 Chapter
13 - Mutual Affinities of Organic Beings: Morphology: Embryology:
Rudimentary Organs
*
CLASSIFICATION, groups subordinate to groups *
Natural system *
Rules and difficulties in classification, explained on the
theory of descent with modification *
Classification of varieties *
Descent always used in classification *
Analogical or adaptive characters *
Affinities, general, complex and radiating *
Extinction separates and defines groups *
MORPHOLOGY, between members of the same class, between parts
of the same individual *
EMBRYOLOGY, laws of, explained by variations not supervening
at an early age, and being inherited at a corresponding
age *
RUDIMENTARY ORGANS; their origin explained *
Summary
From the first dawn of life, all organic beings are found
to resemble each other in descending degrees, so that they
can be classed in groups under groups. This classification
is evidently not arbitrary like the grouping of the stars
in constellations. The existence of groups would have been
of simple signification, if one group had been exclusively
fitted to inhabit the land, and another the water; one to
feed on flesh, another on vegetable matter, and so on; but
the case is widely different in nature; for it is notorious
how commonly members of even the same subgroup have different
habits. In our second and fourth chapters, on Variation
and on Natural Selection, I have attempted to show that
it is the widely ranging, the much diffused and common,
that is the dominant species belonging to the larger genera,
which vary most. The varieties, or incipient species, thus
produced ultimately become converted, as I believe, into
new and distinct species; and these, on the principle of
inheritance, tend to produce other new and dominant species.
Consequently the groups which are now large, and which generally
include many dominant species, tend to go on increasing
indefinitely in size. I further attempted to show that from
the varying descendants of each species trying to occupy
as many and as different 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.
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