- 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
places 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 judgement 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 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 classification. 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 have to
recur. It may even be given as
a general rule, that the less
any part of the organisation
is concerned with special habits,
the more important it becomes
for classification. 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 indications
of its true affinities. We are
least likely in the modifications
of these organs to mistake a
merely adaptive for an essential
character.' So with plants, how
remarkable it is that the organs
of vegetation, on which their
whole life depends, are of little
signification, excepting in the
first main divisions; whereas
the organs of reproduction, with
their product the seed, are of
paramount importance!
We must not, therefore, in
classifying, trust to resemblances
in parts of the organisation,
however important they may be
for the welfare of the being
in relation to the outer world.
Perhaps from this cause it has
partly arisen, that almost all
naturalists lay the greatest
stress on resemblances in organs
of high vital or physiological
importance. No doubt this view
of the classificatory importance
of organs which are important
is generally, but by no means
always, true. But their importance
for classification, I believe,
depends on their greater constancy
throughout large groups of species;
and this constancy depends on
such organs having generally
been subjected to less change
in the adaptation of the species
to their conditions of life.
That the mere physiological importance
of an organ does not determine
the classificatory value, is
almost shown by the one 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 naturalist can
have worked at any group without
being struck with this fact;
and it has been most 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 Connaraceae `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 amongst insects,
in one great division of the
Hymenoptera, the antennae, 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 probably will say
that the antennae in these two
divisions of the same order are
of unequal physiological importance.
Any number of instances could
be given of the varying importance
for classification of the same
important organ within the same
group of beings.
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 high value in classification.
No one will dispute that the
rudimentary teeth in the upper
jaws of young ruminants, and
certain rudimentary 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 rudimentary
florets are of the highest importance
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 jaws in Marsupials
-- the manner in which the wings
of insects are folded mere colour
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 Ornithorhynchus had been
covered with feathers instead
of hair, this external and trifling
character would, I think, have
been considered by naturalists
as important an aid in determining
the degree of affinity of this
strange creature to birds and
reptiles, as an approach in structure
in any one internal and important
organ.
The importance, for classification,
of trifling characters, mainly
depends on their being correlated
with several other characters
of more or less importance. 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 organisation
is universally constant. The
importance of an aggregate of
characters, even when none are
important, alone explains, I
think, that saying of Linnaeus,
that the characters do not give
the genus, but the genus gives
the characters; for this saying
seems founded on an 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.'
But when Aspicarpa produced in
France, during several years,
only 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 amongst the Malpighiaceae.
This case seems to me well to
illustrate the spirit with which
our classifications are sometimes
necessarily founded.
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 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. St. Hilaire. If certain
characters are always found correlated
with others, though no apparent
bond of connexion 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 aërating
it, or those for propagating
the race, are found nearly uniform,
they are considered as highly
serviceable in classification;
but in some groups of animals
all these, the most important
vital organs, are found to offer
characters of quite subordinate
value.
We can see why characters derived
from the embryo should be of
equal importance with those derived
from the adult, for our classifications
of course include all ages of
each species. 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 Edwards and Agassiz, that
embryonic characters are the
most important of any in the
classification of animals; and
this doctrine has very generally
been admitted as true. The same
fact holds good with flowering
plants, of which the two main
divisions have been founded on
characters derived from the embryo,
on the number and position of
the embryonic leaves or cotyledons,
and on the mode of development
of the plumule and radicle. In
our discussion on embryology,
we shall see why such characters
are so valuable, on the view
of classification tacitly including
the idea of descent.
Our classifications are often
plainly influenced by chains
of affinities. Nothing can be
easier than to define a number
of characters common to all birds;
but in the case of crustaceans,
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 onwards, can be recognised
as unequivocally 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.
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 amongst plants and insects,
of a group of forms, 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 research has detected
important structural differences,
at first overlooked, but because
numerous allied species, with
slightly different grades of
difference, have been subsequently
discovered.
All the foregoing rules and
aids and difficulties in classification
are 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, and, in
so far, all true classification
is 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 the other groups, must be
strictly genealogical in order
to be natural; but that the amount of
difference in the several branches
or groups, though allied in the
same degree in blood to their
common progenitor, may differ
greatly, being due to the different
degrees of modification which
they have undergone; and this
is expressed by the forms being
ranked under different genera,
families, sections, or orders.
The reader will best understand
what is meant, if he will take
the trouble of referring to the
diagram in the fourth chapter.
We will suppose the letters A
to L to represent allied genera,
which lived during the Silurian
epoch, and these have descended
from a species which existed
at an unknown anterior period.
