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transmitted to the present day descendants but little modified, these con-
stitute our so-called osculant or aberrant species. The more aberrant any
form is, the greater must be the number of connecting forms which have been
exterminated and utterly lost. And we have evidence of aberrant groups
having suffered severely from extinction, for they are almost always rep-
resented by extremely few species, and such species as do occur are generally
very distinct from each other, which again implies extinction. The genera


Ornithorhynchus and Lepidosiren, for example, would not have been less
aberrant had each been represented by a dozen species, instead of as at
present by a single one, or by two or three. We can, I think, account for this
fact only by looking at aberrant groups as forms which have been conquered
by more successful competitors, with a few members still preserved under
unusually favorable conditions.

Mr. Waterhouse has remarked that when a member belonging to one
group of animals exhibits an affinity to a quite distinct group, this affinity
in most cases is general and not special; thus, according to Mr. Waterhouse,
of all Rodents, the bizcacha is most nearly related to Marsupials; but in the
points in which it approaches this order, its relations are general, that is, not
to any one Marsupial species more than to another. As these points of
affinity are believed to be real and not merely adaptive, they must be due,
in accordance with our view, to inheritance from a common progenitor.
Therefore, we must suppose either that all Rodents, including the bizcacha,
branched off from some ancient Marsupial, which will naturally have been
more or less intermediate in character with respect to all existing Marsupials ;
or that both Rodents and Marsupials branched off from a common
progenitor, and that both groups have since undergone much modification
in divergent directions. On either view we must suppose that the bizcacha
has retained, by inheritance, more of the characters of its ancient progenitor
than have other Rodents; and therefore it will not be specially related to
any one existing Marsupial, but indirectly to all or nearly all Marsupials,
from having partially retained the character of their common progenitor, or
of some early member of the group. On the other hand, of all Marsupials,
as Mr. Waterhouse has remarked, the Phascolomys resembles most nearly,
not any one species, but the general order of Rodents. In this case, however,
it 'may be strongly suspected that the resemblance is only analogical, owing
to the Phascolomys having become adapted to habits like those of a Rodent.
The elder De Candolle has made nearly similar observations on the general
nature of the affinities of distinct families of plants.

On the principle of the multiplication and gradual divergence in char-
acter of the species descended from a common progenitor, together with
their retention by inheritance of some characters in common, we can under-
stand the excessively complex and radiating affinities by which all the mem-
bers of the same family or higher group are connected together. For the
common progenitor of a whole family, now broken up by extinction into
distinct groups and sub-groups, will have transmitted some of its characters,
modified in various ways and degrees, to all the species; and they will con-
sequently be related to each other by circuitous lines of affinity of various
lengths (as may be seen in the diagram so often referred to), mounting up
through many predecessors. As it is difficult to show the blood relationship
between the numerous kindred of any ancient and noble family even by the
aid of a genealogical tree, and almost impossible to do so without this aid,
we can understand the extraordinary difficulty which naturalists have ex-


perienced in describing, without the aid of a diagram, the various affinities
which they perceive between the many Hving and extinct members of the
same great natural class.

Extinction, as we have seen in the fourth chapter, has played an important
part in defining and widening the intervals between the several groups in
each class. We may thus account for the distinctness of whole classes from
each other — for instance, of birds from all other vertebrate animals — ^by the
belief that many ancient forms of life have been utterly lost, through which
the early progenitors of birds were formerly connected with the early progeni-
tors of the other and at that time less differentiated vertebrate classes. There
has been much less extinction of the forms of life which once connected fishes
with Batrachians. There has been still less within some whole classes, for
instance the Crustacea, for here the most wonderfully diverse forms are still
linked together by a long and only partially broken chain of affinities. Ex-
tinction has only defined the groups: it has by no means made them; for if
every form which has ever lived on this earth were suddenly to reappear,
though it would be quite impossible to give definitions by which each group
could be distinguished, still a natural classification, or at least a natural
arrangement, would be possible. We shall see this by turning to the diagram;
the letters, A to L, may represent eleven Silurian genera, some of which have
produced large groups of modified descendants, with every link in each
branch and sub-branch still alive; and the links not greater than those be-
tween existing varieties. In this case it would be quite impossible to give
definitions by which the several members of the several groups could be
distinguished from their more immediate parents and descendants. Yet the
arrangement in the diagram would still hold good and would be natural;
for, on the principle of inheritance, all the forms descended, for instance,
from A, would have something in common. In a tree we can distinguish this
or that branch, though at the actual fork the two unite and blend together.
We could not, as I have said, define the several groups; but we could pick
out types, or forms, representing most of the characters of each group,
whether large or small, and thus give a general idea of the value of the
differences between them. This is what we should be driven to, if we were
ever to succeed in collecting all the forms in any one class which have lived
throughout all time and space. Assuredly we shall never succeed in making so
perfect a collection: nevertheless, in certain classes, we are tending toward
this end; and Milne Edwards has lately insisted, in an able paper, on the
high importance of looking to types, whether or not we can separate and
define the groups to which such types belong.

