Charles Darwin.

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shows that structure has begun to change.

He who believes in separate and innumerable acts of creation may say,'
that in these cases it has pleased the Creator to cause a being of one type to
take the place of one belonging to 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 endeavoring to increase in numbers; and
that if any one being varies ever so little, either in habits or structure, and
thus gains an advantage over some other inhabitant of the same country, it
will seize on the place of that inhabitant, however different that may be from
its own place. Hence it will cause him no surprise that there should be geese
and frigate-birds with webbed feet, living on the dry land and rarely alight-
ing on the water, that there should be long-toed corncrakes, living in mead-
ows instead of in swamps ; that there should be woodpeckers where hardly a
tree grows; that there should be diving thrushes and diving Hymenoptera,
and petrels with the habits of auks.


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
degree. When it was first said that the sun stood still and the world turned
round, the common sense of mankind declared the doctrine false; but the old
saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted
in science. Reason tells me, that if numerous gradations from a simple and
imperfect eye to one complex and perfect can be shown to exist, each grade
being useful to its possessor, as is certainly the case; if further, the eye ever
varies and the variations be inherited, as is likewise certainly the case ; and if
such variations should be useful to any 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, should
not be considered as subversive of the theory. How a nerve comes to be sensi-
tive to light, hardly concerns us more than how life itself originated; but I
may remark that, as some of the lowest organisms in which nerves cannot be
detected, are capable of perceiving light, it does not seem impossible that
certain sensitive elements in their sarcode should become aggregated and de-
veloped into nerves, endowed with this special sensibility.

In searching for the gradations through which an organ in any species has
been perfected, we ought to look exclusively to its lineal progenitors; but
this is scarcely ever possible, and we are forced to look to other species and
genera of the same group, that is to the collateral descendants from the same
parent-form, in order to see what gradations are possible, and for the chance
of some gradations having been transmitted in an unaltered or little altered
condition. But the state of the same organ in distinct classes may incidentally
throw light on the steps by which it has been perfected.

The simplest organ which can be called an eye consists of an optic nerve,
surrounded by pigment-cells and covered by transluc-ent skin, but without
any lens or other refractive body. We may, however, according to M. Jour-
dain, descend even a step lower and find aggregates of pigment-cells, appar-
ently serving as organs of vision, without any nerves, and resting merely on
sarcodic tissue. Eyes of the above simple nature are not capable of distinct
vision, and serve only to distinguish light from darkness. In certain star-fishes,
small depressions in the layer of pigment which surrounds the nerve are filled,
as described by the author just quoted, with transparent gelatinous matter,
projecting with a convex surface, like the cornea in the higher animals. He
suggests that this serves not to form an image, bui only to concentrate the
luminous rays and render their perception more easy. In this concentration of
the rays we gain the first and by far the most important step toward the
formation of a true, picture-forming eye ; for we have only to place the naked
extremity of the optic nerve, which in some of the lower animals lies deeply
buried in the body, and in some near the surface, at the right distance from
the concentrating apparatus, and an image will be formed on it.

In the great class of the Articulata, we may start from an optic nerve
simply coated with pigment, the latter sometimes forming a sort of pupil, but
destitute of lens or other optical contrivance. With insects it is now known
that the numerous facets on the cornea of their great compound eyes form
true lenses, and that the cones include curiously modified nervous filaments.
But these organs in the Articulata are so much diversified that Miiller for-
merly made three main classes with seven subdivisions, besides a fourth main
class of aggregated simple eyes.

When we reflect on these facts, here given much too briefly, with respect
to the wide, diversified, and graduated range of structure in the eyes of the
lower animals; and when we bear in mind how small the number of all liv-
ing forms must be in comparison with those which have become extinct, the
difficulty ceases to be very great in believing that natural selection may have
converted the simple apparatus of an optic nerve, coated with pigment and


invested by transparent membrane, into an optical instrument as perfect as
is possessed by any member of the Articulata class.

