges, though very many of the species are identically the
same, some present varieties, some are ranked as doubtful forms, and
some few are distinct yet closely allied or representative species.
In illustrating what, as I believe, actually took place during the
Glacial period, I assumed that at its commencement the arctic
productions were as uniform round the polar regions as they are at the
present day. But the foregoing remarks on distribution apply not only
to strictly arctic forms, but also to many sub-arctic and to some few
northern temperate forms, for some of these are the same on the lower
mountains and on the plains of North America and Europe; and it may be
reasonably asked how I account for the necessary degree of uniformity
of the sub-arctic and northern temperate forms round the world, at the
commencement of the Glacial period. At the present day, the sub-arctic
and northern temperate productions of the Old and New Worlds are
separated from each other by the Atlantic Ocean and by the extreme
northern part of the Pacific. During the Glacial period, when the
inhabitants of the Old and New Worlds lived further southwards than at
present, they must have been still more completely separated by wider
spaces of ocean. I believe the above difficulty may be surmounted by
looking to still earlier changes of climate of an opposite nature. We
have good reason to believe that during the newer Pliocene period,
before the Glacial epoch, and whilst the majority of the inhabitants
of the world were specifically the same as now, the climate was warmer
than at the present day. Hence we may suppose that the organisms now
living under the climate of latitude 60 deg, during the Pliocene
period lived further north under the Polar Circle, in latitude 66
deg-67 deg; and that the strictly arctic productions then lived on the
broken land still nearer to the pole. Now if we look at a globe, we
shall see that under the Polar Circle there is almost continuous land
from western Europe, through Siberia, to eastern America. And to this
continuity of the circumpolar land, and to the consequent freedom for
intermigration under a more favourable climate, I attribute the
necessary amount of uniformity in the sub-arctic and northern
temperate productions of the Old and New Worlds, at a period anterior
to the Glacial epoch.
Believing, from reasons before alluded to, that our continents have
long remained in nearly the same relative position, though subjected
to large, but partial oscillations of level, I am strongly inclined to
extend the above view, and to infer that during some earlier and still
warmer period, such as the older Pliocene period, a large number of
the same plants and animals inhabited the almost continuous
circumpolar land; and that these plants and animals, both in the Old
and New Worlds, began slowly to migrate southwards as the climate
became less warm, long before the commencement of the Glacial period.
We now see, as I believe, their descendants, mostly in a modified
condition, in the central parts of Europe and the United States. On
this view we can understand the relationship, with very little
identity, between the productions of North America and Europe,--a
relationship which is most remarkable, considering the distance of the
two areas, and their separation by the Atlantic Ocean. We can further
understand the singular fact remarked on by several observers, that
the productions of Europe and America during the later tertiary stages
were more closely related to each other than they are at the present
time; for during these warmer periods the northern parts of the Old
and New Worlds will have been almost continuously united by land,
serving as a bridge, since rendered impassable by cold, for the
inter-migration of their inhabitants.
During the slowly decreasing warmth of the Pliocene period, as soon as
the species in common, which inhabited the New and Old Worlds,
migrated south of the Polar Circle, they must have been completely cut
off from each other. This separation, as far as the more temperate
productions are concerned, took place long ages ago. And as the plants
and animals migrated southward, they will have become mingled in the
one great region with the native American productions, and have had to
compete with them; and in the other great region, with those of the
Old World. Consequently we have here everything favourable for much
modification,--for far more modification than with the Alpine
productions, left isolated, within a much more recent period, on the
several mountain-ranges and on the arctic lands of the two Worlds.
Hence it has come, that when we compare the now living productions of
the temperate regions of the New and Old Worlds, we find very few
identical species (though Asa Gray has lately shown that more plants
are identical than was formerly supposed), but we find in every great
class many forms, which some naturalists rank as geographical races,
and others as distinct species; and a host of closely allied or
representative forms which are ranked by all naturalists as
specifically distinct.
As on the land, so in the waters of the sea, a slow southern migration
of a marine fauna, which during the Pliocene or even a somewhat
earlier period, was nearly uniform along the continuous shores of the
Polar Circle, will account, on the theory of modification, for many
closely allied forms now living in areas completely sundered. Thus, I
think, we can understand the presence of many existing and tertiary
representative forms on the eastern and western shores of temperate
North America; and the still more striking case of many closely allied
crustaceans (as described in Dana's admirable work), of some fish and
other marine animals, in the Mediterranean and in the seas of
Japan,--areas now separated by a continent and by nearly a hemisphere
of equatorial ocean.
These cases of relationship, without identity, of the inhabitants of
seas now disjoined, and likewise of the past and present inhabitants
of the temperate lands of North America and Europe, are inexplicable
on the theory of creation. We cannot say that they have been created
alike, in correspondence with the nearly similar physical conditions
of the areas; for if we compare, for instance, certain parts of South
America with the southern continents of the Old World, we see
countries closely corresponding in all their physical conditions, but
with their inhabitants utterly dissimilar.
But we must return to our more immediate subject, the Glacial period.
I am convinced that Forbes's view may be largely extended. In Europe
we have the plainest evidence of the cold period, from the western
shores of Britain to the Oural range, and southward to the Pyrenees.