Species of three of these genera
(A, F, and I) have transmitted
modified descendants to the present
day, represented by the fifteen
genera (a14 to z14) on the uppermost
horizontal line. Now all these
modified descendants from a single
species, are represented as related
in blood or descent to the same
degree; they may metaphorically
be called cousins to the same
millionth degree; yet they differ
widely and in different degrees
from each other. The forms descended
from A, now broken up into two
or three families, constitute
a distinct order from those descended
from I, also broken up into two
families. Nor can the existing
species, descended from A, be
ranked in the same genus with
the parent A; or those from I,
with the parent I. But the existing
genus F14 may be supposed to
have been but slightly modified;
and it will then rank with the
parent-genus F; just as some
few still living organic beings
belong to Silurian genera. So
that the amount or value of the
differences between organic beings
all related to each other in
the same degree in blood, has
come to be widely different.
Nevertheless their genealogical arrangement remains
strictly true, not only at the
present time, but at each successive
period of descent. All the modified
descendants from A will have
inherited something in common
from their common parent, as
will all the descendants from
I; so will it be with each subordinate
branch of descendants, at each
successive period. If, however,
we choose to suppose that any
of the descendants of A or of
I have been so much modified
as to have more or less completely
lost traces of their parentage,
in this case, their places in
a natural classification will
have been more or less completely
lost, as sometimes seems to have
occurred with existing organisms.
All the descendants of the genus
F, along its whole line of descent,
are supposed to have been but
little modified, and they yet
form a single genus. But this
genus, though much isolated,
will still occupy its proper
intermediate position; for F
originally was intermediate in
character between A and I, and
the several genera descended
from these two genera will have
inherited to a certain extent
their characters. This natural
arrangement is shown, as far
as is possible on paper, in the
diagram, but in much too simple
a manner. If a branching diagram
had not been used, and only the
names of the groups had been
written in a linear series, it
would have been still less possible
to have given a natural arrangement;
and it is notoriously not possible
to represent in a series, on
a flat surface, the affinities
which we discover in nature amongst
the beings of the same group.
Thus, on the view which I hold,
the natural system is genealogical
in its arrangement, like a pedigree;
but the degrees of modification
which the different groups have
undergone, have to be expressed
by ranking them under different
so-called genera, sub-families,
families, sections, orders, and
classes.
It may be worth while to illustrate
this view of classification,
by taking the case of languages.
If we possessed a perfect pedigree
of mankind, a genealogical arrangement
of the races of man would afford
the best classification of the
various languages now spoken
throughout the world; and if
all extinct languages, and all
intermediate and slowly changing
dialects, had to be included,
such an arrangement would, I
think, be the only possible one.
Yet it might be that some very
ancient language had altered
little, and had given rise to
few new languages, whilst others
(owing to the spreading and subsequent
isolation and states of civilisation
of the several races, descended
from a common race) had altered
much, and had given rise to many
new languages and dialects. The
various degrees of difference
in the languages from the same
stock, would have to be expressed
by groups subordinate to groups;
but the proper or even only possible
arrangement would still be genealogical;
and this would be strictly natural,
as it would connect together
all languages, extinct and modern,
by the closest affinities, and
would give the filiation and
origin of each tongue.
In confirmation
of this view, let us glance
at the classification
of varieties, which are believed
or known to have descended from
one species. These are grouped
under species, with sub-varieties
under varieties; and with our
domestic productions, several
other grades of difference are
requisite, as we have seen with
pigeons. The origin of the existence
of groups subordinate to groups,
is the same with varieties as
with species, namely, closeness
of descent with various degrees
of modification. Nearly the same
rules are followed in classifying
varieties, as with species. Authors
have insisted on the necessity
of classing varieties on a natural
instead of an artificial system;
we are cautioned, for instance,
not to class two varieties of
the pine-apple together, merely
because their fruit, though the
most important part, happens
to be nearly identical; no one
puts the swedish and common turnips
together, though the esculent
and thickened stems are so similar.
Whatever part is found to be
most constant, is used in classing
varieties: thus the great agriculturist
Marshall says the horns are very
useful for this purpose with
cattle, because they are less
variable than the shape or colour
of the body, &c.; whereas
with sheep the horns are much
less serviceable, because less
constant. In classing varieties,
I apprehend if we had a real
pedigree, a genealogical classification
would be universally preferred;
and it has been attempted by
some authors. For we might feel
sure, whether there had been
more or less modification, the
principle of inheritance would
keep the forms together which
were allied in the greatest number
of points. In tumbler pigeons,
though some sub-varieties differ
from the others in the important
character of having a longer
beak, yet all are kept together
from having the common habit
of tumbling; but the short-faced
breed has nearly or quite lost
this habit; nevertheless, without
any reasoning or thinking on
the subject, these tumblers are
kept in the same group, because
allied in blood and alike in
some other respects. If it could
be proved that the Hottentot
had descended from the Negro,
I think he would be classed under
the Negro group, however much
he might differ in colour and
other important characters from
negroes.
