On the Origin of Species
by
Charles Darwin
Part 7 out of 9
height, climate, etc., that many of the immigrants should have been
differently modified, though only in a small degree. This long
appeared to me a great difficulty: but it arises in chief part from
the deeply-seated error of considering the physical conditions of a
country as the most important for its inhabitants; whereas it cannot,
I think, be disputed that the nature of the other inhabitants, with
which each has to compete, is at least as important, and generally a
far more important element of success. Now if we look to those
inhabitants of the Galapagos Archipelago which are found in other
parts of the world (laying on one side for the moment the endemic
species, which cannot be here fairly included, as we are considering
how they have come to be modified since their arrival), we find a
considerable amount of difference in the several islands. This
difference might indeed have been expected on the view of the islands
having been stocked by occasional means of transport--a seed, for
instance, of one plant having been brought to one island, and that of
another plant to another island. Hence when in former times an
immigrant settled on any one or more of the islands, or when it
subsequently spread from one island to another, it would undoubtedly
be exposed to different conditions of life in the different islands,
for it would have to compete with different sets of organisms: a
plant, for instance, would find the best-fitted ground more perfectly
occupied by distinct plants in one island than in another, and it
would be exposed to the attacks of somewhat different enemies. If then
it varied, natural selection would probably favour different varieties
in the different islands. Some species, however, might spread and yet
retain the same character throughout the group, just as we see on
continents some species spreading widely and remaining the same.
The really surprising fact in this case of the Galapagos Archipelago,
and in a lesser degree in some analogous instances, is that the new
species formed in the separate islands have not quickly spread to the
other islands. But the islands, though in sight of each other, are
separated by deep arms of the sea, in most cases wider than the
British Channel, and there is no reason to suppose that they have at
any former period been continuously united. The currents of the sea
are rapid and sweep across the archipelago, and gales of wind are
extraordinarily rare; so that the islands are far more effectually
separated from each other than they appear to be on a map.
Nevertheless a good many species, both those found in other parts of
the world and those confined to the archipelago, are common to the
several islands, and we may infer from certain facts that these have
probably spread from some one island to the others. But we often take,
I think, an erroneous view of the probability of closely allied
species invading each other's territory, when put into free
intercommunication. Undoubtedly if one species has any advantage
whatever over another, it will in a very brief time wholly or in part
supplant it; but if both are equally well fitted for their own places
in nature, both probably will hold their own places and keep separate
for almost any length of time. Being familiar with the fact that many
species, naturalised through man's agency, have spread with
astonishing rapidity over new countries, we are apt to infer that most
species would thus spread; but we should remember that the forms which
become naturalised in new countries are not generally closely allied
to the aboriginal inhabitants, but are very distinct species,
belonging in a large proportion of cases, as shown by Alph. de
Candolle, to distinct genera. In the Galapagos Archipelago, many even
of the birds, though so well adapted for flying from island to island,
are distinct on each; thus there are three closely-allied species of
mocking-thrush, each confined to its own island. Now let us suppose
the mocking-thrush of Chatham Island to be blown to Charles Island,
which has its own mocking-thrush: why should it succeed in
establishing itself there? We may safely infer that Charles Island is
well stocked with its own species, for annually more eggs are laid
there than can possibly be reared; and we may infer that the
mocking-thrush peculiar to Charles Island is at least as well fitted
for its home as is the species peculiar to Chatham Island. Sir C.
Lyell and Mr. Wollaston have communicated to me a remarkable fact
bearing on this subject; namely, that Madeira and the adjoining islet
of Porto Santo possess many distinct but representative land-shells,
some of which live in crevices of stone; and although large quantities
of stone are annually transported from Porto Santo to Madeira, yet
this latter island has not become colonised by the Porto Santo
species: nevertheless both islands have been colonised by some
European land-shells, which no doubt had some advantage over the
indigenous species. From these considerations I think we need not
greatly marvel at the endemic and representative species, which
inhabit the several islands of the Galapagos Archipelago, not having
universally spread from island to island. In many other instances, as
in the several districts of the same continent, pre-occupation has
probably played an important part in checking the commingling of
species under the same conditions of life. Thus, the south-east and
south-west corners of Australia have nearly the same physical
conditions, and are united by continuous land, yet they are inhabited
by a vast number of distinct mammals, birds, and plants.
The principle which determines the general character of the fauna and
flora of oceanic islands, namely, that the inhabitants, when not
identically the same, yet are plainly related to the inhabitants of
that region whence colonists could most readily have been
derived,--the colonists having been subsequently modified and better
fitted to their new homes,--is of the widest application throughout
nature. We see this on every mountain, in every lake and marsh. For
Alpine species, excepting in so far as the same forms, chiefly of
plants, have spread widely throughout the world during the recent
Glacial epoch, are related to those of the surrounding lowlands;--thus
we have in South America, Alpine humming-birds, Alpine rodents, Alpine
plants, etc., all of strictly American forms, and it is obvious that a
mountain, as it became slowly upheaved, would naturally be colonised
from the surrounding lowlands. So it is with the inhabitants of lakes
and marshes, excepting in so far as great facility of transport has
given the same general forms to the whole world. We see this same
principle in the blind animals inhabiting the caves of America and of
Europe. Other analogous facts could be given. And it will, I believe,
be universally found to be true, that wherever in two regions, let
them be ever so distant, many closely allied or representative species
occur, there will likewise be found some identical species, showing,
in accordance with the foregoing view, that at some former period
there has been intercommunication or migration between the two
regions. And wherever many closely-allied species occur, there will be
found many forms which some naturalists rank as distinct species, and
some as varieties; these doubtful forms showing us the steps in the
process of modification.
