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Origin of the Species
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 - Analogical or adaptive characters - Affinities, general,
complex and radiating - Extinction separates and defines groups - MORPHOLOGY,
between members of the same class, between parts of the same individual -
EMBRYOLOGY, laws of, explained by variations not supervening at an early age,
and being inherited at a corresponding age - RUDIMENTARY ORGANS; their origin
explained - Summary
From the first dawn of life, all organic beings are found to resemble each other
in descending degrees, so that they can be classed in groups under groups. This
classification is evidently not arbitrary like the grouping of the stars in
constellations. The existence of groups would have been of simple signification,
if one group had been exclusively fitted to inhabit the land, and another the
water; one to feed on flesh, another on vegetable matter, and so on; but the
case is widely different in nature; for it is notorious how commonly members of
even the same subgroup have different habits. In our second and fourth chapters,
on Variation and on Natural Selection, I have attempted to show that it is the
widely ranging, the much diffused and common, that is the dominant species
belonging to the larger genera, which vary most. The varieties, or incipient
species, thus produced ultimately become converted, as I believe, into new and
distinct species; and these, on the principle of inheritance, tend to produce
other new and dominant species. Consequently the groups which are now large, and
which generally include many dominant species, tend to go on increasing
indefinitely in size. I further attempted to show that from the varying
descendants of each species trying to occupy as many and as different places as
possible in the economy of nature, there is a constant tendency in their
characters to diverge. This conclusion was supported by looking at the great
diversity of the forms of life which, in any small area, come into the closest
competition, and by looking to certain facts in naturalisation.
I attempted also to show that there is a constant tendency in the forms which
are increasing in number and diverging in character, to supplant and exterminate
the less divergent, the less improved, and preceding forms. I request the reader
to turn to the diagram illustrating the action, as formerly explained, of these
several principles; and he will see that the inevitable result is that the
modified descendants proceeding from one progenitor become broken up into groups
subordinate to groups. In the diagram each letter on the uppermost line may
represent a genus including several species; and all the genera on this line
form together one class, for all have descended from one ancient but unseen
parent, and, consequently, have inherited something in common. But the three
genera on the left hand have, on this same principle, much in common, and form a
sub-family, distinct from that including the next two genera on the right hand,
which diverged from a common parent at the fifth stage of descent. These five
genera have also much, though less, in common; and they form a family distinct
from that including the three genera still further to the right hand, which
diverged at a still earlier period. And all these genera, descended from (A),
form an order distinct from the genera descended from (I). So that we here have
many species descended from a single progenitor grouped into genera; and the
genera are included in, or subordinate to, sub-families, families, and orders,
all united into one class. Thus, the grand fact in natural history of the
subordination of group under group, which, from its familiarity, does not always
sufficiently strike us, is in my judgement fully explained.
Naturalists try to arrange the species, genera, and families in each class, on
what is called the Natural System. But what is meant by this system? Some
authors look at it merely as a scheme for arranging together those living
objects which are most alike, and for separating those which are most unlike; or
as an artificial means for enunciating, as briefly as possible, general
propositions, that is, by one sentence to give the characters common, for
instance, to all mammals, by another those common to all carnivora, by another
those common to the dog-genus, and then by adding a single sentence, a full
description is given of each kind of dog. The ingenuity and utility of this
system are indisputable. But many naturalists think that something more is meant
by the Natural System; they believe that it reveals the plan of the Creator; but
unless it be specified whether order in time or space, or what else is meant by
the plan of the Creator, it seems to me that nothing is thus added to our
knowledge. Such expressions as that famous one of Linnaeus, and which we often
meet with in a more or less concealed form, that the characters do not make the
genus, but that the genus gives the characters, seem to imply that something
more is included in our classification, than mere resemblance. I believe that
something more is included; and that propinquity of descent, the only known
cause of the similarity of organic beings, is the bond, hidden as it is by
various degrees of modification, which is partially revealed to us by our
classifications.
Let us now consider the rules followed in classification, and the difficulties
which are encountered on the view that classification either gives some unknown
plan of creation, or is simply a scheme for enunciating general propositions and
of placing together the forms most like each other. It might have been thought
(and was in ancient times thought) that those parts of the structure which
determined the habits of life, and the general place of each being in the
economy of nature, would be of very high importance in classification. Nothing
can be more false. No one regards the external similarity of a mouse to a shrew,
of a dugong to a whale, of a whale to a fish, as of any importance. These
resemblances, though so intimately connected with the whole life of the being,
are ranked as merely `adaptive or analogical characters;' but to the
consideration of these resemblances we shall have to recur. It may even be given
as a general rule, that the less any part of the organisation is concerned with
special habits, the more important it becomes for classification. As an
instance: Owen, in speaking of the dugong, says, `The generative organs being
those which are most remotely related to the habits and food of an animal, I
have always regarded as affording very clear indications of its true affinities.
We are least likely in the modifications of these organs to mistake a merely
adaptive for an essential character.' So with plants, how remarkable it is that
the organs of vegetation, on which their whole life depends, are of little
signification, excepting in the first main divisions; whereas the organs of
reproduction, with their product the seed, are of paramount importance!
We must not, therefore, in classifying, trust to resemblances in parts of the
organisation, however important they may be for the welfare of the being in
relation to the outer world. Perhaps from this cause it has partly arisen, that
almost all naturalists lay the greatest stress on resemblances in organs of high
vital or physiological importance. No doubt this view of the classificatory
importance of organs which are important is generally, but by no means always,
true. But their importance for classification, I believe, depends on their
greater constancy throughout large groups of species; and this constancy depends
on such organs having generally been subjected to less change in the adaptation
of the species to their conditions of life. That the mere physiological
importance of an organ does not determine the classificatory value, is almost
shown by the one fact, that in allied groups, in which the same organ, as we
have every reason to suppose, has nearly the same physiological value, its
classificatory value is widely different. No naturalist can have worked at any
group without being struck with this fact; and it has been most fully
acknowledged in the writings of almost every author. It will suffice to quote
the highest authority, Robert Brown, who in speaking of certain organs in the
Proteaceae, says their generic importance, `like that of all their parts, not
only in this but, as I apprehend, in every natural family, is very unequal, and
in some cases seems to be entirely lost.' Again in another work he says, the
genera of the Connaraceae `differ in having one or more ovaria, in the existence
or absence of albumen, in the imbricate or valvular aestivation. Any one of
these characters singly is frequently of more than generic importance, though
here even when all taken together they appear insufficient to separate Cnestis
from Connarus.' To give an example amongst insects, in one great division of the
Hymenoptera, the antennae, as Westwood has remarked, are most constant in
structure; in another division they differ much, and the differences are of
quite subordinate value in classification; yet no one probably will say that the
antennae in these two divisions of the same order are of unequal physiological
importance. Any number of instances could be given of the varying importance for
classification of the same important organ within the same group of beings.
Again, no one will say that rudimentary or atrophied organs are of high
physiological or vital importance; yet, undoubtedly, organs in this condition
are often of high value in classification. No one will dispute that the
rudimentary teeth in the upper jaws of young ruminants, and certain rudimentary
bones of the leg, are highly serviceable in exhibiting the close affinity
between Ruminants and Pachyderms. Robert Brown has strongly insisted on the fact
that the rudimentary florets are of the highest importance in the classification
of the Grasses.
Numerous instances could be given of characters derived from parts which must be
considered of very trifling physiological importance, but which are universally
admitted as highly serviceable in the definition of whole groups. For instance,
whether or not there is an open passage from the nostrils to the mouth, the only
character, according to Owen, which absolutely distinguishes fishes and reptiles
the inflection of the angle of the jaws in Marsupials -- the manner in which the
wings of insects are folded mere colour in certain Algae mere pubescence on
parts of the flower in grasses the nature of the dermal covering, as hair or
feathers, in the Vertebrata. If the Ornithorhynchus had been covered with
feathers instead of hair, this external and trifling character would, I think,
have been considered by naturalists as important an aid in determining the
degree of affinity of this strange creature to birds and reptiles, as an
approach in structure in any one internal and important organ.
The importance, for classification, of trifling characters, mainly depends on
their being correlated with several other characters of more or less importance.