With species in a state of
nature, every naturalist has
in fact brought descent into
his classification; for he includes
in his lowest grade, or that
of a species, the two sexes;
and how enormously these sometimes
differ in the most important
characters, is known to every
naturalist: scarcely a single
fact can be predicated in common
of the males and hermaphrodites
of certain cirripedes, when adult,
and yet no one dreams of separating
them. The naturalist includes
as one species the several larval
stages of the same individual,
however much they may differ
from each other and from the
adult; as he likewise includes
the so-called alternate generations
of Steenstrup, which can only
in a technical sense be considered
as the same individual. He includes
monsters; he includes varieties,
not solely because they closely
resemble the parent-form, but
because they are descended from
it. He who believes that the
cowslip is descended from the
primrose, or conversely, ranks
them together as a single species,
and gives a single definition.
As soon as three Orchidean forms
(Monochanthus, Myanthus, and
Catasetum), which had previously
been ranked as three distinct
genera, were known to be sometimes
produced on the same spike, they
were immediately included as
a single species. But it may
be asked, what ought we to do,
if it could be proved that one
species of kangaroo had been
produced, by a long course of
modification, from a bear? Ought
we to rank this one species with
bears, and what should we do
with the other species? The supposition
is of course preposterous; and
I might answer by the argumentum
ad hominem, and ask what
should be done if a perfect kangaroo
were seen to come out of the
womb of a bear? According to
all analogy, it would be ranked
with bears; but then assuredly
all the other species of the
kangaroo family would have to
be classed under the bear genus.
The whole case is preposterous;
for where there has been close
descent in common, there will
certainly be close resemblance
or affinity.
As descent has universally
been used in classing together
the individuals of the same species,
though the males and females
and larvae are sometimes extremely
different; and as it has been
used in classing varieties which
have undergone a certain, and
sometimes a considerable amount
of modification, may not this
same element of descent have
been unconsciously used in grouping
species under genera, and genera
under higher groups, though in
these cases the modification
has been greater in degree, and
has taken a longer time to complete?
I believe it has thus been unconsciously
used; and only thus can I understand
the several rules and guides
which have been followed by our
best systematists. We have no
written pedigrees; we have to
make out community of descent
by resemblances of any kind.
Therefore we choose those characters
which, as far as we can judge,
are the least likely to have
been modified in relation to
the conditions of life to which
each species has been recently
exposed. Rudimentary structures
on this view are as good as,
or even sometimes better than,
other parts of the organisation.
We care not how trifling a character
may be let it be the mere inflection
of the angle of the jaw, the
manner in which an insect's wing
is folded, whether the skin be
covered by hair or feathers if
it prevail throughout many and
different species, especially
those having very different habits
of life, it assumes high value;
for we can account for its presence
in so many forms with such different
habits, only by its inheritance
from a common parent. We may
err in this respect in regard
to single points of structure,
but when several characters,
let them be ever so trifling,
occur together throughout a large
group of beings having different
habits, we may feel almost sure,
on the theory of descent, that
these characters have been inherited
from a common ancestor. And we
know that such correlated or
aggregated characters have especial
value in classification.
We can understand why a species
or a group of species may depart,
in several of its most important
characteristics, from its allies,
and yet be safely classed with
them. This may be safely done,
and is often done, as long as
a sufficient number of characters,
let them be ever so unimportant,
betrays the hidden bond of community
of descent. Let two forms have
not a single character in common,
yet if these extreme forms are
connected together by a chain
of intermediate groups, we may
at once infer their community
of descent, and we put them all
into the same class. As we find
organs of high physiological
importance those which serve
to preserve life under the most
diverse conditions of existence
are generally the most constant,
we attach especial value to them;
but if these same organs, in
another group or section of a
group, are found to differ much,
we at once value them less in
our classification. We shall
hereafter, I think, clearly see
why embryological characters
are of such high classificatory
importance. Geographical distribution
may sometimes be brought usefully
into play in classing large and
widely-distributed genera, because
all the species of the same genus,
inhabiting any distinct and isolated
region, have in all probability
descended from the same parents.
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 in 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.