This relation between the power and extent of migration of a species,
either at the present time or at some former period under different
physical conditions, and the existence at remote points of the world
of other species allied to it, is shown in another and more general
way. Mr. Gould remarked to me long ago, that in those genera of birds
which range over the world, many of the species have very wide ranges.
I can hardly doubt that this rule is generally true, though it would
be difficult to prove it. Amongst mammals, we see it strikingly
displayed in Bats, and in a lesser degree in the Felidae and Canidae.
We see it, if we compare the distribution of butterflies and beetles.
So it is with most fresh-water productions, in which so many genera
range over the world, and many individual species have enormous
ranges. It is not meant that in world-ranging genera all the species
have a wide range, or even that they have on an AVERAGE a wide range;
but only that some of the species range very widely; for the facility
with which widely-ranging species vary and give rise to new forms will
largely determine their average range. For instance, two varieties of
the same species inhabit America and Europe, and the species thus has
an immense range; but, if the variation had been a little greater, the
two varieties would have been ranked as distinct species, and the
common range would have been greatly reduced. Still less is it meant,
that a species which apparently has the capacity of crossing barriers
and ranging widely, as in the case of certain powerfully-winged birds,
will necessarily range widely; for we should never forget that to
range widely implies not only the power of crossing barriers, but the
more important power of being victorious in distant lands in the
struggle for life with foreign associates. But on the view of all the
species of a genus having descended from a single parent, though now
distributed to the most remote points of the world, we ought to find,
and I believe as a general rule we do find, that some at least of the
species range very widely; for it is necessary that the unmodified
parent should range widely, undergoing modification during its
diffusion, and should place itself under diverse conditions favourable
for the conversion of its offspring, firstly into new varieties and
ultimately into new species.
In considering the wide distribution of certain genera, we should bear
in mind that some are extremely ancient, and must have branched off
from a common parent at a remote epoch; so that in such cases there
will have been ample time for great climatal and geographical changes
and for accidents of transport; and consequently for the migration of
some of the species into all quarters of the world, where they may
have become slightly modified in relation to their new conditions.
There is, also, some reason to believe from geological evidence that
organisms low in the scale within each great class, generally change
at a slower rate than the higher forms; and consequently the lower
forms will have had a better chance of ranging widely and of still
retaining the same specific character. This fact, together with the
seeds and eggs of many low forms being very minute and better fitted
for distant transportation, probably accounts for a law which has long
been observed, and which has lately been admirably discussed by Alph.
de Candolle in regard to plants, namely, that the lower any group of
organisms is, the more widely it is apt to range.
The relations just discussed,--namely, low and slowly-changing
organisms ranging more widely than the high,--some of the species of
widely-ranging genera themselves ranging widely,--such facts, as
alpine, lacustrine, and marsh productions being related (with the
exceptions before specified) to those on the surrounding low lands and
dry lands, though these stations are so different--the very close
relation of the distinct species which inhabit the islets of the same
archipelago,--and especially the striking relation of the inhabitants
of each whole archipelago or island to those of the nearest
mainland,--are, I think, utterly inexplicable on the ordinary view of
the independent creation of each species, but are explicable on the
view of colonisation from the nearest and readiest source, together
with the subsequent modification and better adaptation of the
colonists to their new homes.
SUMMARY OF LAST AND PRESENT CHAPTERS.
In these chapters I have endeavoured to show, that if we make due
allowance for our ignorance of the full effects of all the changes of
climate and of the level of the land, which have certainly occurred
within the recent period, and of other similar changes which may have
occurred within the same period; if we remember how profoundly
ignorant we are with respect to the many and curious means of
occasional transport,--a subject which has hardly ever been properly
experimentised on; if we bear in mind how often a species may have
ranged continuously over a wide area, and then have become extinct in
the intermediate tracts, I think the difficulties in believing that
all the individuals of the same species, wherever located, have
descended from the same parents, are not insuperable. And we are led
to this conclusion, which has been arrived at by many naturalists
under the designation of single centres of creation, by some general
considerations, more especially from the importance of barriers and
from the analogical distribution of sub-genera, genera, and families.
With respect to the distinct species of the same genus, which on my
theory must have spread from one parent-source; if we make the same
allowances as before for our ignorance, and remember that some forms
of life change most slowly, enormous periods of time being thus
granted for their migration, I do not think that the difficulties are
insuperable; though they often are in this case, and in that of the
individuals of the same species, extremely grave.
As exemplifying the effects of climatal changes on distribution, I
have attempted to show how important has been the influence of the
modern Glacial period, which I am fully convinced simultaneously
affected the whole world, or at least great meridional belts. As
showing how diversified are the means of occasional transport, I have
discussed at some little length the means of dispersal of fresh-water
productions.
If the difficulties be not insuperable in admitting that in the long
course of time the individuals of the same species, and likewise of
allied species, have proceeded from some one source; then I think all
the grand leading facts of geographical distribution are explicable on
the theory of migration (generally of the more dominant forms of
life), together with subsequent modification and the multiplication of
new forms. We can thus understand the high importance of barriers,
whether of land or water, which separate our several zoological and
botanical provinces. We can thus understand the localisation of
sub-genera, genera, and families; and how it is that under different
latitudes, for instance in South America, the inhabitants of the
plains and mountains, of the forests, marshes, and deserts, are in so
mysterious a manner linked together by affinity, and are likewise
linked to the extinct beings which formerly inhabited the same
continent. Bearing in mind that the mutual relations of organism to
organism are of the highest importance, we can see why two areas
having nearly the same physical conditions should often be inhabited
by very different forms of life; for according to the length of time
which has elapsed since new inhabitants entered one region; according
to the nature of the communication which allowed certain forms and not
others to enter, either in greater or lesser numbers; according or
not, as those which entered happened to come in more or less direct
competition with each other and with the aborigines; and according as
the immigrants were capable of varying more or less rapidly, there
would ensue in different regions, independently of their physical
conditions, infinitely diversified conditions of life,--there would be
an almost endless amount of organic action and reaction,--and we
should find, as we do find, some groups of beings greatly, and some
only slightly modified,--some developed in great force, some existing
in scanty numbers--in the different great geographical provinces of
the world.