The value indeed of an aggregate of characters is very evident in natural
history. Hence, as has often been remarked, a species may depart from its allies
in several characters, both of high physiological importance and of almost
universal prevalence, and yet leave us in no doubt where it should be ranked.
Hence, also, it has been found, that a classification founded on any single
character, however important that may be, has always failed; for no part of the
organisation is universally constant. The importance of an aggregate of
characters, even when none are important, alone explains, I think, that saying
of Linnaeus, that the characters do not give the genus, but the genus gives the
characters; for this saying seems founded on an appreciation of many trifling
points of resemblance, too slight to be defined. Certain plants, belonging to
the Malpighiaceae, bear perfect and degraded flowers; in the latter, as A. de
Jussieu has remarked, `the greater number of the characters proper to the
species, to the genus, to the family, to the class, disappear, and thus laugh at
our classification.' But when Aspicarpa produced in France, during several
years, only degraded flowers, departing so wonderfully in a number of the most
important points of structure from the proper type of the order, yet M. Richard
sagaciously saw, as Jussieu observes, that this genus should still be retained
amongst the Malpighiaceae. This case seems to me well to illustrate the spirit
with which our classifications are sometimes necessarily founded.
Practically when naturalists are at work, they do not trouble themselves about
the physiological value of the characters which they use in defining a group, or
in allocating any particular species. If they find a character nearly uniform,
and common to a great number of forms, and not common to others, they use it as
one of high value; if common to some lesser number, they use it as of
subordinate value. This principle has been broadly confessed by some naturalists
to be the true one; and by none more clearly than by that excellent botanist,
Aug. St. Hilaire. If certain characters are always found correlated with others,
though no apparent bond of connexion can be discovered between them, especial
value is set on them. As in most groups of animals, important organs, such as
those for propelling the blood, or for aërating it, or those for propagating the
race, are found nearly uniform, they are considered as highly serviceable in
classification; but in some groups of animals all these, the most important
vital organs, are found to offer characters of quite subordinate value.
We can see why characters derived from the embryo should be of equal importance
with those derived from the adult, for our classifications of course include all
ages of each species. But it is by no means obvious, on the ordinary view, why
the structure of the embryo should be more important for this purpose than that
of the adult, which alone plays its full part in the economy of nature. Yet it
has been strongly urged by those great naturalists, Milne Edwards and Agassiz,
that embryonic characters are the most important of any in the classification of
animals; and this doctrine has very generally been admitted as true. The same
fact holds good with flowering plants, of which the two main divisions have been
founded on characters derived from the embryo, on the number and position of the
embryonic leaves or cotyledons, and on the mode of development of the plumule
and radicle. In our discussion on embryology, we shall see why such characters
are so valuable, on the view of classification tacitly including the idea of
descent.
Our classifications are often plainly influenced by chains of affinities.
Nothing can be easier than to define a number of characters common to all birds;
but in the case of crustaceans, such definition has hitherto been found
impossible. There are crustaceans at the opposite ends of the series, which have
hardly a character in common; yet the species at both ends, from being plainly
allied to others, and these to others, and so onwards, can be recognised as
unequivocally belonging to this, and to no other class of the Articulata.
Geographical distribution has often been used, though perhaps not quite
logically, in classification, more especially in very large groups of closely
allied forms. Temminck insists on the utility or even necessity of this practice
in certain groups of birds; and it has been followed by several entomologists
and botanists.
Finally, with respect to the comparative value of the various groups of species,
such as orders, sub-orders, families, sub-families, and genera, they seem to be,
at least at present, almost arbitrary. Several of the best botanists, such as Mr
Bentham and others, have strongly insisted on their arbitrary value. Instances
could be given amongst plants and insects, of a group of forms, first ranked by
practised naturalists as only a genus, and then raised to the rank of a
sub-family or family; and this has been done, not because further research has
detected important structural differences, at first overlooked, but because
numerous allied species, with slightly different grades of difference, have been
subsequently discovered.
All the foregoing rules and aids and difficulties in classification are
explained, if I do not greatly deceive myself, on the view that the natural
system is founded on descent with modification; that the characters which
naturalists consider as showing true affinity between any two or more species,
are those which have been inherited from a common parent, and, in so far, all
true classification is genealogical; that community of descent is the hidden
bond which naturalists have been unconsciously seeking, and not some unknown
plan of creation, or the enunciation of general propositions, and the mere
putting together and separating objects more or less alike.
But I must explain my meaning more fully. I believe that the arrangement
of the groups within each class, in due subordination and relation to the other
groups, must be strictly genealogical in order to be natural; but that the
amount of difference in the several branches or groups, though allied in
the same degree in blood to their common progenitor, may differ greatly, being
due to the different degrees of modification which they have undergone; and this
is expressed by the forms being ranked under different genera, families,
sections, or orders. The reader will best understand what is meant, if he will
take the trouble of referring to the diagram in the fourth chapter. We will
suppose the letters A to L to represent allied genera, which lived during the
Silurian epoch, and these have descended from a species which existed at an
unknown anterior period. Species of three of these genera (A, F, and I) have
transmitted modified descendants to the present day, represented by the fifteen
genera (a14 to z14) on the uppermost horizontal line. Now all these modified
descendants from a single species, are represented as related in blood or
descent to the same degree; they may metaphorically be called cousins to the
same millionth degree; yet they differ widely and in different degrees from each
other. The forms descended from A, now broken up into two or three families,
constitute a distinct order from those descended from I, also broken up into two
families. Nor can the existing species, descended from A, be ranked in the same
genus with the parent A; or those from I, with the parent I. But the existing
genus F14 may be supposed to have been but slightly modified; and it will then
rank with the parent-genus F; just as some few still living organic beings
belong to Silurian genera. So that the amount or value of the differences
between organic beings all related to each other in the same degree in blood,
has come to be widely different. Nevertheless their genealogical arrangement
remains strictly true, not only at the present time, but at each successive
period of descent. All the modified descendants from A will have inherited
something in common from their common parent, as will all the descendants from
I; so will it be with each subordinate branch of descendants, at each successive
period. If, however, we choose to suppose that any of the descendants of A or of
I have been so much modified as to have more or less completely lost traces of
their parentage, in this case, their places in a natural classification will
have been more or less completely lost, as sometimes seems to have occurred with
existing organisms. All the descendants of the genus F, along its whole line of
descent, are supposed to have been but little modified, and they yet form a
single genus. But this genus, though much isolated, will still occupy its proper
intermediate position; for F originally was intermediate in character between A
and I, and the several genera descended from these two genera will have
inherited to a certain extent their characters. This natural arrangement is
shown, as far as is possible on paper, in the diagram, but in much too simple a
manner. If a branching diagram had not been used, and only the names of the
groups had been written in a linear series, it would have been still less
possible to have given a natural arrangement; and it is notoriously not possible
to represent in a series, on a flat surface, the affinities which we discover in
nature amongst the beings of the same group. Thus, on the view which I hold, the
natural system is genealogical in its arrangement, like a pedigree; but the
degrees of modification which the different groups have undergone, have to be
expressed by ranking them under different so-called genera, sub-families,
families, sections, orders, and classes.
It may be worth while to illustrate this view of classification, by taking the
case of languages. If we possessed a perfect pedigree of mankind, a genealogical
arrangement of the races of man would afford the best classification of the
various languages now spoken throughout the world; and if all extinct languages,
and all intermediate and slowly changing dialects, had to be included, such an
arrangement would, I think, be the only possible one. Yet it might be that some
very ancient language had altered little, and had given rise to few new
languages, whilst others (owing to the spreading and subsequent isolation and
states of civilisation of the several races, descended from a common race) had
altered much, and had given rise to many new languages and dialects. The various
degrees of difference in the languages from the same stock, would have to be
expressed by groups subordinate to groups; but the proper or even only possible
arrangement would still be genealogical; and this would be strictly natural, as
it would connect together all languages, extinct and modern, by the closest
affinities, and would give the filiation and origin of each tongue.