We may infer, from the frozen mammals and nature of the mountain
vegetation, that Siberia was similarly affected. Along the Himalaya,
at points 900 miles apart, glaciers have left the marks of their
former low descent; and in Sikkim, Dr. Hooker saw maize growing on
gigantic ancient moraines. South of the equator, we have some direct
evidence of former glacial action in New Zealand; and the same plants,
found on widely separated mountains in this island, tell the same
story. If one account which has been published can be trusted, we have
direct evidence of glacial action in the south-eastern corner of
Australia.
Looking to America; in the northern half, ice-borne fragments of rock
have been observed on the eastern side as far south as lat. 36 deg-37
deg, and on the shores of the Pacific, where the climate is now so
different, as far south as lat. 46 deg; erratic boulders have, also,
been noticed on the Rocky Mountains. In the Cordillera of Equatorial
South America, glaciers once extended far below their present level.
In central Chile I was astonished at the structure of a vast mound of
detritus, about 800 feet in height, crossing a valley of the Andes;
and this I now feel convinced was a gigantic moraine, left far below
any existing glacier. Further south on both sides of the continent,
from lat. 41 deg to the southernmost extremity, we have the clearest
evidence of former glacial action, in huge boulders transported far
from their parent source.
We do not know that the Glacial epoch was strictly simultaneous at
these several far distant points on opposite sides of the world. But
we have good evidence in almost every case, that the epoch was
included within the latest geological period. We have, also, excellent
evidence, that it endured for an enormous time, as measured by years,
at each point. The cold may have come on, or have ceased, earlier at
one point of the globe than at another, but seeing that it endured for
long at each, and that it was contemporaneous in a geological sense,
it seems to me probable that it was, during a part at least of the
period, actually simultaneous throughout the world. Without some
distinct evidence to the contrary, we may at least admit as probable
that the glacial action was simultaneous on the eastern and western
sides of North America, in the Cordillera under the equator and under
the warmer temperate zones, and on both sides of the southern
extremity of the continent. If this be admitted, it is difficult to
avoid believing that the temperature of the whole world was at this
period simultaneously cooler. But it would suffice for my purpose, if
the temperature was at the same time lower along certain broad belts
of longitude.
On this view of the whole world, or at least of broad longitudinal
belts, having been simultaneously colder from pole to pole, much light
can be thrown on the present distribution of identical and allied
species. In America, Dr. Hooker has shown that between forty and fifty
of the flowering plants of Tierra del Fuego, forming no inconsiderable
part of its scanty flora, are common to Europe, enormously remote as
these two points are; and there are many closely allied species. On
the lofty mountains of equatorial America a host of peculiar species
belonging to European genera occur. On the highest mountains of
Brazil, some few European genera were found by Gardner, which do not
exist in the wide intervening hot countries. So on the Silla of
Caraccas the illustrious Humboldt long ago found species belonging to
genera characteristic of the Cordillera. On the mountains of
Abyssinia, several European forms and some few representatives of the
peculiar flora of the Cape of Good Hope occur. At the Cape of Good
Hope a very few European species, believed not to have been introduced
by man, and on the mountains, some few representative European forms
are found, which have not been discovered in the intertropical parts
of Africa. On the Himalaya, and on the isolated mountain-ranges of the
peninsula of India, on the heights of Ceylon, and on the volcanic
cones of Java, many plants occur, either identically the same or
representing each other, and at the same time representing plants of
Europe, not found in the intervening hot lowlands. A list of the
genera collected on the loftier peaks of Java raises a picture of a
collection made on a hill in Europe! Still more striking is the fact
that southern Australian forms are clearly represented by plants
growing on the summits of the mountains of Borneo. Some of these
Australian forms, as I hear from Dr. Hooker, extend along the heights
of the peninsula of Malacca, and are thinly scattered, on the one hand
over India and on the other as far north as Japan.
On the southern mountains of Australia, Dr. F. Muller has discovered
several European species; other species, not introduced by man, occur
on the lowlands; and a long list can be given, as I am informed by Dr.
Hooker, of European genera, found in Australia, but not in the
intermediate torrid regions. In the admirable 'Introduction to the
Flora of New Zealand,' by Dr. Hooker, analogous and striking facts are
given in regard to the plants of that large island. Hence we see that
throughout the world, the plants growing on the more lofty mountains,
and on the temperate lowlands of the northern and southern
hemispheres, are sometimes identically the same; but they are much
oftener specifically distinct, though related to each other in a most
remarkable manner.
This brief abstract applies to plants alone: some strictly analogous
facts could be given on the distribution of terrestrial animals. In
marine productions, similar cases occur; as an example, I may quote a
remark by the highest authority, Professor Dana, that "it is certainly
a wonderful fact that New Zealand should have a closer resemblance in
its crustacea to Great Britain, its antipode, than to any other part
of the world." Sir J. Richardson, also, speaks of the reappearance on
the shores of New Zealand, Tasmania, etc., of northern forms of fish.
Dr. Hooker informs me that twenty-five species of Algae are common to
New Zealand and to Europe, but have not been found in the intermediate
tropical seas.
It should be observed that the northern species and forms found in the
southern parts of the southern hemisphere, and on the mountain-ranges
of the intertropical regions, are not arctic, but belong to the
northern temperate zones. As Mr. H. C. Watson has recently remarked,
"In receding from polar towards equatorial latitudes, the Alpine or
mountain floras really become less and less arctic." Many of the forms
living on the mountains of the warmer regions of the earth and in the
southern hemisphere are of doubtful value, being ranked by some
naturalists as specifically distinct, by others as varieties; but some
are certainly identical, and many, though closely related to northern
forms, must be ranked as distinct species.