On these same principles, we can understand, as I have endeavoured to
show, why oceanic islands should have few inhabitants, but of these a
great number should be endemic or peculiar; and why, in relation to
the means of migration, one group of beings, even within the same
class, should have all its species endemic, and another group should
have all its species common to other quarters of the world. We can see
why whole groups of organisms, as batrachians and terrestrial mammals,
should be absent from oceanic islands, whilst the most isolated
islands possess their own peculiar species of aerial mammals or bats.
We can see why there should be some relation between the presence of
mammals, in a more or less modified condition, and the depth of the
sea between an island and the mainland. We can clearly see why all the
inhabitants of an archipelago, though specifically distinct on the
several islets, should be closely related to each other, and likewise
be related, but less closely, to those of the nearest continent or
other source whence immigrants were probably derived. We can see why
in two areas, however distant from each other, there should be a
correlation, in the presence of identical species, of varieties, of
doubtful species, and of distinct but representative species.
As the late Edward Forbes often insisted, there is a striking
parallelism in the laws of life throughout time and space: the laws
governing the succession of forms in past times being nearly the same
with those governing at the present time the differences in different
areas. We see this in many facts. The endurance of each species and
group of species is continuous in time; for the exceptions to the rule
are so few, that they may fairly be attributed to our not having as
yet discovered in an intermediate deposit the forms which are therein
absent, but which occur above and below: so in space, it certainly is
the general rule that the area inhabited by a single species, or by a
group of species, is continuous; and the exceptions, which are not
rare, may, as I have attempted to show, be accounted for by migration
at some former period under different conditions or by occasional
means of transport, and by the species having become extinct in the
intermediate tracts. Both in time and space, species and groups of
species have their points of maximum development. Groups of species,
belonging either to a certain period of time, or to a certain area,
are often characterised by trifling characters in common, as of
sculpture or colour. In looking to the long succession of ages, as in
now looking to distant provinces throughout the world, we find that
some organisms differ little, whilst others belonging to a different
class, or to a different order, or even only to a different family of
the same order, differ greatly. In both time and space the lower
members of each class generally change less than the higher; but there
are in both cases marked exceptions to the rule. On my theory these
several relations throughout time and space are intelligible; for
whether we look to the forms of life which have changed during
successive ages within the same quarter of the world, or to those
which have changed after having migrated into distant quarters, in
both cases the forms within each class have been connected by the same
bond of ordinary generation; and the more nearly any two forms are
related in blood, the nearer they will generally stand to each other
in time and space; in both cases the laws of variation have been the
same, and modifications have been accumulated by the same power of
natural selection.
CHAPTER 13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY:
EMBRYOLOGY: RUDIMENTARY ORGANS.
CLASSIFICATION, groups subordinate to groups.
Natural system.
Rules and difficulties in classification, explained on the theory of
descent with modification.
Classification of varieties.
Descent always used in classification.
Analogical or adaptive characters.
Affinities, general, complex and radiating.
Extinction separates and defines groups.
MORPHOLOGY, between members of the same class, between parts of the
same individual.
EMBRYOLOGY, laws of, explained by variations not supervening at an
early age, and being inherited at a corresponding age.
RUDIMENTARY ORGANS; their origin explained.
Summary.
From the first dawn of life, all organic beings are found to resemble
each other in descending degrees, so that they can be classed in
groups under groups. This classification is evidently not arbitrary
like the grouping of the stars in constellations. The existence of
groups would have been of simple signification, if one group had been
exclusively fitted to inhabit the land, and another the water; one to
feed on flesh, another on vegetable matter, and so on; but the case is
widely different in nature; for it is notorious how commonly members
of even the same subgroup have different habits. In our second and
fourth chapters, on Variation and on Natural Selection, I have
attempted to show that it is the widely ranging, the much diffused and
common, that is the dominant species belonging to the larger genera,
which vary most. The varieties, or incipient species, thus produced
ultimately become converted, as I believe, into new and distinct
species; and these, on the principle of inheritance, tend to produce
other new and dominant species. Consequently the groups which are now
large, and which generally include many dominant species, tend to go
on increasing indefinitely in size. I further attempted to show that
from the varying descendants of each species trying to occupy as many
and as different places as possible in the economy of nature, there is
a constant tendency in their characters to diverge. This conclusion
was supported by looking at the great diversity of the forms of life
which, in any small area, come into the closest competition, and by
looking to certain facts in naturalisation.
I attempted also to show that there is a constant tendency in the
forms which are increasing in number and diverging in character, to
supplant and exterminate the less divergent, the less improved, and
preceding forms. I request the reader to turn to the diagram
illustrating the action, as formerly explained, of these several
principles; and he will see that the inevitable result is that the
modified descendants proceeding from one progenitor become broken up
into groups subordinate to groups. In the diagram each letter on the
uppermost line may represent a genus including several species; and
all the genera on this line form together one class, for all have
descended from one ancient but unseen parent, and, consequently, have
inherited something in common. But the three genera on the left hand
have, on this same principle, much in common, and form a sub-family,
distinct from that including the next two genera on the right hand,
which diverged from a common parent at the fifth stage of descent.
