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which affected even the equatorial regions, and which, during the alterna-
tions of the cold in the north and the south, allowed the productions of
opposite hemispheres to mingle, and left some of them stranded on the
mountain-summits in all parts of the world. 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 all the individuals of the same species, and likewise of the several
species belonging to the same genus, have proceeded from some one source;
then all the grand leading facts of geographical distribution are explicable
on the theory of migration, together with subsequent modification and the
multiplication of new forms. We can thus understand the high importance
of barriers, whether of land or water, in not only separating but in apparently
forming the several zoological and botanical provinces. We can thus under-
stand the concentration of related species within the same areas; and how
it is that under different latitudes, for instance, in South America, the in-
habitants of the plains and mountains, of the forests, marshes and deserts,
are linked together in so mysterious a manner, and are likewise linked to
the extinct beings which formerly inhabited the same continent. Bearing in
mind that the mutual relation of organism to organism is 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 the colonists
entered one of the regions, or both; according to the nature of the com-
munication which allowed certain forms and not others to enter, either in
greater or lesser numbers; according or not as those which entered hap-
pened to come into more or less direct competition with each other and
with the aborigines; and according as the immigrants were capable of vary-
ing more or less rapidly, there would ensue in the two or more regions, in-
dependently of their physical conditions, infinitely diversified conditions of
life; there would be an almost endless amount of organic action and reac-
tion, and we should find some groups of beings greatly, and some only
slightly, modified; some developed in great force, some existing in scanty
numbers — and this we do find in the several great geographical provinces
of the world.

On these same principles we can understand, as I have endeavored to
show, why oceanic islands should have few inhabitants, but that, of these, a
large proportion should be endemic or peculiar; and why, in relation to the
means of migration, one group of beings should have all its species peculiar,



GEOGRAPHICAL DISTRIBUTION 301

and another group, even within the same class, should have all its species
the same with those in an adjoining quarter of the world. We can see why
whole groups of organisms, as batrachians and terrestrial mammals, should
be absent from oceanic islands, while the most isolated islands should possess
their own peculiar species of aerial mammals or bats. We can see why, in
islands, there should be some relation between the presence of mammals,
in a more or less modified condition, and the depth of the sea between such
islands 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 should likewise be related, but less closely,
to those of the nearest continent, or other source whence immigrants might
have been derived. We can see why, if there exist very closely allied or
representative species in two areas, however distant from each other, some
identical species will almost always there be found.

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 succes-
sion 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 apparent exceptions to the rule are so few that they may
fairly be attributed to our not having as yet discovered in an intermediate
deposit certain forms which are absent in it, but which occur both above
and below: so in space, it certainly is the general rule that the area in-
habited 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 ac-
counted for by former migrations under different circumstances, or through
occasional means of transport, or 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, liv-
ing during the same period of time, or living within the same area, are
often characterized by trifling features in common, as of sculpture or color.
In looking to the long succession of past ages, as in looking to distant
provinces throughout the world, we find that species in certain classes differ
little from each other, while those in another class, or only in a different
section of the same order, differ greatly from each other. In both time and
space the lowly organized members of each class generally change less than
the highly organized; but there are in both cases marked exceptions to the
rule. According to our theory, these several relations throughout time and
space are intelligible; for whether we look to the allied forms of life which
have changed during successive ages, or to those which have changed after
having migrated into distant quarters, in both cases they are connected by
the same bond of ordinary generation; in both cases the laws of variation
have been the same, and modifications have been accumulated by the same
means of natural selection.



CHAPTER XIV

Mutual Affinities of Organic Beings: Morphology — Embryology — Rudimen-
tary Organs

Classification, Groups Subordinate to Groups — ^Natural System — ^Rules and Difficulties
in Classification, explained on the Theory of Descent with Modification — Classi-
fication 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 Vari- '
ations not supervening at an Early Age, and being inherited at a Corresponding
Age — Rudimentary Organs, their Origin explained — Summary.

