- 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 judgement
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 the
existence of varieties, which
they think sufficiently distinct
to be worthy of record in systematic
works. No one can draw any clear
distinction between individual
differences and slight varieties;
or between more plainly marked
varieties and subspecies, and
species. Let it be observed how
naturalists differ in the rank
which they assign to the many
representative forms in Europe
and North America.
If then we
have under nature variability
and a powerful agent
always ready to act and select,
why should we doubt that variations
in any way useful to beings,
under their excessively complex
relations of life, would be preserved,
accumulated, and inherited? Why,
if man can by patience select
variations most useful to himself,
should nature fail in selecting
variations useful, under changing
conditions of life, to her living
products? What limit can be put
to this power, acting during
long ages and rigidly scrutinising
the whole constitution, structure,
and habits of each creature, —favouring
the good and rejecting the bad?
I can see no limit to this power,
in slowly and beautifully adapting
each form to the most complex
relations of life. The theory
of natural selection, even if
we looked no further than this,
seems to me to be in itself probable.
I have already recapitulated,
as fairly as I could, the opposed
difficulties and objections:
now let us turn to the special
facts and arguments in favour
of the theory.
On the view that species are
only strongly marked and permanent
varieties, and that each species
first existed as a variety, we
can see why it is that no line
of demarcation can be drawn between
species, commonly supposed to
have been produced by special
acts of creation, and varieties
which are acknowledged to have
been produced by secondary laws.
On this same view we can understand
how it is that in each region
where many species of a genus
have been produced, and where
they now flourish, these same
species should present many varieties;
for where the manufactory of
species has been active, we might
expect, as a general rule, to
find it still in action; and
this is the case if varieties
be incipient species. Moreover,
the species of the large genera,
which afford the greater number
of varieties or incipient species,
retain to a certain degree the
character of varieties; for they
differ from each other by a less
amount of difference than do
the species of smaller genera.
The closely allied species also
of the larger genera apparently
have restricted ranges, and they
are clustered in little groups
round other species -- in which
respects they resemble varieties.
These are strange relations on
the view of each species having
been independently created, but
are intelligible if all species
first existed as varieties.
As each species tends by its
geometrical ratio of reproduction
to increase inordinately in number;
and as the modified descendants
of each species will be enabled
to increase by so much the more
as they become more diversified
in habits and structure, so as
to be enabled to seize on many
and widely different places in
the economy of nature, there
will be a constant tendency in
natural selection to preserve
the most divergent offspring
of any one species. Hence during
a long-continued course of modification,
the slight differences, characteristic
of varieties of the same species,
tend to be augmented into the
greater differences characteristic
of species of the same genus.
New and improved varieties will
inevitably supplant and exterminate
the older, less improved and
intermediate varieties; and thus
species are rendered to a large
extent defined and distinct objects.
Dominant species belonging to
the larger groups tend to give
birth to new and dominant forms;
so that each large group tends
to become still larger, and at
the same time more divergent
in character. But as all groups
cannot thus succeed in increasing
in size, for the world would
not hold them, the more dominant
groups beat the less dominant.
This tendency in the large groups
to go on increasing in size and
diverging in character, together
with the almost inevitable contingency
of much extinction, explains
the arrangement of all the forms
of life, in groups subordinate
to groups, all within a few great
classes, which we now see everywhere
around us, and which has prevailed
throughout all time. This grand
fact of the grouping of all organic
beings seems to me utterly inexplicable
on the theory of creation.
As natural selection acts solely
by accumulating slight, successive,
favourable variations, it can
produce no great or sudden modification;
it can act only by very short
and slow steps. Hence the canon
of `Natura non facit saltum,'
which every fresh addition to
our knowledge tends to make more
strictly correct, is on this
theory simply intelligible. We
can plainly see why nature is
prodigal in variety, though niggard
in innovation. But why this should
be a law of nature if each species
has been independently created,
no man can explain.
