- Difficulties
on the theory of descent with
modification
- Transitions
- Absence
or rarity of transitional
varieties
- Transitions
in habits of life
- Diversified
habits in the same species
- Species with
habits widely different
from those of
their allies
- Organs
of extreme perfection
- Means
of transition
- Cases
of difficulty
- Natura
non facit saltum
- Organs
of small importance
- Organs
not in all cases absolutely perfect
- The
law of Unity of Type and of the Conditions of Existence
embraced by the theory of Natural Selection
LONG before having arrived
at this part of my work, a crowd
of difficulties will have occurred
to the reader. Some of them are
so grave that to this day I can
never reflect on them without
being staggered; but, to the
best of my judgment, the greater
number are only apparent, and
those that are real are not,
I think, fatal to my theory.
These difficulties and objections
may be classed under the following
heads:-Firstly, why, if species
have descended from other species
by insensibly fine gradations,
do we not everywhere see innumerable
transitional forms? Why is not
all nature in confusion instead
of the species being, as we see
them, well defined?
Secondly, is it possible that
an animal having, for instance,
the structure and habits of a
bat, could have been formed by
the modification of some animal
with wholly different habits?
Can we believe that natural selection
could produce, on the one hand,
organs of trifling importance,
such as the tail of a giraffe,
which serves as a fly-flapper,
and, on the other hand, organs
of such wonderful structure,
as the eye, of which we hardly
as yet fully understand the inimitable
perfection?
Thirdly, can instincts be acquired
and modified through natural
selection? What shall we say
to so marvellous an instinct
as that which leads the bee to
make cells, which have practically
anticipated the discoveries of
profound mathematicians?
Fourthly, how can we account
for species, when crossed, being
sterile and producing sterile
offspring, whereas, when varieties
are crossed, their fertility
is unimpaired?
The two first heads shall be
here discussed Instinct and Hybridism
in separate chapters.
On the absence or rarity
of transitional varieties. As
natural selection acts solely
by the preservation of profitable
modifications, each new form
will tend in a fully-stocked
country to take the place of,
and finally to exterminate,
its own less improved parent
or other less-favoured forms
with which it comes into competition.
Thus extinction and natural
selection will, as we have
seen, go hand in hand. Hence,
if we look at each species
as descended from some other
unknown form, both the parent
and all the transitional varieties
will generally have been exterminated
by the very process of formation
and perfection of the new form.
But, as by this theory innumerable
transitional forms must have
existed, why do we not find them
embedded in countless numbers
in the crust of the earth? It
will be much more convenient
to discuss this question in the
chapter on the Imperfection of
the geological record; and I
will here only state that I believe
the answer mainly lies in the
record being incomparably less
perfect than is generally supposed;
the imperfection of the record
being chiefly due to organic
beings not inhabiting profound
depths of the sea, and to their
remains being embedded and preserved
to a future age only in masses
of sediment sufficiently thick
and extensive to withstand an
enormous amount of future degradation;
and such fossiliferous masses
can be accumulated only where
much sediment is deposited on
the shallow bed of the sea, whilst
it slowly subsides. These contingencies
will concur only rarely, and
after enormously long intervals.
Whilst the bed of the sea is
stationary or is rising, or when
very little sediment is being
deposited, there will be blanks
in our geological history. The
crust of the earth is a vast
museum; but the natural collections
have been made only at intervals
of time immensely remote.
But it may be urged that when
several closely-allied species
inhabit the same territory we
surely ought to find at the present
time many transitional forms.
Let us take a simple case: in
travelling from north to south
over a continent, we generally
meet at successive intervals
with closely allied or representative
species, evidently filling nearly
the same place in the natural
economy of the land. These representative
species often meet and interlock;
and as the one becomes rarer
and rarer, the other becomes
more and more frequent, till
the one replaces the other. But
if we compare these species where
they intermingle, they are generally
as absolutely distinct from each
other in every detail of structure
as are specimens taken from the
metropolis inhabited by each.
By my theory these allied species
have descended from a common
parent; and during the process
of modification, each has become
adapted to the conditions of
life of its own region, and has
supplanted and exterminated its
original parent and all the transitional
varieties between its past and
present states. Hence we ought
not to expect at the present
time to meet with numerous transitional
varieties in each region, though
they must have existed there,
and may be embedded there in
a fossil condition. But in the
intermediate region, having intermediate
conditions of life, why do we
not now find closely-linking
intermediate varieties? This
difficulty for a long time quite
confounded me. But I think it
can be in large part explained.
In the first place we should
be extremely cautious in inferring,
because an area is now continuous,
that it has been continuous during
a long period. Geology would
lead us to believe that almost
every continent has been broken
up into islands even during the
later tertiary periods; and in
such islands distinct species
might have been separately formed
without the possibility of intermediate
varieties existing in the intermediate
zones. By changes in the form
of the land and of climate, marine
areas now continuous must often
have existed within recent times
in a far less continuous and
uniform condition than at present.
But I will pass over this way
of escaping from the difficulty;
for I believe that many perfectly
defined species have been formed
on strictly continuous areas;
though I do not doubt that the
formerly broken condition of
areas now continuous has played
an important part in the formation
of new species, more especially
with freely-crossing and wandering
animals.
In looking at species as they
are now distributed over a wide
area, we generally find them
tolerably numerous over a large
territory, then becoming somewhat
abruptly rarer and rarer on the
confines, and finally disappearing.
Hence the neutral territory between
two representative species is
generally narrow in comparison
with the territory proper to
each. We see the same fact in
ascending mountains, and sometimes
it is quite remarkable how abruptly,
as Alph. De Candolle has observed,
a common alpine species disappears.
The same fact has been noticed
by Forbes in sounding the depths
of the sea with the dredge. To
those who look at climate and
the physical conditions of life
as the all-important elements
of distribution, these facts
ought to cause surprise, as climate
and height or depth graduate
away insensibly. But when we
bear in mind that almost every
species, even in its metropolis,
would increase immensely in numbers,
were it not for other competing
species; that nearly all either
prey on or serve as prey for
others; in short, that each organic
being is either directly or indirectly
related in the most important
manner to other organic beings,
we must see that the range of
the inhabitants of any country
by no means exclusively depends
on insensibly changing physical
conditions, but in large part
on the presence of other species,
on which it depends, or by which
it is destroyed, or with which
it comes into competition; and
as these species are already
defined objects (however they
may have become so), not blending
one into another by insensible
gradations, the range of any
one species, depending as it
does on the range of others,
will tend to be sharply defined.
