On the Origin of Species by Means of Natural Selection
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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 aerial 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.
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