These five genera have also much, though less, in common; and they
form a family distinct from that including the three genera still
further to the right hand, which diverged at a still earlier period.
And all these genera, descended from (A), form an order distinct from
the genera descended from (I). So that we here have many species
descended from a single progenitor grouped into genera; and the genera
are included in, or subordinate to, sub-families, families, and
orders, all united into one class. Thus, the grand fact in natural
history of the subordination of group under group, which, from its
familiarity, does not always sufficiently strike us, is in my judgment
fully explained.
Naturalists try to arrange the species, genera, and families in each
class, on what is called the Natural System. But what is meant by this
system? Some authors look at it merely as a scheme for arranging
together those living objects which are most alike, and for separating
those which are most unlike; or as an artificial means for
enunciating, as briefly as possible, general propositions,--that is,
by one sentence to give the characters common, for instance, to all
mammals, by another those common to all carnivora, by another those
common to the dog-genus, and then by adding a single sentence, a full
description is given of each kind of dog. The ingenuity and utility of
this system are indisputable. But many naturalists think that
something more is meant by the Natural System; they believe that it
reveals the plan of the Creator; but unless it be specified whether
order in time or space, or what else is meant by the plan of the
Creator, it seems to me that nothing is thus added to our knowledge.
Such expressions as that famous one of Linnaeus, and which we often
meet with in a more or less concealed form, that the characters do not
make the genus, but that the genus gives the characters, seem to imply
that something more is included in our classification, than mere
resemblance. I believe that something more is included; and that
propinquity of descent,--the only known cause of the similarity of
organic beings,--is the bond, hidden as it is by various degrees of
modification, which is partially revealed to us by our
classifications.
Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification
either gives some unknown plan of creation, or is simply a scheme for
enunciating general propositions and of placing together the forms
most like each other. It might have been thought (and was in ancient
times thought) that those parts of the structure which determined the
habits of life, and the general place of each being in the economy of
nature, would be of very high importance in classification. Nothing
can be more false. No one regards the external similarity of a mouse
to a shrew, of a dugong to a whale, of a whale to a fish, as of any
importance. These resemblances, though so intimately connected with
the whole life of the being, are ranked as merely "adaptive or
analogical characters;" but to the consideration of these resemblances
we shall have to recur. It may even be given as a general rule, that
the less any part of the organisation is concerned with special
habits, the more important it becomes for classification. As an
instance: Owen, in speaking of the dugong, says, "The generative
organs being those which are most remotely related to the habits and
food of an animal, I have always regarded as affording very clear
indications of its true affinities. We are least likely in the
modifications of these organs to mistake a merely adaptive for an
essential character." So with plants, how remarkable it is that the
organs of vegetation, on which their whole life depends, are of little
signification, excepting in the first main divisions; whereas the
organs of reproduction, with their product the seed, are of paramount
importance!
We must not, therefore, in classifying, trust to resemblances in parts
of the organisation, however important they may be for the welfare of
the being in relation to the outer world. Perhaps from this cause it
has partly arisen, that almost all naturalists lay the greatest stress
on resemblances in organs of high vital or physiological importance.
No doubt this view of the classificatory importance of organs which
are important is generally, but by no means always, true. But their
importance for classification, I believe, depends on their greater
constancy throughout large groups of species; and this constancy
depends on such organs having generally been subjected to less change
in the adaptation of the species to their conditions of life. That the
mere physiological importance of an organ does not determine its
classificatory value, is almost shown by the one fact, that in allied
groups, in which the same organ, as we have every reason to suppose,
has nearly the same physiological value, its classificatory value is
widely different. No naturalist can have worked at any group without
being struck with this fact; and it has been most fully acknowledged
in the writings of almost every author. It will suffice to quote the
highest authority, Robert Brown, who in speaking of certain organs in
the Proteaceae, says their generic importance, "like that of all their
parts, not only in this but, as I apprehend, in every natural family,
is very unequal, and in some cases seems to be entirely lost." Again
in another work he says, the genera of the Connaraceae "differ in
having one or more ovaria, in the existence or absence of albumen, in
the imbricate or valvular aestivation. Any one of these characters
singly is frequently of more than generic importance, though here even
when all taken together they appear insufficient to separate Cnestis
from Connarus." To give an example amongst insects, in one great
division of the Hymenoptera, the antennae, as Westwood has remarked,
are most constant in structure; in another division they differ much,
and the differences are of quite subordinate value in classification;
yet no one probably will say that the antennae in these two divisions
of the same order are of unequal physiological importance. Any number
of instances could be given of the varying importance for
classification of the same important organ within the same group of
beings.
Again, no one will say that rudimentary or atrophied organs are of
high physiological or vital importance; yet, undoubtedly, organs in
this condition are often of high value in classification. No one will
dispute that the rudimentary teeth in the upper jaws of young
ruminants, and certain rudimentary bones of the leg, are highly
serviceable in exhibiting the close affinity between Ruminants and
Pachyderms. Robert Brown has strongly insisted on the fact that the
rudimentary florets are of the highest importance in the
classification of the Grasses.
Numerous instances could be given of characters derived from parts
which must be considered of very trifling physiological importance,
but which are universally admitted as highly serviceable in the
definition of whole groups. For instance, whether or not there is an
open passage from the nostrils to the mouth, the only character,
according to Owen, which absolutely distinguishes fishes and
reptiles--the inflection of the angle of the jaws in Marsupials--the
manner in which the wings of insects are folded--mere colour in
certain Algae--mere pubescence on parts of the flower in grasses--the
nature of the dermal covering, as hair or feathers, in the Vertebrata.