Moreover, each species on the
confines of its range, where
it exists in lessened numbers,
will, during fluctuations in
the number of its enemies or
of its prey, or in the seasons,
be extremely liable to utter
extermination; and thus its geographical
range will come to be still more
sharply defined.
If I am right in believing
that allied or representative
species, when inhabiting a continuous
area, are generally so distributed
that each has a wide range, with
a comparatively narrow neutral
territory between them, in which
they become rather suddenly rarer
and rarer; then, as varieties
do not essentially differ from
species, the same rule will probably
apply to both; and if we in imagination
adapt a varying species to a
very large area, we shall have
to adapt two varieties to two
large areas, and a third variety
to a narrow intermediate zone.
The intermediate variety, consequently,
will exist in lesser numbers
from inhabiting a narrow and
lesser area; and practically,
as far as I can make out, this
rule holds good with varieties
in a state of nature. I have
met with striking instances of
the rule in the case of varieties
intermediate between well-marked
varieties in the genus Balanus.
And it would appear from information
given me by Mr Watson, Dr Asa
Gray, and Mr Wollaston, that
generally when varieties intermediate
between two other forms occur,
they are much rarer numerically
than the forms which they connect.
Now, if we may trust these facts
and inferences, and therefore
conclude that varieties linking
two other varieties together
have generally existed in lesser
numbers than the forms which
they connect, then, I think,
we can understand why intermediate
varieties should not endure for
very long periods; why as a general
rule they should be exterminated
and disappear, sooner than the
forms which they originally linked
together.
For any form existing in lesser
numbers would, as already remarked,
run a greater chance of being
exterminated than one existing
in large numbers; and in this
particular case the intermediate
form would be eminently liable
to the inroads of closely allied
forms existing on both sides
of it. But a far more important
consideration, as I believe,
is that, during the process of
further modification, by which
two varieties are supposed on
my theory to be converted and
perfected into two distinct species,
the two which exist in larger
numbers from inhabiting larger
areas, will have a great advantage
over the intermediate variety,
which exists in smaller numbers
in a narrow and intermediate
zone. For forms existing in larger
numbers will always have a better
chance, within any given period,
of presenting further favourable
variations for natural selection
to seize on, than will the rarer
forms which exist in lesser numbers.
Hence, the more common forms,
in the race for life, will tend
to beat and supplant the less
common forms, for these will
be more slowly modified and improved.
It is the same principle which,
as I believe, accounts for the
common species in each country,
as shown in the second chapter,
presenting on an average a greater
number of well-marked varieties
than do the rarer species. I
may illustrate what I mean by
supposing three varieties of
sheep to be kept, one adapted
to an extensive mountainous region;
a second to a comparatively narrow,
hilly tract; and a third to wide
plains at the base; and that
the inhabitants are all trying
with equal steadiness and skill
to improve their stocks by selection;
the chances in this case will
be strongly in favour of the
great holders on the mountains
or on the plains improving their
breeds more quickly than the
small holders on the intermediate
narrow, hilly tract; and consequently
the improved mountain or plain
breed will soon take the place
of the less improved hill breed;
and thus the two breeds, which
originally existed in greater
numbers, will come into close
contact with each other, without
the interposition of the supplanted,
intermediate hill-variety.
To sum up, I believe that species
come to be tolerably well-defined
objects, and do not at any one
period present an inextricable
chaos of varying and intermediate
links: firstly, because new varieties
are very slowly formed, for variation
is a very slow process, and natural
selection can do nothing until
favourable variations chance
to occur, and until a place in
the natural polity of the country
can be better filled by some
modification of some one or more
of its inhabitants. And such
new places will depend on slow
changes of climate, or on the
occasional immigration of new
inhabitants, and, probably, in
a still more important degree,
on some of the old inhabitants
becoming slowly modified, with
the new forms thus produced and
the old ones acting and reacting
on each other. So that, in any
one region and at any one time,
we ought only to see a few species
presenting slight modifications
of structure in some degree permanent;
and this assuredly we do see.
Secondly, areas now continuous
must often have existed within
the recent period in isolated
portions, in which many forms,
more especially amongst the classes
which unite for each birth and
wander much, may have separately
been rendered sufficiently distinct
to rank as representative species.
In this case, intermediate varieties
between the several representative
species and their common parent,
must formerly have existed in
each broken portion of the land,
but these links will have been
supplanted and exterminated during
the process of natural selection,
so that they will no longer exist
in a living state.
Thirdly, when two or more varieties
have been formed in different
portions of a strictly continuous
area, intermediate varieties
will, it is probable, at first
have been formed in the intermediate
zones, but they will generally
have had a short duration. For
these intermediate varieties
will, from reasons already assigned
(namely from what we know of
the actual distribution of closely
allied or representative species,
and likewise of acknowledged
varieties), exist in the intermediate
zones in lesser numbers than
the varieties which they tend
to connect. From this cause alone
the intermediate varieties will
be liable to accidental extermination;
and during the process of further
modification through natural
selection, they will almost certainly
be beaten and supplanted by the
forms which they connect; for
these from existing in greater
numbers will, in the aggregate,
present more variation, and thus
be further improved through natural
selection and gain further advantages.
Lastly, looking not to any
one time, but to all time, if
my theory be true, numberless
intermediate varieties, linking
most closely all the species
of the same group together, must
assuredly have existed; but the
very process of natural selection
constantly tends, as has been
so often remarked, to exterminate
the parent forms and the intermediate
links. Consequently evidence
of their former existence could
be found only amongst fossil
remains, which are preserved,
as we shall in a future chapter
attempt to show, in an extremely
imperfect and intermittent record.
On the origin and transitions
of organic beings with peculiar
habits and structure. It
has been asked by the opponents
of such views as I hold, how,
for instance, a land carnivorous
animal could have been converted
into one with aquatic habits;
for how could the animal in
its transitional state have
subsisted? It would be easy
to show that within the same
group carnivorous animals exist
having every intermediate grade
between truly aquatic and strictly
terrestrial habits; and as
each exists by a struggle for
life, it is clear that each
is well adapted in its habits
to its place in nature. Look
at the Mustela vison of North
America, which has webbed feet
and which resembles an otter
in its fur, short legs, and
form of tail; during summer
this animal dives for and preys
on fish, but during the long
winter it leaves the frozen
waters, and preys like other
polecats on mice and land animals.
If a different case had been
taken, and it had been asked
how an insectivorous quadruped
could possibly have been converted
into a flying bat, the question
would have been far more difficult,
and I could have given no answer.
Yet I think such difficulties
have very little weight.
