On the Origin of Species by Means of Natural Selection
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We can understand, on these views, the very important distinction between
real affinities and analogical or adaptive resemblances. Lamarck first
called attention to this distinction, and he has been ably followed by
Macleay and others. The resemblance, in the shape of the body and in the
fin-like anterior limbs, between the dugong, which is a pachydermatous
animal, and the whale, and between both these mammals and fishes, is
analogical. Amongst insects there are innumerable instances: thus
Linnaeus, misled by external appearances, actually classed an homopterous
insect as a moth. We see something of the same kind even in our domestic
varieties, as in the thickened stems of the common and swedish turnip. The
resemblance of the greyhound and racehorse is hardly more fanciful than the
analogies which have been drawn by some authors between very distinct
animals. On my view of characters being of real importance for
classification, only in so far as they reveal descent, we can clearly
understand why analogical or adaptive character, although of the utmost
importance to the welfare of the being, are almost valueless to the
systematist. For animals, belonging to two most distinct lines of descent,
may readily become adapted to similar conditions, and thus assume a close
external resemblance; but such resemblances will not reveal--will rather
tend to conceal their blood-relationship to their proper lines of descent.
We can also understand the apparent paradox, that the very same characters
are analogical when one class or order is compared with another, but give
true affinities when the members of the same class or order are compared
one with another: thus the shape of the body and fin-like limbs are only
analogical when whales are compared with fishes, being adaptations in both
classes for swimming through the water; but the shape of the body and
fin-like limbs serve as characters exhibiting true affinity between the
several members of the whale family; for these cetaceans agree in so many
characters, great and small, that we cannot doubt that they have inherited
their general shape of body and structure of limbs from a common ancestor.
So it is with fishes.
As members of distinct classes have often been adapted by successive slight
modifications to live under nearly similar circumstances,--to inhabit for
instance the three elements of land, air, and water,--we can perhaps
understand how it is that a numerical parallelism has sometimes been
observed between the sub-groups in distinct classes. A naturalist, struck
by a parallelism of this nature in any one class, by arbitrarily raising or
sinking the value of the groups in other classes (and all our experience
shows that this valuation has hitherto been arbitrary), could easily extend
the parallelism over a wide range; and thus the septenary, quinary,
quaternary, and ternary classifications have probably arisen.
As the modified descendants of dominant species, belonging to the larger
genera, tend to inherit the advantages, which made the groups to which they
belong large and their parents dominant, they are almost sure to spread
widely, and to seize on more and more places in the economy of nature. The
larger and more dominant groups thus tend to go on increasing in size; and
they consequently supplant many smaller and feebler groups. Thus we can
account for the fact that all organisms, recent and extinct, are included
under a few great orders, under still fewer classes, and all in one great
natural system. As showing how few the higher groups are in number, and
how widely spread they are throughout the world, the fact is striking, that
the discovery of Australia has not added a single insect belonging to a new
order; and that in the vegetable kingdom, as I learn from Dr. Hooker, it
has added only two or three orders of small size.
In the chapter on geological succession I attempted to show, on the
principle of each group having generally diverged much in character during
the long-continued process of modification, how it is that the more ancient
forms of life often present characters in some slight degree intermediate
between existing groups. A few old and intermediate parent-forms having
occasionally transmitted to the present day descendants but little
modified, will give to us our so-called osculant or aberrant groups. The
more aberrant any form is, the greater must be the number of connecting
forms which on my theory have been exterminated and utterly lost. And we
have some evidence of aberrant forms having suffered severely from
extinction, for they are generally represented 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 by a single one; but such
richness in species, as I find after some investigation, does not commonly
fall to the lot of aberrant genera. We can, I think, account for this fact
only by looking at aberrant forms as failing groups conquered by more
successful competitors, with a few members preserved by some unusual
coincidence of favourable circumstances.
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,
and not to any one marsupial species more than to another. As the points
of affinity of the bizcacha to Marsupials are believed to be real and not
merely adaptive, they are due on my theory to inheritance in common.
Therefore we must suppose either that all Rodents, including the bizcacha,
branched off from some very ancient Marsupial, which will have had a
character in some degree intermediate 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 may suppose that the bizcacha has
retained, by inheritance, more of the character 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 an 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 orders of plants.