Now let us see what light can be thrown on the foregoing facts, on the
belief, supported as it is by a large body of geological evidence,
that the whole world, or a large part of it, was during the Glacial
period simultaneously much colder than at present. The Glacial period,
as measured by years, must have been very long; and when we remember
over what vast spaces some naturalised plants and animals have spread
within a few centuries, this period will have been ample for any
amount of migration. As the cold came slowly on, all the tropical
plants and other productions will have retreated from both sides
towards the equator, followed in the rear by the temperate
productions, and these by the arctic; but with the latter we are not
now concerned. The tropical plants probably suffered much extinction;
how much no one can say; perhaps formerly the tropics supported as
many species as we see at the present day crowded together at the Cape
of Good Hope, and in parts of temperate Australia. As we know that
many tropical plants and animals can withstand a considerable amount
of cold, many might have escaped extermination during a moderate fall
of temperature, more especially by escaping into the warmest spots.
But the great fact to bear in mind is, that all tropical productions
will have suffered to a certain extent. On the other hand, the
temperate productions, after migrating nearer to the equator, though
they will have been placed under somewhat new conditions, will have
suffered less. And it is certain that many temperate plants, if
protected from the inroads of competitors, can withstand a much warmer
climate than their own. Hence, it seems to me possible, bearing in
mind that the tropical productions were in a suffering state and could
not have presented a firm front against intruders, that a certain
number of the more vigorous and dominant temperate forms might have
penetrated the native ranks and have reached or even crossed the
equator. The invasion would, of course, have been greatly favoured by
high land, and perhaps by a dry climate; for Dr. Falconer informs me
that it is the damp with the heat of the tropics which is so
destructive to perennial plants from a temperate climate. On the other
hand, the most humid and hottest districts will have afforded an
asylum to the tropical natives. The mountain-ranges north-west of the
Himalaya, and the long line of the Cordillera, seem to have afforded
two great lines of invasion: and it is a striking fact, lately
communicated to me by Dr. Hooker, that all the flowering plants, about
forty-six in number, common to Tierra del Fuego and to Europe still
exist in North America, which must have lain on the line of march. But
I do not doubt that some temperate productions entered and crossed
even the LOWLANDS of the tropics at the period when the cold was most
intense,--when arctic forms had migrated some twenty-five degrees of
latitude from their native country and covered the land at the foot of
the Pyrenees. At this period of extreme cold, I believe that the
climate under the equator at the level of the sea was about the same
with that now felt there at the height of six or seven thousand feet.
During this the coldest period, I suppose that large spaces of the
tropical lowlands were clothed with a mingled tropical and temperate
vegetation, like that now growing with strange luxuriance at the base
of the Himalaya, as graphically described by Hooker.
Thus, as I believe, a considerable number of plants, a few terrestrial
animals, and some marine productions, migrated during the Glacial
period from the northern and southern temperate zones into the
intertropical regions, and some even crossed the equator. As the
warmth returned, these temperate forms would naturally ascend the
higher mountains, being exterminated on the lowlands; those which had
not reached the equator, would re-migrate northward or southward
towards their former homes; but the forms, chiefly northern, which had
crossed the equator, would travel still further from their homes into
the more temperate latitudes of the opposite hemisphere. Although we
have reason to believe from geological evidence that the whole body of
arctic shells underwent scarcely any modification during their long
southern migration and re-migration northward, the case may have been
wholly different with those intruding forms which settled themselves
on the intertropical mountains, and in the southern hemisphere. These
being surrounded by strangers will have had to compete with many new
forms of life; and it is probable that selected modifications in their
structure, habits, and constitutions will have profited them. Thus
many of these wanderers, though still plainly related by inheritance
to their brethren of the northern or southern hemispheres, now exist
in their new homes as well-marked varieties or as distinct species.
It is a remarkable fact, strongly insisted on by Hooker in regard to
America, and by Alph. de Candolle in regard to Australia, that many
more identical plants and allied forms have apparently migrated from
the north to the south, than in a reversed direction. We see, however,
a few southern vegetable forms on the mountains of Borneo and
Abyssinia. I suspect that this preponderant migration from north to
south is due to the greater extent of land in the north, and to the
northern forms having existed in their own homes in greater numbers,
and having consequently been advanced through natural selection and
competition to a higher stage of perfection or dominating power, than
the southern forms. And thus, when they became commingled during the
Glacial period, the northern forms were enabled to beat the less
powerful southern forms. Just in the same manner as we see at the
present day, that very many European productions cover the ground in
La Plata, and in a lesser degree in Australia, and have to a certain
extent beaten the natives; whereas extremely few southern forms have
become naturalised in any part of Europe, though hides, wool, and
other objects likely to carry seeds have been largely imported into
Europe during the last two or three centuries from La Plata, and
during the last thirty or forty years from Australia. Something of the
same kind must have occurred on the intertropical mountains: no doubt
before the Glacial period they were stocked with endemic Alpine forms;
but these have almost everywhere largely yielded to the more dominant
forms, generated in the larger areas and more efficient workshops of
the north. In many islands the native productions are nearly equalled
or even outnumbered by the naturalised; and if the natives have not
been actually exterminated, their numbers have been greatly reduced,
and this is the first stage towards extinction. A mountain is an
island on the land; and the intertropical mountains before the Glacial
period must have been completely isolated; and I believe that the
productions of these islands on the land yielded to those produced
within the larger areas of the north, just in the same way as the
productions of real islands have everywhere lately yielded to
continental forms, naturalised by man's agency.