Here, as on other occasions,
I lie under a heavy disadvantage,
for out of the many striking
cases which I have collected,
I can give only one or two instances
of transitional habits and structures
in closely allied species of
the same genus; and of diversified
habits, either constant or occasional,
in the same species. And it seems
to me that nothing less than
a long list of such cases is
sufficient to lessen the difficulty
in any particular case like that
of the bat.
Look at the family of squirrels;
here we have the finest gradation
from animals with their tails
only slightly flattened, and
from others, as Sir J. Richardson
has remarked, with the posterior
part of their bodies rather wide
and with the skin on their flanks
rather full, to the so-called
flying squirrels; and flying
squirrels have their limbs and
even the base of the tail united
by a broad expanse of skin, which
serves as a parachute and allows
them to glide through the air
to an astonishing distance from
tree to tree. We cannot doubt
that each structure is of use
to each kind of squirrel in its
own country, by enabling it to
escape birds or beasts of prey,
or to collect food more quickly,
or, as there is reason to believe,
by lessening the danger from
occasional falls. But it does
not follow from this fact that
the structure of each squirrel
is the best that it is possible
to conceive under all natural
conditions. Let the climate and
vegetation change, let other
competing rodents or new beasts
of prey immigrate, or old ones
become modified, and all analogy
would lead us to believe that
some at least of the squirrels
would decrease in numbers or
become exterminated, unless they
also became modified and improved
in structure in a corresponding
manner. Therefore, I can see
no difficulty, more especially
under changing conditions of
life, in the continued preservation
of individuals with fuller and
fuller flank-membranes, each
modification being useful, each
being propagated, until by the
accumulated effects of this process
of natural selection, a perfect
so-called flying squirrel was
produced.
Now look at the Galeopithecus
or flying lemur, which formerly
was falsely ranked amongst bats.
It has an extremely wide flank-membrane,
stretching from the corners of
the jaw to the tail, and including
the limbs and the elongated fingers:
the flank membrane is, also,
furnished with an extensor muscle.
Although no graduated links of
structure, fitted for gliding
through the air, now connect
the Galeopithecus with the other
Lemuridae, yet I can see no difficulty
in supposing that such links
formerly existed, and that each
had been formed by the same steps
as in the case of the less perfectly
gliding squirrels; and that each
grade of structure had been useful
to its possessor. Nor can I see
any insuperable difficulty in
further believing it possible
that the membrane-connected fingers
and fore-arm of the Galeopithecus
might be greatly lengthened by
natural selection; and this,
as far as the organs of flight
are concerned, would convert
it into a bat. In bats which
have the wing-membrane extended
from the top of the shoulder
to the tail, including the hind-legs,
we perhaps see traces of an apparatus
originally constructed for gliding
through the air rather than for
flight.
If about a dozen genera of
birds had become extinct or were
unknown, who would have ventured
to have surmised that birds might
have existed which used their
wings solely as flappers, like
the logger-headed duck (Micropterus
of Eyton); as fins in the water
and front legs on the land, like
the penguin; as sails, like the
ostrich; and functionally for
no purpose, like the Apteryx.
Yet the structure of each of
these birds is good for it, under
the conditions of life to which
it is exposed, for each has to
live by a struggle; but it is
not necessarily the best possible
under all possible conditions.
It must not be inferred from
these remarks that any of the
grades of wing-structure here
alluded to, which perhaps may
all have resulted from disuse,
indicate the natural steps by
which birds have acquired their
perfect power of flight; but
they serve, at least, to show
what diversified means of transition
are possible.
Seeing that a few members of
such water-breathing classes
as the Crustacea and Mollusca
are adapted to live on the land,
and seeing that we have flying
birds and mammals, flying insects
of the most diversified types,
and formerly had flying reptiles,
it is conceivable that flying-fish,
which now glide far through the
air, slightly rising and turning
by the aid of their fluttering
fins, might have been modified
into perfectly winged animals.
If early transitional state they
had been inhabitants of the open
ocean, and had used their incipient
organs of flight exclusively,
as far as we know, to escape
being devoured by other fish?
When we see any structure highly
perfected for any particular
habit, as the wings of a bird
for flight, we should bear in
mind that animals displaying
early transitional grades of
the structure will seldom continue
to exist to the present day,
for they will have been supplanted
by the very process of perfection
through natural selection. Furthermore,
we may conclude that transitional
grades between structures fitted
for very different habits of
life will rarely have been developed
at an early period in great numbers
and under many subordinate forms.
Thus, to return to our imaginary
illustration of the flying-fish,
it does not seem probable that
fishes capable of true flight
would have been developed under
many subordinate forms, for taking
prey of many kinds in many ways,
on the land and in the water,
until their organs of flight
had come to a high stage of perfection,
so as to have given them a decided
advantage over other animals
in the battle for life. Hence
the chance of discovering species
with transitional grades of structure
in a fossil condition will always
be less, from their having existed
in lesser numbers, than in the
case of species with fully developed
structures.
I will now give two or three
instances of diversified and
of changed habits in the individuals
of the same species. When either
case occurs, it would be easy
for natural selection to fit
the animal, by some modification
of its structure, for its changed
habits, or exclusively for one
of its several different habits.
But it is difficult to tell,
and immaterial for us, whether
habits generally change first
and structure afterwards; or
whether slight modifications
of structure lead to changed
habits; both probably often change
almost simultaneously. Of cases
of changed habits it will suffice
merely to allude to that of the
many British insects which now
feed on exotic plants, or exclusively
on artificial substances. Of
diversified habits innumerable
instances could be given: I have
often watched a tyrant flycatcher
(Saurophagus sulphuratus) in
South America, hovering over
one spot and then proceeding
to another, like a kestrel, and
at other times standing stationary
on the margin of water, and then
dashing like a kingfisher at
a fish. In our own country the
larger titmouse (Parus major)
may be seen climbing branches,
almost like a creeper; it often,
like a shrike, kills small birds
by blows on the head; and I have
many times seen and heard it
hammering the seeds of the yew
on a branch, and thus breaking
them like a nuthatch. In North
America the black bear was seen
by Hearne swimming for hours
with widely open mouth, thus
catching, like a whale, insects
in the water. Even in so extreme
a case as this, if the supply
of insects were constant, and
if better adapted competitors
did not already exist in the
country, I can see no difficulty
in a race of bears being rendered,
by natural selection, more and
more aquatic in their structure
and habits, with larger and larger
mouths, till a creature was produced
as monstrous as a whale.
As we sometimes see individuals
of a species following habits
widely different from those both
of their own species and of the
other species of the same genus,
we might expect, on my theory,
that such individuals would occasionally
have given rise to new species,
having anomalous habits, and
with their structure either slightly
or considerably modified from
that of their proper type. And
such instances do occur in nature.