On the principle of the multiplication and gradual divergence in character
of the species descended from a common parent, together with their
retention by inheritance of some characters in common, we can understand
the excessively complex and radiating affinities by which all the members
of the same family or higher group are connected together. For the common
parent of a whole family of species, 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; and the several
species will consequently 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 this without this aid, we can understand the extraordinary difficulty
which naturalists have experienced in describing, without the aid of a
diagram, the various affinities which they perceive between the many living
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 even 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 progenitors of the other vertebrate classes. There has been
less entire extinction of the forms of life which once connected fishes
with batrachians. There has been still less in some other classes, as in
that of the Crustacea, for here the most wonderfully diverse forms are
still tied together by a long, but broken, chain of affinities. Extinction
has only separated 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 from other groups, as all would blend together by steps as
fine as those between the finest existing varieties, nevertheless 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. Every intermediate link between these eleven
genera and their primordial parent, and every intermediate link in each
branch and sub-branch of their descendants, may be supposed to be still
alive; and the links to be as fine as those between the finest varieties.
In this case it would be quite impossible to give any definition by which
the several members of the several groups could be distinguished from their
more immediate parents; or these parents from their ancient and unknown
progenitor. Yet the natural arrangement in the diagram would still hold
good; and, on the principle of inheritance, all the forms descended from A,
or from I, would have something in common. In a tree we can specify 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 class which have lived
throughout all time and space. We shall certainly never succeed in making
so perfect a collection: nevertheless, in certain classes, we are tending
in this direction; 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 results from the
struggle for existence, and which almost inevitably induces extinction and
divergence of character in the many descendants from one dominant
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, although having few characters in common, under one species;
we use descent in classing acknowledged varieties, however different they
may be from their parent; and I believe this element of descent is the
hidden bond of connexion 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 between the descendants from a common parent, expressed by the
terms genera, families, orders, &c., we can understand the rules which we
are compelled to follow in our classification. We can understand why we
value certain resemblances far more than others; why we are permitted to
use rudimentary and useless organs, or others of trifling physiological
importance; why, in comparing one group with a distinct group, 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 in one great system; 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 affinities between the members 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.
Morphology. -- 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 organisation. 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 name of Morphology. This is the most
interesting department of natural history, and may 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 the horse, the paddle of
the porpoise, and the wing of the bat, should all be constructed on the
same pattern, and should include the same bones, in the same relative
positions? Geoffroy St. Hilaire has insisted strongly on the high
importance of relative connexion in homologous organs: the parts may
change to almost any extent in form and size, and yet they always remain
connected together in the same 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 different
purposes, are formed by infinitely numerous modifications of an upper lip,
mandibles, and two pairs of maxillae. Analogous laws govern the
construction of the mouths and limbs 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 so pleased the Creator to construct each animal
and plant.
The explanation is manifest on the theory of the natural selection of
successive slight modifications,--each modification being profitable in
some way to the modified form, but often affecting by correlation of growth
other parts of the organisation. In changes of this nature, there will be
little or no tendency to modify the original pattern, or to transpose
parts. The bones of a limb might be shortened and widened to any extent,
and become gradually enveloped in thick membrane, so as to serve as a fin;
or a webbed foot might have all its bones, or certain bones, lengthened to
any extent, and the membrane connecting them increased to any extent, so as
to serve as a wing: yet in all this great amount of modification there
will be no tendency to alter the framework of bones or the relative
connexion of the several parts. If we suppose that the ancient progenitor,
the archetype as it may be called, of all mammals, 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 whole class. So with the mouths
of insects, we have only to suppose that their common progenitor had an
upper lip, mandibles, and two pair of maxillae, these parts being perhaps
very simple in form; and then natural selection will account for the
infinite diversity in structure and function 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 atrophy and
ultimately by the complete abortion of certain parts, by the soldering
together 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 extinct gigantic sea-lizards, and in the mouths of
certain suctorial crustaceans, the general pattern seems to have been thus
to a certain extent obscured.