Many other facts are, as it
seems to me, explicable on this
theory. How strange it is that
a bird, under the form of woodpecker,
should have been created to prey
on insects on the ground; that
upland geese, which never or
rarely swim, should have been
created with webbed feet; that
a thrush should have been created
to dive and feed on sub-aquatic
insects; and that a petrel should
have been created with habits
and structure fitting it for
the life of an auk or grebe!
and so on in endless other cases.
But on the view of each species
constantly trying to increase
in number, with natural selection
always ready to adapt the slowly
varying descendants of each to
any unoccupied or ill-occupied
place in nature, these facts
cease to be strange, or perhaps
might even have been anticipated.
As natural selection acts by
competition, it adapts the inhabitants
of each country only in relation
to the degree of perfection of
their associates; so that we
need feel no surprise at the
inhabitants of any one country,
although on the ordinary view
supposed to have been specially
created and adapted for that
country, being beaten and supplanted
by the naturalised productions
from another land. Nor ought
we to marvel if all the contrivances
in nature be not, as far as we
can judge, absolutely perfect;
and if some of them be abhorrent
to our ideas of fitness. We need
not marvel at the sting of the
bee causing the bee's own death;
at drones being produced in such
vast numbers for one single act,
and being then slaughtered by
their sterile sisters; at the
astonishing waste of pollen by
our fir-trees; at the instinctive
hatred of the queen bee for her
own fertile daughters; at ichneumonidae
feeding within the live bodies
of caterpillars; and at other
such cases. The wonder indeed
is, on the theory of natural
selection, that more cases of
the want of absolute perfection
have not been observed.
The complex and little known
laws governing variation are
the same, as far as we can see,
with the laws which have governed
the production of so-called specific
forms. In both cases physical
conditions seem to have produced
but little direct effect; yet
when varieties enter any zone,
they occasionally assume some
of the characters of the species
proper to that zone. In both
varieties and species, use and
disuse seem to have produced
some effect; for it is difficult
to resist this conclusion when
we look, for instance, at the
logger-headed duck, which has
wings incapable of flight, in
nearly the same condition as
in the domestic duck; or when
we look at the burrowing tucutucu,
which is occasionally blind,
and then at certain moles, which
are habitually blind and have
their eyes covered with skin;
or when we look at the blind
animals inhabiting the dark caves
of America and Europe. In both
varieties and species correction
of growth seems to have played
a most important part, so that
when one part has been modified
other parts are necessarily modified.
In both varieties and species
reversions to long-lost characters
occur. How inexplicable on the
theory of creation is the occasional
appearance of stripes on the
shoulder and legs of the several
species of the horse-genus and
in their hybrids! How simply
is this fact explained if we
believe that these species have
descended from a striped progenitor,
in the same manner as the several
domestic breeds of pigeon have
descended from the blue and barred
rock-pigeon!
On the ordinary view of each
species having been independently
created, why should the specific
characters, or those by which
the species of the same genus
differ from each other, be more
variable than the generic characters
in which they all agree? Why,
for instance, should the colour
of a flower be more likely to
vary in any one species of a
genus, if the other species,
supposed to have been created
independently, have differently
coloured flowers, than if all
the species of the genus have
the same coloured flowers? If
species are only well-marked
varieties, of which the characters
have become in a high degree
permanent, we can understand
this fact; for they have already
varied since they branched off
from a common progenitor in certain
characters, by which they have
come to be specifically distinct
from each other; and therefore
these same characters would be
more likely still to be variable
than the generic characters which
have been inherited without change
for an enormous period. It is
inexplicable on the theory of
creation why a part developed
in a very unusual manner in any
one species of a genus, and therefore,
as we may naturally infer, of
great importance to the species,
should be eminently liable to
variation; but, on my view, this
part has undergone, since the
several species branched off
from a common progenitor, an
unusual amount of variability
and modification, and therefore
we might expect this part generally
to be still variable. But a part
may be developed in the most
unusual manner, like the wing
of a bat, and yet not be more
variable than any other structure,
if the part be common to many
subordinate forms, that is, if
it has been inherited for a very
long period; for in this case
it will have been rendered constant
by long-continued natural selection.