Can a more striking instance
of adaptation be given than that
of a woodpecker for climbing
trees and for seizing insects
in the chinks of the bark? Yet
in North America there are woodpeckers
which feed largely on fruit,
and others with elongated wings
which chase insects on the wing;
and on the plains of La Plata,
where not a tree grows, there
is a woodpecker, which in every
essential part of its organisation,
even in its colouring, in the
harsh tone of its voice, and
undulatory flight, told me plainly
of its close blood-relationship
to our common species; yet it
is a woodpecker which never climbs
a tree!
Petrels are
the most aërial
and oceanic of birds, yet in
the quiet Sounds of Tierra del
Fuego, the Puffinuria berardi,
in its general habits, in its
astonishing power of diving,
its manner of swimming, and of
flying when unwillingly it takes
flight, would be mistaken by
any one for an auk or grebe;
nevertheless, it is essentially
a petrel, but with many parts
of its organisation profoundly
modified. On the other hand,
the acutest observer by examining
the dead body of the water-ouzel
would never have suspected its
sub-aquatic habits; yet this
anomalous member of the strictly
terrestrial thrush family wholly
subsists by diving, grasping
the stones with its feet and
using its wings under water.
He who believes that each being
has been created as we now see
it, must occasionally have felt
surprise when he has met with
an animal having habits and structure
not at all in agreement. What
can be plainer than that the
webbed feet of ducks and geese
are formed for swimming; yet
there are upland geese with webbed
feet which rarely or never go
near the water; and no one except
Audubon has seen the frigate-bird,
which has all its four toes webbed,
alight on the surface of the
sea. On the other hand, grebes
and coots are eminently aquatic,
although their toes are only
bordered by membrane. What seems
plainer than that the long toes
of grallatores are formed for
walking over swamps and floating
plants, yet the water-hen is
nearly as aquatic as the coot;
and the landrail nearly as terrestrial
as the quail or partridge. In
such cases, and many others could
be given, habits have changed
without a corresponding change
of structure. The webbed feet
of the upland goose may be said
to have become rudimentary in
function, though not in structure.
In the frigate-bird, the deeply-scooped
membrane between the toes shows
that structure has begun to change.
He who believes in separate
and innumerable acts of creation
will say, that in these cases
it has pleased the Creator to
cause a being of one type to
take the place of one of another
type; but this seems to me only
restating the fact in dignified
language. He who believes in
the struggle for existence and
in the principle of natural selection,
will acknowledge that every organic
being is constantly endeavouring
to increase in numbers; and that
if any one being vary ever so
little, either in habits or structure,
and thus gain an advantage over
some other inhabitant of the
country, it will seize on the
place of that inhabitant, however
different it may be from its
own place. Hence it will cause
him no surprise that there should
be geese and frigate-birds with
webbed feet, either living on
the dry land or most rarely alighting
on the water; that there should
be long-toed corncrakes living
in meadows instead of in swamps;
that there should be woodpeckers
where not a tree grows; that
there should be diving thrushes,
and petrels with the habits of
auks.
Organs of extreme perfection
and complication. To suppose
that the eye, with all its
inimitable contrivances for
adjusting the focus to different
distances, for admitting different
amounts of light, and for the
correction of spherical and
chromatic aberration, could
have been formed by natural
selection, seems, I freely
confess, absurd in the highest
possible degree. Yet reason
tells me, that if numerous
gradations from a perfect and
complex eye to one very imperfect
and simple, each grade being
useful to its possessor, can
be shown to exist; if further,
the eye does vary ever so slightly,
and the variations be inherited,
which is certainly the case;
and if any variation or modification
in the organ be ever useful
to an animal under changing
conditions of life, then the
difficulty of believing that
a perfect and complex eye could
be formed by natural selection,
though insuperable by our imagination,
can hardly be considered real.
How a nerve comes to be sensitive
to light, hardly concerns us
more than how life itself first
originated; but I may remark
that several facts make me
suspect that any sensitive
nerve may be rendered sensitive
to light, and likewise to those
coarser vibrations of the air
which produce sound.
In looking for the gradations
by which an organ in any species
has been perfected, we ought
to look exclusively to its lineal
ancestors; but this is scarcely
ever possible, and we are forced
in each case to look to species
of the same group, that is to
the collateral descendants from
the same original parent-form,
in order to see what gradations
are possible, and for the chance
of some gradations having been
transmitted from the earlier
stages of descent, in an unaltered
or little altered condition.
Amongst existing Vertebrata,
we find but a small amount of
gradation in the structure of
the eye, and from fossil species
we can learn nothing on this
head. In this great class we
should probably have to descend
far beneath the lowest known
fossiliferous stratum to discover
the earlier stages, by which
the eye has been perfected.
In the Articulata we can commence
a series with an optic nerve
merely coated with pigment, and
without any other mechanism;
and from this low stage, numerous
gradations of structure, branching
off in two fundamentally different
lines, can be shown to exist,
until we reach a moderately high
stage of perfection. In certain
crustaceans, for instance, there
is a double cornea, the inner
one divided into facets, within
each of which there is a lens
shaped swelling. In other crustaceans
the transparent cones which are
coated by pigment, and which
properly act only by excluding
lateral pencils of light, are
convex at their upper ends and
must act by convergence; and
at their lower ends there seems
to be an imperfect vitreous substance.
With these facts, here far too
briefly and imperfectly given,
which show that there is much
graduated diversity in the eyes
of living crustaceans, and bearing
in mind how small the number
of living animals is in proportion
to those which have become extinct,
I can see no very great difficulty
(not more than in the case of
many other structures) in believing
that natural selection has converted
the simple apparatus of an optic
nerve merely coated with pigment
and invested by transparent membrane,
into an optical instrument as
perfect as is possessed by any
member of the great Articulate
class.
He who will go thus far, if
he find on finishing this treatise
that large bodies of facts, otherwise
inexplicable, can be explained
by the theory of descent, ought
not to hesitate to go further,
and to admit that a structure
even as perfect as the eye of
an eagle might be formed by natural
selection, although in this case
he does not know any of the transitional
grades. His reason ought to conquer
his imagination; though I have
felt the difficulty far too keenly
to be surprised at any degree
of hesitation in extending the
principle of natural selection
to such startling lengths.
It is scarcely possible to
avoid comparing the eye to a
telescope. We know that this
instrument has been perfected
by the long-continued efforts
of the highest human intellects;
and we naturally infer that the
eye has been formed by a somewhat
analogous process. But may not
this inference be presumptuous?