I am far from supposing that all difficulties are removed on the view
here given in regard to the range and affinities of the allied species
which live in the northern and southern temperate zones and on the
mountains of the intertropical regions. Very many difficulties remain
to be solved. I do not pretend to indicate the exact lines and means
of migration, or the reason why certain species and not others have
migrated; why certain species have been modified and have given rise
to new groups of forms, and others have remained unaltered. We cannot
hope to explain such facts, until we can say why one species and not
another becomes naturalised by man's agency in a foreign land; why one
ranges twice or thrice as far, and is twice or thrice as common, as
another species within their own homes.
I have said that many difficulties remain to be solved: some of the
most remarkable are stated with admirable clearness by Dr. Hooker in
his botanical works on the antarctic regions. These cannot be here
discussed. I will only say that as far as regards the occurrence of
identical species at points so enormously remote as Kerguelen Land,
New Zealand, and Fuegia, I believe that towards the close of the
Glacial period, icebergs, as suggested by Lyell, have been largely
concerned in their dispersal. But the existence of several quite
distinct species, belonging to genera exclusively confined to the
south, at these and other distant points of the southern hemisphere,
is, on my theory of descent with modification, a far more remarkable
case of difficulty. For some of these species are so distinct, that we
cannot suppose that there has been time since the commencement of the
Glacial period for their migration, and for their subsequent
modification to the necessary degree. The facts seem to me to indicate
that peculiar and very distinct species have migrated in radiating
lines from some common centre; and I am inclined to look in the
southern, as in the northern hemisphere, to a former and warmer
period, before the commencement of the Glacial period, when the
antarctic lands, now covered with ice, supported a highly peculiar and
isolated flora. I suspect that before this flora was exterminated by
the Glacial epoch, a few forms were widely dispersed to various points
of the southern hemisphere by occasional means of transport, and by
the aid, as halting-places, of existing and now sunken islands, and
perhaps at the commencement of the Glacial period, by icebergs. By
these means, as I believe, the southern shores of America, Australia,
New Zealand have become slightly tinted by the same peculiar forms of
vegetable life.
Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate
on geographical distribution. I believe that the world has recently
felt one of his great cycles of change; and that on this view,
combined with modification through natural selection, a multitude of
facts in the present distribution both of the same and of allied forms
of life can be explained. The living waters may be said to have flowed
during one short period from the north and from the south, and to have
crossed at the equator; but to have flowed with greater force from the
north so as to have freely inundated the south. As the tide leaves its
drift in horizontal lines, though rising higher on the shores where
the tide rises highest, so have the living waters left their living
drift on our mountain-summits, in a line gently rising from the arctic
lowlands to a great height under the equator. The various beings thus
left stranded may be compared with savage races of man, driven up and
surviving in the mountain-fastnesses of almost every land, which serve
as a record, full of interest to us, of the former inhabitants of the
surrounding lowlands.
CHAPTER 12. GEOGRAPHICAL DISTRIBUTION--continued.
Distribution of fresh-water productions.
On the inhabitants of oceanic islands.
Absence of Batrachians and of terrestrial Mammals.
On the relation of the inhabitants of islands to those of the nearest
mainland.
On colonisation from the nearest source with subsequent modification.
Summary of the last and present chapters.
As lakes and river-systems are separated from each other by barriers
of land, it might have been thought that fresh-water productions would
not have ranged widely within the same country, and as the sea is
apparently a still more impassable barrier, that they never would have
extended to distant countries. But the case is exactly the reverse.
Not only have many fresh-water species, belonging to quite different
classes, an enormous range, but allied species prevail in a remarkable
manner throughout the world. I well remember, when first collecting in
the fresh waters of Brazil, feeling much surprise at the similarity of
the fresh-water insects, shells, etc., and at the dissimilarity of the
surrounding terrestrial beings, compared with those of Britain.
But this power in fresh-water productions of ranging widely, though so
unexpected, can, I think, in most cases be explained by their having
become fitted, in a manner highly useful to them, for short and
frequent migrations from pond to pond, or from stream to stream; and
liability to wide dispersal would follow from this capacity as an
almost necessary consequence. We can here consider only a few cases.
In regard to fish, I believe that the same species never occur in the
fresh waters of distant continents. But on the same continent the
species often range widely and almost capriciously; for two
river-systems will have some fish in common and some different. A few
facts seem to favour the possibility of their occasional transport by
accidental means; like that of the live fish not rarely dropped by
whirlwinds in India, and the vitality of their ova when removed from
the water. But I am inclined to attribute the dispersal of fresh-water
fish mainly to slight changes within the recent period in the level of
the land, having caused rivers to flow into each other. Instances,
also, could be given of this having occurred during floods, without
any change of level. We have evidence in the loess of the Rhine of
considerable changes of level in the land within a very recent
geological period, and when the surface was peopled by existing land
and fresh-water shells. The wide difference of the fish on opposite
sides of continuous mountain-ranges, which from an early period must
have parted river-systems and completely prevented their inosculation,
seems to lead to this same conclusion. With respect to allied
fresh-water fish occurring at very distant points of the world, no
doubt there are many cases which cannot at present be explained: but
some fresh-water fish belong to very ancient forms, and in such cases
there will have been ample time for great geographical changes, and
consequently time and means for much migration. In the second place,
salt-water fish can with care be slowly accustomed to live in fresh
water; and, according to Valenciennes, there is hardly a single group
of fishes confined exclusively to fresh water, so that we may imagine
that a marine member of a fresh-water group might travel far along the
shores of the sea, and subsequently become modified and adapted to the
fresh waters of a distant land.
