The Origin of Species
Chapter 13: Mutual Affinities of Organic Beings: Morphology: Embryology:
Rudimentary Organs
by Charles Darwin
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 - bAnalogical 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.