How, then, can we explain these
several facts in embryology,
namely the very general, but
not universal difference in structure
between the embryo and the adult;
of parts in the same individual
embryo, which ultimately become
very unlike and serve for diverse
purposes, being at this early
period of growth alike; of embryos
of different species within the
same class, generally, but not
universally, resembling each
other; of the structure of the
embryo not being closely related
to its conditions of existence,
except when the embryo becomes
at any period of life active
and has to provide for itself;
of the embryo apparently having
sometimes a higher organisation
than the mature animal, into
which it is developed. I believe
that all these facts can be explained,
as follows, on the view of descent
with modification.
It is commonly assumed, perhaps
from monstrosities often affecting
the embryo at a very early period,
that slight variations necessarily
appear at an equally early period.
But we have little evidence on
this head indeed the evidence
rather points the other way;
for it is notorious that breeders
of cattle, horses, and various
fancy animals, cannot positively
tell, until some time after the
animal has been born, what its
merits or form will ultimately
turn out. We see this plainly
in our own children; we cannot
always tell whether the child
will be tall or short, or what
its precise features will be.
The question is not, at what
period of life any variation
has been caused, but at what
period it is fully displayed.
The cause may have acted, and
I believe generally has acted,
even before the embryo is formed;
and the variation may be due
to the male and female sexual
elements having been affected
by the conditions to which either
parent, or their ancestors, have
been exposed. Nevertheless an
effect thus caused at a very
early period, even before the
formation of the embryo, may
appear late in life; as when
an hereditary disease, which
appears in old age alone, has
been communicated to the offspring
from the reproductive element
of one parent. Or again, as when
the horns of cross-bred cattle
have been affected by the shape
of the horns of either parent.
For the welfare of a very young
animal, as long as it remains
in its mother's womb, or in the
egg, or as long as it is nourished
and protected by its parent,
it must be quite unimportant
whether most of its characters
are fully acquired a little earlier
or later in life. It would not
signify, for instance, to a bird
which obtained its food best
by having a long beak, whether
or not it assumed a beak of this
particular length, as long as
it was fed by its parents. Hence,
I conclude, that it is quite
possible, that each of the many
successive modifications, by
which each species has acquired
its present structure, may have
supervened at a not very early
period of life; and some direct
evidence from our domestic animals
supports this view. But in other
cases it is quite possible that
each successive modification,
or most of them, may have appeared
at an extremely early period.
I have stated in the first
chapter, that there is some evidence
to render it probable, that at
whatever age any variation first
appears in the parent, it tends
to reappear at a corresponding
age in the offspring. Certain
variations can only appear at
corresponding ages, for instance,
peculiarities in the caterpillar,
cocoon, or imago states of the
silk-moth; or, again, in the
horns of almost full-grown cattle.
But further than this, variations
which, for all that we can see,
might have appeared earlier or
later in life, tend to appear
at a corresponding age in the
offspring and parent. I am far
from meaning that this is invariably
the case; and I could give a
good many cases of variations
(taking the word in the largest
sense) which have supervened
at an earlier age in the child
than in the parent.
These two principles, if their
truth be admitted, will, I believe,
explain all the above specified
leading facts in embryology.
But first let us look at a few
analogous cases in domestic varieties.
Some authors who have written
on Dogs, maintain that the greyhound
and bulldog, though appearing
so different, are really varieties
most closely allied, and have
probably descended from the same
wild stock; hence I was curious
to see how far their puppies
differed from each other: I was
told by breeders that they differed
just as much as their parents,
and this, judging by the eye,
seemed almost to be the case;
but on actually measuring the
old dogs and their six-days old
puppies, I found that the puppies
had not nearly acquired their
full amount of proportional difference.
So, again, I was told that the
foals of cart and race-horses
differed as much as the full-grown
animals; and this surprised me
greatly, as I think it probable
that the difference between these
two breeds has been wholly caused
by selection under domestication;
but having had careful measurements
made of the dam and of a three-days
old colt of a race and heavy
cart-horse, I find that the colts
have by no means acquired their
full amount of proportional difference.
As the evidence appears to
me conclusive, that the several
domestic breeds of pigeon have
descended from one wild species,
I compared young pigeons of various
breeds, within twelve hours after
being hatched; I carefully measured
the proportions (but will not
here give details) of the beak,
width of mouth, length of nostril
and of eyelid, size of feet and
length of leg, in the wild stock,
in pouters, fantails, runts,
barbs, dragons, carriers, and
tumblers. Now some of these birds,
when mature, differ so extraordinarily
in length and form of beak, that
they would, I cannot doubt, be
ranked in distinct genera, had
they been natural productions.
But when the nestling birds of
these several breeds were placed
in a row, though most of them
could be distinguished from each
other, yet their proportional
differences in the above specified
several points were incomparably
less than in the full-grown birds.