Glancing at instincts, marvellous
as some are, they offer no greater
difficulty than does corporeal
structure on the theory of the
natural selection of successive,
slight, but profitable modifications.
We can thus understand why nature
moves by graduated steps in endowing
different animals of the same
class with their several instincts.
I have attempted to show how
much light the principle of gradation
throws on the admirable architectural
powers of the hive-bee. Habit
no doubt sometimes comes into
play in modifying instincts;
but it certainly is not indispensable,
as we see, in the case of neuter
insects, which leave no progeny
to inherit the effects of long-continued
habit. On the view of all the
species of the same genus having
descended from a common parent,
and having inherited much in
common, we can understand how
it is that allied species, when
placed under considerably different
conditions of life, yet should
follow nearly the same instincts;
why the thrush of South America,
for instance, lines her nest
with mud like our British species.
On the view of instincts having
been slowly acquired through
natural selection we need not
marvel at some instincts being
apparently not perfect and liable
to mistakes, and at many instincts
causing other animals to suffer.
If species be only well-marked
and permanent varieties, we can
at once see why their crossed
offspring should follow the same
complex laws in their degrees
and kinds of resemblance to their
parents, -- in being absorbed
into each other by successive
crosses, and in other such points,
-- as do the crossed offspring
of acknowledged varieties. On
the other hand, these would be
strange facts if species have
been independently created, and
varieties have been produced
by secondary laws.
If we admit
that the geological record
is imperfect in an extreme
degree, then such facts as the
record gives, support the theory
of descent with modification.
New species have come on the
stage slowly and at successive
intervals; and the amount of
change, after equal intervals
of time, is widely different
in different groups. The extinction
of species and of whole groups
of species, which has played
so conspicuous a part in the
history of the organic world,
almost inevitably follows on
the principle of natural selection;
for old forms will be supplanted
by new and improved forms. Neither
single species nor groups of
species reappear when the chain
of ordinary generation has once
been broken. The gradual diffusion
of dominant forms, with the slow
modification of their descendants,
causes the forms of life, after
long intervals of time, to appear
as if they had changed simultaneously
throughout the world. The fact
of the fossil remains of each
formation being in some degree
intermediate in character between
the fossils in the formations
above and below, is simply explained
by their intermediate position
in the chain of descent. The
grand fact that all extinct organic
beings belong to the same system
with recent beings, falling either
into the same or into intermediate
groups, follows from the living
and the extinct being the offspring
of common parents. As the groups
which have descended from an
ancient progenitor have generally
diverged in character, the progenitor
with its early descendants will
often be intermediate in character
in comparison with its later
descendants; and thus we can
see why the more ancient a fossil
is, the oftener it stands in
some degree intermediate between
existing and allied groups. Recent
forms are generally looked at
as being, in some vague sense,
higher than ancient and extinct
forms; and they are in so far
higher as the later and more
improved forms have conquered
the older and less improved organic
beings in the struggle for life.
Lastly, the law of the n='448'> long
endurance of allied forms on
the same continent, —of
marsupials in Australia, of edentata
in America, and other such cases,
-- is intelligible, for within
a confined country, the recent
and the extinct will naturally
be allied by descent.