He who will go thus far, ought not to hesitate to go one step further, if he
finds on finishing this volume that large bodies of facts, otherwise inexplicable,
can be explained by the theory of modification through natural selection; he
ought to admit that a structure even as perfect as an eagle's eye might thus
be formed, although in this case he does not know the transitional states. It
has been objected that in order to modify the eye and still preserve it as a
perfect instrument, many changes would have to be effected simultaneously,
which, it is assumed, could not be done through natural selection; but as I
have attempted to show in my work on the variation of domestic animals,
it is not necessary to suppose that the modifications were all simultaneous,
if they were extremely slight and gradual. Different kinds of modification
would, also, serve for the same general purpose: as Mr. Wallace has re-
marked, "If a lens has too short or too long a focus, it may be amended either
by an alteration of curvature, or an alteration of density; if the curvature
be irregular, and the rays do not converge to a point, then any increased
regularity of curvature will be an improvement. So the contraction of the
iris and the muscular movements of the eye are neither of them essential
to vision, but only improvements which might have been added and per-
fected at any stage of the construction of the instrument." Within the highest
division of the animal kingdom, namely, the Vertebrata, we can start from
an eye so simple, that it consists, as in the lancelet, of a little sack of trans-
parent skin, furnished with a nerve and lined with pigment, but destitute of
any other apparatus. In fishes and reptiles, as Owen has remarked, the range
of graduation of dioptic structures is very great. It is a significant fact that
even in man, according to the high authority of Virchow, the beautiful crys-
talline lens is formed in the embryo by an accumulation of epidermic cells,
lying in a sac-like fold of the skin; and the vitreous body is formed from
embryonic subcutaneous tissue. To arrive, however, at a just conclusion re-
garding the formation of the eye, with all its marvellous yet not absolutely
perfect characters, it is indispensable that the reason should conquer the
imagination; but I have felt the difficulty far too keenly to be surprised at
others hesitating to extend the principle of natural selection to so startling
a length.

It is scarcely possible to avoid comparing the eye with 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 in-
tellectual powers like those of men? If we must compare the eye to an optical
instrument, we ought in imagination to take a thick layer of transparent
tissue, with spaces filled with fluid, and with a nerve sensitive to light be-
neath, 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 thick-
nesses, 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, represented by natural selection or the survival of the fittest, always
intently watching each slight alteration in the transparent layers; and care-
fully preserving each which, under varied circumstances, in any way or de-
gree, tends to produce a distincter image. We must suppose each new state
of the instrument to be multiplied by the million; each to be preserved until
a better one is produced, and then the old ones to be all destroyed. In living
bodies, variation will cause the slight alteration, generation will multiply
them almost infinitely, and natural selection will pick out with unerring skill
each improvement. Let this process go on for 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, around which, according
to the theory, there has been much extinction. Or again, if we take an organ
common to all the members of a class, for in this latter case the organ must
have been originally formed at a 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 among the lower animals of the same organ performing at
the same time wholly distinct functions; thus in the larva of the dragon-fly
and in the fish Cobites the alimentary canal respires, digests and excretes. 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
specialize, if any advantage were thus gained, the whole or part of an organ,
which had previously performed two functions, for one function alone, and
thus by insensible steps greatly change its nature. Many plants are known
which regularly produce at the same time diff"erently constructed flowers;
and if such plants were to produce one kind alone, a great change would be
eff"ected with comparative suddenness in the character of the species. It is,
however, probable that the two sorts of flowers borne by the same plant were
originally differentiated by finely graduated steps, which may still be fol-
lowed in some few cases. \

Again, two distinct organs, or the same organ under two very different
forms, may simultaneously perform in the same individual the same func-
tion, and this is an extremely important means of transition: to give one


instance — there are fish wdth gills or branchiae that breathe the air dissolved
in the water, at the same time that they breathe free air in their swim-blad-
ders, this latter organ being divided by highly vascular partitions and ha\dng
a ductus pneumaticus for the supply of air. To give another instance from the
vegetable kingdom: plants climb by three distinct means, by spirally twin-
ing, by clasping a support with their sensitive tendrils, and by the emission
of aerial rootlets; these three means are usually found in distinct groups,
but some few species exhibit two of the means, or even all three, combined
in the same individual. In all such cases one of the two organs might readily
be modified and perfected so as to perform all the work, being aided during
the progress of modification by the other organ; and then this other organ
might be modified for some other and quite distinct purpose, or be wholly

