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.
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.
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