If the Ornithorhynchus had been covered with feathers instead of hair,
this external and trifling character would, I think, have been
considered by naturalists as important an aid in determining the
degree of affinity of this strange creature to birds and reptiles, as
an approach in structure in any one internal and important organ.
The importance, for classification, of trifling characters, mainly
depends on their being correlated with several other characters of
more or less importance. The value indeed of an aggregate of
characters is very evident in natural history. Hence, as has often
been remarked, a species may depart from its allies in several
characters, both of high physiological importance and of almost
universal prevalence, and yet leave us in no doubt where it should be
ranked. Hence, also, it has been found, that a classification founded
on any single character, however important that may be, has always
failed; for no part of the organisation is universally constant. The
importance of an aggregate of characters, even when none are
important, alone explains, I think, that saying of Linnaeus, that the
characters do not give the genus, but the genus gives the characters;
for this saying seems founded on an appreciation of many trifling
points of resemblance, too slight to be defined. Certain plants,
belonging to the Malpighiaceae, bear perfect and degraded flowers; in
the latter, as A. de Jussieu has remarked, "the greater number of the
characters proper to the species, to the genus, to the family, to the
class, disappear, and thus laugh at our classification." But when
Aspicarpa produced in France, during several years, only degraded
flowers, departing so wonderfully in a number of the most important
points of structure from the proper type of the order, yet M. Richard
sagaciously saw, as Jussieu observes, that this genus should still be
retained amongst the Malpighiaceae. This case seems to me well to
illustrate the spirit with which our classifications are sometimes
necessarily founded.
Practically when naturalists are at work, they do not trouble
themselves about the physiological value of the characters which they
use in defining a group, or in allocating any particular species. If
they find a character nearly uniform, and common to a great number of
forms, and not common to others, they use it as one of high value; if
common to some lesser number, they use it as of subordinate value.
This principle has been broadly confessed by some naturalists to be
the true one; and by none more clearly than by that excellent
botanist, Aug. St. Hilaire. If certain characters are always found
correlated with others, though no apparent bond of connexion can be
discovered between them, especial value is set on them. As in most
groups of animals, important organs, such as those for propelling the
blood, or for aerating it, or those for propagating the race, are
found nearly uniform, they are considered as highly serviceable in
classification; but in some groups of animals all these, the most
important vital organs, are found to offer characters of quite
subordinate value.
We can see why characters derived from the embryo should be of equal
importance with those derived from the adult, for our classifications
of course include all ages of each species. But it is by no means
obvious, on the ordinary view, why the structure of the embryo should
be more important for this purpose than that of the adult, which alone
plays its full part in the economy of nature. Yet it has been strongly
urged by those great naturalists, Milne Edwards and Agassiz, that
embryonic characters are the most important of any in the
classification of animals; and this doctrine has very generally been
admitted as true. The same fact holds good with flowering plants, of
which the two main divisions have been founded on characters derived
from the embryo,--on the number and position of the embryonic leaves
or cotyledons, and on the mode of development of the plumule and
radicle. In our discussion on embryology, we shall see why such
characters are so valuable, on the view of classification tacitly
including the idea of descent.
Our classifications are often plainly influenced by chains of
affinities. Nothing can be easier than to define a number of
characters common to all birds; but in the case of crustaceans, such
definition has hitherto been found impossible. There are crustaceans
at the opposite ends of the series, which have hardly a character in
common; yet the species at both ends, from being plainly allied to
others, and these to others, and so onwards, can be recognised as
unequivocally belonging to this, and to no other class of the
Articulata.
Geographical distribution has often been used, though perhaps not
quite logically, in classification, more especially in very large
groups of closely allied forms. Temminck insists on the utility or
even necessity of this practice in certain groups of birds; and it has
been followed by several entomologists and botanists.
Finally, with respect to the comparative value of the various groups
of species, such as orders, sub-orders, families, sub-families, and
genera, they seem to be, at least at present, almost arbitrary.
Several of the best botanists, such as Mr. Bentham and others, have
strongly insisted on their arbitrary value. Instances could be given
amongst plants and insects, of a group of forms, first ranked by
practised naturalists as only a genus, and then raised to the rank of
a sub-family or family; and this has been done, not because further
research has detected important structural differences, at first
overlooked, but because numerous allied species, with slightly
different grades of difference, have been subsequently discovered.
All the foregoing rules and aids and difficulties in classification
are explained, if I do not greatly deceive myself, on the view that
the natural system is founded on descent with modification; that the
characters which naturalists consider as showing true affinity between
any two or more species, are those which have been inherited from a
common parent, and, in so far, all true classification is
genealogical; that community of descent is the hidden bond which
naturalists have been unconsciously seeking, and not some unknown plan
of creation, or the enunciation of general propositions, and the mere
putting together and separating objects more or less alike.
But I must explain my meaning more fully. I believe that the
ARRANGEMENT of the groups within each class, in due subordination and
relation to the other groups, must be strictly genealogical in order
to be natural; but that the AMOUNT of difference in the several
branches or groups, though allied in the same degree in blood to their
common progenitor, may differ greatly, being due to the different
degrees of modification which they have undergone; and this is
expressed by the forms being ranked under different genera, families,
sections, or orders. The reader will best understand what is meant, if
he will take the trouble of referring to the diagram in the fourth
chapter. We will suppose the letters A to L to represent allied
genera, which lived during the Silurian epoch, and these have
descended from a species which existed at an unknown anterior period.
