This relation between the power and extent of migration of a species,
either at the present time or at some former period under different
physical conditions, and the existence at remote points of the world of
other species allied to it, is shown in another and more general way. Mr.
Gould remarked to me long ago, that in those genera of birds which range
over the world, many of the species have very wide ranges. I can hardly
doubt that this rule is generally true, though it would be difficult to
prove it. Amongst mammals, we see it strikingly displayed in Bats, and in
a lesser degree in the Felidae and Canidae. We see it, if we compare the
distribution of butterflies and beetles. So it is with most fresh-water
productions, in which so many genera range over the world, and many
individual species have enormous ranges. It is not meant that in
world-ranging genera all the species have a wide range, or even that they
have on an average a wide range; but only that some of the species range
very widely; for the facility with which widely-ranging species vary and
give rise to new forms will largely determine their average range. For
instance, two varieties of the same species inhabit America and Europe, and
the species thus has an immense range; but, if the variation had been a
little greater, the two varieties would have been ranked as distinct
species, and the common range would have been greatly reduced. Still less
is it meant, that a species which apparently has the capacity of crossing
barriers and ranging widely, as in the case of certain powerfully-winged
birds, will necessarily range widely; for we should never forget that to
range widely implies not only the power of crossing barriers, but the more
important power of being victorious in distant lands in the struggle for
life with foreign associates. But on the view of all the species of a
genus having descended from a single parent, though now distributed to the
most remote points of the world, we ought to find, and I believe as a
general rule we do find, that some at least of the species range very
widely; for it is necessary that the unmodified parent should range widely,
undergoing modification during its diffusion, and should place itself under
diverse conditions favourable for the conversion of its offspring, firstly
into new varieties and ultimately into new species.
In considering the wide distribution of certain genera, we should bear in
mind that some are extremely ancient, and must have branched off from a
common parent at a remote epoch; so that in such cases there will have been
ample time for great climatal and geographical changes and for accidents of
transport; and consequently for the migration of some of the species into
all quarters of the world, where they may have become slightly modified in
relation to their new conditions. There is, also, some reason to believe
from geological evidence that organisms low in the scale within each great
class, generally change at a slower rate than the higher forms; and
consequently the lower forms will have had a better chance of ranging
widely and of still retaining the same specific character. This fact,
together with the seeds and eggs of many low forms being very minute and
better fitted for distant transportation, probably accounts for a law which
has long been observed, and which has lately been admirably discussed by
Alph. de Candolle in regard to plants, namely, that the lower any group of
organisms is, the more widely it is apt to range.
The relations just discussed,--namely, low and slowly-changing organisms
ranging more widely than the high,--some of the species of widely-ranging
genera themselves ranging widely,--such facts, as alpine, lacustrine, and
marsh productions being related (with the exceptions before specified) to
those on the surrounding low lands and dry lands, though these stations are
so different--the very close relation of the distinct species which inhabit
the islets of the same archipelago,--and especially the striking relation
of the inhabitants of each whole archipelago or island to those of the
nearest mainland,--are, I think, utterly inexplicable on the ordinary view
of the independent creation of each species, but are explicable on the view
of colonisation from the nearest and readiest source, together with the
subsequent modification and better adaptation of the colonists to their new
homes.
Summary of last and present Chapters -- In these chapters I have
endeavoured to show, that if we make due allowance for our ignorance of the
full effects of all the changes of climate and of the level of the land,
which have certainly occurred within the recent period, and of other
similar changes which may have occurred within the same period; if we
remember how profoundly ignorant we are with respect to the many and
curious means of occasional transport,--a subject which has hardly ever
been properly experimentised on; if we bear in mind how often a species may
have ranged continuously over a wide area, and then have become extinct in
the intermediate tracts, I think the difficulties in believing that all the
individuals of the same species, wherever located, have descended from the
same parents, are not insuperable. And we are led to this conclusion,
which has been arrived at by many naturalists under the designation of
single centres of creation, by some general considerations, more especially
from the importance of barriers and from the analogical distribution of
sub-genera, genera, and families.
With respect to the distinct species of the same genus, which on my theory
must have spread from one parent-source; if we make the same allowances as
before for our ignorance, and remember that some forms of life change most
slowly, enormous periods of time being thus granted for their migration, I
do not think that the difficulties are insuperable; though they often are
in this case, and in that of the individuals of the same species, extremely
grave.