Have we any right to assume that
the Creator works by intellectual
powers like those of man? If
we must compare the eye to an
optical instrument, we ought
in imagination to take a thick
layer of transparent tissue,
with a nerve sensitive to light
beneath, and then suppose every
part of this layer to be continually
changing slowly in density, so
as to separate into layers of
different densities and thicknesses,
placed at different distances
from each other, and with the
surfaces of each layer slowly
changing in form. Further we
must suppose that there is a
power always intently watching
each slight accidental alteration
in the transparent layers; and
carefully selecting each alteration
which, under varied circumstances,
may in any way, or in any degree,
tend to produce a distincter
image. We must suppose each new
state of the instrument to be
multiplied by the million; and
each to be preserved till a better
be produced, and then the old
ones to be destroyed. In living
bodies, variation will cause
the slight alterations, generation
will multiply them almost infinitely,
and natural selection will pick
out with unerring skill each
improvement. Let this process
go on for millions on millions
of years; and during each year
on millions of individuals of
many kinds; and may we not believe
that a living optical instrument
might thus be formed as superior
to one of glass, as the works
of the Creator are to those of
man?
If it could be demonstrated
that any complex organ existed,
which could not possibly have
been formed by numerous, successive,
slight modifications, my theory
would absolutely break down.
But I can find out no such case.
No doubt many organs exist of
which we do not know the transitional
grades, more especially if we
look to much-isolated species,
round which, according to my
theory, there has been much extinction.
Or again, if we look to an organ
common to all the members of
a large class, for in this latter
case the organ must have been
first formed at an extremely
remote period, since which all
the many members of the class
have been developed; and in order
to discover the early transitional
grades through which the organ
has passed, we should have to
look to very ancient ancestral
forms, long since become extinct.
We should be extremely cautious
in concluding that an organ could
not have been formed by transitional
gradations of some kind. Numerous
cases could be given amongst
the lower animals of the same
organ performing at the same
time wholly distinct functions;
thus the alimentary canal respires,
digests, and excretes in the
larva of the dragon-fly and in
the fish Cobites. In the Hydra,
the animal may be turned inside
out, and the exterior surface
will then digest and the stomach
respire. In such cases natural
selection might easily specialise,
if any advantage were thus gained,
a part or organ, which had performed
two functions, for one function
alone, and thus wholly change
its nature by insensible steps.
Two distinct organs sometimes
perform simultaneously the same
function in the same individual;
to give one instance, there are
fish with gills or branchiae
that breathe the air dissolved
in the water, at the same time
that they breathe free air in
their swimbladders, this latter
organ having a ductus pneumaticus
for its supply, and being divided
by highly vascular partitions.
In these cases, one of the two
organs might with ease be modified
and perfected so as to perform
all the work by itself, being
aided during the process of modification
by the other organ; and then
this other organ might be modified
for some other and quite distinct
purpose, or be quite obliterated.
The illustration of the swimbladder
in fishes is a good one, because
it shows us clearly the highly
important fact that an organ
originally constructed for one
purpose, namely flotation, may
be converted into one for a wholly
different purpose, namely respiration.
The swimbladder has, also, been
worked in as an accessory to
the auditory organs of certain
fish, or, for I do not know which
view is now generally held, a
part of the auditory apparatus
has been worked in as a complement
to the swimbladder. All physiologists
admit that the swimbladder is
homologous, or 'ideally similar,'
in position and structure with
the lungs of the higher vertebrate
animals: hence there seems to
me to be no great difficulty
in believing that natural selection
has actually converted a swimbladder
into a lung, or organ used exclusively
for respiration.
I can, indeed, hardly doubt
that all vertebrate animals having
true lungs have descended by
ordinary generation from an ancient
prototype, of which we know nothing,
furnished with a floating apparatus
or swimbladder. We can thus,
as I infer from Professor Owen's
interesting description of these
parts, understand the strange
fact that every particle of food
and drink which we swallow has
to pass over the orifice of the
trachea, with some risk of falling
into the lungs, notwithstanding
the beautiful contrivance by
which the glottis is closed.
In the higher Vertebrata the
branchiae have wholly disappeared
the slits on the sides of the
neck and the loop-like course
of the arteries still marking
in the embryo their former position.
But it is conceivable that the
now utterly lost branchiae might
have been gradually worked in
by natural selection for some
quite distinct purpose: in the
same manner as, on the view entertained
by some naturalists that the
branchiae and dorsal scales of
Annelids are homologous with
the wings and wing-covers of
insects, it is probable that
organs which at a very ancient
period served for respiration
have been actually converted
into organs of flight.
In considering transitions
of organs, it is so important
to bear in mind the probability
of conversion from one function
to another, that I will give
one more instance. Pedunculated
cirripedes have two minute folds
of skin, called by me the ovigerous
frena, which serve, through the
means of a sticky secretion,
to retain the eggs until they
are hatched within the sack.
These cirripedes have no branchiae,
the whole surface of the body
and sack, including the small
frena, serving for respiration.
The Balanidae or sessile cirripedes,
on the other hand, have no ovigerous
frena, the eggs lying loose at
the bottom of the sack, in the
well-enclosed shell; but they
have large folded branchiae.
Now I think no one will dispute
that the ovigerous frena in the
one family are strictly homologous
with the branchiae of the other
family; indeed, they graduate
into each other. Therefore I
do not doubt that little folds
of skin, which originally served
as ovigerous frena, but which,
likewise, very slightly aided
the act of respiration, have
been gradually converted by natural
selection into branchiae, simply
through an increase in their
size and the obliteration of
their adhesive glands. If all
pedunculated cirripedes had become
extinct, and they have already
suffered far more extinction
than have sessile cirripedes,
who would ever have imagined
that the branchiae in this latter
family had originally existed
as organs for preventing the
ova from being washed out of
the sack?
Although we must be extremely
cautious in concluding that any
organ could not possibly have
been produced by successive transitional
gradations, yet, undoubtedly,
grave cases of difficulty occur,
some of which will be discussed
in my future work.
One of the gravest is that
of neuter insects, which are
often very differently constructed
from either the males or fertile
females; but this case will be
treated of in the next chapter.
The electric organs of fishes
offer another case of special
difficulty; it is impossible
to conceive by what steps these
wondrous organs have been produced;
but, as Owen and others have
remarked, their intimate structure
closely resembles that of common
muscle; and as it has lately
been shown that Rays have an
organ closely analogous to the
electric apparatus, and yet do
not, as Matteuchi asserts, discharge
any electricity, we must own
that we are far too ignorant
to argue that no transition of
any kind is possible.
The electric organs offer another
and even more serious difficulty;
for they occur in only about
a dozen fishes, of which several
are widely remote in their affinities.