Looking to geographical distribution,
if we admit that there has been
during the long course of ages
much migration from one part
of the world to another, owing
to former climatal and geographical
changes and to the many occasional
and unknown means of dispersal,
then we can understand, on the
theory of descent with modification,
most of the great leading facts
in Distribution. We can see why
there should be so striking a
parallelism in the distribution
of organic beings throughout
space, and in their geological
succession throughout time; for
in both cases the beings have
been connected by the bond of
ordinary generation, and the
means of modification have been
the same. We see the full meaning
of the wonderful fact, which
must have struck every traveller,
namely, that on the same continent,
under the most diverse conditions,
under heat and cold, on mountain
and lowland, on deserts and marshes,
most of the inhabitants within
each great class are plainly
related; for they will generally
be descendants of the same progenitors
and early colonists. On this
same principle of former migration,
combined in most cases with modification,
we can understand, by the aid
of the Glacial period, the identity
of some few plants, and the close
alliance of many others, on the
most distant mountains, under
the most different climates;
and likewise the close alliance
of some of the inhabitants of
the sea in the northern and southern
temperate zones, though separated
by the whole intertropical ocean.
Although two areas may present
the same physical conditions
of life, we need feel no surprise
at their inhabitants being widely
different, if they have been
for a long period completely
separated from each other; for
as the relation of organism to
organism is the most important
of all relations, and as the
two areas will have received
colonists from some third source
or from each other, at various
periods and in different proportions,
the course of modification in
the two areas will inevitably
be different.
On this view of migration,
with subsequent modification,
we can see why oceanic islands
should be inhabited by few species,
but of these, that many should
be peculiar. We can clearly see
why those animals which cannot
cross wide spaces of ocean, as
frogs and terrestrial mammals,
should not inhabit oceanic islands;
and why, on the other hand, new
and peculiar species of bats,
which can traverse the ocean,
should so often be found on islands
far distant from any continent.
Such facts as the presence of
peculiar species of bats, and
the absence of all other mammals,
on oceanic islands, are utterly
inexplicable on the theory of
independent acts of creation.
The existence of closely allied
or representative species in
any two areas, implies, on the
theory of descent with modification,
that the same parents formerly
inhabited both areas; and we
almost invariably find that wherever
many closely allied species inhabit
two areas, some identical species
common to both still exist. Wherever
many closely allied yet distinct
species occur, many doubtful
forms and varieties of the same
species likewise occur. It is
a rule of high generality that
the inhabitants of each area
are related to the inhabitants
of the nearest source whence
immigrants might have been derived.
We see this in nearly all the
plants and animals of the Galapagos
archipelago, of Juan Fernandez,
and of the other American islands
being related in the most striking
manner to the plants and animals
of the neighbouring American
mainland; and those of the Cape
de Verde archipelago and other
African islands to the African
mainland. It must be admitted
that these facts receive no explanation
on the theory of creation.
The fact, as we have seen,
that all past and present organic
beings constitute one grand natural
system, with group subordinate
to group, and with extinct groups
often falling in between recent
groups, is intelligible on the
theory of natural selection with
its contingencies of extinction
and divergence of character.
On these same principles we see
how it is, that the mutual affinities
of the species and genera within
each class are so complex and
circuitous. We see why certain
characters are far more serviceable
than others for classification;
-- why adaptive characters, though
of paramount importance to the
being, are of hardly any importance
in classification; why characters
derived from rudimentary parts,
though of no service to the being,
are often of high classificatory
value; and why embryological
characters are the most valuable
of all. The real affinities of
all organic beings are due to
inheritance or community of descent.
The natural system is a genealogical
arrangement, in which we have
to discover the lines of descent
by the most permanent characters,
however slight their vital importance
may be.
The framework of bones being
the same in the hand of a man,
wing of a bat, fin of the porpoise,
and leg of the horse, -- the
same number of vertebrae forming
the neck of the giraffe and of
the elephant, -- and innumerable
other such facts, at once explain
themselves on the theory of descent
with slow and slight successive
modifications. The similarity
of pattern in the wing and leg
of a bat, though used for such
different purposes, -- in the
jaws and legs of a crab, -- in
the petals, stamens, and pistils
of a flower, is likewise intelligible
on the view of the gradual modification
of parts or organs, which were
alike in the early progenitor
of each class. On the principle
of successive variations not
always supervening at an early
age, and being inherited at a
corresponding not early period
of life, we can clearly see why
the embryos of mammals, birds,
reptiles, and fishes should be
so closely alike, and should
be so unlike the adult forms.