Species of three of these genera (A, F, and I) have transmitted
modified descendants to the present day, represented by the fifteen
genera (a14 to z14) on the uppermost horizontal line. Now all these
modified descendants from a single species, are represented as related
in blood or descent to the same degree; they may metaphorically be
called cousins to the same millionth degree; yet they differ widely
and in different degrees from each other. The forms descended from A,
now broken up into two or three families, constitute a distinct order
from those descended from I, also broken up into two families. Nor can
the existing species, descended from A, be ranked in the same genus
with the parent A; or those from I, with the parent I. But the
existing genus F14 may be supposed to have been but slightly modified;
and it will then rank with the parent-genus F; just as some few still
living organic beings belong to Silurian genera. So that the amount or
value of the differences between organic beings all related to each
other in the same degree in blood, has come to be widely different.
Nevertheless their genealogical ARRANGEMENT remains strictly true, not
only at the present time, but at each successive period of descent.
All the modified descendants from A will have inherited something in
common from their common parent, as will all the descendants from I;
so will it be with each subordinate branch of descendants, at each
successive period. If, however, we choose to suppose that any of the
descendants of A or of I have been so much modified as to have more or
less completely lost traces of their parentage, in this case, their
places in a natural classification will have been more or less
completely lost,--as sometimes seems to have occurred with existing
organisms. All the descendants of the genus F, along its whole line of
descent, are supposed to have been but little modified, and they yet
form a single genus. But this genus, though much isolated, will still
occupy its proper intermediate position; for F originally was
intermediate in character between A and I, and the several genera
descended from these two genera will have inherited to a certain
extent their characters. This natural arrangement is shown, as far as
is possible on paper, in the diagram, but in much too simple a manner.
If a branching diagram had not been used, and only the names of the
groups had been written in a linear series, it would have been still
less possible to have given a natural arrangement; and it is
notoriously not possible to represent in a series, on a flat surface,
the affinities which we discover in nature amongst the beings of the
same group. Thus, on the view which I hold, the natural system is
genealogical in its arrangement, like a pedigree; but the degrees of
modification which the different groups have undergone, have to be
expressed by ranking them under different so-called genera,
sub-families, families, sections, orders, and classes.
It may be worth while to illustrate this view of classification, by
taking the case of languages. If we possessed a perfect pedigree of
mankind, a genealogical arrangement of the races of man would afford
the best classification of the various languages now spoken throughout
the world; and if all extinct languages, and all intermediate and
slowly changing dialects, had to be included, such an arrangement
would, I think, be the only possible one. Yet it might be that some
very ancient language had altered little, and had given rise to few
new languages, whilst others (owing to the spreading and subsequent
isolation and states of civilisation of the several races, descended
from a common race) had altered much, and had given rise to many new
languages and dialects. The various degrees of difference in the
languages from the same stock, would have to be expressed by groups
subordinate to groups; but the proper or even only possible
arrangement would still be genealogical; and this would be strictly
natural, as it would connect together all languages, extinct and
modern, by the closest affinities, and would give the filiation and
origin of each tongue.
In confirmation of this view, let us glance at the classification of
varieties, which are believed or known to have descended from one
species. These are grouped under species, with sub-varieties under
varieties; and with our domestic productions, several other grades of
difference are requisite, as we have seen with pigeons. The origin of
the existence of groups subordinate to groups, is the same with
varieties as with species, namely, closeness of descent with various
degrees of modification. Nearly the same rules are followed in
classifying varieties, as with species. Authors have insisted on the
necessity of classing varieties on a natural instead of an artificial
system; we are cautioned, for instance, not to class two varieties of
the pine-apple together, merely because their fruit, though the most
important part, happens to be nearly identical; no one puts the
swedish and common turnips together, though the esculent and thickened
stems are so similar. Whatever part is found to be most constant, is
used in classing varieties: thus the great agriculturist Marshall says
the horns are very useful for this purpose with cattle, because they
are less variable than the shape or colour of the body, etc.; 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, etc., 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 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.
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, etc.,--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, etc., 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, etc., 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 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.
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.
CHAPTER 14. RECAPITULATION AND CONCLUSION.
Recapitulation of the difficulties on the theory of Natural Selection.
Recapitulation of the general and special circumstances in its favour.
Causes of the general belief in the immutability of species.
How far the theory of natural selection may be extended.
Effects of its adoption on the study of Natural history.
Concluding remarks.
As this whole volume is one long argument, it may be convenient to the
reader to have the leading facts and inferences briefly recapitulated.
That many and grave objections may be advanced against the theory of
descent with modification through natural selection, I do not deny. I
have endeavoured to give to them their full force. Nothing at first
can appear more difficult to believe than that the more complex organs
and instincts should have been perfected, not by means superior to,
though analogous with, human reason, but by the accumulation of
innumerable slight variations, each good for the individual possessor.
Nevertheless, this difficulty, though appearing to our imagination
insuperably great, cannot be considered real if we admit the following
propositions, namely,--that gradations in the perfection of any organ
or instinct, which we may consider, either do now exist or could have
existed, each good of its kind,--that all organs and instincts are, in
ever so slight a degree, variable,--and, lastly, that there is a
struggle for existence leading to the preservation of each profitable
deviation of structure or instinct. The truth of these propositions
cannot, I think, be disputed.
It is, no doubt, extremely difficult even to conjecture by what
gradations many structures have been perfected, more especially
amongst broken and failing groups of organic beings; but we see so
many strange gradations in nature, as is proclaimed by the canon,
"Natura non facit saltum," that we ought to be extremely cautious in
saying that any organ or instinct, or any whole being, could not have
arrived at its present state by many graduated steps. There are, it
must be admitted, cases of special difficulty on the theory of natural
selection; and one of the most curious of these is the existence of
two or three defined castes of workers or sterile females in the same
community of ants; but I have attempted to show how this difficulty
can be mastered.