Generally when the same organ
appears in several members of
the same class, especially if
in members having very different
habits of life, we may attribute
its presence to inheritance from
a common ancestor; and its absence
in some of the members to its
loss through disuse or natural
selection. But if the electric
organs had been inherited from
one ancient progenitor thus provided,
we might have expected that all
electric fishes would have been
specially related to each other.
Nor does geology at all lead
to the belief that formerly most
fishes had electric organs, which
most of their modified descendants
have lost. The presence of luminous
organs in a few insects, belonging
to different families and orders,
offers a parallel case of difficulty.
Other cases could be given; for
instance in plants, the very
curious contrivance of a mass
of pollen-grains, borne on a
foot-stalk with a sticky gland
at the end, is the same in Orchis
and Asclepias, genera almost
as remote as possible amongst
flowering plants. In all these
cases of two very distinct species
furnished with apparently the
same anomalous organ, it should
be observed that, although the
general appearance and function
of the organ may be the same,
yet some fundamental difference
can generally be detected. I
am inclined to believe that in
nearly the same way as two men
have sometimes independently
hit on the very same invention,
so natural selection, working
for the good of each being and
taking advantage of analogous
variations, has sometimes modified
in very nearly the same manner
two parts in two organic beings,
which owe but little of their
structure in common to inheritance
from the same ancestor.
Although in many cases it is
most difficult to conjecture
by what transitions an organ
could have arrived at its present
state; yet, considering that
the proportion of living and
known forms to the extinct and
unknown is very small, I have
been astonished how rarely an
organ can be named, towards which
no transitional grade is known
to lead. The truth of this remark
is indeed shown by that old canon
in natural history of 'Natura
non facit saltum.' We meet with
this admission in the writings
of almost every experienced naturalist;
or, as Milne Edwards has well
expressed it, nature is prodigal
in variety, but niggard in innovation.
Why, on the theory of Creation,
should this be so? Why should
all the parts and organs of many
independent beings, each supposed
to have been separately created
for its proper place in nature,
be so invariably linked together
by graduated steps? Why should
not Nature have taken a leap
from structure to structure?
On the theory of natural selection,
we can clearly understand why
she should not; for natural selection
can act only by taking advantage
of slight successive variations;
she can never take a leap, but
must advance by the shortest
and slowest steps.
Organs of little apparent
importance. As natural
selection acts by life and
death, by the preservation
of individuals with any favourable
variation, and by the destruction
of those with any unfavourable
deviation of structure, I have
sometimes felt much difficulty
in understanding the origin
of simple parts, of which the
importance does not seem sufficient
to cause the preservation of
successively varying individuals.
I have sometimes felt as much
difficulty, though of a very
different kind, on this head,
as in the case of an organ
as perfect and complex as the
eye.
In the first place, we are
much too ignorant in regard to
the whole economy of any one
organic being, to say what slight
modifications would be of importance
or not. In a former chapter I
have given instances of most
trifling characters, such as
the down on fruit and the colour
of the flesh, which, from determining
the attacks of insects or from
being correlated with constitutional
differences, might assuredly
be acted on by natural selection.
The tail of the giraffe looks
like an artificially constructed
fly-flapper; and it seems at
first incredible that this could
have been adapted for its present
purpose by successive slight
modifications, each better and
better, for so trifling an object
as driving away flies; yet we
should pause before being too
positive even in this case, for
we know that the distribution
and existence of cattle and other
animals in South America absolutely
depends on their power of resisting
the attacks of insects: so that
individuals which could by any
means defend themselves from
these small enemies, would be
able to range into new pastures
and thus gain a great advantage.
It is not that the larger quadrupeds
are actually destroyed (except
in some rare cases) by the flies,
but they are incessantly harassed
and their strength reduced, so
that they are more subject to
disease, or not so well enabled
in a coming dearth to search
for food, or to escape from beasts
of prey.
Organs now of trifling importance
have probably in some cases been
of high importance to an early
progenitor, and, after having
been slowly perfected at a former
period, have been transmitted
in nearly the same state, although
now become of very slight use;
and any actually injurious deviations
in their structure will always
have been checked by natural
selection. Seeing how important
an organ of locomotion the tail
is in most aquatic animals, its
general presence and use for
many purposes in so many land
animals, which in their lungs
or modified swim-bladders betray
their aquatic origin, may perhaps
be thus accounted for. A well-developed
tail having been formed in an
aquatic animal, it might subsequently
come to be worked in for all
sorts of purposes, as a fly-flapper,
an organ of prehension, or as
an aid in turning, as with the
dog, though the aid must be slight,
for the hare, with hardly any
tail, can double quickly enough.
In the second
place, we may sometimes attribute
importance
to characters which are really
of very little importance, and
which have originated from quite
secondary causes, independently
of natural selection. We should
remember that climate, food, &c.,
probably have some little direct
influence on the organisation;
that characters reappear from
the law of reversion;, that correlation
of growth will have had a most
important influence in modifying
various structures; and finally,
that sexual selection will often
have largely modified the external
characters of animals having
a will, to give one male an advantage
in fighting with another or in
charming the females. Moreover
when a modification of structure
has primarily arisen from the
above or other unknown causes,
it may at first have been of
no advantage to the species,
but may subsequently have been
taken advantage of by the descendants
of the species under new conditions
of life and with newly acquired
habits.
To give a few instances to
illustrate these latter remarks.
If green woodpeckers alone had
existed, and we did not know
that there were many black and
pied kinds, I dare say that we
should have thought that the
green colour was a beautiful
adaptation to hide this tree-frequenting
bird from its enemies; and consequently
that it was a character of importance
and might have been acquired
through natural selection; as
it is, I have no doubt that the
colour is due to some quite distinct
cause, probably to sexual selection.
A trailing bamboo in the Malay
Archipelago climbs the loftiest
trees by the aid of exquisitely
constructed hooks clustered around
the ends of the branches, and
this contrivance, no doubt, is
of the highest service to the
plant; but as we see nearly similar
hooks on many trees which are
not climbers the hooks on the
bamboo may have arisen from unknown
laws of growth, and have been
subsequently taken advantage
of by the plant undergoing further
modification and becoming a climber.
The naked skin on the head of
a vulture is generally looked
at as a direct adaptation for
wallowing in putridity; and so
it may be, or it may possibly
be due to the direct action of
putrid matter; but we should
be very cautious in drawing any
such inference, when we see that
the skin on the head of the clean-feeding
male turkey is likewise naked.
The sutures in the skulls of
young mammals have been advanced
as a beautiful adaptation for
aiding parturition, and no doubt
they facilitate, or may be indispensable
for this act; but as sutures
occur in the skulls of young
birds and reptiles, which have
only to escape from a broken
egg, we may infer that this structure
has arisen from the laws of growth,
and has been taken advantage
of in the parturition of the
higher animals.