We may cease marvelling at the
embryo of an air-breathing mammal
or bird having branchial slits
and arteries running in loops,
like those in a fish which has
to breathe the air dissolved
in water, by the aid of well-developed
branchiae.
Disuse, aided sometimes by
natural selection, will often
tend to reduce an organ, when
it has become useless by changed
habits or under changed conditions
of life; and we can clearly understand
on this view the meaning of rudimentary
organs. But disuse and selection
will generally act on each creature,
when it has come to maturity
and has to play its full part
in the struggle for existence,
and will thus have little power
of acting on an organ during
early life; hence the organ will
not be much reduced or rendered
rudimentary at this early age.
The calf, for instance, has inherited
teeth, which never cut through
the gums of the upper jaw, from
an early progenitor having well-developed
teeth; and we may believe, that
the teeth in the mature animal
were reduced, during successive
generations, by disuse or by
the tongue and palate having
been fitted by natural selection
to browse without their aid;
whereas in the calf, the teeth
have been left untouched by selection
or disuse, and on the principle
of inheritance at corresponding
ages have been inherited from
a remote period to the present
day. On the view of each organic
being and each separate organ
having been specially created,
how utterly inexplicable it is
that parts, like the teeth in
the embryonic calf or like the
shrivelled wings under the soldered
wing-covers of some beetles,
should thus so frequently bear
the plain stamp of inutility!
Nature may be said to have taken
pains to reveal, by rudimentary
organs and by homologous structures,
her scheme of modification, which
it seems that we wilfully will
not understand.
I have now recapitulated the
chief facts and considerations
which have thoroughly convinced
me that species have changed,
and are still slowly changing
by the preservation and accumulation
of successive slight favourable
variations. Why, it may be asked,
have all the most eminent living
naturalists and geologists rejected
this view of the mutability of
species? It cannot be asserted
that organic beings in a state
of nature are subject to no variation;
it cannot be proved that the
amount of variation in the course
of long ages is a limited quantity;
no clear distinction has been,
or can be, drawn between species
and well-marked varieties. It
cannot be maintained that species
when intercrossed are invariably
sterile, and varieties invariably
fertile; or that sterility is
a special endowment and sign
of creation. The belief that
species were immutable productions
was almost unavoidable as long
as the history of the world was
thought to be of short duration;
and now that we have acquired
some idea of the lapse of time,
we are too apt to assume, without
proof, that the geological record
is so perfect that it would have
afforded us plain evidence of
the mutation of species, if they
had undergone mutation.
But the chief cause of our
natural unwillingness to admit
that one species has given birth
to other and distinct species,
is that we are always slow in
admitting any great change of
which we do not see the intermediate
steps. The difficulty is the
same as that felt by so many
geologists, when Lyell first
insisted that long lines of inland
cliffs had been formed, and great
valleys excavated, by the slow
action of the coast-waves. The
mind cannot possibly grasp the
full meaning of the term of a
hundred million years; it cannot
add up and perceive the full
effects of many slight variations,
accumulated during an almost
infinite number of generations.
Although I
am fully convinced of the truth
of the views given
in this volume under the form
of an abstract, I by no means
expect to convince experienced
naturalists whose minds are stocked
with a multitude of facts all
viewed, during a long course
of years, from a point of view
directly opposite to mine. It
is so easy to hide our ignorance
under such expressions as the
`plan of creation,' `unity of
design,' &c., and to think
that we give an explanation when
we only restate a fact. Any one
whose disposition leads him to
attach more weight to unexplained
difficulties than to the explanation
of a certain number of facts
will certainly reject my theory.
A few naturalists, endowed with
much flexibility of mind, and
who have already begun to doubt
on the immutability of species,
may be influenced by this volume;
but I look with confidence to
the future, to young and rising
naturalists, who will be able
to view both sides of the question
with impartiality. Whoever is
led to believe that species are
mutable will do good service
by conscientiously expressing
his conviction; for only thus
can the load of prejudice by
which this subject is overwhelmed
be removed.
