Chapter XIII Mutual
Affinities of Organic Beings:
Morphology: Embryology: Rudimentary Organs
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.
2 I attempted also to show that there is a constant
tendency in the forms which are increasing in number and diverging in
character, to supplant and exterminate the less divergent, the less improved,
and preceding forms. I request the reader to turn to the diagram illustrating
the action, as formerly explained, of these several principles; and he will
see that the inevitable result is that the modified descendants proceeding
from one progenitor become broken up into groups subordinate to groups. In the
diagram each letter on the uppermost line may represent a genus including
several species; and all the genera on this line form together one class, for
all have descended from one ancient but unseen parent, and, consequently, have
inherited something in common. But the three genera on the left hand have, on
this same principle, much in common, and form a sub-family, distinct from that
including the next two genera on the right hand, which diverged from a common
parent at the fifth stage of descent. These five genera have also much, though
less, in common; and they form a family distinct from that including the three
genera still further to the right hand, which diverged at a still earlier
period. And all these genera, descended from (A), form an order distinct from
the genera descended from (I). So that we here have many species descended
from a single progenitor grouped into genera; and the genera are included in,
or subordinate to, sub-families, families, and orders, all united into one
class. Thus, the grand fact in natural history of the subordination of group
under group, which, from its familiarity, does not always sufficiently strike
us, is in my judgment fully explained.
3 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.
4 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!
5 We must not, therefore, in classifying, trust to
resemblances in parts of the organisation, however important they may be for
the welfare of the being in relation to the outer world. Perhaps from this
cause it has partly arisen, that almost all naturalists lay the greatest
stress on resemblances in organs of high vital or physiological importance. No
doubt this view of the classificatory importance of organs which are important
is generally, but by no means always, true. But their importance for
classification, I believe, depends on their greater constancy throughout large
groups of species; and this constancy depends on such organs having generally
been subjected to less change in the adaptation of the species to their
conditions of life. That the mere physiological importance of an organ does
not determine its classificatory value, is almost shown by the one fact, that
in allied groups, in which the same organ, as we have every reason to suppose,
has nearly the same physiological value, its classificatory value is widely
different. No naturalist can have worked at any group without being struck
with this fact; and it has been most fully acknowledged in the writings of
almost every author. It will suffice to quote the highest authority, Robert
Brown, who in speaking of certain organs in the Proteaceae, says their generic
importance, 'like that of all their parts, not only in this but, as I
apprehend, in every natural family, is very unequal, and in some cases seems
to be entirely lost.' Again in another work he says, the genera of the
Connaraceae 'differ in having one or more ovaria, in the existence or absence
of albumen, in the imbricate or valvular aestivation. Any one of these
characters singly is frequently of more than generic importance, though here
even when all taken together they appear insufficient to separate Cnestis from
Connarus.' To give an example amongst insects, in one great division of the
Hymenoptera, the antennae, as Westwood has remarked, are most constant in
structure; in another division they differ much, and the differences are of
quite subordinate value in classification; yet no one probably will say that
the antennae in these two divisions of the same order are of unequal
physiological importance. Any number of instances could be given of the
varying importance for classification of the same important organ within the
same group of beings.
6 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.
7 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.
8 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.
9 Practically when naturalists are at work, they do not
trouble themselves about the physiological value of the characters which they
use in defining a group, or in allocating any particular species. If they find
a character nearly uniform, and common to a great number of forms, and not
common to others, they use it as one of high value; if common to some lesser
number, they use it as of subordinate value. This principle has been broadly
confessed by some naturalists to be the true one; and by none more clearly
than by that excellent botanist, Aug. St. Hilaire. If certain characters are
always found correlated with others, though no apparent bond of connexion can
be discovered between them, especial value is set on them. As in most groups
of animals, important organs, such as those for propelling the blood, or for
aerating it, or those for propagating the race, are found nearly uniform, they
are considered as highly serviceable in classification; but in some groups of
animals all these, the most important vital organs, are found to offer
characters of quite subordinate value.
10 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.
11 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.
12 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.
13 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.
14 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.
15 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.
16 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.
17 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.
18 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.
19 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.
20 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.
21 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.
22 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.
23 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.
24 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.
25 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.
26 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.
27 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.
28 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.
29 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.
30 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.
31 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.
32 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 on 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.
33 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?
34 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.
35 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.
36 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.
37 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.
38 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.
39 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.
40 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.
41 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.
42 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.
43 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.
44 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.
45 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.
46 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.
47 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.
48 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.
49 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.
50 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.
51 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.
52 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!
53 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 Kolreuter 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.
54 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.
55 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.
56 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.
57 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.
58 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.
59 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.
60 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.
61 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.
62 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.
63 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.
64 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.