Some species of fresh-water shells have a very wide range, and allied
species, which, on my theory, are descended from a common parent and
must have proceeded from a single source, prevail throughout the
world. Their distribution at first perplexed me much, as their ova are
not likely to be transported by birds, and they are immediately killed
by sea water, as are the adults. I could not even understand how some
naturalised species have rapidly spread throughout the same country.
But two facts, which I have observed--and no doubt many others remain
to be observed--throw some light on this subject. When a duck suddenly
emerges from a pond covered with duck-weed, I have twice seen these
little plants adhering to its back; and it has happened to me, in
removing a little duck-weed from one aquarium to another, that I have
quite unintentionally stocked the one with fresh-water shells from the
other. But another agency is perhaps more effectual: I suspended a
duck's feet, which might represent those of a bird sleeping in a
natural pond, in an aquarium, where many ova of fresh-water shells
were hatching; and I found that numbers of the extremely minute and
just hatched shells crawled on the feet, and clung to them so firmly
that when taken out of the water they could not be jarred off, though
at a somewhat more advanced age they would voluntarily drop off. These
just hatched molluscs, though aquatic in their nature, survived on the
duck's feet, in damp air, from twelve to twenty hours; and in this
length of time a duck or heron might fly at least six or seven hundred
miles, and would be sure to alight on a pool or rivulet, if blown
across sea to an oceanic island or to any other distant point. Sir
Charles Lyell also informs me that a Dyticus has been caught with an
Ancylus (a fresh-water shell like a limpet) firmly adhering to it; and
a water-beetle of the same family, a Colymbetes, once flew on board
the 'Beagle,' when forty-five miles distant from the nearest land: how
much farther it might have flown with a favouring gale no one can
tell.
With respect to plants, it has long been known what enormous ranges
many fresh-water and even marsh-species have, both over continents and
to the most remote oceanic islands. This is strikingly shown, as
remarked by Alph. de Candolle, in large groups of terrestrial plants,
which have only a very few aquatic members; for these latter seem
immediately to acquire, as if in consequence, a very wide range. I
think favourable means of dispersal explain this fact. I have before
mentioned that earth occasionally, though rarely, adheres in some
quantity to the feet and beaks of birds. Wading birds, which frequent
the muddy edges of ponds, if suddenly flushed, would be the most
likely to have muddy feet. Birds of this order I can show are the
greatest wanderers, and are occasionally found on the most remote and
barren islands in the open ocean; they would not be likely to alight
on the surface of the sea, so that the dirt would not be washed off
their feet; when making land, they would be sure to fly to their
natural fresh-water haunts. I do not believe that botanists are aware
how charged the mud of ponds is with seeds: I have tried several
little experiments, but will here give only the most striking case: I
took in February three table-spoonfuls of mud from three different
points, beneath water, on the edge of a little pond; this mud when dry
weighed only 6 3/4 ounces; I kept it covered up in my study for six
months, pulling up and counting each plant as it grew; the plants were
of many kinds, and were altogether 537 in number; and yet the viscid
mud was all contained in a breakfast cup! Considering these facts, I
think it would be an inexplicable circumstance if water-birds did not
transport the seeds of fresh-water plants to vast distances, and if
consequently the range of these plants was not very great. The same
agency may have come into play with the eggs of some of the smaller
fresh-water animals.
Other and unknown agencies probably have also played a part. I have
stated that fresh-water fish eat some kinds of seeds, though they
reject many other kinds after having swallowed them; even small fish
swallow seeds of moderate size, as of the yellow water-lily and
Potamogeton. Herons and other birds, century after century, have gone
on daily devouring fish; they then take flight and go to other waters,
or are blown across the sea; and we have seen that seeds retain their
power of germination, when rejected in pellets or in excrement, many
hours afterwards. When I saw the great size of the seeds of that fine
water-lily, the Nelumbium, and remembered Alph. de Candolle's remarks
on this plant, I thought that its distribution must remain quite
inexplicable; but Audubon states that he found the seeds of the great
southern water-lily (probably, according to Dr. Hooker, the Nelumbium
luteum) in a heron's stomach; although I do not know the fact, yet
analogy makes me believe that a heron flying to another pond and
getting a hearty meal of fish, would probably reject from its stomach
a pellet containing the seeds of the Nelumbium undigested; or the
seeds might be dropped by the bird whilst feeding its young, in the
same way as fish are known sometimes to be dropped.
In considering these several means of distribution, it should be
remembered that when a pond or stream is first formed, for instance,
on a rising islet, it will be unoccupied; and a single seed or egg
will have a good chance of succeeding. Although there will always be a
struggle for life between the individuals of the species, however few,
already occupying any pond, yet as the number of kinds is small,
compared with those on the land, the competition will probably be less
severe between aquatic than between terrestrial species; consequently
an intruder from the waters of a foreign country, would have a better
chance of seizing on a place, than in the case of terrestrial
colonists. We should, also, remember that some, perhaps many,
fresh-water productions are low in the scale of nature, and that we
have reason to believe that such low beings change or become modified
less quickly than the high; and this will give longer time than the
average for the migration of the same aquatic species. We should not
forget the probability of many species having formerly ranged as
continuously as fresh-water productions ever can range, over immense
areas, and having subsequently become extinct in intermediate regions.