In confirmation of this view, let us glance at the classification of varieties,
which are believed or known to have descended from one species. These are
grouped under species, with sub-varieties under varieties; and with our domestic
productions, several other grades of difference are requisite, as we have seen
with pigeons. The origin of the existence of groups subordinate to groups, is
the same with varieties as with species, namely, closeness of descent with
various degrees of modification. Nearly the same rules are followed in
classifying varieties, as with species. Authors have insisted on the necessity
of classing varieties on a natural instead of an artificial system; we are
cautioned, for instance, not to class two varieties of the pine-apple together,
merely because their fruit, though the most important part, happens to be nearly
identical; no one puts the swedish and common turnips together, though the
esculent and thickened stems are so similar. Whatever part is found to be most
constant, is used in classing varieties: thus the great agriculturist Marshall
says the horns are very useful for this purpose with cattle, because they are
less variable than the shape or colour of the body, &c.; whereas with sheep the
horns are much less serviceable, because less constant. In classing varieties, I
apprehend if we had a real pedigree, a genealogical classification would be
universally preferred; and it has been attempted by some authors. For we might
feel sure, whether there had been more or less modification, the principle of
inheritance would keep the forms together which were allied in the greatest
number of points. In tumbler pigeons, though some sub-varieties differ from the
others in the important character of having a longer beak, yet all are kept
together from having the common habit of tumbling; but the short-faced breed has
nearly or quite lost this habit; nevertheless, without any reasoning or thinking
on the subject, these tumblers are kept in the same group, because allied in
blood and alike in some other respects. If it could be proved that the Hottentot
had descended from the Negro, I think he would be classed under the Negro group,
however much he might differ in colour and other important characters from
negroes.
With species in a state of nature, every naturalist has in fact brought descent
into his classification; for he includes in his lowest grade, or that of a
species, the two sexes; and how enormously these sometimes differ in the most
important characters, is known to every naturalist: scarcely a single fact can
be predicated in common of the males and hermaphrodites of certain cirripedes,
when adult, and yet no one dreams of separating them. The naturalist includes as
one species the several larval stages of the same individual, however much they
may differ from each other and from the adult; as he likewise includes the
so-called alternate generations of Steenstrup, which can only in a technical
sense be considered as the same individual. He includes monsters; he includes
varieties, not solely because they closely resemble the parent-form, but because
they are descended from it. He who believes that the cowslip is descended from
the primrose, or conversely, ranks them together as a single species, and gives
a single definition. As soon as three Orchidean forms (Monochanthus, Myanthus,
and Catasetum), which had previously been ranked as three distinct genera, were
known to be sometimes produced on the same spike, they were immediately included
as a single species. But it may be asked, what ought we to do, if it could be
proved that one species of kangaroo had been produced, by a long course of
modification, from a bear? Ought we to rank this one species with bears, and
what should we do with the other species? The supposition is of course
preposterous; and I might answer by the argumentum ad hominem, and ask
what should be done if a perfect kangaroo were seen to come out of the womb of a
bear? According to all analogy, it would be ranked with bears; but then
assuredly all the other species of the kangaroo family would have to be classed
under the bear genus. The whole case is preposterous; for where there has been
close descent in common, there will certainly be close resemblance or affinity.
As descent has universally been used in classing together the individuals of the
same species, though the males and females and larvae are sometimes extremely
different; and as it has been used in classing varieties which have undergone a
certain, and sometimes a considerable amount of modification, may not this same
element of descent have been unconsciously used in grouping species under
genera, and genera under higher groups, though in these cases the modification
has been greater in degree, and has taken a longer time to complete? I believe
it has thus been unconsciously used; and only thus can I understand the several
rules and guides which have been followed by our best systematists. We have no
written pedigrees; we have to make out community of descent by resemblances of
any kind. Therefore we choose those characters which, as far as we can judge,
are the least likely to have been modified in relation to the conditions of life
to which each species has been recently exposed. Rudimentary structures on this
view are as good as, or even sometimes better than, other parts of the
organisation. We care not how trifling a character may be let it be the mere
inflection of the angle of the jaw, the manner in which an insect's wing is
folded, whether the skin be covered by hair or feathers if it prevail throughout
many and different species, especially those having very different habits of
life, it assumes high value; for we can account for its presence in so many
forms with such different habits, only by its inheritance from a common parent.
We may err in this respect in regard to single points of structure, but when
several characters, let them be ever so trifling, occur together throughout a
large group of beings having different habits, we may feel almost sure, on the
theory of descent, that these characters have been inherited from a common
ancestor. And we know that such correlated or aggregated characters have
especial value in classification.
We can understand why a species or a group of species may depart, in several of
its most important characteristics, from its allies, and yet be safely classed
with them. This may be safely done, and is often done, as long as a sufficient
number of characters, let them be ever so unimportant, betrays the hidden bond
of community of descent. Let two forms have not a single character in common,
yet if these extreme forms are connected together by a chain of intermediate
groups, we may at once infer their community of descent, and we put them all
into the same class. As we find organs of high physiological importance those
which serve to preserve life under the most diverse conditions of existence are
generally the most constant, we attach especial value to them; but if these same
organs, in another group or section of a group, are found to differ much, we at
once value them less in our classification. We shall hereafter, I think, clearly
see why embryological characters are of such high classificatory importance.
Geographical distribution may sometimes be brought usefully into play in
classing large and widely-distributed genera, because all the species of the
same genus, inhabiting any distinct and isolated region, have in all probability
descended from the same parents.
We can understand, on these views, the very important distinction between real
affinities and analogical or adaptive resemblances. Lamarck first called
attention to this distinction, and he has been ably followed by Macleay and
others. The resemblance, in the shape of the body and in the fin-like anterior
limbs, between the dugong, which is a pachydermatous animal, and the whale, and
between both these mammals and fishes, is analogical. Amongst insects there are
innumerable instances: thus Linnaeus, misled by external appearances, actually
classed an homopterous insect as a moth. We see something of the same kind even
in our domestic varieties, as in the thickened stems of the common and swedish
turnip. The resemblance of the greyhound and racehorse is hardly more fanciful
than the analogies which have been drawn by some authors between very distinct
animals. On my view of characters being of real importance for classification,
only in so far as they reveal descent, we can clearly understand why analogical
or adaptive character, although of the utmost importance to the welfare of the
being, are almost valueless to the systematist. For animals, belonging to two
most distinct lines of descent, may readily become adapted to similar
conditions, and thus assume a close external resemblance; but such resemblances
will not reveal will rather tend to conceal their blood-relationship to their
proper lines of descent. We can also understand the apparent paradox, that the
very same characters are analogical when one class or order is compared with
another, but give true affinities when the members of the same class or order
are compared one with another: thus the shape of the body and fin-like limbs are
only analogical when whales are compared with fishes, being adaptations in both
classes for swimming through the water; but the shape of the body and fin-like
limbs serve as characters exhibiting true affinity between the several members
of the whale family; for these cetaceans agree in so many characters, great and
small, that we cannot doubt that they have inherited their general shape of body
and structure of limbs from a common ancestor. So it is with fishes.
As members of distinct classes have often been adapted by successive slight
modifications to live under nearly similar circumstances, to inhabit for
instance the three elements of land, air, and water, we can perhaps understand
how it is that a numerical parallelism has sometimes been observed between the
sub-groups in distinct classes. A naturalist, struck by a parallelism of this
nature in any one class, by arbitrarily raising or sinking the value of the
groups in other classes (and all our experience shows that this valuation has
hitherto been arbitrary), could easily extend the parallelism over a wide range;
and thus the septenary, quinary, quaternary, and ternary classifications have
probably arisen.
As the modified descendants of dominant species, belonging to the larger genera,
tend to inherit the advantages, which made the groups to which they belong large
and their parents dominant, they are almost sure to spread widely, and to seize
on more and more places in the economy of nature. The larger and more dominant
groups thus tend to go on increasing in size; and they consequently supplant
many smaller and feebler groups. Thus we can account for the fact that all
organisms, recent and extinct, are included under a few great orders, under
still fewer classes, and all in one great natural system. As showing how few the
higher groups are in number, and how widely spread they are throughout the
world, the fact is striking, that the discovery of Australia has not added a
single insect belonging to a new order; and that in the vegetable kingdom, as I
learn from Dr. Hooker, it has added only two or three orders of small size.