With respect to the almost universal sterility of species when first
crossed, which forms so remarkable a contrast with the almost
universal fertility of varieties when crossed, I must refer the reader
to the recapitulation of the facts given at the end of the eighth
chapter, which seem to me conclusively to show that this sterility is
no more a special endowment than is the incapacity of two trees to be
grafted together, but that it is incidental on constitutional
differences in the reproductive systems of the intercrossed species.
We see the truth of this conclusion in the vast difference in the
result, when the same two species are crossed reciprocally; that is,
when one species is first used as the father and then as the mother.
The fertility of varieties when intercrossed and of their mongrel
offspring cannot be considered as universal; nor is their very general
fertility surprising when we remember that it is not likely that
either their constitutions or their reproductive systems should have
been profoundly modified. Moreover, most of the varieties which have
been experimentised on have been produced under domestication; and as
domestication apparently tends to eliminate sterility, we ought not to
expect it also to produce sterility.
The sterility of hybrids is a very different case from that of first
crosses, for their reproductive organs are more or less functionally
impotent; whereas in first crosses the organs on both sides are in a
perfect condition. As we continually see that organisms of all kinds
are rendered in some degree sterile from their constitutions having
been disturbed by slightly different and new conditions of life, we
need not feel surprise at hybrids being in some degree sterile, for
their constitutions can hardly fail to have been disturbed from being
compounded of two distinct organisations. This parallelism is
supported by another parallel, but directly opposite, class of facts;
namely, that the vigour and fertility of all organic beings are
increased by slight changes in their conditions of life, and that the
offspring of slightly modified forms or varieties acquire from being
crossed increased vigour and fertility. So that, on the one hand,
considerable changes in the conditions of life and crosses between
greatly modified forms, lessen fertility; and on the other hand,
lesser changes in the conditions of life and crosses between less
modified forms, increase fertility.
Turning to geographical distribution, the difficulties encountered on
the theory of descent with modification are grave enough. All the
individuals of the same species, and all the species of the same
genus, or even higher group, must have descended from common parents;
and therefore, in however distant and isolated parts of the world they
are now found, they must in the course of successive generations have
passed from some one part to the others. We are often wholly unable
even to conjecture how this could have been effected. Yet, as we have
reason to believe that some species have retained the same specific
form for very long periods, enormously long as measured by years, too
much stress ought not to be laid on the occasional wide diffusion of
the same species; for during very long periods of time there will
always be a good chance for wide migration by many means. A broken or
interrupted range may often be accounted for by the extinction of the
species in the intermediate regions. It cannot be denied that we are
as yet very ignorant of the full extent of the various climatal and
geographical changes which have affected the earth during modern
periods; and such changes will obviously have greatly facilitated
migration. As an example, I have attempted to show how potent has been
the influence of the Glacial period on the distribution both of the
same and of representative species throughout the world. We are as yet
profoundly ignorant of the many occasional means of transport. With
respect to distinct species of the same genus inhabiting very distant
and isolated regions, as the process of modification has necessarily
been slow, all the means of migration will have been possible during a
very long period; and consequently the difficulty of the wide
diffusion of species of the same genus is in some degree lessened.
As on the theory of natural selection an interminable number of
intermediate forms must have existed, linking together all the species
in each group by gradations as fine as our present varieties, it may
be asked, Why do we not see these linking forms all around us? Why are
not all organic beings blended together in an inextricable chaos? With
respect to existing forms, we should remember that we have no right to
expect (excepting in rare cases) to discover DIRECTLY connecting links
between them, but only between each and some extinct and supplanted
form. Even on a wide area, which has during a long period remained
continuous, and of which the climate and other conditions of life
change insensibly in going from a district occupied by one species
into another district occupied by a closely allied species, we have no
just right to expect often to find intermediate varieties in the
intermediate zone. For we have reason to believe that only a few
species are undergoing change at any one period; and all changes are
slowly effected. I have also shown that the intermediate varieties
which will at first probably exist in the intermediate zones, will be
liable to be supplanted by the allied forms on either hand; and the
latter, from existing in greater numbers, will generally be modified
and improved at a quicker rate than the intermediate varieties, which
exist in lesser numbers; so that the intermediate varieties will, in
the long run, be supplanted and exterminated.
On this doctrine of the extermination of an infinitude of connecting
links, between the living and extinct inhabitants of the world, and at
each successive period between the extinct and still older species,
why is not every geological formation charged with such links? Why
does not every collection of fossil remains afford plain evidence of
the gradation and mutation of the forms of life? We meet with no such
evidence, and this is the most obvious and forcible of the many
objections which may be urged against my theory. Why, again, do whole
groups of allied species appear, though certainly they often falsely
appear, to have come in suddenly on the several geological stages? Why
do we not find great piles of strata beneath the Silurian system,
stored with the remains of the progenitors of the Silurian groups of
fossils? For certainly on my theory such strata must somewhere have
been deposited at these ancient and utterly unknown epochs in the
world's history.
I can answer these questions and grave objections only on the
supposition that the geological record is far more imperfect than most
geologists believe. It cannot be objected that there has not been time
sufficient for any amount of organic change; for the lapse of time has
been so great as to be utterly inappreciable by the human intellect.
The number of specimens in all our museums is absolutely as nothing
compared with the countless generations of countless species which
certainly have existed. We should not be able to recognise a species
as the parent of any one or more species if we were to examine them
ever so closely, unless we likewise possessed many of the intermediate
links between their past or parent and present states; and these many
links we could hardly ever expect to discover, owing to the
imperfection of the geological record. Numerous existing doubtful
forms could be named which are probably varieties; but who will
pretend that in future ages so many fossil links will be discovered,
that naturalists will be able to decide, on the common view, whether
or not these doubtful forms are varieties? As long as most of the
links between any two species are unknown, if any one link or
intermediate variety be discovered, it will simply be classed as
another and distinct species. Only a small portion of the world has
been geologically explored. Only organic beings of certain classes can
be preserved in a fossil condition, at least in any great number.