Finally, we have seen that natural selection, which follows from the
struggle for existence, and which almost inevitably leads to extinction and
divergence of character in the descendants from any one parent species,
explains that great and universal feature in the affinities of all organic beings,
namely, their subordination in group under group. We use the element of
descent in classing the individuals of both sexes and of all ages under one
species, although they may have but few characters in common; we use


descent in classing acknowledged varieties, however different they may be
from their parents; and I believe that this element of descent is the hidden
bond of connection which naturalists have sought under the term of the
Natural System. On this idea of the natural system being, in so far as it has
been perfected, genealogical in its arrangement, with the grades of difference
expressed by the terms genera, families, orders, etc., we can understand the
rules which we are compelled to follow in our classification. We can under-
stand why we value certain resemblances far more than others; why we use
rudimentary and useless organs, or others of trifling physiological im-
portance; why, in finding the relations between one group and another, we
summarily reject analogical or adaptive characters, and yet use these same
characters within the limits of the same group. We can clearly see how it is
that all living and extinct forms can be grouped together within a few great
classes; and how the several members of each class are connected together
by the most complex and radiating lines of affinities. We shall never,
probably, disentangle the inextricable web of the affinities between the mem-
bers of any one class; but when we have a distinct object in view, and do
not look to some unknown plan of creation, we may hope to make sure but
slow progress.

Professor Hackel in his "Generelle Morphologic," and in other works, has
recently brought his great knowledge and abilities to bear on what he calls
phylogeny, or the lines of descent of all organic beings. In drawing up the
several series he trusts chiefly to embryological characters, but receives aid
from homologous and rudimentary organs, as well as from the successive
periods at which the various forms of life are believed to have first appeared
in our geological formations. He has thus boldly made a great beginning,
and shows us how classification will in the future be treated.


We have seen that the members of the same class, independently of their
habits of life, resemble each other in the general plan of their organization.
This resemblance is often expressed by the term "unity of type"; or by saying
that the several parts and organs in the different species of the class are
homologous. The whole subject is included under the general term of
Morphology. This is one of the most interesting departments of natural
history, and may almost be said to be its very soul. What can be more curious
than that the hand of a man, formed for grasping, that of a mole for digging,
the leg of a horse, the paddle of the porpoise, and the wing of the bat,
should all be constructed on the same pattern, and should include similar
bones, in the same relative positions? How curious it is, to give a subordinate
though striking instance, that the hind feet of the kangaroo, which are so
well fitted for bounding over the open plains — those of the climbing, leaf-
eating koala, equally well fitted for grasping the branches of trees — those of
the ground-dwelling, insect or root eating, bandicoots — and those of some
other Australian marsupials — should all be constructed on the same extraor-


dinary type, namely with the bones of the second and third digits extremely
slender and enveloped within the same skin, so that they appear like a single
toe furnished with two claws. Notwithstanding the similarity of pattern, it
is obvious that the hind feet of these several animals are used for as widely
different purposes as it is possible to conceive. The case is rendered all the
more striking by the American opossums, which follow nearly the same
habits of life as some of their Australian relatives, having feet constructed on
the ordinary plan. Professor Flower, from whom these statements are taken,
remarks in conclusion : "We may call this conformity to type, without getting
much nearer to an explanation of the phenomenon" ; and he then adds, "but
is it not powerfully suggestive of true relationship, of inheritance from a
common ancestor?"

Geoffroy Saint-Hilaire has strongly insisted on the high importance of
relative position or connection in homologous parts; they may differ to
almost any extent in form and size, and yet remain connected together in
the same invariable order. We never find, for instance, the bones of the arm
and forearm, or of the thigh and leg, transposed. Hence, the same names
can be given to the homologous bones in widely different animals. We see
the same great law in the construction of the mouths of insects: what can
be more different than the immensely long spiral proboscis of a sphinx-moth,
the curious folded one of a bee or bug, and the great jaws of a beetle? Yet
all these organs, serving for such widely different purposes, are formed by
infinitely numerous modifications of an upper lip, mandibles, and two pairs
of maxillae. The same law governs the construction of the mouths and Hmbs
of crustaceans. So it is with the flowers of plants.

Nothing can be more hopeless than to attempt to explain this similarity
of pattern in members of the same class, by utility or by the doctrine of final
causes. The hopelessness of the attempt has been expressly admitted by Owen
in his most interesting work on the "Nature of Limbs." On the ordinary view
of the independent creation of each being, we can only say that so it is; that
it has pleased the Creator to construct all the animals and plants in each
great class on a uniform plan; but this is not a scientific explanation.