In the chapter on geological succession I attempted to show, on the principle of
each group having generally diverged much in character during the long-continued
process of modification, how it is that the more ancient forms of life often
present characters in some slight degree intermediate between existing groups. A
few old and intermediate parent-forms having occasionally transmitted to the
present day descendants but little modified, will give to us our so-called
osculant or aberrant groups. The more aberrant any form is, the greater must be
the number of connecting forms which on my theory have been exterminated and
utterly lost. And we have some evidence of aberrant forms having suffered
severely from extinction, for they are generally represented by extremely few
species; and such species as do occur are generally very distinct from each
other, which again implies extinction. The genera Ornithorhynchus and
Lepidosiren, for example, would not have been less aberrant had each been
represented by a dozen species instead of by a single one; but such richness in
species, as I find after some investigation, does not commonly fall to the lot
of aberrant genera. We can, I think, account for this fact only by looking at
aberrant forms as failing groups conquered by more successful competitors, with
a few members preserved by some unusual coincidence of favourable circumstances.
Mr. Waterhouse has remarked that, when a member belonging to one group of
animals exhibits an affinity to a quite distinct group, this affinity in most
cases is general and not special: thus, according to Mr. Waterhouse, of all
Rodents, the bizcacha is most nearly related to Marsupials; but in the points in
which it approaches this order, its relations are general, and not to any one
marsupial species more than to another. As the points of affinity of the
bizcacha to Marsupials are believed to be real and not merely adaptive, they are
due on my theory to inheritance in common. Therefore we must suppose either that
all Rodents, including the bizcacha, branched off from some very ancient
Marsupial, which will have had a character in some degree intermediate with
respect to all existing Marsupials; or that both Rodents and Marsupials branched
off from a common progenitor, and that both groups have since undergone much
modification in divergent directions. On either view we may suppose that the
bizcacha has retained, by inheritance, more of the character of its ancient
progenitor than have other Rodents; and therefore it will not be specially
related to any one existing Marsupial, but indirectly to all or nearly all
Marsupials, from having partially retained the character of their common
progenitor, or of an early member of the group. On the other hand, of all
Marsupials, as Mr. Waterhouse has remarked, the phascolomys resembles most
nearly, not any one species, but the general order of Rodents. In this case,
however, it may be strongly suspected that the resemblance is only analogical,
owing to the phascolomys having become adapted to habits like those of a Rodent.
The elder De Candolle has made nearly similar observations on the general nature
of the affinities of distinct orders of plants.
On the principle of the multiplication and gradual divergence in character of
the species descended from a common parent, together with their retention by
inheritance of some characters in common, we can understand the excessively
complex and radiating affinities by which all the members of the same family or
higher group are connected together. For the common parent of a whole family of
species, now broken up by extinction into distinct groups and sub-groups, will
have transmitted some of its characters, modified in various ways and degrees,
to all; and the several species will consequently be related to each other by
circuitous lines of affinity of various lengths (as may be seen in the diagram
so often referred to), mounting up through many predecessors. As it is difficult
to show the blood-relationship between the numerous kindred of any ancient and
noble family, even by the aid of a genealogical tree, and almost impossible to
do this without this aid, we can understand the extraordinary difficulty which
naturalists have experienced in describing, without the aid of a diagram, the
various affinities which they perceive between the many living and extinct
members of the same great natural class.
Extinction, as we have seen in the fourth chapter, has played an important part
in defining and widening the intervals between the several groups in each class.
We may thus account even for the distinctness of whole classes from each other
for instance, of birds from all other vertebrate animals by the belief that many
ancient forms of life have been utterly lost, through which the early
progenitors of birds were formerly connected with the early progenitors of the
other vertebrate classes. There has been less entire extinction of the forms of
life which once connected fishes with batrachians. There has been still less in
some other classes, as in that of the Crustacea, for here the most wonderfully
diverse forms are still tied together by a long, but broken, chain of
affinities. Extinction has only separated groups: it has by no means made them;
for if every form which has ever lived on this earth were suddenly to reappear,
though it would be quite impossible to give definitions by which each group
could be distinguished from other groups, as all would blend together by steps
as fine as those between the finest existing varieties, nevertheless a natural
classification, or at least a natural arrangement, would be possible. We shall
see this by turning to the diagram: the letters, A to L, may represent eleven
Silurian genera, some of which have produced large groups of modified
descendants. Every intermediate link between these eleven genera and their
primordial parent, and every intermediate link in each branch and sub-branch of
their descendants, may be supposed to be still alive; and the links to be as
fine as those between the finest varieties. In this case it would be quite
impossible to give any definition by which the several members of the several
groups could be distinguished from their more immediate parents; or these
parents from their ancient and unknown progenitor. Yet the natural arrangement
in the diagram would still hold good; and, on the principle of inheritance, all
the forms descended from A, or from I, would have something in common. In a tree
we can specify this or that branch, though at the actual fork the two unite and
blend together. We could not, as I have said, define the several groups; but we
could pick out types, or forms, representing most of the characters of each
group, whether large or small, and thus give a general idea of the value of the
differences between them. This is what we should be driven to, if we were ever
to succeed in collecting all the forms in any class which have lived throughout
all time and space. We shall certainly never succeed in making so perfect a
collection: nevertheless, in certain classes, we are tending in this direction;
and Milne Edwards has lately insisted, in an able paper, on the high importance
of looking to types, whether or not we can separate and define the groups to
which such types belong.
Finally, we have seen that natural selection, which results from the struggle
for existence, and which almost inevitably induces extinction and divergence of
character in the many descendants from one dominant parent-species, explains
that great and universal feature in the affinities of all organic beings,
namely, their subordination in group under group. We use the element of descent
in classing the individuals of both sexes and of all ages, although having few
characters in common, under one species; we use descent in classing acknowledged
varieties, however different they may be from their parent; and I believe this
element of descent is the hidden bond of connexion which naturalists have sought
under the term of the Natural System. On this idea of the natural system being,
in so far as it has been perfected, genealogical in its arrangement, with the
grades of difference between the descendants from a common parent, expressed by
the terms genera, families, orders, &c., we can understand the rules which we
are compelled to follow in our classification. We can understand why we value
certain resemblances far more than others; why we are permitted to use
rudimentary and useless organs, or others of trifling physiological importance;
why, in comparing one group with a distinct group, we summarily reject
analogical or adaptive characters, and yet use these same characters within the
limits of the same group. We can clearly see how it is that all living and
extinct forms can be grouped together in one great system; and how the several
members of each class are connected together by the most complex and radiating
lines of affinities. We shall never, probably, disentangle the inextricable web
of affinities between the members of any one class; but when we have a distinct
object in view, and do not look to some unknown plan of creation, we may hope to
make sure but slow progress.
Morphology
We have seen that the members of the same class, independently of their habits
of life, resemble each other in the general plan of their organisation. This
resemblance is often expressed by the term `unity of type;' or by saying that
the several parts and organs in the different species of the class are
homologous. The whole subject is included under the general name of Morphology.
This is the most interesting department of natural history, and may be said to
be its very soul. What can be more curious than that the hand of a man, formed
for grasping, that of a mole for digging, the leg of the horse, the paddle of
the porpoise, and the wing of the bat, should all be constructed on the same
pattern, and should include the same bones, in the same relative positions?
Geoffroy St Hilaire has insisted strongly on the high importance of relative
connexion in homologous organs: the parts may change to almost any extent in
form and size, and yet they always remain connected together in the same order.
We never find, for instance, the bones of the arm and forearm, or of the thigh
and leg, transposed. Hence the same names can be given to the homologous bones
in widely different animals. We see the same great law in the construction of
the mouths of insects: what can be more different than the immensely long spiral
proboscis of a sphinx-moth, the curious folded one of a bee or bug, and the
great jaws of a beetle? yet all these organs, serving for such different
purposes, are formed by infinitely numerous modifications of an upper lip,
mandibles, and two pairs of maxillae. Analogous laws govern the construction of
the mouths and limbs of crustaceans. So it is with the flowers of plants.