But the wide distribution of fresh-water plants and of the lower
animals, whether retaining the same identical form or in some degree
modified, I believe mainly depends on the wide dispersal of their
seeds and eggs by animals, more especially by fresh-water birds, which
have large powers of flight, and naturally travel from one to another
and often distant piece of water. Nature, like a careful gardener,
thus takes her seeds from a bed of a particular nature, and drops them
in another equally well fitted for them.
ON THE INHABITANTS OF OCEANIC ISLANDS.
We now come to the last of the three classes of facts, which I have
selected as presenting the greatest amount of difficulty, on the view
that all the individuals both of the same and of allied species have
descended from a single parent; and therefore have all proceeded from
a common birthplace, notwithstanding that in the course of time they
have come to inhabit distant points of the globe. I have already
stated that I cannot honestly admit Forbes's view on continental
extensions, which, if legitimately followed out, would lead to the
belief that within the recent period all existing islands have been
nearly or quite joined to some continent. This view would remove many
difficulties, but it would not, I think, explain all the facts in
regard to insular productions. In the following remarks I shall not
confine myself to the mere question of dispersal; but shall consider
some other facts, which bear on the truth of the two theories of
independent creation and of descent with modification.
The species of all kinds which inhabit oceanic islands are few in
number compared with those on equal continental areas: Alph. de
Candolle admits this for plants, and Wollaston for insects. If we look
to the large size and varied stations of New Zealand, extending over
780 miles of latitude, and compare its flowering plants, only 750 in
number, with those on an equal area at the Cape of Good Hope or in
Australia, we must, I think, admit that something quite independently
of any difference in physical conditions has caused so great a
difference in number. Even the uniform county of Cambridge has 847
plants, and the little island of Anglesea 764, but a few ferns and a
few introduced plants are included in these numbers, and the
comparison in some other respects is not quite fair. We have evidence
that the barren island of Ascension aboriginally possessed under
half-a-dozen flowering plants; yet many have become naturalised on it,
as they have on New Zealand and on every other oceanic island which
can be named. In St. Helena there is reason to believe that the
naturalised plants and animals have nearly or quite exterminated many
native productions. He who admits the doctrine of the creation of each
separate species, will have to admit, that a sufficient number of the
best adapted plants and animals have not been created on oceanic
islands; for man has unintentionally stocked them from various sources
far more fully and perfectly than has nature.
Although in oceanic islands the number of kinds of inhabitants is
scanty, the proportion of endemic species (i.e. those found nowhere
else in the world) is often extremely large. If we compare, for
instance, the number of the endemic land-shells in Madeira, or of the
endemic birds in the Galapagos Archipelago, with the number found on
any continent, and then compare the area of the islands with that of
the continent, we shall see that this is true. This fact might have
been expected on my theory, for, as already explained, species
occasionally arriving after long intervals in a new and isolated
district, and having to compete with new associates, will be eminently
liable to modification, and will often produce groups of modified
descendants. 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 on the species which do not become modified
having immigrated with facility and in a body, so that their mutual
relations have not been much disturbed. Thus in the Galapagos Islands
nearly every land-bird, but only two out of the eleven marine birds,
are peculiar; and it is obvious that marine birds could arrive at
these islands more easily 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 one endemic land bird; and we know from Mr. J.
M. Jones's admirable account of Bermuda, that very many North American
birds, during their great annual migrations, visit either periodically
or occasionally this island. Madeira does not possess one peculiar
bird, and many European and African birds are almost every year blown
there, as I am informed by Mr. E. V. Harcourt. So that these two
islands of Bermuda and Madeira have been stocked by birds, which for
long ages have struggled together in their former homes, and have
become mutually adapted to each other; and when settled in their new
homes, each kind will have been kept by the others to their proper
places and habits, and will consequently have been little liable to
modification. Madeira, again, is inhabited by a wonderful number of
peculiar land-shells, whereas not one species of sea-shell is confined
to its shores: now, though we do not know how seashells are dispersed,
yet we can see that their eggs or larvae, perhaps attached to seaweed
or floating timber, or to the feet of wading-birds, might be
transported far more easily than land-shells, across three or four
hundred miles of open sea. The different orders of insects in Madeira
apparently present analogous facts.
Oceanic islands are sometimes deficient in certain classes, and their
places are apparently occupied by the other inhabitants; in the
Galapagos Islands reptiles, and in New Zealand gigantic wingless
birds, take the place of mammals. In the plants of the Galapagos
Islands, Dr. Hooker has shown that the proportional numbers of the
different orders are very different from what they are elsewhere. Such
cases are generally accounted for by the physical conditions of the
islands; but this explanation seems to me not a little doubtful.
Facility of immigration, I believe, has been at least as important as
the nature of the conditions.
Many remarkable little facts could be given with respect to the
inhabitants of remote islands. For instance, in certain islands not
tenanted by mammals, some of the endemic plants have beautifully
hooked seeds; yet few relations are more striking than the adaptation
of hooked seeds for transportal by the wool and fur of quadrupeds.
This case presents no difficulty on my view, for a hooked seed might
be transported to an island by some other means; and the plant then
becoming slightly modified, but still retaining its hooked seeds,
would form an endemic species, having as useless an appendage as any
rudimentary organ,--for instance, as the shrivelled wings under the
soldered elytra 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, though it would have no chance of successfully
competing in stature with a fully developed tree, when established on
an island and having to compete with herbaceous plants alone, might
readily gain an advantage by growing taller and taller and overtopping
the other plants. If so, natural selection would often tend to add to
the stature of herbaceous plants when growing on an island, to
whatever order they belonged, and thus convert them first into bushes
and ultimately into trees.