CLASSIFICATION

From the most remote period in the history of the world, organic beings have
been found to resemble each other in descending degrees, so that they can
be classed in groups under groups. This classification is not arbitrary like
the grouping of the stars in constellations. The existence of groups would
have been of simple significance, 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, for it is notori-
ous how commonly members of even the same sub-group have different
habits. In the second and fourth chapters, on Variation and on Natural
Selection, I have attempted to show that within each country it is the widely
ranging, the much difl["used and common, that is the dominant species, be-
longing to the larger genera in each class, which vary most. The varieties,
or incipient species, thus produced, ultimately become converted into new
and distinct species; and these, on the principle of inheritance, tend to pro-
duce other new and dominant species. Consequently the groups which are
now large, and which generally include many dominant species, tend to go on
increasing in size. I further attempted to show that from the varying descend-
ants of each species trying to occupy as many and as different places as pos-
sible in the economy of nature, they constantly tend to diverge in character.
This latter conclusion is supported by observing the great diversity of forms,
which, in any small area, come into the closest competition, and by certain
facts in naturalization.

I attempted also to show that there is a steady tendency in the forms which
are increasing in number and diverging in character, to supplant and exter-
minate the preceding, less divergent, and less improved 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 up-
permost line may represent a genus including several species; and the whole
of the genera along this upper line form together one class, for all are de-
scended from one ancient parent, and, consequently, have inherited some-

302



MUTUAL AFFINITIES OF ORGANIC BEINGS 303

thing 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 con-
taining the next two genera on the right hand, which diverged from a com-
mon parent at the fifth stage of descent. These five genera have also much in
common, though less than when grooiped in sub-families; and they form a
family distinct from that containing the three genera still farther to the right
hand, which diverged at an 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 into sub-families, families, and orders, all under
one great class. The grand fact of the natural subordination of organic beings
in groups under groups, which, from its familiarity, does not always suffi-
ciently strike us, is in my judgment thus explained. No doubt organic beings,
like all other objects, can be classed in many ways, either artificially by single
characters, or more naturally by a number of characters. We know, for in-
stance, that minerals and the elemental substances can be thus arranged. In
this case there is of course no relation to genealogical succession, and no cause
can at present be assigned for their falling into groups. But with organic
beings the case is diff'erent, and the view above given accords with their
natural arrangement in group under group; and no other explanation has
ever been attempted.

Naturalists, as we have seen, 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 Hving objects which are most alike, and for separating those which are
most unlike; or as an artificial method of enunciating, as briefly as possible,
general propositions — that is, by one sentence to give the characters com-
mon, 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 some-
thing 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 both, or what else is meant by the plan of the Creator, it seems to me that
nothing is thus added to our knowledge. Expressions such as that famous
one by Linnaeus, which we often meet with in a more or less concealed form,
namely, that the characters do not make the genus, but that the genus gives
the characters, seem to imply that some deeper bond is included in our clas-
sifications than mere resemblance. I believe that this is the case, and that
community of descent — the one known cause of close similarity in organic
beings — is the bond, which, though observed by various degrees of modifica-
tion, 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 un-
known plan of creation, or is simply a scheme for enunciating general propo-
sitions and of placing together the forms most like each other. It might have



304 THE ORIGIN OF SPECIES

been thought (and was in ancient times thought) that those parts of the
structure which determined the habits of life, and the general place of each
being in the economy of nature, would be of very high importance in classi-
fication. Nothing can be more false. No one regards the external similarity
of a mouse to a shrew, of a dugong to a whale, of a whale to a fish, as of
any importance. These resemblances, though so intimately connected with
the whole life of the being, are ranked as merely "adaptive or analogical
characters": but to the consideration of these resemblances we shall recur.
It may even be given as a general rule, that the less any part of the organiza-
tion is concerned with special habits, the more important it becomes for clas-
sification. As an instance: Owen, in speaking of the dugong, says, "The
generative organs, being those which are most remotely related to the habits
and food of an animal, I have always regarded as affording very clear indi-
cations of its true affinities. We are least likely in the modifications of these
organs to mistake a merely adaptive for an essential character." With plants
how remarkable it is that the organs of vegetation, on which their nutrition
and life depend, are of little signification; whereas the organs of reproduc-
tion, with their product the seed and embryo, are of paramount importance!
So again, in formerly discussing certain morphological characters which are
not functionally important, we have seen that they are often of the highest
service in classification. This depends on their constancy throughout many
allied groups; and their constancy chiefly depends on any slight deviations
not having been preserved and accumulated by natural selection, which acts
only on serviceable characters.