Nothing can be more hopeless than to attempt to explain this similarity of
pattern in members of the same class, by utility or by the doctrine of final
causes. The hopelessness of the attempt has been expressly admitted by Owen in
his most interesting work on the `Nature of Limbs.' On the ordinary view of the
independent creation of each being, we can only say that so it is; that it has
so pleased the Creator to construct each animal and plant.
The explanation is manifest on the theory of the natural selection of successive
slight modifications, each modification being profitable in some way to the
modified form, but often affecting by correlation of growth other parts of the
organisation. In changes of this nature, there will be little or no tendency to
modify the original pattern, or to transpose parts. The bones of a limb might be
shortened and widened to any extent, and become gradually enveloped in thick
membrane, so as to serve as a fin; or a webbed foot might have all its bones, or
certain bones, lengthened to any extent, and the membrane connecting them
increased to any extent, so as to serve as a wing: yet in all this great amount
of modification there will be no tendency to alter the framework of bones or the
relative connexion of the several parts. If we suppose that the ancient
progenitor, the archetype as it may be called, of all mammals, had its limbs
constructed on the existing general pattern, for whatever purpose they served,
we can at once perceive the plain signification of the homologous construction
of the limbs throughout the whole class. So with the mouths of insects, we have
only to suppose that their common progenitor had an upper lip, mandibles, and
two pair of maxillae, these parts being perhaps very simple in form; and then
natural selection will account for the infinite diversity in structure and
function of the mouths of insects. Nevertheless, it is conceivable that the
general pattern of an organ might become so much obscured as to be finally lost,
by the atrophy and ultimately by the complete abortion of certain parts, by the
soldering together of other parts, and by the doubling or multiplication of
others, variations which we know to be within the limits of possibility. In the
paddles of the extinct gigantic sea-lizards, and in the mouths of certain
suctorial crustaceans, the general pattern seems to have been thus to a certain
extent obscured.
There is another and equally curious branch of the present subject; namely, the
comparison not of the same part in different members of a class, but of the
different parts or organs in the same individual. Most physiologists believe
that the bones of the skull are homologous with that is correspond in number and
in relative connexion with the elemental parts of a certain number of vertebrae.
The anterior and posterior limbs in each member of the vertebrate and articulate
classes are plainly homologous. We see the same law in comparing the wonderfully
complex jaws and legs in crustaceans. It is familiar to almost every one, that
in a flower the relative position of the sepals, petals, stamens, and pistils,
as well as their intimate structure, are intelligible in the view that they
consist of metamorphosed leaves, arranged in a spire. In monstrous plants, we
often get direct evidence of the possibility of one organ being transformed into
another; and we can actually see in embryonic crustaceans and in many other
animals, and in flowers, that organs which when mature become extremely
different, are at an early stage of growth exactly alike.
How inexplicable are these facts on the ordinary view of creation! Why should
the brain be enclosed in a box composed of such numerous and such
extraordinarily shaped pieces of bone? As Owen has remarked, the benefit derived
from the yielding of the separate pieces in the act of parturition of mammals,
will by no means explain the same construction in the skulls of birds. Why
should similar bones have been created in the formation of the wing and leg of a
bat, used as they are for such totally different purposes? Why should one
crustacean, which has an extremely complex mouth formed of many parts,
consequently always have fewer legs; or conversely, those with many legs have
simpler mouths? Why should the sepals, petals, stamens, and pistils in any
individual flower, though fitted for such widely different purposes, be all
constructed on the same pattern ?
On the theory of natural selection, we can satisfactorily answer these
questions. In the vertebrata, we see a series of internal vertebrae bearing
certain processes and appendages; in the articulata, we see the body divided
into a series of segments, bearing external appendages; and in flowering plants,
we see a series of successive spiral whorls of leaves. An indefinite repetition
of the same part or organ is the common characteristic (as Owen has observed) of
all low or little-modified forms; therefore we may readily believe that the
unknown progenitor of the vertebrata possessed many vertebrae; the unknown
progenitor of the articulata, many segments; and the unknown progenitor of
flowering plants, many spiral whorls of leaves. We have formerly seen that parts
many times repeated are eminently liable to vary in number and structure;
consequently it is quite probable that natural selection, during a
long-continued course of modification, should have seized on a certain number of
the primordially similar elements, many times repeated, and have adapted them to
the most diverse purposes. And as the whole amount of modification will have
been effected by slight successive steps, we need not wonder at discovering in
such parts or organs, a certain degree of fundamental resemblance, retained by
the strong principle of inheritance.
In the great class of molluscs, though we can homologise the parts of one
species with those of another and distinct species, we can indicate but few
serial homologies; that is, we are seldom enabled to say that one part or organ
is homologous with another in the same individual. And we can understand this
fact; for in molluscs, even in the lowest members of the class, we do not find
nearly so much indefinite repetition of any one part, as we find in the other
great classes of the animal and vegetable kingdoms.
Naturalists frequently speak of the skull as formed of metamorphosed vertebrae:
the jaws of crabs as metamorphosed legs; the stamens and pistils of flowers as
metamorphosed leaves; but it would in these cases probably be more correct, as
Professor Huxley has remarked, to speak of both skull and vertebrae, both jaws
and legs, &c., as having been metamorphosed, not one from the other, but from
some common element. Naturalists, however, use such language only in a
metaphorical sense: they are far from meaning that during a long course of
descent, primordial organs of any kind vertebrae in the one case and legs in the
other have actually been modified into skulls or jaws. Yet so strong is the
appearance of a modification of this nature having occurred, that naturalists
can hardly avoid employing language having this plain signification. On my view
these terms may be used literally; and the wonderful fact of the jaws, for
instance, of a crab retaining numerous characters, which they would probably
have retained through inheritance, if they had really been metamorphosed during
a long course of descent from true legs, or from some simple appendage, is
explained.
Embryology
It has already been casually remarked that certain organs in the individual,
which when mature become widely different and serve for different purposes, are
in the embryo exactly alike. The embryos, also, of distinct animals within the
same class are often strikingly similar: a better proof of this cannot be given,
than a circumstance mentioned by Agassiz, namely, that having forgotten to
ticket the embryo of some vertebrate animal, he cannot now tell whether it be
that of a mammal, bird, or reptile. The vermiform larvae of moths, flies,
beetles, &c., resemble each other much more closely than do the mature insects;
but in the case of larvae, the embryos are active, and have been adapted for
special lines of life. A trace of the law of embryonic resemblance, sometimes
lasts till a rather late age: thus birds of the same genus, and of closely
allied genera, often resemble each other in their first and second plumage; as
we see in the spotted feathers in the thrush group. In the cat tribe, most of
the species are striped or spotted in lines; and stripes can be plainly
distinguished in the whelp of the lion. We occasionally though rarely see
something of this kind in plants: thus the embryonic leaves of the ulex or
furze, and the first leaves of the phyllodineous acaceas, are pinnate or divided
like the ordinary leaves of the leguminosae.
The points of structure, in which the embryos of widely different animals of the
same class resemble each other, often have no direct relation to their
conditions of existence. We cannot, for instance, suppose that in the embryos of
the vertebrata the peculiar loop-like course of the arteries near the branchial
slits are related to similar conditions, in the young mammal which is nourished
in the womb of its mother, in the egg of the bird which is hatched in a nest,
and in the spawn of a frog under water. We have no more reason to believe in
such a relation, than we have to believe that the same bones in the hand of a
man, wing of a bat, and fin of a porpoise, are related to similar conditions of
life. No one will suppose that the stripes on the whelp of a lion, or the spots
on the young blackbird, are of any use to these animals, or are related to the
conditions to which they are exposed.
The case, however, is different when an animal during any part of its embryonic
career is active, and has to provide for itself. The period of activity may come
on earlier or later in life; but whenever it comes on, the adaptation of the
larva to its conditions of life is just as perfect and as beautiful as in the
adult animal. From such special adaptations, the similarity of the larvae or
active embryos of allied animals is sometimes much obscured; and cases could be
given of the larvae of two species, or of two groups of species, differing quite
as much, or even more, from each other than do their adult parents. In most
cases, however, the larvae, though active, still obey more or less closely the
law of common embryonic resemblance. Cirripedes afford a good instance of this:
even the illustrious Cuvier did not perceive that a barnacle was, as it
certainly is, a crustacean; but a glance at the larva shows this to be the case
in an unmistakeable manner. So again the two main divisions of cirripedes, the
pedunculated and sessile, which differ widely in external appearance, have
larvae in all their several stages barely distinguishable.