The illustration of the swim-bladder in fishes is a good one, because it
shows us clearly the highly important fact that an organ originally con-
structed for one purpose, namely, flotation, may be converted into one for
a widely different purpose, namely respiration. The swim-bladder has, also,
been worked in as an accessory to the auditory organs of certain fishes. All
physiologists admit that the swim-bladder is homologous, or "ideally similar"
in position and structure with the lungs of the higher vertebrate animals:
hence there is no reason to doubt that the swim-bladder has actually been
converted into lungs, or an organ used exclusively for respiration.

According to this view it may be inferred that all vertebrate animals with
true lungs are descended by ordinary generation from an ancient and un-
known prototype, which was furnished with a floating apparatus or swim-
bladder. "VVe can thus, as I infer from Owen's interesting description of these
parts, understand the strange fact that every particle of food and drink which
w^e swallow has to pass over the orifice of the trachea, with some risk of fall-
ing into the lungs, notvvithstanding the beautiful contrivance by which the
glottis is closed. In the higher vertebrata the branchiae have wholly disap-
peared — but in the embrv'o the slits on the sides of the neck and the loop-
like course of the arteries still mark their former position. But it is conceiv-
able that the now utterly lost branchiae might have been gradually worked
in by natural selection for some distinct purpose : for instance, Landois has
shown that the wings of insects are developed from the trachea; it is there-
fore highly probable that in this great class organs which once served for
respiration, have been actually converted into organs for flight.

In considering transitions of organs, it is so important to bear in mind the
probability' of conversions from one function to another, that I will give an-
other instance. Pedunculated cirripedes have two minute folds of skin, called
by me the ovigerous frena, which serve, through the means of a sticky secre-
tion, to retain the eggs until they are hatched within the sack. These cirri-
pedes have no branchiae, the whole surface of the body and of the sack, to-
gether with 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, within the well-enclosed shell; but they have, in the


same relative position with the frena, large, much-folded membranes, which
freely communicate with the circulatory lacunae of the sack and body, and
which have been considered by all naturalists to act as 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 it need not be doubted that the two httle folds of skin,
which originally served as ovigerous frena, but which, likewise, very slightly
aided in 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 be-
come extinct, and they have 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?

There is another possible mode of transition, namely, through the accelera-
tion or retardation of the period of reproduction. This has lately been in-
sisted on by Professor Cope and others in the United States. It is now known
that some animals are capable of reproduction at a very early age, before
they have acquired their perfect characters; and if this power became thor-
oughly well developed in a species, it seems probable that the adult stage of
development would sooner or later be lost; and in this case, especially if the
larva differed much from the mature form, the character of the species would
be greatly changed and degraded. Again, not a few animals, after arriving at
maturity, go on changing in character during nearly their whole lives. With
mammals, for instance, the form of the skull is often much altered with age,
of which Dr. Murie has given some striking instances with seals. Every one
knows how the horns of stags become more and more branched, and the
plumes of some birds become more finely developed, as they grow older.
Professor Cope states that the teeth of certain lizards change much in shape
with advancing years. With crustaceans not only many trivial, but some im-
portant, parts assume a new character, as recorded by Fritz Miiller, after
maturity. In all such cases — and many could be given — if the age for repro-
duction were retarded, the character of the species, at least in its adult state,
would be modified; nor is it improbable that the previous and earher stages
of development would in some cases be hurried through and finally lost.
Whether species have often or ever been modified through this comparatively
sudden mode of transition, I can form no opinion; but if this has occurred,
it is probable that the differences between the young and the mature, and
between the mature and the old, were primordially acquired by graduated


Although we must be extremely cautious in concluding that any organ
could not have been produced by successive, small, transitional gradations,
yet undoubtedly serious cases of difficulty occur.