Several eminent naturalists
have of late published their
belief that a multitude of reputed
species in each genus are not
real species; but that other
species are real, that is, have
been independently created. This
seems to me a strange conclusion
to arrive at. They admit that
a multitude of forms, which till
lately they themselves thought
were special creations, and which
are still thus looked at by the
majority of naturalists, and
which consequently have every
external characteristic feature
of true species, -- they admit
that these have been produced
by variation, but they refuse
to extend the same view to other
and very slightly different forms.
Nevertheless they do not pretend
that they can define, or even
conjecture, which are the created
forms of life, and which are
those produced by secondary laws.
They admit variation as a vera
causa in one case, they arbitrarily
reject it in another, without
assigning any distinction in
the two cases. The day will come
when this will be given as a
curious illustration of the blindness
of preconceived opinion. These
authors seem no more startled
at a miraculous act of creation
than at an ordinary birth. But
do they really believe that at
innumerable periods in the earth's
history certain elemental atoms
have been commanded suddenly
to flash into living tissues?
Do they believe that at each
supposed act of creation one
individual or many were produced?
Were all the infinitely numerous
kinds of animals and plants created
as eggs or seed, or as full grown?
and in the case of mammals, were
they created bearing the false
marks of nourishment from the
mother's womb? Although naturalists
very properly demand a full explanation
of every difficulty from those
who believe in the mutability
of species, on their own side
they ignore the whole subject
of the first appearance of species
in what they consider reverent
silence.
It may be asked how far I extend
the doctrine of the modification
of species. The question is difficult
to answer, because the more distinct
the forms are which we may consider,
by so much the arguments fall
away in force. But some arguments
of the greatest weight extend
very far. All the members of
whole classes can be connected
together by chains of affinities,
and all can be classified on
the same principle, in groups
subordinate to groups. Fossil
remains sometimes tend to fill
up very wide intervals between
existing orders. Organs in a
rudimentary condition plainly
show that an early progenitor
had the organ in a fully developed
state; and this in some instances
necessarily implies an enormous
amount of modification in the
descendants. Throughout whole
classes various structures are
formed on the same pattern, and
at an embryonic age the species
closely resemble each other.
Therefore I cannot doubt that
the theory of descent with modification
embraces all the members of the
same class. I believe that animals
have descended from at most only
four or five progenitors, and
plants from an equal or lesser
number.
Analogy would lead me one step
further, namely, to the belief
that all animals and plants have
descended from some one prototype.
But analogy may be a deceitful
guide. Nevertheless all living
things have much in common, in
their chemical composition, their
germinal vesicles, their cellular
structure, and their laws of
growth and reproduction. We see
this even in so trifling a circumstance
as that the same poison often
similarly affects plants and
animals; or that the poison secreted
by the gall-fly produces monstrous
growths on the wild rose or oak-tree.
Therefore I should infer from
analogy that probably all the
organic beings which have ever
lived on this earth have descended
from some one primordial form,
into which life was first breathed.