The explanation is to a large extent simple, on the theory of the selection
of successive slight modifications, each being profitable in some way to the
modified form, but often affecting by correlation other parts of the organiza-
tion. In changes of this nature, there will be little or no tendency to alter the
original pattern, or to transpose the parts. The bones of a limb might be
shortened and flattened to any extent, becoming at the same time enveloped
in thick membrane, so as to serve as a fin; or a webbed hand might have
all its bones, or certain bones, lengthened to any extent, with the membrane
connecting them increased, so as to serve as a wing; yet all these modifica-
tions would not tend to alter the framework of the bones or the relative
connection of the parts. If we suppose that an early progenitor — the arche-
type, as it may be called — of all mammals, birds, and reptiles, had its limbs
constructed on the existing general pattern, for whatever purpose they
served, we can at once perceive the plain signification of the homologous


construction of the limbs throughout the class. So with the mouths of insects,
we have only to suppose that their common progenitor had an upper lip,
mandibles, and two pairs of maxillae, these parts being perhaps very simple in
form; and then natural selection will account for the definite diversity in
the structure and functions of the mouths of insects. Nevertheless, it is
conceivable that the general pattern of an organ might become so much
obscured as to be finally lost, by the reduction and ultimately by the complete
abortion of certain parts, by the fusion of other parts, and by the doubling
or multiplication of others, variations which we know to be within the limits
of possibility. In the paddles of the gigantic extinct sea-lizards, and in the
mouths of certain suctorial crustaceans, the general pattern seems thus to
have become partially obscured.

There is another and equally curious branch of our subject; namely,
serial homologies, or the comparison of the different parts or organs in the
same individual, and not of the same parts or organs in different members
of the same class. Most physiologists believe that the bones of the skull are
homologous — that is, correspond in number and relative connection — ^with
the elemental parts of a certain number of vertebrae. The anterior and
posterior limbs in all the higher vertebrate classes are plainly homologous.
So it is with the wonderfully complex jaws and legs of crustaceans. It is
familiar to almost every one, that in a flower the relative position of the
sepals, petals, stamens, and pistils, as well as their intimate structure, are
intelligible on the view that they consist of metamorphosed leaves arranged
in a spire. In monstrous plants, we often get direct evidence of the possibility
of one organ being transformed into another; and we can actually see, during
the early or embryonic stages of development in flowers, as well as in
crustaceans and many other animals, that organs which when mature be-
come extremely diff'erent are at first exactly alike.

How inexplicable are the cases of serial homologies on the ordinary
view of creation! Why should the brain be enclosed in a box composed of
such numerous and such extraordinarily shaped pieces of bone, apparently
representing vertebrae? As Owen has remarked, the benefit derived from the
yielding of the separate pieces in the act of parturition by mammals, will by
no means explain the same construction in the skulls of birds and reptiles.
Why should similar bones have been created to form the wing and the leg
of a bat, used as they are for such totally different purposes, namely, flying
and walking? Why should one crustacean, which has an extremely complex
mouth formed of many parts, consequently always have fewer legs; or con-
versely, those with many legs have simpler mouths? Why should the sepals,
petals, stamens, and pistils, in each flower, though fitted for such distinct
purposes, be all constructed on the same pattern?

On the theory of natural selection, we can, to a certain extent, answer
these questions. We need not here consider how the bodies of some animals
first became divided into a series of segments, or how they became divided
into right and left sides, with corresponding organs, for such questions are
almost beyond investigation. It is, however, probable that some serial struc-


tures are the result of cells multiplying by division, entailing the multiplica-
tion of the parts developed from such cells. It must suffice for our purpose
to bear in mind that an indefinite repetition of the same part or organ is
the common characteristic, as Owen has remarked, of all low or little
specialized forms; therefore the unknown progenitor of the Vertebrata prob-
ably possessed many vertebrae; the unknown progenitor of the Articulata,
many segments; and the unknown progenitor of flowering plants, many
leaves arranged in one or more spires. We have also formerly seen that parts
many times repeated are eminently liable to vary, not only in number, but in
form. Consequently such parts, being already present in considerable num-
bers, and being highly variable, would naturally afford the materials for
adaptation to the most different purposes; yet they would generally retain,
through the force of inheritance, plain traces of their original or fundamental
resemblance. They would retain this resemblance all the more, as the varia-
tions, which afforded the basis for their subsequent modification through
natural selection, would tend from the first to be similar; the parts being
at an early stage of growth alike, and being subjected to nearly the same
conditions. Such parts, whether more or less modified, unless their common
origin became wholly obscured, would be serially homologous.