Some characteristic points of
difference for instance, that
of the width of mouth -- could
hardly be detected in the young.
But there was one remarkable
exception to this rule, for the
young of the short-faced tumbler
differed from the young of the
wild rock-pigeon and of the other
breeds, in all its proportions,
almost exactly as much as in
the adult state.
The two principles above given
seem to me to explain these facts
in regard to the later embryonic
stages of our domestic varieties.
Fanciers select their horses,
dogs, and pigeons, for breeding,
when they are nearly grown up:
they are indifferent whether
the desired qualities and structures
have been acquired earlier or
later in life, if the full-grown
animal possesses them. And the
cases just given, more especially
that of pigeons, seem to show
that the characteristic differences
which give value to each breed,
and which have been accumulated
by man's selection, have not
generally first appeared at an
early period of life, and have
been inherited by the offspring
at a corresponding not early
period. But the case of the short-faced
tumbler, which when twelve hours
old had acquired its proper proportions,
proves that this is not the universal
rule; for here the characteristic
differences must either have
appeared at an earlier period
than usual, or, if not so, the
differences must have been inherited,
not at the corresponding, but
at an earlier age.
Now let us apply these facts
and the above two principles
which latter, though not proved
true, can be shown to be in some
degree probable to species in
a state of nature. Let us take
a genus of birds, descended on
my theory from some one parent-species,
and of which the several new
species have become modified
through natural selection in
accordance with their diverse
habits. Then, from the many slight
successive steps of variation
having supervened at a rather
late age, and having been inherited
at a corresponding age, the young
of the new species of our supposed
genus will manifestly tend to
resemble each other much more
closely than do the adults, just
as we have seen in the case of
pigeons. We may extend this view
to whole families or even classes.
The fore-limbs, for instance,
which served as legs in the parent-species,
may become, by a long course
of modification, adapted in one
descendant to act as hands, in
another as paddles, in another
as wings; and on the above two
principles namely of each successive
modification supervening at a
rather late age, and being inherited
at a corresponding late age the
fore-limbs in the embryos of
the several descendants of the
parent-species will still resemble
each other closely, for they
will not have been modified.
But in each individual new species,
the embryonic fore-limbs will
differ greatly from the fore-limbs
in the mature animal; the limbs
in the latter having undergone
much modification at a rather
late period of life, and having
thus been converted into hands,
or paddles, or wings. Whatever
influence long-continued exercise
or use on the one hand, and disuse
on the other, may have in modifying
an organ, such influence will
mainly affect the mature animal,
which has come to its full powers
of activity and has to gain its
own living; and the effects thus
produced will be inherited at
a corresponding mature age. Whereas
the young will remain unmodified,
or be modified in a lesser degree,
by the effects of use and disuse.
In certain
cases the successive steps
of variation might supervene,
from causes of which we are wholly
ignorant, at a very early period
of life, or each step might be
inherited at an earlier period
than that at which it first appeared.
In either case (as with the short-faced
tumbler) the young or embryo
would closely resemble the mature
parent-form. We have seen that
this is the rule of development
in certain whole groups of animals,
as with cuttle-fish and spiders,
and with a few members of the
great class of insects, as with
Aphis. With respect to the final
cause of the young in these cases
not undergoing any metamorphosis,
or closely resembling their parents
from their earliest age, we can
see that this would result from
the two following contingencies;
firstly, from the young, during
a course of modification carried
on for many generations, having
to provide for their own wants
at a very early stage of development,
and secondly, from their following
exactly the same habits of life
with their parents; for in this
case, it would be indispensable
for the existence of the species,
that the child should be modified
at a very early age in the same
manner with its parents, in accordance
with their similar habits. Some
further explanation, however,
of the embryo not undergoing
any metamorphosis is perhaps
requisite. If, on the other hand,
it profited the young to follow
habits of life in any degree
different from those of their
parent, and consequently to be
constructed in a slightly different
manner, then, on the principle
of inheritance at corresponding
ages, the active young or larvae
might easily be rendered by natural
selection different to any conceivable
extent from their parents. Such
differences might, also, become
correlated with successive stages
of development; so that the larvae,
in the first stage, might differ
greatly from the larvae in the
second stage, as we have seen
to be the case with cirripedes.
The adult might become fitted
for sites or habits, in which
organs of locomotion or of the
senses, &c., would be useless;
and in this case the final metamorphosis
would be said to be retrograde.