When the views entertained
in this volume on the origin
of species, or when analogous
views are generally admitted,
we can dimly foresee that there
will be a considerable revolution
in natural history. Systematists
will be able to pursue their
labours as at present; but they
will not be incessantly haunted
by the shadowy doubt whether
this or that form be in essence
a species. This I feel sure,
and I speak after experience,
will be no slight relief. The
endless disputes whether or not
some fifty species of British
brambles are true species will
cease. Systematists will have
only to decide (not that this
will be easy) whether any form
be sufficiently constant and
distinct from other forms, to
be capable of definition; and
if definable, whether the differences
be sufficiently important to
deserve a specific name. This
latter point will become a far
more essential consideration
than it is at present; for differences,
however slight, between any two
forms, if not blended by intermediate
gradations, are looked at by
most naturalists as sufficient
to raise both forms to the rank
of species. Hereafter we shall
be compelled to acknowledge that
the only distinction between
species and well-marked varieties
is, that the latter are known,
or believed, to be connected
at the present day by intermediate
gradations, whereas species were
formerly thus connected. Hence,
without quite rejecting the consideration
of the present existence of intermediate
gradations between any two forms,
we shall be led to weigh more
carefully and to value higher
the actual amount of difference
between them. It is quite possible
that forms now generally acknowledged
to be merely varieties may hereafter
be thought worthy of specific
names, as with the primrose and
cowslip; and in this case scientific
and common language will come
into accordance. In short, we
shall have to treat species in
the same manner as those naturalists
treat genera, who admit that
genera are merely artificial
combinations made for convenience.
This may not be a cheering prospect;
but we shall at least be freed
from the vain search for the
undiscovered and undiscoverable
essence of the term species.
The other and
more general departments of
natural history
will rise greatly in interest.
The terms used by naturalists
of affinity, relationship, community
of type, paternity, morphology,
adaptive characters, rudimentary
and aborted organs, &c.,
will cease to be metaphorical,
and will have a plain signification.
When we no longer look at an
organic being as a savage looks
at a ship, as at something wholly
beyond his comprehension; when
we regard every production of
nature as one which has had a
history; when we contemplate
every complex structure and instinct
as the summing up of many contrivances,
each useful to the possessor,
nearly in the same way as when
we look at any great mechanical
invention as the summing up of
the labour, the experience, the
reason, and even the blunders
of numerous workmen; when we
thus view each organic being,
how far more interesting, I speak
from experience, will the study
of natural history become!
A grand and almost untrodden
field of inquiry will be opened,
on the causes and laws of variation,
on correlation of growth, on
the effects of use and disuse,
on the direct action of external
conditions, and so forth. The
study of domestic productions
will rise immensely in value.
A new variety raised by man will
be a far more important and interesting
subject for study than one more
species added to the infinitude
of already recorded species.
Our classifications will come
to be, as far as they can be
so made, genealogies; and will
then truly give what may be called
the plan of creation. The rules
for classifying will no doubt
become simpler when we have a
definite object in view. We possess
no pedigrees or armorial bearings;
and we have to discover and trace
the many diverging lines of descent
in our natural genealogies, by
characters of any kind which
have long been inherited. Rudimentary
organs will speak infallibly
with respect to the nature of
long-lost structures. Species
and groups of species, which
are called aberrant, and which
may fancifully be called living
fossils, will aid us in forming
a picture of the ancient forms
of life. Embryology will reveal
to us the structure, in some
degree obscured, of the prototypes
of each great class.
When we can feel assured that
all the individuals of the same
species, and all the closely
allied species of most genera,
have within a not very remote
period descended from one parent,
and have migrated from some one
birthplace; and when we better
know the many means of migration,
then, by the light which geology
now throws, and will continue
to throw, on former changes of
climate and of the level of the
land, we shall surely be enabled
to trace in an admirable manner
the former migrations of the
inhabitants of the whole world.
Even at present, by comparing
the differences of the inhabitants
of the sea on the opposite sides
of a continent, and the nature
of the various inhabitants of
that continent in relation to
their apparent means of immigration,
some light can be thrown on ancient
geography.
The noble science of Geology
loses glory from the extreme
imperfection of the record. The
crust of the earth with its embedded
remains must not be looked at
as a well-filled museum, but
as a poor collection made at
hazard and at rare intervals.
The accumulation of each great
fossiliferous formation will
be recognised as having depended
on an unusual concurrence of
circumstances, and the blank
intervals between the successive
stages as having been of vast
duration. But we shall be able
to gauge with some security the
duration of these intervals by
a comparison of the preceding
and succeeding organic forms.