There is another and equally curious branch of the present subject; namely,
the comparison not of the same part in different members of a class, but of
the different parts or organs in the same individual. Most physiologists
believe that the bones of the skull are homologous with--that is correspond
in number and in relative connexion with--the elemental parts of a certain
number of vertebrae. The anterior and posterior limbs in each member of
the vertebrate and articulate classes are plainly homologous. We see the
same law in comparing the wonderfully complex jaws and legs in 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 in embryonic crustaceans and in many other animals,
and in flowers, that organs, which when mature become extremely different,
are at an early stage of growth exactly alike.
How inexplicable are these facts 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? As Owen has remarked, the benefit
derived from the yielding of the separate pieces in the act of parturition
of mammals, will by no means explain the same construction in the skulls of
birds. Why should similar bones have been created in the formation of the
wing and leg of a bat, used as they are for such totally different
purposes? Why should one crustacean, which has an extremely complex mouth
formed of many parts, consequently always have fewer legs; or conversely,
those with many legs have simpler mouths? Why should the sepals, petals,
stamens, and pistils in any individual flower, though fitted for such
widely different purposes, be all constructed on the same pattern?
On the theory of natural selection, we can satisfactorily answer these
questions. In the vertebrata, we see a series of internal vertebrae
bearing certain processes and appendages; in the articulata, we see the
body divided into a series of segments, bearing external appendages; and in
flowering plants, we see a series of successive spiral whorls of leaves.
An indefinite repetition of the same part or organ is the common
characteristic (as Owen has observed) of all low or little-modified forms;
therefore we may readily believe that the unknown progenitor of the
vertebrata possessed many vertebrae; the unknown progenitor of the
articulata, many segments; and the unknown progenitor of flowering plants,
many spiral whorls of leaves. We have formerly seen that parts many times
repeated are eminently liable to vary in number and structure; consequently
it is quite probable that natural selection, during a long-continued course
of modification, should have seized on a certain number of the primordially
similar elements, many times repeated, and have adapted them to the most
diverse purposes. And as the whole amount of modification will have been
effected by slight successive steps, we need not wonder at discovering in
such parts or organs, a certain degree of fundamental resemblance, retained
by the strong principle of inheritance.
In the great class of molluscs, though we can homologise the parts of one
species with those of another and distinct species, we can indicate but few
serial homologies; that is, we are seldom enabled to say that one part or
organ is homologous with another in the same individual. And we can
understand this fact; for in molluscs, 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.
Naturalists frequently speak of the skull as formed of metamorphosed
vertebrae: the jaws of crabs as metamorphosed legs; the stamens and
pistils of flowers as metamorphosed leaves; but it would in these cases
probably be more correct, as Professor Huxley has remarked, to speak of
both skull and vertebrae, both jaws and legs, &c.,--as having been
metamorphosed, not one from the other, but from some common element.
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 modified into skulls or jaws. Yet so strong is the
appearance of a modification of this nature having occurred, that
naturalists can hardly avoid employing language having this plain
signification. On my view these terms may be used literally; and the
wonderful fact of the jaws, for instance, of a crab retaining numerous
characters, which they would probably have retained through inheritance, if
they had really been metamorphosed during a long course of descent from
true legs, or from some simple appendage, is explained.
Embryology. -- It has already been casually remarked that certain organs in
the individual, which when mature become widely different and serve for
different purposes, are in the embryo exactly alike. The embryos, also, of
distinct animals within the same class are often strikingly similar: a
better proof of this cannot be given, than a circumstance mentioned by
Agassiz, namely, that having forgotten to ticket the embryo of some
vertebrate animal, he cannot now tell whether it be that of a mammal, bird,
or reptile. The vermiform larvae of moths, flies, beetles, &c., resemble
each other much more closely than do the mature insects; but in the case of
larvae, the embryos are active, and have been adapted for special lines of
life. A trace of the law of embryonic resemblance, sometimes lasts till a
rather late age: thus birds of the same genus, and of closely allied
genera, often resemble each other in their first and second plumage; as we
see in the spotted feathers in the thrush group. In the cat tribe, most of
the species are striped or spotted in lines; and stripes can be plainly
distinguished in the whelp of the lion. We occasionally though rarely see
something of this kind in plants: thus the embryonic leaves of the ulex or
furze, and the first leaves of the phyllodineous acaceas, are pinnate or
divided like the ordinary leaves of the leguminosae.