One of the most serious is that of neuter insects, which are often 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, for it is impossible to conceive by what steps these
wondrous organs have been produced. But this is not surprising, for we do
not even know of what use they are. In the gymnotus and torpedo they no
doubt serve as powerful means of defence, and perhaps for securing prey;
yet in the ray, as observed by Matteucci, an analogous organ in the tail tin
manifests but little electricity, even when the animal is greatly irritated ; so
little that it can hardly be of any use for the above purposes. Moreover, in
the ray, besides the organ just referred to, there is, as Dr. R. McDonnell has
shown, another organ near the head, not known to be electrical, but which
appears to be the real homologue of the electric battery in the torpedo. It is
generally admitted that there exists between these organs and ordinary muscle '
a close analogy, in intimate structure, in the distribution of the nerves, and I
in the manner in which they are acted on by various reagents. It should, also, ,
be especially observed that muscular contraction is accompanied by an elec-
trical discharge; and, as Dr. Radcliffe insists, "in the electrical apparatus of '
the torpedo during rest, there would seem to be a charge in every respect like ;
that which is met with in muscle and nerve during the rest, and the dis- •
charge of the torpedo, instead of being peculiar, may be only another form i
of the discharge which attends upon the action of muscle and motor nerve." '
Beyond this we cannot at present go in the way of explanation; but as we
know so little about the uses of these organs, and as we know nothing about
the habits and structure of the progenitors of the existing electric fishes, it
would be extremely bold to maintain that no serviceable transitions are pos-
sible by which these organs might have been gradually developed.

These organs appear at first to offer another and far more serious diffi-^
culty; for they occur in about a dozen kinds of fish, of which several are;
widely remote in their affinities. When the same organ is found in several
members of the same class, especially if in members having very different
habits of life, we may generally attribute its presence to inheritance from a
common ancestor; and its absence in some of the members to loss through
disuse or natural selection. So that, if the electric organs had been inherited
from some one ancient progenitor, we might have expected that all electric
fishes would have been specially related to each other; but this is far from
the case. Nor does geology at all lead to the belief that most fishes formerly
possessed electric organs, which their modffied descendants have now lost.
But when we look at the subject more closely, we find in the several fishes
provided with electric organs, that these are situated in different parts of the
body, that they differ in construction, as in the arrangement of the plates,
and, according to Pacini, in the process or means by which the electricity is
excited — and lastly, in being supplied with nerves proceeding from different
sources, and this is perhaps the most important of all the differences. Hence
in the several fishes furnished with electric organs, these cannot be consid-
ered as homologous, but only as analogous in function. Consequently there
is no reason to suppose that they have been inherited from a common pro-


genitor; for had this been the case they would have closely resembled each
other in all respects. Thus the difficulty of an organ, apparently the same,
arising in several remotely allied species, disappears, leaving only the lesser
yet still great difficulty: namely, by what graduated steps these organs have
been developed in each separate group of fishes.

The luminous organs which occur in a few insects, belonging to widely
different families, and which are situated in different parts of the body, offer,
under our present state of ignorance, a difficulty almost exactly parallel with
that of the electric organs. Other similar cases could be given; for instance
in plants, the very curious contrivance of a mass of pollen-grains, borne on a
foot-stalk with an adhesive gland, is apparently the same in Orchis and As-
clepias, genera almost as remote as is possible among flowering plants; but
here again the parts are not homologous. In all cases of beings, far removed
from each other in the scale of organization, which are furnished with similar
and peculiar organs, it will be found that although the general appearance
and function of the organs may be the same, yet fundamental differences be-
tween them can always be detected. For instance, the eyes of Cephalopods or
cuttle-fish and of vertebrate animals appear wonderfully alike; and in such
widely sundered groups, no part of this resemblance can be due to inheritance
from a common progenitor. Mr. Mivart has advanced this case as one of
special difficulty, but I am unable to see the force of his argument. An organ
for vision must be formed of transparent tissue, and must include some sort
of lens for throwing an image at the back of a darkened chamber. Beyond
this superficial resemblance, there is hardly any real similarity between the
eyes of cuttle-fish and vertebrates, as may be seen by consulting Hensen's ad-
mirable memoir on these organs in the Cephalopoda. It is impossible for me

Online LibraryCharles DarwinThe origin of species → online text (page 17 of 50)