We are profoundly ignorant
of the causes producing slight
and unimportant variations; and
we are immediately made conscious
of this by reflecting on the
differences in the breeds of
our domesticated animals in different
countries, more especially in
the less civilized countries
where there has been but little
artificial selection. Careful
observers are convinced that
a damp climate affects the growth
of the hair, and that with the
hair the horns are correlated.
Mountain breeds always differ
from lowland breeds; and a mountainous
country would probably affect
the hind limbs from exercising
them more, and possibly even
the form of the pelvis; and then
by the law of homologous variation,
the front limbs and even the
head would probably be affected.
The shape, also, of the pelvis
might affect by pressure the
shape of the head of the young
in the womb. The laborious breathing
necessary in high regions would,
we have some reason to believe,
increase the size of the chest;
and again correlation would come
into play. Animals kept by savages
in different countries often
have to struggle for their own
subsistence, and would be exposed
to a certain extent to natural
selection, and individuals with
slightly different constitutions
would succeed best under different
climates; and there is reason
to believe that constitution
and colour are correlated. A
good observer, also, states that
in cattle susceptibility to the
attacks of flies is correlated
with colour, as is the liability
to be poisoned by certain plants;
so that colour would be thus
subjected to the action of natural
selection. But we are far too
ignorant to speculate on the
relative importance of the several
known and unknown laws of variation;
and I have here alluded to them
only to show that, if we are
unable to account for the characteristic
differences of our domestic breeds,
which nevertheless we generally
admit to have arisen through
ordinary generation, we ought
not to lay too much stress on
our ignorance of the precise
cause of the slight analogous
differences between species.
I might have adduced for this
same purpose the differences
between the races of man, which
are so strongly marked; I may
add that some little light can
apparently be thrown on the origin
of these differences, chiefly
through sexual selection of a
particular kind, but without
here entering on copious details
my reasoning would appear frivolous.
The foregoing
remarks lead me to say a few
words on the
protest lately made by some naturalists,
against the utilitarian doctrine
that every detail of structure
has been produced for the good
of its possessor. They believe
that very many structures have
been created for beauty in the
eyes of man, or for mere variety.
This doctrine, if true, would
be absolutely fatal to my theory.
Yet I fully admit that many structures
are of no direct use to their
possessors. Physical conditions
probably have had some little
effect on structure, quite independently
of any good thus gained. Correlation
of growth has no doubt played
a most important part, and a
useful modification of one part
will often have entailed on other
parts diversified changes of
no direct use. So again characters
which formerly were useful, or
which formerly had arisen from
correlation of growth, or from
other unknown cause, may reappear
from the law of reversion, though
now of no direct use. The effects
of sexual selection, when displayed
in beauty to charm the females,
can be called useful only in
rather a forced sense. But by
far the most important consideration
is that the chief part of the
organisation of every being is
simply due to inheritance; and
consequently, though each being
assuredly is well fitted for
its place in nature, many structures
now have no direct relation to
the habits of life of each species.
Thus, we can hardly believe that
the webbed feet of the upland
goose or of the frigate-bird
are of special use to these birds;
we cannot believe that the same
bones in the arm of the monkey,
in the fore leg of the horse,
in the wing of the bat, and in
the flipper of the seal, are
of special use to these animals.
We may safely attribute these
structures to inheritance. But
to the progenitor of the upland
goose and of the frigate-bird,
webbed feet no doubt were as
useful as they now are to the
most aquatic of existing birds.
So we may believe that the progenitor
of the seal had not a flipper,
but a foot with five toes fitted
for walking or grasping; and
we may further venture to believe
that the several bones in the
limbs of the monkey, horse, and
bat, which have been inherited
from a common progenitor, were
formerly of more special use
to that progenitor, or its progenitors,
than they now are to these animals
having such widely diversified
habits. Therefore we may infer
that these several bones might
have been acquired through natural
selection, subjected formerly,
as now, to the several laws of
inheritance, reversion, correlation
of growth, &c. Hence every
detail of structure in every
living creature (making some
little allowance for the direct
action of physical conditions)
may be viewed, either as having
been of special use to some ancestral
form, or as being now of special
use to the descendants of this
form either directly, or indirectly
through the complex laws of growth.
Natural selection cannot possibly
produce any modification in any
one species exclusively for the
good of another species; though
throughout nature one species
incessantly takes advantage of,
and profits by, the structure
of another. But natural selection
can and does often produce structures
for the direct injury of other
species, as we see in the fang
of the adder, and in the ovipositor
of the ichneumon, by which its
eggs are deposited in the living
bodies of other insects. If it
could be proved that any part
of the structure of any one species
had been formed for the exclusive
good of another species, it would
annihilate my theory, for such
could not have been produced
through natural selection. Although
many statements may be found
in works on natural history to
this effect, I cannot find even
one which seems to me of any
weight. It is admitted that the
rattlesnake has a poison-fang
for its own defence and for the
destruction of its prey; but
some authors suppose that at
the same time this snake is furnished
with a rattle for its own injury,
namely, to warn its prey to escape.
I would almost as soon believe
that the cat curls the end of
its tail when preparing to spring,
in order to warn the doomed mouse.
But I have not space here to
enter on this and other such
cases.
Natural selection will never
produce in a being anything injurious
to itself, for natural selection
acts solely by and for the good
of each. No organ will be formed,
as Paley has remarked, for the
purpose of causing pain or for
doing an injury to its possessor.
If a fair balance be struck between
the good and evil caused by each
part, each will be found on the
whole advantageous. After the
lapse of time, under changing
conditions of life, if any part
comes to be injurious, it will
be modified; or if it be not
so, the being will become extinct,
as myriads have become extinct.
Natural selection tends only
to make each organic being as
perfect as, or slightly more
perfect than, the other inhabitants
of the same country with which
it has to struggle for existence.
And we see that this is the degree
of perfection attained under
nature. The endemic productions
of New Zealand, for instance,
are perfect one compared with
another; but they are now rapidly
yielding before the advancing
legions of plants and animals
introduced from Europe. Natural
selection will not produce absolute
perfection, nor do we always
meet, as far as we can judge,
with this high standard under
nature. The correction for the
aberration of light is said,
on high authority, not to be
perfect even in that most perfect
organ, the eye. If our reason
leads us to admire with enthusiasm
a multitude of inimitable contrivances
in nature, this same reason tells
us, though we may easily err
on both sides, that some other
contrivances are less perfect.
