On the Origin of Species by Means of Natural Selection
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Chapter XIII
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 judgment 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 its 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 aerating 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.
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