The embryo in the course of development generally rises in organisation: I use
this expression, though I am aware that it is hardly possible to define clearly
what is meant by the organisation being higher or lower. But no one probably
will dispute that the butterfly is higher than the caterpillar. In some cases,
however, the mature animal is generally considered as lower in the scale than
the larva, as with certain parasitic crustaceans. To refer once again to
cirripedes: the larvae in the first stage have three pairs of legs, a very
simple single eye, and a probosciformed mouth, with which they feed largely, for
they increase much in size. In the second stage, answering to the chrysalis
stage of butterflies, they have six pairs of beautifully constructed natatory
legs, a pair of magnificent compound eyes, and extremely complex antennae; but
they have a closed and imperfect mouth, and cannot feed: their function at this
stage is, to search by their well-developed organs of sense, and to reach by
their active powers of swimming, a proper place on which to become attached and
to undergo their final metamorphosis. When this is completed they are fixed for
life: their legs are now converted into prehensile organs; they again obtain a
well-constructed mouth; but they have no antennae, and their two eyes are now
reconverted into a minute, single, and very simple eye-spot. In this last and
complete state, cirripedes may be considered as either more highly or more lowly
organised than they were in the larval condition. But in some genera the larvae
become developed either into hermaphrodites having the ordinary structure, or
into what I have called complemental males: and in the latter, the development
has assuredly been retrograde; for the male is a mere sack, which lives for a
short time, and is destitute of mouth, stomach, or other organ of importance,
excepting for reproduction.
We are so much accustomed to see differences in structure between the embryo and
the adult, and likewise a close similarity in the embryos of widely different
animals within the same class, that we might be led to look at these facts as
necessarily contingent in some manner on growth. But there is no obvious reason
why, for instance, the wing of a bat, or the fin of a porpoise, should not have
been sketched out with all the parts in proper proportion, as soon as any
structure became visible in the embryo. And in some whole groups of animals and
in certain members of other groups, the embryo does not at any period differ
widely from the adult: thus Owen has remarked in regard to cuttle-fish, `there
is no metamorphosis; the cephalopodic character is manifested long before the
parts of the embryo are completed;' and again in spiders, `there is nothing
worthy to be called a metamorphosis.' The larvae of insects, whether adapted to
the most diverse and active habits, or quite inactive, being fed by their
parents or placed in the midst of proper nutriment, yet nearly all pass through
a similar worm-like stage of development; but in some few cases, as in that of
Aphis, if we look to the admirable drawings by Professor Huxley of the
development of this insect, we see no trace of the vermiform stage.
How, then, can we explain these several facts in embryology, namely the very
general, but not universal difference in structure between the embryo and the
adult; of parts in the same individual embryo, which ultimately become very
unlike and serve for diverse purposes, being at this early period of growth
alike; of embryos of different species within the same class, generally, but not
universally, resembling each other; of the structure of the embryo not being
closely related to its conditions of existence, except when the embryo becomes
at any period of life active and has to provide for itself; of the embryo
apparently having sometimes a higher organisation than the mature animal, into
which it is developed. I believe that all these facts can be explained, as
follows, on the view of descent with modification.
It is commonly assumed, perhaps from monstrosities often affecting the embryo at
a very early period, that slight variations necessarily appear at an equally
early period. But we have little evidence on this head indeed the evidence
rather points the other way; for it is notorious that breeders of cattle,
horses, and various fancy animals, cannot positively tell, until some time after
the animal has been born, what its merits or form will ultimately turn out. We
see this plainly in our own children; we cannot always tell whether the child
will be tall or short, or what its precise features will be. The question is
not, at what period of life any variation has been caused, but at what period it
is fully displayed. The cause may have acted, and I believe generally has acted,
even before the embryo is formed; and the variation may be due to the male and
female sexual elements having been affected by the conditions to which either
parent, or their ancestors, have been exposed. Nevertheless an effect thus
caused at a very early period, even before the formation of the embryo, may
appear late in life; as when an hereditary disease, which appears in old age
alone, has been communicated to the offspring from the reproductive element of
one parent. Or again, as when the horns of cross-bred cattle have been affected
by the shape of the horns of either parent. For the welfare of a very young
animal, as long as it remains in its mother's womb, or in the egg, or as long as
it is nourished and protected by its parent, it must be quite unimportant
whether most of its characters are fully acquired a little earlier or later in
life. It would not signify, for instance, to a bird which obtained its food best
by having a long beak, whether or not it assumed a beak of this particular
length, as long as it was fed by its parents. Hence, I conclude, that it is
quite possible, that each of the many successive modifications, by which each
species has acquired its present structure, may have supervened at a not very
early period of life; and some direct evidence from our domestic animals
supports this view. But in other cases it is quite possible that each successive
modification, or most of them, may have appeared at an extremely early period.
I have stated in the first chapter, that there is some evidence to render it
probable, that at whatever age any variation first appears in the parent, it
tends to reappear at a corresponding age in the offspring. Certain variations
can only appear at corresponding ages, for instance, peculiarities in the
caterpillar, cocoon, or imago states of the silk-moth; or, again, in the horns
of almost full-grown cattle. But further than this, variations which, for all
that we can see, might have appeared earlier or later in life, tend to appear at
a corresponding age in the offspring and parent. I am far from meaning that this
is invariably the case; and I could give a good many cases of variations (taking
the word in the largest sense) which have supervened at an earlier age in the
child than in the parent.
These two principles, if their truth be admitted, will, I believe, explain all
the above specified leading facts in embryology. But first let us look at a few
analogous cases in domestic varieties. Some authors who have written on Dogs,
maintain that the greyhound and bulldog, though appearing so different, are
really varieties most closely allied, and have probably descended from the same
wild stock; hence I was curious to see how far their puppies differed from each
other: I was told by breeders that they differed just as much as their parents,
and this, judging by the eye, seemed almost to be the case; but on actually
measuring the old dogs and their six-days old puppies, I found that the puppies
had not nearly acquired their full amount of proportional difference. So, again,
I was told that the foals of cart and race-horses differed as much as the
full-grown animals; and this surprised me greatly, as I think it probable that
the difference between these two breeds has been wholly caused by selection
under domestication; but having had careful measurements made of the dam and of
a three-days old colt of a race and heavy cart-horse, I find that the colts have
by no means acquired their full amount of proportional difference.
As the evidence appears to me conclusive, that the several domestic breeds of
pigeon have descended from one wild species, I compared young pigeons of various
breeds, within twelve hours after being hatched; I carefully measured the
proportions (but will not here give details) of the beak, width of mouth, length
of nostril and of eyelid, size of feet and length of leg, in the wild stock, in
pouters, fantails, runts, barbs, dragons, carriers, and tumblers. Now some of
these birds, when mature, differ so extraordinarily in length and form of beak,
that they would, I cannot doubt, be ranked in distinct genera, had they been
natural productions. But when the nestling birds of these several breeds were
placed in a row, though most of them could be distinguished from each other, yet
their proportional differences in the above specified several points were
incomparably less than in the full-grown birds. Some characteristic points of
difference for instance, that of the width of mouth -- could hardly be detected
in the young. But there was one remarkable exception to this rule, for the young
of the short-faced tumbler differed from the young of the wild rock-pigeon and
of the other breeds, in all its proportions, almost exactly as much as in the
adult state.
The two principles above given seem to me to explain these facts in regard to
the later embryonic stages of our domestic varieties. Fanciers select their
horses, dogs, and pigeons, for breeding, when they are nearly grown up: they are
indifferent whether the desired qualities and structures have been acquired
earlier or later in life, if the full-grown animal possesses them. And the cases
just given, more especially that of pigeons, seem to show that the
characteristic differences which give value to each breed, and which have been
accumulated by man's selection, have not generally first appeared at an early
period of life, and have been inherited by the offspring at a corresponding not
early period. But the case of the short-faced tumbler, which when twelve hours
old had acquired its proper proportions, proves that this is not the universal
rule; for here the characteristic differences must either have appeared at an
earlier period than usual, or, if not so, the differences must have been
inherited, not at the corresponding, but at an earlier age.