In the great class of mollusks, though the parts in distinct species can
be shown to be homologous, only a few serial homologies, such as the valves
of Chitons, can be indicated; that is, we are seldom enabled to say that one
part is homologous with another part in the same individual. And we can
understand this fact ; for in mollusks, even in the lowest members of the class,
we do not find nearly so much indefinite repetition of any one part as we
find in the other great classes of the animal and vegetable kingdoms.

But morphology is a much more complex subject than it at first appears,
as has lately been well shown in a remarkable paper by Mr. E. Ray
Lankester, who has drawn an important distinction between certain classes of
cases which have all been equally ranked by naturalists as homologous. He
proposes to call the structures which resemble each other in distinct animals,
owing to their descent from a common progenitor with subsequent modifi-
cation, homogenous; and the resemblances which cannot thus be accounted
for, he proposes to call homoplastic. For instance, he believes that the hearts
of birds and mammals are as a whole homogenous — that is, have been
derived from a common progenitor; but that the four cavities of the heart
in the two classes are homoplastic — that is, have been independently de-
veloped. Mr. Lankester also adduces the close resemblance of the parts on
the right and left sides of the body, and in the successive segments of the
same individual animal ; and here we have parts commonly called homologous
which bear no relation to the descent of distinct species from a common
progenitor. Homoplastic structures are the same with those which I have
classed, though in a very imperfect manner, as analogous modifications or
resemblances. Their formation may be attributed in part to distinct organ-
isms, or to distinct parts of the same organism, having varied in an analogous
manner; and in part to similar modifications, having been preserved for the


same general purpose or function, of which many instances have been given.
Naturalists frequently speak of the skull as formed of metamorphosed
vertebrae; the jaws of crabs as metamorphosed legs; the stamens and pistils
in flowers as metamorphosed leaves; but it would in most cases be more
correct, as Professor Huxley has remarked, to speak of both skull and
vertebrae, jaws and legs, etc., as having been metamorphosed, not one from
the other, as they now exist, but from some common and simpler element.
Most naturalists, however, use such language only in a metaphorical sense;
they are far from meaning that during a long course of descent, primordial
organs of any kind — vertebrae in the one case and legs in the other — have
actually been converted into skulls or jaws. Yet so strong is the appearance
of this having occurred, that naturalists can hardly avoid employing language
having this plain signification. According to the views here maintained, such
language may be used literally; and the wonderful fact of the jaws, for
instance, of a crab, retaining numerous characters, which they probably
would have retained through inheritance, if they had really been met-
amorphosed from true though extremely simple legs, is in part explained.


This is one of the most important subjects in the whole round of natural
history. The metamorphoses of insects, with which every one is familiar, are
generally eflfected abruptly by a few stages; but the transformations are in
reality numerous and gradual, though concealed. A certain ephemerous
insect (Chloeon) during its development moults, as shown by Sir J. Lubbock,
above twenty times, and each time undergoes a certain amount of change;
and in this case we see the act of metamorphosis performed in a primary
and gradual manner. Many insects, and especially certain crustaceans, show
us what wonderful changes of structure can be effected during development.
Such changes, however, reach their acme in the so-called alternate genera-
tions of some of the lower animals. It is, for instance, an astonishing fact
that a delicate branching coralline, studded with polypi, and attached to a
submarine rock, should produce, first by budding and then by transverse
division, a host of huge floating jelly-fishes; and that these should produce
eggs, from which are hatched swimming animalcules, which attach them-
selves to rocks and become developed into branching corallines ; and so on in
an endless cycle. The belief in the essential identity of the process of alternate
generation and of ordinary metamorphosis has been greatly strengthened
by Wagner's discovery of the larva or maggot of a fly, namely the Cecidomyia,
producing asexually other larvae, and these others which finally are developed
into mature males and females, propagating their kind in the ordinary
manner by eggs.

It may be worth notice, that when Wagner's remarkable discovery was
first announced, I was asked how was it possible to account for the larvae of
this fly having acquired the power of asexual reproduction. As long as the
case remained unique, no answer could be given. But already Grimm has


shown that another fly, a Chironomus, reproduces itself in nearly the same
manner, and he believes that this occurs frequently in the order. It is the
pupa, and not the larva, of the Chironomus which has this power; and
Grimm further shows that this case, to a certain extent, "unites that of the
Cecidomyia, with the parthenogenesis of the Goccidae"; the term par-
thenogenesis implying that the mature females of the Coccidse are capable
of producing fertile eggs without the concourse of the male. Certain animals
belonging to several classes are now known to have the power of ordinary
reproduction at an unusually early age; and we have only to accelerate

Online LibraryCharles DarwinThe origin of species → online text (page 43 of 50)