With respect to the absence of whole orders on oceanic islands, Bory
St. Vincent long ago remarked that Batrachians (frogs, toads, newts)
have never been found on any of the many islands with which the great
oceans are studded. I have taken pains to verify this assertion, and I
have found it strictly true. I have, however, been assured that a frog
exists on the mountains of the great island of New Zealand; but I
suspect that this exception (if the information be correct) may be
explained through glacial agency. This general absence of frogs,
toads, and newts on so many oceanic islands cannot be accounted for by
their physical conditions; indeed it seems that islands are peculiarly
well fitted for these animals; for frogs have been introduced into
Madeira, the Azores, and Mauritius, and have multiplied so as to
become a nuisance. But as these animals and their spawn are known to
be immediately killed by sea-water, on my view we can see that there
would be great difficulty in their transportal across the sea, and
therefore why they do not exist on any oceanic island. 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, but have not finished my search; as yet I have not
found a single instance, free from doubt, of a terrestrial mammal
(excluding domesticated animals kept by the natives) inhabiting an
island situated above 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 connected with the mainland; moreover,
icebergs formerly brought boulders to its western shores, and they may
have formerly transported foxes, as so frequently now happens in the
arctic regions. Yet it cannot be said that small islands will not
support small mammals, for they occur in many parts of the world on
very small islands, if close to a continent; and hardly an island can
be named on which our smaller quadrupeds have not become naturalised
and greatly multiplied. It cannot be said, on the ordinary view of
creation, that there has not been time for the creation of mammals;
many volcanic islands are sufficiently ancient, as shown by the
stupendous degradation which they have suffered and by their tertiary
strata: there has also been time for the production of endemic species
belonging to other classes; and on continents it is thought that
mammals appear and disappear at a quicker rate than other and lower
animals. Though terrestrial mammals do not occur on oceanic islands,
aerial mammals do occur on almost every island. New Zealand possesses
two bats found nowhere else in the world: Norfolk Island, the Viti
Archipelago, the Bonin Islands, the Caroline and Marianne
Archipelagoes, and Mauritius, all possess their peculiar bats. Why, it
may be asked, has the supposed creative force produced bats and no
other mammals on remote islands? On my view this question can easily
be answered; for no terrestrial mammal can be transported across a
wide space of sea, but bats can fly across. Bats have been seen
wandering by day far over the Atlantic Ocean; and two North American
species either regularly or occasionally visit Bermuda, at the
distance of 600 miles from the mainland. I hear from Mr. Tomes, who
has specially studied this family, that many of the same species have
enormous ranges, and are found on continents and on far distant
islands. Hence we have only to suppose that such wandering species
have been modified through natural selection in their new homes in
relation to their new position, and we can understand the presence of
endemic bats on islands, with the absence of all terrestrial mammals.
Besides the absence of terrestrial mammals in relation to the
remoteness of islands from continents, there is also a relation, to a
certain extent independent of distance, between the depth of the sea
separating an island from the neighbouring mainland, and the presence
in both of the same mammiferous species or of allied species in a more
or less modified condition. Mr. Windsor Earl has made some striking
observations on this head in regard to the great Malay Archipelago,
which is traversed near Celebes by a space of deep ocean; and this
space separates two widely distinct mammalian faunas. On either side
the islands are situated on moderately deep submarine banks, and they
are inhabited by closely allied or identical quadrupeds. No doubt some
few anomalies occur in this great archipelago, and there is much
difficulty in forming a judgment in some cases owing to the probable
naturalisation of certain mammals through man's agency; but we shall
soon have much light thrown on the natural history of this archipelago
by the admirable zeal and researches of Mr. Wallace. I have not as yet
had time to follow up this subject in all other quarters of the world;
but as far as I have gone, the relation generally holds good. We see
Britain separated by a shallow channel from Europe, and the mammals
are the same on both sides; we meet with analogous facts on many
islands separated by similar channels from Australia. The West Indian
Islands stand on a deeply submerged bank, nearly 1000 fathoms in
depth, and here we find American forms, but the species and even the
genera are distinct. As the amount of modification in all cases
depends to a certain degree on the lapse of time, and as during
changes of level it is obvious that islands separated by shallow
channels are more likely to have been continuously united within a
recent period to the mainland than islands separated by deeper
channels, we can understand the frequent relation between the depth of
the sea and the degree of affinity of the mammalian inhabitants of
islands with those of a neighbouring continent,--an inexplicable
relation on the view of independent acts of creation.
All the foregoing remarks on the inhabitants of oceanic
islands,--namely, the scarcity of kinds--the richness in endemic forms
in particular classes or sections of classes,--the absence of whole
groups, as of batrachians, and of terrestrial mammals notwithstanding
the presence of aerial bats,--the singular proportions of certain
orders of plants,--herbaceous forms having been developed into trees,
etc.,--seem to me to accord better with the view of occasional means
of transport having been largely efficient in the long course of time,
than with the view of all our oceanic islands having been formerly
connected by continuous land with the nearest continent; for on this
latter view the migration would probably have been more complete; and
if modification be admitted, all the forms of life would have been
more equally modified, in accordance with the paramount importance of
the relation of organism to organism.