That the mere physiological importance of an organ does not determine its
classificatory value, is almost proved by the fact, that in allied groups, in
which the same organ, as we have every reason to suppose, has nearly the
same physiological value, its classificatory value is widely different. No natu-
ralist can have worked long at any group without being struck with this fact;
and it has been fully acknowledged in the writings of almost every author.
It will suffice to quote the highest authority, Robert Brown, who, in speaking
of certain organs in the Proteaceae, says their generic importance, "like that
of all their parts, not only in this, but, as I apprehend, in every natural
family, is very unequal, and in some cases seems to be entirely lost." Again,
in another work he says, the genera of the Connaracese "differ in having
one or more ovaria, in the existence or absence of albumen, in the imbricate
or valvular aestivation. Any one of these characters singly is frequently of
more than generic importance, though here, even when all taken together,,
they appear insufficient to separate Cnestis from Connarus." To give an
example among insects : in one great division of the Hymenoptera, the anten-
nae, as Westwood has remarked, are most constant in structure; in another
division they differ much, and the differences are of quite subordinate value
in classification ; yet no one will say that the antennae in these two divisions of
the same order are of unequal physiological importance. Any number of in-
stances could be given of the varying importance for classification of the same^
important organ within the same group of beings.



MUTUAL AFFINITIES OF ORGANIC BEINGS 305

Again, no one will say that rudimentary or atrophied organs are of high
physiological or vital importance; yet, undoubtedly, organs in this condition
are often of much value in classification. No one will dispute that the rudi-
mentary teeth in the upper jaws of young ruminants, and certain rudimen-
tary bones of the leg, are highly serviceable in exhibiting the close affinity
between ruminants and pachyderms. Robert Brown has strongly insisted on
the fact that the position of the rudimentary florets is of the highest im-
portance in the classification of the grasses.

Numerous instances could be given of characters derived from parts which
must be considered of very trifling physiological importance, but which are
universally admitted as highly serviceable in the definition of whole groups.
For instance, whether or not there is an open passage from the nostrils to the
mouth, the only character, according to Owen, which absolutely distinguishes
fishes and reptiles — the inflection of the angle of the lower jaw in Marsupials
— the manner in which the wings of insects are folded — mere color in certain
Algae — mere pubescence on parts of the flower in grasses — the nature of the
dermal covering, as hair or feathers, in the Vertebrata. If the Ornithorhyn-
chus had been covered with feathers instead of hair, this external and trifling
character would have been considered by naturalists as an important aid in
determining the degree of affinity of this strange creature to birds.

The importance, for classification, of trifling characters, mainly depends
on their being correlated with many other characters of more or less im-
portance. The value indeed of an aggregate of characters is very evident in
natural history. Hence, as has often been remarked, a species may depart
from its allies in several characters, both of high physiological importance,
and of almost universal prevalence, and yet leave us in no doubt where it
should be ranked. Hence, also, it has been found that a classification founded
on any single character, however important that may be, has always failed;
for no part of the organization is invariably constant. The importance of an
aggregate of characters, even when none are important, alone explains the
aphorism enunciated by Linnaeus, namely, that the characters do not give
the genus, but the genus gives the character; for this seems founded on the
appreciation of many trifling points of resemblance, too slight to be defined.
Certain plants belonging to the Malpighiaceae bear perfect and degraded
flowers; in the latter, as A. de Jussieu has remarked, "The greater number of
the characters proper to the species, to the genus, to the family, to the class,
disappear, and thus laugh at our classification." When Aspicarpa produced
in France, during several years, only these degraded flowers, departing so
wonderfully in a number of the most important points of structure from the
proper type of the order, yet M. Richard sagaciously saw, as Jussieu observes,
that this genus should still be retained among the Malpighiaceae. This case
well illustrates the spirit of our classifications.