We must be cautious in attempting
to correlate as strictly contemporaneous
two formations, which include
few identical species, by the
general succession of their forms
of life. As species are produced
and exterminated by slowly acting
and still existing causes, and
not by miraculous acts of creation
and by catastrophes; and as the
most important of all causes
of organic change is one which
is almost independent of altered
and perhaps suddenly altered
physical conditions, namely,
the mutual relation of organism
to organism, -- the improvement
of one being entailing the improvement
or the extermination of others;
it follows, that the amount of
organic change in the fossils
of consecutive formations probably
serves as a fair measure of the
lapse of actual time. A number
of species, however, keeping
in a body might remain for a
long period unchanged, whilst
within this same period, several
of these species, by migrating
into new countries and coming
into competition with foreign
associates, might become modified;
so that we must not overrate
the accuracy of organic change
as a measure of time. During
early periods of the earth's
history, when the forms of life
were probably fewer and simpler,
the rate of change was probably
slower; and at the first dawn
of life, when very few forms
of the simplest structure existed,
the rate of change may have been
slow in an extreme degree. The
whole history of the world, as
at present known, although of
a length quite incomprehensible
by us, will hereafter be recognised
as a mere fragment of time, compared
with the ages which have elapsed
since the first creature, the
progenitor of innumerable extinct
and living descendants, was created.
In the distant future I see
open fields for far more important
researches. Psychology will be
based on a new foundation, that
of the necessary acquirement
of each mental power and capacity
by gradation. Light will be thrown
on the origin of man and his
history.
Authors of the highest eminence
seem to be fully satisfied with
the view that each species has
been independently created. To
my mind it accords better with
what we know of the laws impressed
on matter by the Creator, that
the production and extinction
of the past and present inhabitants
of the world should have been
due to secondary causes, like
those determining the birth and
death of the individual. When
I view all beings not as special
creations, but as the lineal
descendants of some few beings
which lived long before the first
bed of the Silurian system was
deposited, they seem to me to
become ennobled. Judging from
the past, we may safely infer
that not one living species will
transmit its unaltered likeness
to a distant futurity. And of
the species now living very few
will transmit progeny of any
kind to a far distant futurity;
for the manner in which all organic
beings are grouped, shows that
the greater number of species
of each genus, and all the species
of many genera, have left no
descendants, but have become
utterly extinct. We can so far
take a prophetic glance into
futurity as to foretel that it
will be the common and widely-spread
species, belonging to the larger
and dominant groups, which will
ultimately prevail and procreate
new and dominant species. As
all the living forms of life
are the lineal descendants of
those which lived long before
the Silurian epoch, we may feel
certain that the ordinary succession
by generation has never once
been broken, and that no cataclysm
has desolated the whole world.
Hence we may look with some confidence
to a secure future of equally
inappreciable length. And as
natural selection works solely
by and for the good of each being,
all corporeal and mental endowments
will tend to progress towards
perfection.
It is interesting to contemplate
an entangled bank, clothed with
many plants of many kinds, with
birds singing on the bushes,
with various insects flitting
about, and with worms crawling
through the damp earth, and to
reflect that these elaborately
constructed forms, so different
from each other, and dependent
on each other in so complex a
manner, have all been produced
by laws acting around us. These
laws, taken in the largest sense,
being Growth with Reproduction;
inheritance which is almost implied
by reproduction; Variability
from the indirect and direct
action of the external conditions
of life, and from use and disuse;
a Ratio of Increase so high as
to lead to a Struggle for Life,
and as a consequence to Natural
Selection, entailing Divergence
of Character and the Extinction
of less-improved forms. Thus,
from the war of nature, from
famine and death, the most exalted
object which we are capable of
conceiving, namely, the production
of the higher animals, directly
follows. There is grandeur in
this view of life, with its several
powers, having been originally
breathed into a few forms or
into one; and that, whilst this
planet has gone cycling on according
to the fixed law of gravity,
from so simple a beginning endless
forms most beautiful and most
wonderful have been, and are
being, evolved. |