As exemplifying the effects of climatal changes on distribution, I have
attempted to show how important has been the influence of the modern
Glacial period, which I am fully convinced simultaneously affected the
whole world, or at least great meridional belts. As showing how
diversified are the means of occasional transport, I have discussed at some
little length the means of dispersal of fresh-water productions.
If the difficulties be not insuperable in admitting that in the long course
of time the individuals of the same species, and likewise of allied
species, have proceeded from some one source; then I think all the grand
leading facts of geographical distribution are explicable on the theory of
migration (generally of the more dominant forms of life), together with
subsequent modification and the multiplication of new forms. We can thus
understand the high importance of barriers, whether of land or water, which
separate our several zoological and botanical provinces. We can thus
understand the localisation of sub-genera, genera, and families; and how it
is that under different latitudes, for instance in South America, the
inhabitants of the plains and mountains, of the forests, marshes, and
deserts, are in so mysterious a manner linked together by affinity, and are
likewise linked to the extinct beings which formerly inhabited the same
continent. Bearing in mind that the mutual relations of organism to
organism are of the highest importance, we can see why two areas having
nearly the same physical conditions should often be inhabited by very
different forms of life; for according to the length of time which has
elapsed since new inhabitants entered one region; according to the nature
of the communication which allowed certain forms and not others to enter,
either in greater or lesser numbers; according or not, as those which
entered happened to come in more or less direct competition with each other
and with the aborigines; and according as the immigrants were capable of
varying more or less rapidly, there would ensue in different regions,
independently of their physical conditions, infinitely diversified
conditions of life,--there would be an almost endless amount of organic
action and reaction,--and we should find, as we do find, some groups of
beings greatly, and some only slightly modified,--some developed in great
force, some existing in scanty numbers--in the different great geographical
provinces of the world.
On these same principles, we can understand, as I have endeavoured to show,
why oceanic islands should have few inhabitants, but of these a great
number should be endemic or peculiar; and why, in relation to the means of
migration, one group of beings, even within the same class, should have all
its species endemic, and another group should have all its species common
to other quarters of the world. We can see why whole groups of organisms,
as batrachians and terrestrial mammals, should be absent from oceanic
islands, whilst the most isolated islands possess their own peculiar
species of aerial mammals or bats. We can see why there should be some
relation between the presence of mammals, in a more or less modified
condition, and the depth of the sea between an island and the mainland. We
can clearly see why all the inhabitants of an archipelago, though
specifically distinct on the several islets, should be closely related to
each other, and likewise be related, but less closely, to those of the
nearest continent or other source whence immigrants were probably derived.
We can see why in two areas, however distant from each other, there should
be a correlation, in the presence of identical species, of varieties, of
doubtful species, and of distinct but representative species.
As the late Edward Forbes often insisted, there is a striking parallelism
in the laws of life throughout time and space: the laws governing the
succession of forms in past times being nearly the same with those
governing at the present time the differences in different areas. We see
this in many facts. The endurance of each species and group of species is
continuous in time; for the exceptions to the rule are so few, that they
may fairly be attributed to our not having as yet discovered in an
intermediate deposit the forms which are therein absent, but which occur
above and below: so in space, it certainly is the general rule that the
area inhabited by a single species, or by a group of species, is
continuous; and the exceptions, which are not rare, may, as I have
attempted to show, be accounted for by migration at some former period
under different conditions or by occasional means of transport, and by the
species having become extinct in the intermediate tracts. Both in time and
space, species and groups of species have their points of maximum
development. Groups of species, belonging either to a certain period of
time, or to a certain area, are often characterised by trifling characters
in common, as of sculpture or colour. In looking to the long succession of
ages, as in now looking to distant provinces throughout the world, we find
that some organisms differ little, whilst others belonging to a different
class, or to a different order, or even only to a different family of the
same order, differ greatly. In both time and space the lower members of
each class generally change less than the higher; but there are in both
cases marked exceptions to the rule. On my theory these several relations
throughout time and space are intelligible; for whether we look to the
forms of life which have changed during successive ages within the same
quarter of the world, or to those which have changed after having migrated
into distant quarters, in both cases the forms within each class have been
connected by the same bond of ordinary generation; and the more nearly any
two forms are related in blood, the nearer they will generally stand to
each other in time and space; in both cases the laws of variation have been
the same, and modifications have been accumulated by the same power of
natural selection.