Can we consider the sting of
the wasp or of the bee as perfect,
which, when used against many
attacking animals, cannot be
withdrawn, owing to the backward
serratures, and so inevitably
causes the death of the insect
by tearing out its viscera?
If we look at the sting of
the bee, as having originally
existed in a remote progenitor
as a boring and serrated instrument,
like that in so many members
of the same great order, and
which has been modified but not
perfected for its present purpose,
with the poison originally adapted
to cause galls subsequently intensified,
we can perhaps understand how
it is that the use of the sting
should so often cause the insect's
own death: for if on the whole
the power of stinging be useful
to the community, it will fulfil
all the requirements of natural
selection, though it may cause
the death of some few members.
If we admire the truly wonderful
power of scent by which the males
of many insects find their females,
can we admire the production
for this single purpose of thousands
of drones, which are utterly
useless to the community for
any other end, and which are
ultimately slaughtered by their
industrious and sterile sisters?
It may be difficult, but we ought
to admire the savage instinctive
hatred of the queen-bee, which
urges her instantly to destroy
the young queens her daughters
as soon as born, or to perish
herself in the combat; for undoubtedly
this is for the good of the community;
and maternal love or maternal
hatred, though the latter fortunately
is most rare, is all the same
to the inexorable principle of
natural selection. If we admire
the several ingenious contrivances,
by which the flowers of the orchis
and of many other plants are
fertilised through insect agency,
can we consider as equally perfect
the elaboration by our fir-trees
of dense clouds of pollen, in
order that a few granules may
be wafted by a chance breeze
on to the ovules?
Summary of Chapter. We
have in this chapter discussed
some of the difficulties and
objections which may be urged
against my theory. Many of them
are very grave; but I think that
in the discussion light has been
thrown on several facts, which
on the theory of independent
acts of creation are utterly
obscure. We have seen that species
at any one period are not indefinitely
variable, and are not linked
together by a multitude of intermediate
gradations, partly because the
process of natural selection
will always be very slow, and
will act, at any one time, only
on a very few forms; and partly
because the very process of natural
selection almost implies the
continual supplanting and extinction
of preceding and intermediate
gradations. Closely allied species,
now living on a continuous area,
must often have been formed when
the area was not continuous,
and when the conditions of life
did not insensibly graduate away
from one part to another. When
two varieties are formed in two
districts of a continuous area,
an intermediate variety will
often be formed, fitted for an
intermediate zone; but from reasons
assigned, the intermediate variety
will usually exist in lesser
numbers than the two forms which
it connects; consequently the
two latter, during the course
of further modification, from
existing in greater numbers,
will have a great advantage over
the less numerous intermediate
variety, and will thus generally
succeed in supplanting and exterminating
it.
We have seen in this chapter
how cautious we should be in
concluding that the most different
habits of life could not graduate
into each other; that a bat,
for instance, could not have
been formed by natural selection
from an animal which at first
could only glide through the
air.
We have seen that a species
may under new conditions of life
change its habits, or have diversified
habits, with some habits very
unlike those of its nearest congeners.
Hence we can understand bearing
in mind that each organic being
is trying to live wherever it
can live, how it has arisen that
there are upland geese with webbed
feet, ground woodpeckers, diving
thrushes, and petrels with the
habits of auks.
Although the belief that an
organ so perfect as the eye could
have been formed by natural selection,
is more than enough to stagger
any one; yet in the case of any
organ, if we know of a long series
of gradations in complexity,
each good for its possessor,
then, under changing conditions
of life, there is no logical
impossibility in the acquirement
of any conceivable degree of
perfection through natural selection.
In the cases in which we know
of no intermediate or transitional
states, we should be very cautious
in concluding that none could
have existed, for the homologies
of many organs and their intermediate
states show that wonderful metamorphoses
in function are at least possible.
For instance, a swim-bladder
has apparently been converted
into an air-breathing lung. The
same organ having performed simultaneously
very different functions, and
then having been specialised
for one function; and two very
distinct organs having performed
at the same time the same function,
the one having been perfected
whilst aided by the other, must
often have largely facilitated
transitions.
We are far too ignorant, in
almost every case, to be enabled
to assert that any part or organ
is so unimportant for the welfare
of a species, that modifications
in its structure could not have
been slowly accumulated by means
of natural selection. But we
may confidently believe that
many modifications, wholly due
to the laws of growth, and at
first in no way advantageous
to a species, have been subsequently
taken advantage of by the still
further modified descendants
of this species. We may, also,
believe that a part formerly
of high importance has often
been retained (as the tail of
an aquatic animal by its terrestrial
descendants), though it has become
of such small importance that
it could not, in its present
state, have been acquired by
natural selection, a power which
acts solely by the preservation
of profitable variations in the
struggle for life.
Natural selection will produce
nothing in one species for the
exclusive good or injury of another;
though it may well produce parts,
organs, and excretions highly
useful or even indispensable,
or highly injurious to another
species, but in all cases at
the same time useful to the owner.
Natural selection in each well-stocked
country, must act chiefly through
the competition of the inhabitants
one with another, and consequently
will produce perfection, or strength
in the battle for life, only
according to the standard of
that country. Hence the inhabitants
of one country, generally the
smaller one, will often yield,
as we see they do yield, to the
inhabitants of another and generally
larger country. For in the larger
country there will have existed
more individuals, and more diversified
forms, and the competition will
have been severer, and thus the
standard of perfection will have
been rendered higher. Natural
selection will not necessarily
produce absolute perfection;
nor, as far as we can judge by
our limited faculties, can absolute
perfection be everywhere found.
On the theory of natural selection
we can clearly understand the
full meaning of that old canon
in natural history, 'Natura non
facit saltum.' This canon, if
we look only to the present inhabitants
of the world, is not strictly
correct, but if we include all
those of past times, it must
by my theory be strictly true.
It is generally acknowledged
that all organic beings have
been formed on two great laws
Unity of Type, and the Conditions
of Existence. By unity of type
is meant that fundamental agreement
in structure, which we see in
organic beings of the same class,
and which is quite independent
of their habits of life. On my
theory, unity of type is explained
by unity of descent. The expression
of conditions of existence, so
often insisted on by the illustrious
Cuvier, is fully embraced by
the principle of natural selection.
For natural selection acts by
either now adapting the varying
parts of each being to its organic
and inorganic conditions of life;
or by having adapted them during
long-past periods of time: the
adaptations being aided in some
cases by use and disuse, being
slightly affected by the direct
action of the external conditions
of life, and being in all cases
subjected to the several laws
of growth. Hence, in fact, the
law of the Conditions of Existence
is the higher law; as it includes,
through the inheritance of former
adaptations, that of Unity of
Type. |