Widely ranging species vary most, and varieties are often at first
local,--both causes rendering the discovery of intermediate links less
likely. Local varieties will not spread into other and distant regions
until they are considerably modified and improved; and when they do
spread, if discovered in a geological formation, they will appear as
if suddenly created there, and will be simply classed as new species.
Most formations have been intermittent in their accumulation; and
their duration, I am inclined to believe, has been shorter than the
average duration of specific forms. Successive formations are
separated from each other by enormous blank intervals of time; for
fossiliferous formations, thick enough to resist future degradation,
can be accumulated only where much sediment is deposited on the
subsiding bed of the sea. During the alternate periods of elevation
and of stationary level the record will be blank. During these latter
periods there will probably be more variability in the forms of life;
during periods of subsidence, more extinction.
With respect to the absence of fossiliferous formations beneath the
lowest Silurian strata, I can only recur to the hypothesis given in
the ninth chapter. That the geological record is imperfect all will
admit; but that it is imperfect to the degree which I require, few
will be inclined to admit. If we look to long enough intervals of
time, geology plainly declares that all species have changed; and they
have changed in the manner which my theory requires, for they have
changed slowly and in a graduated manner. We clearly see this in the
fossil remains from consecutive formations invariably being much more
closely related to each other, than are the fossils from formations
distant from each other in time.
Such is the sum of the several chief objections and difficulties which
may justly be urged against my theory; and I have now briefly
recapitulated the answers and explanations which can be given to them.
I have felt these difficulties far too heavily during many years to
doubt their weight. But it deserves especial notice that the more
important objections relate to questions on which we are confessedly
ignorant; nor do we know how ignorant we are. We do not know all the
possible transitional gradations between the simplest and the most
perfect organs; it cannot be pretended that we know all the varied
means of Distribution during the long lapse of years, or that we know
how imperfect the Geological Record is. Grave as these several
difficulties are, in my judgment they do not overthrow the theory of
descent with modification.
Now let us turn to the other side of the argument. Under domestication
we see much variability. This seems to be mainly due to the
reproductive system being eminently susceptible to changes in the
conditions of life; so that this system, when not rendered impotent,
fails to reproduce offspring exactly like the parent-form. Variability
is governed by many complex laws,--by correlation of growth, by use
and disuse, and by the direct action of the physical conditions of
life. There is much difficulty in ascertaining how much modification
our domestic productions have undergone; but we may safely infer that
the amount has been large, and that modifications can be inherited for
long periods. As long as the conditions of life remain the same, we
have reason to believe that a modification, which has already been
inherited for many generations, may continue to be inherited for an
almost infinite number of generations. On the other hand we have
evidence that variability, when it has once come into play, does not
wholly cease; for new varieties are still occasionally produced by our
most anciently domesticated productions.
Man does not actually produce variability; he only unintentionally
exposes organic beings to new conditions of life, and then nature acts
on the organisation, and causes variability. But man can and does
select the variations given to him by nature, and thus accumulate them
in any desired manner. He thus adapts animals and plants for his own
benefit or pleasure. He may do this methodically, or he may do it
unconsciously by preserving the individuals most useful to him at the
time, without any thought of altering the breed. It is certain that he
can largely influence the character of a breed by selecting, in each
successive generation, individual differences so slight as to be quite
inappreciable by an uneducated eye. This process of selection has been
the great agency in the production of the most distinct and useful
domestic breeds. That many of the breeds produced by man have to a
large extent the character of natural species, is shown by the
inextricable doubts whether very many of them are varieties or
aboriginal species.
There is no obvious reason why the principles which have acted so
efficiently under domestication should not have acted under nature. In
the preservation of favoured individuals and races, during the
constantly-recurrent Struggle for Existence, we see the most powerful
and ever-acting means of selection. The struggle for existence
inevitably follows from the high geometrical ratio of increase which
is common to all organic beings. This high rate of increase is proved
by calculation, by the effects of a succession of peculiar seasons,
and by the results of naturalisation, as explained in the third
chapter. More individuals are born than can possibly survive. A grain
in the balance will determine which individual shall live and which
shall die,--which variety or species shall increase in number, and
which shall decrease, or finally become extinct. As the individuals of
the same species come in all respects into the closest competition
with each other, the struggle will generally be most severe between
them; it will be almost equally severe between the varieties of the
same species, and next in severity between the species of the same
genus. But the struggle will often be very severe between beings most
remote in the scale of nature. The slightest advantage in one being,
at any age or during any season, over those with which it comes into
competition, or better adaptation in however slight a degree to the
surrounding physical conditions, will turn the balance.
With animals having separated sexes there will in most cases be a
struggle between the males for possession of the females. The most
vigorous individuals, or those which have most successfully struggled
with their conditions of life, will generally leave most progeny. But
success will often depend on having special weapons or means of
defence, or on the charms of the males; and the slightest advantage
will lead to victory.
As geology plainly proclaims that each land has undergone great
physical changes, we might have expected that organic beings would
have varied under nature, in the same way as they generally have
varied under the changed conditions of domestication. And if there be
any variability under nature, it would be an unaccountable fact if
natural selection had not come into play. It has often been asserted,
but the assertion is quite incapable of proof, that the amount of
variation under nature is a strictly limited quantity. Man, though
acting on external characters alone and often capriciously, can
produce within a short period a great result by adding up mere
individual differences in his domestic productions; and every one
admits that there are at least individual differences in species under
nature. But, besides such differences, all naturalists have admitted
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