As all the organic beings,
extinct and recent, which have
ever lived on this earth have
to be classed together, and as
all have been connected by the
finest gradations, the best,
or indeed, if our collections
were nearly perfect, the only
possible arrangement, would be
genealogical. Descent being on
my view the hidden bond of connexion
which naturalists have been seeking
under the term of the natural
system. On this view we can understand
how it is that, in the eyes of
most naturalists, the structure
of the embryo is even more important
for classification than that
of the adult. For the embryo
is the animal in its less modified
state; and in so far it reveals
the structure of its progenitor.
In two groups of animal, however
much they may at present differ
from each other in structure
and habits, if they pass through
the same or similar embryonic
stages, we may feel assured that
they have both descended from
the same or nearly similar parents,
and are therefore in that degree
closely related. Thus, community
in embryonic structure reveals
community of descent. It will
reveal this community of descent,
however much the structure of
the adult may have been modified
and obscured; we have seen, for
instance, that cirripedes can
at once be recognised by their
larvae as belonging to the great
class of crustaceans. As the
embryonic state of each species
and group of species partially
shows us the structure of their
less modified ancient progenitors,
we can clearly see why ancient
and extinct forms of life should
resemble the embryos of their
descendants, our existing species.
Agassiz believes this to be a
law of nature; but I am bound
to confess that I only hope to
see the law hereafter proved
true. It can be proved true in
those cases alone in which the
ancient state, now supposed to
be represented in many embryos,
has not been obliterated, either
by the successive variations
in a long course of modification
having supervened at a very early
age, or by the variations having
been inherited at an earlier
period than that at which they
first appeared. It should also
be borne in mind, that the supposed
law of resemblance of ancient
forms of life to the embryonic
stages of recent forms, may be
true, but yet, owing to the geological
record not extending far enough
back in time, may remain for
a long period, or for ever, incapable
of demonstration.
Thus, as it seems to me, the
leading facts in embryology,
which are second in importance
to none in natural history, are
explained on the principle of
slight modifications not appearing,
in the many descendants from
some one ancient progenitor,
at a very early period in the
life of each, though perhaps
caused at the earliest, and being
inherited at a corresponding
not early period. Embryology
rises greatly in interest, when
we thus look at the embryo as
a picture, more or less obscured,
of the common parent-form of
each great class of animals.
Rudimentary, atrophied, or
aborted organs
Organs or parts in this strange
condition, bearing the stamp
of inutility, are extremely common
throughout nature. For instance,
rudimentary mammae are very general
in the males of mammals: I presume
that the `bastard-wing' in birds
may be safely considered as a
digit in a rudimentary state:
in very many snakes one lobe
of the lungs is rudimentary;
in other snakes there are rudiments
of the pelvis and hind limbs.
Some of the cases of rudimentary
organs are extremely curious;
for instance, the presence of
teeth in foetal whales, which
when grown up have not a tooth
in their heads; and the presence
of teeth, which never cut through
the gums, in the upper jaws of
our unborn calves. It has even
been stated on good authority
that rudiments of teeth can be
detected in the beaks of certain
embryonic birds. Nothing can
be plainer than that wings are
formed for flight, yet in how
many insects do we see wings
so reduced in size as to be utterly
incapable of flight, and not
rarely lying under wing-cases,
firmly soldered together!
The meaning
of rudimentary organs is often
quite unmistakeable:
for instance there are beetles
of the same genus (and even of
the same species) resembling
each other most closely in all
respects, one of which will have
full-sized wings, and another
mere rudiments of membrane; and
here it is impossible to doubt,
that the rudiments represent
wings. Rudimentary organs sometimes
retain their potentiality, and
are merely not developed: this
seems to be the case with the
mammae of male mammals, for many
instances are on record of these
organs having become well developed
in full-grown males, and having
secreted milk. So again there
are normally four developed and
two rudimentary teats in the
udders of the genus Bos, but
in our domestic cows the two
sometimes become developed and
give milk. In individual plants
of the same species the petals
sometimes occur as mere rudiments,
and sometimes in a well-developed
state. In plants with separated
sexes, the male flowers often
have a rudiment of a pistil;
and Kölreuter found that
by crossing such male plants
with an hermaphrodite species,
the rudiment of the pistil in
the hybrid offspring was much
increased in size; and this shows
that the rudiment and the perfect
pistil are essentially alike
in nature.
An organ serving for two purposes,
may become rudimentary or utterly
aborted for one, even the more
important purpose;, and remain
perfectly efficient for the other.