Now let us apply these facts and the above two principles which latter, though
not proved true, can be shown to be in some degree probable to species in a
state of nature. Let us take a genus of birds, descended on my theory from some
one parent-species, and of which the several new species have become modified
through natural selection in accordance with their diverse habits. Then, from
the many slight successive steps of variation having supervened at a rather late
age, and having been inherited at a corresponding age, the young of the new
species of our supposed genus will manifestly tend to resemble each other much
more closely than do the adults, just as we have seen in the case of pigeons. We
may extend this view to whole families or even classes. The fore-limbs, for
instance, which served as legs in the parent-species, may become, by a long
course of modification, adapted in one descendant to act as hands, in another as
paddles, in another as wings; and on the above two principles namely of each
successive modification supervening at a rather late age, and being inherited at
a corresponding late age the fore-limbs in the embryos of the several
descendants of the parent-species will still resemble each other closely, for
they will not have been modified. But in each individual new species, the
embryonic fore-limbs will differ greatly from the fore-limbs in the mature
animal; the limbs in the latter having undergone much modification at a rather
late period of life, and having thus been converted into hands, or paddles, or
wings. Whatever influence long-continued exercise or use on the one hand, and
disuse on the other, may have in modifying an organ, such influence will mainly
affect the mature animal, which has come to its full powers of activity and has
to gain its own living; and the effects thus produced will be inherited at a
corresponding mature age. Whereas the young will remain unmodified, or be
modified in a lesser degree, by the effects of use and disuse.
In certain cases the successive steps of variation might supervene, from causes
of which we are wholly ignorant, at a very early period of life, or each step
might be inherited at an earlier period than that at which it first appeared. In
either case (as with the short-faced tumbler) the young or embryo would closely
resemble the mature parent-form. We have seen that this is the rule of
development in certain whole groups of animals, as with cuttle-fish and spiders,
and with a few members of the great class of insects, as with Aphis. With
respect to the final cause of the young in these cases not undergoing any
metamorphosis, or closely resembling their parents from their earliest age, we
can see that this would result from the two following contingencies; firstly,
from the young, during a course of modification carried on for many generations,
having to provide for their own wants at a very early stage of development, and
secondly, from their following exactly the same habits of life with their
parents; for in this case, it would be indispensable for the existence of the
species, that the child should be modified at a very early age in the same
manner with its parents, in accordance with their similar habits. Some further
explanation, however, of the embryo not undergoing any metamorphosis is perhaps
requisite. If, on the other hand, it profited the young to follow habits of life
in any degree different from those of their parent, and consequently to be
constructed in a slightly different manner, then, on the principle of
inheritance at corresponding ages, the active young or larvae might easily be
rendered by natural selection different to any conceivable extent from their
parents. Such differences might, also, become correlated with successive stages
of development; so that the larvae, in the first stage, might differ greatly
from the larvae in the second stage, as we have seen to be the case with
cirripedes. The adult might become fitted for sites or habits, in which organs
of locomotion or of the senses, &c., would be useless; and in this case the
final metamorphosis would be said to be retrograde.
As all the organic beings, extinct and recent, which have ever lived on this
earth have to be classed together, and as all have been connected by the finest
gradations, the best, or indeed, if our collections were nearly perfect, the
only possible arrangement, would be genealogical. Descent being on my view the
hidden bond of connexion which naturalists have been seeking under the term of
the natural system. On this view we can understand how it is that, in the eyes
of most naturalists, the structure of the embryo is even more important for
classification than that of the adult. For the embryo is the animal in its less
modified state; and in so far it reveals the structure of its progenitor. In two
groups of animal, however much they may at present differ from each other in
structure and habits, if they pass through the same or similar embryonic stages,
we may feel assured that they have both descended from the same or nearly
similar parents, and are therefore in that degree closely related. Thus,
community in embryonic structure reveals community of descent. It will reveal
this community of descent, however much the structure of the adult may have been
modified and obscured; we have seen, for instance, that cirripedes can at once
be recognised by their larvae as belonging to the great class of crustaceans. As
the embryonic state of each species and group of species partially shows us the
structure of their less modified ancient progenitors, we can clearly see why
ancient and extinct forms of life should resemble the embryos of their
descendants, our existing species. Agassiz believes this to be a law of nature;
but I am bound to confess that I only hope to see the law hereafter proved true.
It can be proved true in those cases alone in which the ancient state, now
supposed to be represented in many embryos, has not been obliterated, either by
the successive variations in a long course of modification having supervened at
a very early age, or by the variations having been inherited at an earlier
period than that at which they first appeared. It should also be borne in mind,
that the supposed law of resemblance of ancient forms of life to the embryonic
stages of recent forms, may be true, but yet, owing to the geological record not
extending far enough back in time, may remain for a long period, or for ever,
incapable of demonstration.
Thus, as it seems to me, the leading facts in embryology, which are second in
importance to none in natural history, are explained on the principle of slight
modifications not appearing, in the many descendants from some one ancient
progenitor, at a very early period in the life of each, though perhaps caused at
the earliest, and being inherited at a corresponding not early period.
Embryology rises greatly in interest, when we thus look at the embryo as a
picture, more or less obscured, of the common parent-form of each great class of
animals.
Rudimentary, atrophied, or aborted organs
Organs or parts in this strange condition, bearing the stamp of inutility, are
extremely common throughout nature. For instance, rudimentary mammae are very
general in the males of mammals: I presume that the `bastard-wing' in birds may
be safely considered as a digit in a rudimentary state: in very many snakes one
lobe of the lungs is rudimentary; in other snakes there are rudiments of the
pelvis and hind limbs. Some of the cases of rudimentary organs are extremely
curious; for instance, the presence of teeth in foetal whales, which when grown
up have not a tooth in their heads; and the presence of teeth, which never cut
through the gums, in the upper jaws of our unborn calves. It has even been
stated on good authority that rudiments of teeth can be detected in the beaks of
certain embryonic birds. Nothing can be plainer than that wings are formed for
flight, yet in how many insects do we see wings so reduced in size as to be
utterly incapable of flight, and not rarely lying under wing-cases, firmly
soldered together!
The meaning of rudimentary organs is often quite unmistakeable: for instance
there are beetles of the same genus (and even of the same species) resembling
each other most closely in all respects, one of which will have full-sized
wings, and another mere rudiments of membrane; and here it is impossible to
doubt, that the rudiments represent wings. Rudimentary organs sometimes retain
their potentiality, and are merely not developed: this seems to be the case with
the mammae of male mammals, for many instances are on record of these organs
having become well developed in full-grown males, and having secreted milk. So
again there are normally four developed and two rudimentary teats in the udders
of the genus Bos, but in our domestic cows the two sometimes become developed
and give milk. In individual plants of the same species the petals sometimes
occur as mere rudiments, and sometimes in a well-developed state. In plants with
separated sexes, the male flowers often have a rudiment of a pistil; and
Kölreuter found that by crossing such male plants with an hermaphrodite species,
the rudiment of the pistil in the hybrid offspring was much increased in size;
and this shows that the rudiment and the perfect pistil are essentially alike in
nature.
An organ serving for two purposes, may become rudimentary or utterly aborted for
one, even the more important purpose;, and remain perfectly efficient for the
other. Thus in plants, the office of the pistil is to allow the pollen-tubes to
reach the ovules protected in the ovarium at its base. The pistil consists of a
stigma supported on the style; but in some Compositae, the male florets, which
of course cannot be fecundated, have a pistil, which is in a rudimentary state,
for it is not crowned with a stigma; but the style remains well developed, and
is clothed with hairs as in other compositae, for the purpose of brushing the
pollen out of the surrounding anthers. Again, an organ may become rudimentary
for its proper purpose, and be used for a distinct object: in certain fish the
swim-bladder seems to be rudimentary for its proper function of giving buoyancy,
but has become converted into a nascent breathing organ or lung. Other similar
instances could be given.