Practically, when naturalists are at work, they do not trouble themselves
about the physiological value of the characters which they use in defining a
group or in allocating any particular species. If they find a character nearly
uniform^ and common to a great number of forms, and not common to



306 THE ORIGIN OF SPECIES

others, they use it as one of high value; if common to some lesser number,
they use it as of subordinate value. This principle has been broadly confessed
by some naturalists to be the true one ; and by none more clearly than by that
excellent botanist, Aug. Saint-Hilaire. If several trifling characters are
always found in combination, though no apparent bond of connection can be
discovered between them, especial value is set on them. As in most groups of
animals, important organs, such as those for propelling the blood, or for
aerating it, or those for propagating the race, are found nearly uniform, they
are considered as highly serviceable in classification; but in some groups all
these, the most important vital organs, are found to offer characters of quite
subordinate value. Thus, as Fritz Miiller has lately remarked, in the same
group of crustaceans, Cypridina is furnished with a heart, while in two closely
allied genera, namely Cypris and Cytherea, there is no such organ; one species
of Cypridina has well-developed branchiae, while another species is destitute
of them.

We can see why characters derived from the embryo should be of equal
importance with those derived from the adult, for a natural classification of
course includes all ages. But it is by no means obvious, on the ordinary view,
why the structure of the embryo should be more important for this purpose
than that of the adult, which alone plays its full part in the economy of
nature. Yet it has been strongly urged by those great naturalists, Milne Ed-
wards and Agassiz, that embryological characters are the most important
of all; and this doctrine has very generally been admitted as true. Neverthe-
less, their importance has sometimes been exaggerated, owing to the adap-
tive characters of larvae not having been excluded; in order to show this,
Fritz Miiller arranged, by the aid of such characters alone, the great class of
crustaceans, and the arrangement did not prove a natural one. But there
can be no doubt that embryonic, excluding larval, characters, are of the
highest value for classification, not only with animals but with plants. Thus
the main divisions of flowering plants are founded on diflferences in the
embryo — on the number and position of the cotyledons, and on the mode of
development of the plumule and radicle. We shall immediately see why these
characters possess so high a value in classification; namely, from the natural
system being genealogical in its arrangement.

Our classifications are often plainly influenced by chains of affinities. Noth-
ing can be easier than to define a number of characters common to all birds;
but with crustaceans, any such definition has hitherto been found impossible.
There are crustaceans at the opposite ends of the series, which have hardly
a character in common; yet the species at both ends, from being plainly allied
to others, and these to others, and so onward, can be recognized as unequivo-
cally belonging to this, and to no other class of the Articulata.

Geographical distribution has often been used, though perhaps not quite
logically, in classification, more especially in very large groups of closely allied
forms. Temminck insists on the utility or even necessity of this practice in
certain groups of birds ; and it has been followed by several entomologists and
botanists.



MUTUAL AFFINITIES OF ORGANIC BEINGS 307

Finally, with respect to the comparative value of the various groups of
species, such as orders, sub-orders, families, sub-families, and genera, they
seem to be, at least at present, almost arbitrary. Several of the best botanists,
such as Mr. Bentham, and others, have strongly insisted on their arbitrary
value. Instances could be given among plants and insects, of a group first
ranked by practised naturalists as only a genus, and then raised to the rank
of a sub-family or family; and this has been done, not because further re-
search has detected important structural differences, at first overlooked, but
because numerous allied species, with slightly difTerent grades of difference,
have been subsequently discovered.

All the foregoing rules and aids and difficulties in classification may be
explained, if I do not greatly deceive myself, on the view that the Natural
System is founded on descent with modification — that the characters which
naturalists consider as showing true affinity between any two or more species,
are those which have been inherited from a common parent, all true classifi-
cation being genealogical — that community of descent is the hidden bond
which naturalists have been unconsciously seeking, and not some unknown
plan of creation, or the enunciation of general propositions, and the mere
putting together and separating objects more or less alike.

But I must explain my meaning more fully. I believe that the arrangement
of the groups within each class, in due subordination and relation to each



Online LibraryCharles DarwinThe origin of species → online text (page 41 of 50)