Thus in plants, the office of
the pistil is to allow the pollen-tubes
to reach the ovules protected
in the ovarium at its base. The
pistil consists of a stigma supported
on the style; but in some Compositae,
the male florets, which of course
cannot be fecundated, have a
pistil, which is in a rudimentary
state, for it is not crowned
with a stigma; but the style
remains well developed, and is
clothed with hairs as in other
compositae, for the purpose of
brushing the pollen out of the
surrounding anthers. Again, an
organ may become rudimentary
for its proper purpose, and be
used for a distinct object: in
certain fish the swim-bladder
seems to be rudimentary for its
proper function of giving buoyancy,
but has become converted into
a nascent breathing organ or
lung. Other similar instances
could be given.
Rudimentary organs in the individuals
of the same species are very
liable to vary in degree of development
and in other respects. Moreover,
in closely allied species, the
degree to which the same organ
has been rendered rudimentary
occasionally differs much. This
latter fact is well exemplified
in the state of the wings of
the female moths in certain groups.
Rudimentary organs may be utterly
aborted; and this implies, that
we find in an animal or plant
no trace of an organ, which analogy
would lead us to expect to find,
and which is occasionally found
in monstrous individuals of the
species. Thus in the snapdragon
(antirrhinum) we generally do
not find a rudiment of a fifth
stamen; but this may sometimes
be seen. In tracing the homologies
of the same part in different
members of a class, nothing is
more common, or more necessary,
than the use and discovery of
rudiments. This is well shown
in the drawings given by Owen
of the bones of the leg of the
horse, ox, and rhinoceros.
It is an important fact that
rudimentary organs, such as teeth
in the upper jaws of whales and
ruminants, can often be detected
in the embryo, but afterwards
wholly disappear. It is also,
I believe, a universal rule,
that a rudimentary part or organ
is of greater size relatively
to the adjoining parts in the
embryo, than in the adult; so
that the organ at this early
age is less rudimentary, or even
cannot be said to be in any degree
rudimentary. Hence, also, a rudimentary
organ in the adult, is often
said to have retained its embryonic
condition.
I have now given the leading
facts with respect to rudimentary
organs. In reflecting on them,
every one must be struck with
astonishment: for the same reasoning
power which tells us plainly
that most parts and organs are
exquisitely adapted for certain
purposes, tells us with equal
plainness that these rudimentary
or atrophied organs, are imperfect
and useless. In works on natural
history rudimentary organs are
generally said to have been created
`for the sake of symmetry,' or
in order `to complete the scheme
of nature;' but this seems to
me no explanation, merely a restatement
of the fact. Would it be thought
sufficient to say that because
planets revolve in elliptic courses
round the sun, satellites follow
the same course round the planets,
for the sake of symmetry, and
to complete the scheme of nature?
An eminent physiologist accounts
for the presence of rudimentary
organs, by supposing that they
serve to excrete matter in excess,
or injurious to the system; but
can we suppose that the minute
papilla, which often represents
the pistil in male flowers, and
which is formed merely of cellular
tissue, can thus act? Can we
suppose that the formation of
rudimentary teeth which are subsequently
absorbed, can be of any service
to the rapidly growing embryonic
calf by the excretion of precious
phosphate of lime? When a man's
fingers have been amputated,
imperfect nails sometimes appear
on the stumps: I could as soon
believe that these vestiges of
nails have appeared, not from
unknown laws of growth, but in
order to excrete horny matter,
as that the rudimentary nails
on the fin of the manatee were
formed for this purpose.
On my view of descent with
modification, the origin of rudimentary
organs is simple. We have plenty
of cases of rudimentary organs
in our domestic productions,
as the stump of a tail in tailless
breeds, the vestige of an ear
in earless breeds, -- the reappearance
of minute dangling horns in hornless
breeds of cattle, more especially,
according to Youatt, in young
animals, and the state of the
whole flower in the cauliflower.
We often see rudiments of various
parts in monsters. But I doubt
whether any of these cases throw
light on the origin of rudimentary
organs in a state of nature,
further than by showing that
rudiments can be produced; for
I doubt whether species under
nature ever undergo abrupt changes.
I believe that disuse has been
the main agency; that it has
led in successive generations
to the gradual reduction of various
organs, until they have become
rudimentary, as in the case of
the eyes of animals inhabiting
dark caverns, and of the wings
of birds inhabiting oceanic islands,
which have seldom been forced
to take flight, and have ultimately
lost the power of flying. Again,
an organ useful under certain
conditions, might become injurious
under others, as with the wings
of beetles living on small and
exposed islands; and in this
case natural selection would
continue slowly to reduce the
organ, until it was rendered
harmless and rudimentary.