Chapter XIII
Mutual Affinities of Organic Beings:
Morphology: Embryology: Rudimentary Organs
Classification, groups subordinate to groups -- Natural system -- Rules and
difficulties in classification, explained on the theory of descent with
modification -- Classification of varieties -- Descent always used in
classification -- Analogical or adaptive characters -- Affinities, general,
complex and radiating -- Extinction separates and defines groups --
Morphology, between members of the same class, between parts of the same
individual -- Embryology, laws of, explained by variations not supervening
at an early age, and being inherited at a corresponding age -- Rudimentary
Organs; their origin explained -- Summary.
From the first dawn of life, all organic beings are found to resemble each
other in descending degrees, so that they can be classed in groups under
groups. This classification is evidently not arbitrary like the grouping
of the stars in constellations. The existence of groups would have been of
simple signification, if one group had been exclusively fitted to inhabit
the land, and another the water; one to feed on flesh, another on vegetable
matter, and so on; but the case is widely different in nature; for it is
notorious how commonly members of even the same subgroup have different
habits. In our second and fourth chapters, on Variation and on Natural
Selection, I have attempted to show that it is the widely ranging, the much
diffused and common, that is the dominant species belonging to the larger
genera, which vary most. The varieties, or incipient species, thus
produced ultimately become converted, as I believe, into new and distinct
species; and these, on the principle of inheritance, tend to produce other
new and dominant species. Consequently the groups which are now large, and
which generally include many dominant species, tend to go on increasing
indefinitely in size. I further attempted to show that from the varying
descendants of each species trying to occupy as many and as different
pla
ces as possible in the economy of nature, there is a constant tendency
in their characters to diverge. This conclusion was supported by looking
at the great diversity of the forms of life which, in any small area, come
into the closest competition, and by looking to certain facts in
naturalisation.
I attempted also to show that there is a constant tendency in the forms
which are increasing in number and diverging in character, to supplant and
exterminate the less divergent, the less improved, and preceding forms. I
request the reader to turn to the diagram illustrating the action, as
formerly explained, of these several principles; and he will see that the
inevitable result is that the modified descendants proceeding from one
progenitor become broken up into groups subordinate to groups. In the
diagram each letter on the uppermost line may represent a genus including
several species; and all the genera on this line form together one class,
for all have descended from one ancient but unseen parent, and,
consequently, have inherited something in common. But the three genera on
the left hand have, on this same principle, much in common, and form a
sub-family, distinct from that including the next two genera on the right
hand, which diverged from a common parent at the fifth stage of descent.
These five genera have also much, though less, in common; and they form a
family distinct from that including the three genera still further to the
right hand, which diverged at a still earlier period. And all these
genera, descended from (A), form an order distinct from the genera
descended from (I). So that we here have many species descended from a
single progenitor grouped into genera; and the genera are included in, or
subordinate to, sub-families, families, and orders, all united into one
class. Thus, the grand fact in natural history of the subordination of
group under group, which, from its familiarity, does not always
sufficiently strike us, is in my judgment fully explained.
Naturalists try to arrange the species, genera, and families in each class,
on what is called the Natural System. But what is meant by this system?
Some authors look at it merely as a scheme for arranging together those
living objects which are most alike, and for separating those which are
most unlike; or as an artificial means for enunciating, as briefly as
possible, general propositions,--that is, by one sentence to give the
characters common, for instance, to all mammals, by another those common to
all carnivora, by another those common to the dog-genus, and then by adding
a single sentence, a full description is given of each kind of dog. The
ingenuity and utility of this system are indisputable. But many
naturalists think that something more is meant by the Natural System; they
believe that it reveals the plan of the Creator; but unless it be specified
whether order in time or space, or what else is meant by the plan of the
Creator, it seems to me that nothing is thus added to our knowledge. Such
expressions as that famous one of Linnaeus, and which we often meet with in
a more or less concealed form, that the characters do not make the genus,
but that the genus gives the characters, seem to imply that something more
is included in our classification, than mere resemblance. I believe that
something more is included; and that propinquity of descent,--the only
known cause of the similarity of organic beings,--is the bond, hidden as it
is by various degrees of modification, which is partially revealed to us by
our classifications.
Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification either
gives some unknown plan of creation, or is simply a scheme for enunciating
general propositions and of placing together the forms most like each
other. It might have been thought (and was in ancient times thought) that