The points of structure, in which the embryos of widely different animals
of the same class resemble each other, often have no direct relation to
their conditions of existence. We cannot, for instance, suppose that in
the embryos of the vertebrata the peculiar loop-like course of the arteries
near the branchial slits are related to similar conditions,--in the young
mammal which is nourished in the womb of its mother, in the egg of the bird
which is hatched in a nest, and in the spawn of a frog under water. We
have no more reason to believe in such a relation, than we have to believe
that the same bones in the hand of a man, wing of a bat, and fin of a
porpoise, are related to similar conditions of life. No one will suppose
that the stripes on the whelp of a lion, or the spots on the young
blackbird, are of any use to these animals, or are related to the
conditions to which they are exposed.
The case, however, is different when an animal during any part of its
embryonic career is active, and has to provide for itself. The period of
activity may come on earlier or later in life; but whenever it comes on,
the adaptation of the larva to its conditions of life is just as perfect
and as beautiful as in the adult animal. From such special adaptations,
the similarity of the larvae or active embryos of allied animals is
sometimes much obscured; and cases could be given of the larvae of two
species, or of two groups of species, differing quite as much, or even
more, from each other than do their adult parents. In most cases, however,
the larvae, though active, still obey more or less closely the law of
common embryonic resemblance. Cirripedes afford a good instance of this:
even the illustrious Cuvier did not perceive that a barnacle was, as it
certainly is, a crustacean; but a glance at the larva shows this to be the
case in an unmistakeable manner. So again the two main divisions of
cirripedes, the pedunculated and sessile, which differ widely in external
appearance, have larvae in all their several stages barely distinguishable.
The embryo in the course of development generally rises in organisation: I
use this expression, though I am aware that it is hardly possible to define
clearly what is meant by the organisation being higher or lower. But no
one probably will dispute that the butterfly is higher than the
caterpillar. In some cases, however, the mature animal is generally
considered as lower in the scale than the larva, as with certain parasitic
crustaceans. To refer once again to cirripedes: the larvae in the first
stage have three pairs of legs, a very simple single eye, and a
probosciformed mouth, with which they feed largely, for they increase much
in size. In the second stage, answering to the chrysalis stage of
butterflies, they have six pairs of beautifully constructed natatory legs,
a pair of magnificent compound eyes, and extremely complex antennae; but
they have a closed and imperfect mouth, and cannot feed: their function at
this stage is, to search by their well-developed organs of sense, and to
reach by their active powers of swimming, a proper place on which to become
attached and to undergo their final metamorphosis. When this is completed
they are fixed for life: their legs are now converted into prehensile
organs; they again obtain a well-constructed mouth; but they have no
antennae, and their two eyes are now reconverted into a minute, single, and
very simple eye-spot. In this last and complete state, cirripedes may be
considered as either more highly or more lowly organised than they were in
the larval condition. But in some genera the larvae become developed
either into hermaphrodites having the ordinary structure, or into what I
have called complemental males: and in the latter, the development has
assuredly been retrograde; for the male is a mere sack, which lives for a
short time, and is destitute of mouth, stomach, or other organ of
importance, excepting for reproduction.
We are so much accustomed to see differences in structure between the
embryo and the adult, and likewise a close similarity in the embryos of
widely different animals within the same class, that we might be led to
look at these facts as necessarily contingent in some manner on growth.
But there is no obvious reason why, for instance, the wing of a bat, or the
fin of a porpoise, should not have been sketched out with all the parts in
proper proportion, as soon as any structure became visible in the embryo.
And in some whole groups of animals and in certain members of other groups,
the embryo does not at any period differ widely from the adult: thus Owen
has remarked in regard to cuttle-fish, 'there is no metamorphosis; the
cephalopodic character is manifested long before the parts of the embryo
are completed;' and again in spiders, 'there is nothing worthy to be called
a metamorphosis.' The larvae of insects, whether adapted to the most
diverse and active habits, or quite inactive, being fed by their parents or
placed in the midst of proper nutriment, yet nearly all pass through a
similar worm-like stage of development; but in some few cases, as in that
of Aphis, if we look to the admirable drawings by Professor Huxley of the
development of this insect, we see no trace of the vermiform stage.
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