I do not deny that there are many and grave difficulties in
understanding how several of the inhabitants of the more remote
islands, whether still retaining the same specific form or modified
since their arrival, could have reached their present homes. But the
probability of many islands having existed as halting-places, of which
not a wreck now remains, must not be overlooked. I will here give a
single instance of one of the cases of difficulty. Almost all oceanic
islands, even the most isolated and smallest, are inhabited by
land-shells, generally by endemic species, but sometimes by species
found elsewhere. Dr. Aug. A. Gould has given several interesting cases
in regard to the land-shells of the islands of the Pacific. Now it is
notorious that land-shells are very easily killed by salt; their eggs,
at least such as I have tried, sink in sea-water and are killed by it.
Yet there must be, on my view, some unknown, but highly efficient
means for their transportal. Would the just-hatched young occasionally
crawl on and adhere to the feet of birds roosting on the ground, and
thus get transported? It occurred to me that land-shells, when
hybernating and having a membranous diaphragm over the mouth of the
shell, might be floated in chinks of drifted timber across moderately
wide arms of the sea. And I found that several species did in this
state withstand uninjured an immersion in sea-water during seven days:
one of these shells was the Helix pomatia, and after it had again
hybernated I put it in sea-water for twenty days, and it perfectly
recovered. As this species has a thick calcareous operculum, I removed
it, and when it had formed a new membranous one, I immersed it for
fourteen days in sea-water, and it recovered and crawled away: but
more experiments are wanted on this head. The most striking and
important fact for us in regard to the inhabitants of islands, is
their affinity to those of the nearest mainland, without being
actually the same species. Numerous instances could be given of this
fact. I will give only one, that of the Galapagos Archipelago,
situated under the equator, between 500 and 600 miles from the shores
of South America. Here almost every product of the land and water
bears the unmistakeable stamp of the American continent. There are
twenty-six land birds, and twenty-five of these are ranked by Mr.
Gould as distinct species, supposed to have been created here; yet the
close affinity of most of these birds to American species in every
character, in their habits, gestures, and tones of voice, was
manifest. So it is with the other animals, and with nearly all the
plants, as shown by Dr. Hooker in his admirable memoir on the Flora of
this archipelago. The naturalist, looking at the inhabitants of these
volcanic islands in the Pacific, distant several hundred miles from
the continent, yet feels that he is standing on American land. Why
should this be so? why should the species which are supposed to have
been created in the Galapagos Archipelago, and nowhere else, bear so
plain a stamp of affinity to those created in America? There is
nothing in the conditions of life, in the geological nature of the
islands, in their height or climate, or in the proportions in which
the several classes are associated together, which resembles closely
the conditions of the South American coast: in fact there is a
considerable dissimilarity in all these respects. On the other hand,
there is a considerable degree of resemblance in the volcanic nature
of the soil, in climate, height, and size of the islands, between the
Galapagos and Cape de Verde Archipelagos: but what an entire and
absolute difference in their inhabitants! The inhabitants of the Cape
de Verde Islands are related to those of Africa, like those of the
Galapagos to America. I believe this grand fact can receive no sort of
explanation on the ordinary view of independent creation; whereas on
the view here maintained, it is obvious that the Galapagos Islands
would be likely to receive colonists, whether by occasional means of
transport or by formerly continuous land, from America; and the Cape
de Verde Islands from Africa; and that such colonists would be liable
to modification;--the principle of inheritance still betraying their
original birthplace.
Many analogous facts could be given: indeed it is an almost universal
rule that the endemic productions of islands are related to those of
the nearest continent, or of other near islands. The exceptions are
few, and most of them can be explained. Thus the plants of Kerguelen
Land, though standing nearer to Africa than to America, are related,
and that very closely, as we know from Dr. Hooker's account, to those
of America: but on the view that this island has been mainly stocked
by seeds brought with earth and stones on icebergs, drifted by the
prevailing currents, this anomaly disappears. New Zealand in its
endemic plants is much more closely related to Australia, the nearest
mainland, than to any other region: and this is what might have been
expected; but it is also plainly related to South America, which,
although the next nearest continent, is so enormously remote, that the
fact becomes an anomaly. But this difficulty almost disappears on the
view that both New Zealand, South America, and other southern lands
were long ago partially stocked from a nearly intermediate though
distant point, namely from the antarctic islands, when they were
clothed with vegetation, before the commencement of the Glacial
period. The affinity, which, though feeble, I am assured by Dr. Hooker
is real, between the flora of the south-western corner of Australia
and of the Cape of Good Hope, is a far more remarkable case, and is at
present inexplicable: but this affinity is confined to the plants, and
will, I do not doubt, be some day explained.
The law which causes the inhabitants of an archipelago, though
specifically distinct, to be closely allied to those of the nearest
continent, we sometimes see displayed on a small scale, yet in a most
interesting manner, within the limits of the same archipelago. Thus
the several islands of the Galapagos Archipelago are tenanted, as I
have elsewhere shown, in a quite marvellous manner, by very closely
related species; so that the inhabitants of each separate island,
though mostly distinct, are related in an incomparably closer degree
to each other than to the inhabitants of any other part of the world.
And this is just what might have been expected on my view, for the
islands are situated so near each other that they would almost
certainly receive immigrants from the same original source, or from
each other. But this dissimilarity between the endemic inhabitants of
the islands may be used as an argument against my views; for it may be
asked, how has it happened in the several islands situated within
sight of each other, having the same geological nature, the same
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