Any change in function, which
can be effected by insensibly
small steps, is within the power
of natural selection; so that
an organ rendered, during changed
habits of life, useless or injurious
for one purpose, might easily
be modified and used for another
purpose. Or an organ might be
retained for one alone of its
former functions. An organ, when
rendered useless, may well be
variable, for its variations
cannot be checked by natural
selection. At whatever period
of life disuse or selection reduces
an organ, and this will generally
be when the being has come to
maturity and to its full powers
of action, the principle of inheritance
at corresponding ages will reproduce
the organ in its reduced state
at the same age, and consequently
will seldom affect or reduce
it in the embryo. Thus we can
understand the greater relative
size of rudimentary organs in
the embryo, and their lesser
relative size in the adult. But
if each step of the process of
reduction were to be inherited,
not at the corresponding age,
but at an extremely early period
of life (as we have good reason
to believe to be possible) the
rudimentary part would tend to
be wholly lost, and we should
have a case of complete abortion.
The principle, also, of economy,
explained in a former chapter,
by which the materials forming
any part or structure, if not
useful to the possessor, will
be saved as far as is possible,
will probably often come into
play; and this will tend to cause
the entire obliteration of a
rudimentary organ.
As the presence of rudimentary
organs is thus due to the tendency
in every part of the organisation,
which has long existed, to be
inherited we can understand,
on the genealogical view of classification,
how it is that systematists have
found rudimentary parts as useful
as, or even sometimes more useful
than, parts of high physiological
importance. Rudimentary organs
may be compared with the letters
in a word, still retained in
the spelling, but become useless
in the pronunciation, but which
serve as a clue in seeking for
its derivation. On the view of
descent with modification, we
may conclude that the existence
of organs in a rudimentary, imperfect,
and useless condition, or quite
aborted, far from presenting
a strange difficulty, as they
assuredly do on the ordinary
doctrine of creation, might even
have been anticipated, and can
be accounted for by the laws
of inheritance.
Summary
In this chapter I have attempted
to show, that the subordination
of group to group in all organisms
throughout all time; that the
nature of the relationship, by
which all living and extinct
beings are united by complex,
radiating, and circuitous lines
of affinities into one grand
system; the rules followed and
the difficulties encountered
by naturalists in their classifications;
the value set upon characters,
if constant and prevalent, whether
of high vital importance, or
of the most trifling importance,
or, as in rudimentary organs,
of no importance; the wide opposition
in value between analogical or
adaptive characters, and characters
of true affinity; and other such
rules; all naturally follow on
the view of the common parentage
of those forms which are considered
by naturalists as allied, together
with their modification through
natural selection, with its contingencies
of extinction and divergence
of character. In considering
this view of classification,
it should be borne in mind that
the element of descent has been
universally used in ranking together
the sexes, ages, and acknowledged
varieties of the same species,
however different they may be
in structure. If we extend the
use of this element of descent,
the only certainly known cause
of similarity in organic beings,
we shall understand what is meant
by the natural system: it is
genealogical in its attempted
arrangement, with the grades
of acquired difference marked
by the terms varieties, species,
genera, families, orders, and
classes.
On this same view of descent
with modification, all the great
facts in Morphology become intelligible,
whether we look to the same pattern
displayed in the homologous organs,
to whatever purpose applied,
of the different species of a
class; or to the homologous parts
constructed on the same pattern
in each individual animal and
plant.
On the principle of successive
slight variations, not necessarily
or generally supervening at a
very early period of life, and
being inherited at a corresponding
period, we can understand the
great leading facts in Embryology;
namely, the resemblance in an
individual embryo of the homologous
parts, which when matured will
become widely different from
each other in structure and function;
and the resemblance in different
species of a class of the homologous
parts or organs, though fitted
in the adult members for purposes
as different as possible. Larvae
are active embryos, which have
become specially modified in
relation to their habits of life,
through the principle of modifications
being inherited at corresponding
ages. On this same principle
and bearing in mind, that when
organs are reduced in size, either
from disuse or selection, it
will generally be at that period
of life when the being has to
provide for its own wants, and
bearing in mind how strong is
the principle of inheritance
the occurrence of rudimentary
organs and their final abortion,
present to us no inexplicable
difficulties; on the contrary,
their presence might have been
even anticipated. The importance
of embryological characters and
of rudimentary organs in classification
is intelligible, on the view
that an arrangement is only so
far natural as it is genealogical.
Finally, the several classes
of facts which have been considered
in this chapter, seem to me to
proclaim so plainly, that the
innumerable species, genera,
and families of organic beings,
with which this world is peopled,
have all descended, each within
its own class or group, from
common parents, and have all
been modified in the course of
descent, that I should without
hesitation adopt this view, even
if it were unsupported by other
facts or arguments. |