Rudimentary organs in the individuals of the same species are very liable to
vary in degree of development and in other respects. Moreover, in closely allied
species, the degree to which the same organ has been rendered rudimentary
occasionally differs much. This latter fact is well exemplified in the state of
the wings of the female moths in certain groups. Rudimentary organs may be
utterly aborted; and this implies, that we find in an animal or plant no trace
of an organ, which analogy would lead us to expect to find, and which is
occasionally found in monstrous individuals of the species. Thus in the
snapdragon (antirrhinum) we generally do not find a rudiment of a fifth stamen;
but this may sometimes be seen. In tracing the homologies of the same part in
different members of a class, nothing is more common, or more necessary, than
the use and discovery of rudiments. This is well shown in the drawings given by
Owen of the bones of the leg of the horse, ox, and rhinoceros.
It is an important fact that rudimentary organs, such as teeth in the upper jaws
of whales and ruminants, can often be detected in the embryo, but afterwards
wholly disappear. It is also, I believe, a universal rule, that a rudimentary
part or organ is of greater size relatively to the adjoining parts in the
embryo, than in the adult; so that the organ at this early age is less
rudimentary, or even cannot be said to be in any degree rudimentary. Hence,
also, a rudimentary organ in the adult, is often said to have retained its
embryonic condition.
I have now given the leading facts with respect to rudimentary organs. In
reflecting on them, every one must be struck with astonishment: for the same
reasoning power which tells us plainly that most parts and organs are
exquisitely adapted for certain purposes, tells us with equal plainness that
these rudimentary or atrophied organs, are imperfect and useless. In works on
natural history rudimentary organs are generally said to have been created `for
the sake of symmetry,' or in order `to complete the scheme of nature;' but this
seems to me no explanation, merely a restatement of the fact. Would it be
thought sufficient to say that because planets revolve in elliptic courses round
the sun, satellites follow the same course round the planets, for the sake of
symmetry, and to complete the scheme of nature? An eminent physiologist accounts
for the presence of rudimentary organs, by supposing that they serve to excrete
matter in excess, or injurious to the system; but can we suppose that the minute
papilla, which often represents the pistil in male flowers, and which is formed
merely of cellular tissue, can thus act? Can we suppose that the formation of
rudimentary teeth which are subsequently absorbed, can be of any service to the
rapidly growing embryonic calf by the excretion of precious phosphate of lime?
When a man's fingers have been amputated, imperfect nails sometimes appear on
the stumps: I could as soon believe that these vestiges of nails have appeared,
not from unknown laws of growth, but in order to excrete horny matter, as that
the rudimentary nails on the fin of the manatee were formed for this purpose.
On my view of descent with modification, the origin of rudimentary organs is
simple. We have plenty of cases of rudimentary organs in our domestic
productions, as the stump of a tail in tailless breeds, the vestige of an ear in
earless breeds, -- the reappearance of minute dangling horns in hornless breeds
of cattle, more especially, according to Youatt, in young animals, and the state
of the whole flower in the cauliflower. We often see rudiments of various parts
in monsters. But I doubt whether any of these cases throw light on the origin of
rudimentary organs in a state of nature, further than by showing that rudiments
can be produced; for I doubt whether species under nature ever undergo abrupt
changes. I believe that disuse has been the main agency; that it has led in
successive generations to the gradual reduction of various organs, until they
have become rudimentary, as in the case of the eyes of animals inhabiting dark
caverns, and of the wings of birds inhabiting oceanic islands, which have seldom
been forced to take flight, and have ultimately lost the power of flying. Again,
an organ useful under certain conditions, might become injurious under others,
as with the wings of beetles living on small and exposed islands; and in this
case natural selection would continue slowly to reduce the organ, until it was
rendered harmless and rudimentary.
Any change in function, which can be effected by insensibly small steps, is
within the power of natural selection; so that an organ rendered, during changed
habits of life, useless or injurious for one purpose, might easily be modified
and used for another purpose. Or an organ might be retained for one alone of its
former functions. An organ, when rendered useless, may well be variable, for its
variations cannot be checked by natural selection. At whatever period of life
disuse or selection reduces an organ, and this will generally be when the being
has come to maturity and to its full powers of action, the principle of
inheritance at corresponding ages will reproduce the organ in its reduced state
at the same age, and consequently will seldom affect or reduce it in the embryo.
Thus we can understand the greater relative size of rudimentary organs in the
embryo, and their lesser relative size in the adult. But if each step of the
process of reduction were to be inherited, not at the corresponding age, but at
an extremely early period of life (as we have good reason to believe to be
possible) the rudimentary part would tend to be wholly lost, and we should have
a case of complete abortion. The principle, also, of economy, explained in a
former chapter, by which the materials forming any part or structure, if not
useful to the possessor, will be saved as far as is possible, will probably
often come into play; and this will tend to cause the entire obliteration of a
rudimentary organ.
As the presence of rudimentary organs is thus due to the tendency in every part
of the organisation, which has long existed, to be inherited we can understand,
on the genealogical view of classification, how it is that systematists have
found rudimentary parts as useful as, or even sometimes more useful than, parts
of high physiological importance. Rudimentary organs may be compared with the
letters in a word, still retained in the spelling, but become useless in the
pronunciation, but which serve as a clue in seeking for its derivation. On the
view of descent with modification, we may conclude that the existence of organs
in a rudimentary, imperfect, and useless condition, or quite aborted, far from
presenting a strange difficulty, as they assuredly do on the ordinary doctrine
of creation, might even have been anticipated, and can be accounted for by the
laws of inheritance.
Summary
In this chapter I have attempted to show, that the subordination of group to
group in all organisms throughout all time; that the nature of the relationship,
by which all living and extinct beings are united by complex, radiating, and
circuitous lines of affinities into one grand system; the rules followed and the
difficulties encountered by naturalists in their classifications; the value set
upon characters, if constant and prevalent, whether of high vital importance, or
of the most trifling importance, or, as in rudimentary organs, of no importance;
the wide opposition in value between analogical or adaptive characters, and
characters of true affinity; and other such rules; all naturally follow on the
view of the common parentage of those forms which are considered by naturalists
as allied, together with their modification through natural selection, with its
contingencies of extinction and divergence of character. In considering this
view of classification, it should be borne in mind that the element of descent
has been universally used in ranking together the sexes, ages, and acknowledged
varieties of the same species, however different they may be in structure. If we
extend the use of this element of descent, the only certainly known cause of
similarity in organic beings, we shall understand what is meant by the natural
system: it is genealogical in its attempted arrangement, with the grades of
acquired difference marked by the terms varieties, species, genera, families,
orders, and classes.
On this same view of descent with modification, all the great facts in
Morphology become intelligible, whether we look to the same pattern displayed in
the homologous organs, to whatever purpose applied, of the different species of
a class; or to the homologous parts constructed on the same pattern in each
individual animal and plant.
On the principle of successive slight variations, not necessarily or generally
supervening at a very early period of life, and being inherited at a
corresponding period, we can understand the great leading facts in Embryology;
namely, the resemblance in an individual embryo of the homologous parts, which
when matured will become widely different from each other in structure and
function; and the resemblance in different species of a class of the homologous
parts or organs, though fitted in the adult members for purposes as different as
possible. Larvae are active embryos, which have become specially modified in
relation to their habits of life, through the principle of modifications being
inherited at corresponding ages. On this same principle and bearing in mind,
that when organs are reduced in size, either from disuse or selection, it will
generally be at that period of life when the being has to provide for its own
wants, and bearing in mind how strong is the principle of inheritance the
occurrence of rudimentary organs and their final abortion, present to us no
inexplicable difficulties; on the contrary, their presence might have been even
anticipated. The importance of embryological characters and of rudimentary
organs in classification is intelligible, on the view that an arrangement is
only so far natural as it is genealogical.
Finally, the several classes of facts which have been considered in this
chapter, seem to me to proclaim so plainly, that the innumerable species,
genera, and families of organic beings, with which this world is peopled, have
all descended, each within its own class or group, from common parents, and have
all been modified in the course of descent, that I should without hesitation
adopt this view, even if it were unsupported by other facts or arguments.
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