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here to enter on details, but I may specify a few of the points of difference.
The crystalline lens in the higher cuttle-fish consists of two parts, placed one
behind the other like two lenses, both having a very different structure and
disposition to what occurs in the vertebrata. The retina is wholly different,
with an actual inversion of the elemental parts, and with a large nervous
ganglion included within the membranes of the eye. The relations of the
muscles are as different as it is possible to conceive, and so in other points.
Hence it is not a little difficult to decide how far even the same terms ought
to be employed in describing the eyes of the Cephalopoda and Vertebrata.
It is, of course, open to any one to deny that the eye in either case could have
been developed through the natural selection of successive slight variations;
but if this be admitted in the one case it is clearly possible in the other; and
fundamental differences of structure in the visual organs of two groups might
have been anticipated, in accordance with this view of their manner of
formation. As two men have sometimes independently hit on the same inven-
tion, so in the several foregoing cases it appears that natural selection, work-
ing for the good of each being, and taking advantage of all favorable varia-
tions, has produced similar organs, as far as function is concerned, in dis-
tinct organic beings, which owe none of their structure in common to in-
heritance from a common progenitor.



128 THE ORIGIN OF SPECIES

Fritz Miiller, in order to test the conclusions arrived at in this volume, has
followed out with much care a nearly similar line of argument. Several
families of crustaceans include a few species, possessing an air-breathing ap-
paratus and fitted to live out of the water. In two of these families, which
were more especially examined by Miiller, and which are nearly related to
each other, the species agree most closely in all important characters : namely
in their sense organs, circulating systems, in the position of the tufts of hair
within their complex stomachs, and lastly in the whole structure of the water-
breathing branchiae, even to the microscopical hooks by which they are
cleansed. Hence it might have been expected that in the few species belong-
ing to both families which live on the land, the equally important air-breath-
ing apparatus would have been the same ; for why should this one apparatus,
given for the same purpose, have been made to differ, while all the other
important organs were closely similar, or rather, identical.

Fritz Miiller argues that this close similarity in so many points of struc-
ture must, in accordance with the views advanced by me, be accounted for
by inheritance from a common progenitor. But as the vast majority of the
species in the above two families, as well as most other crustaceans, are aquatic
in their habits, it is improbable in the highest degree that their common
progenitor should have been adapted for breathing air. Miiller was thus led
carefully to examine the apparatus in the air-breathing species ; and he found
it to differ in each in several important points, as in the position of the
orifices, in the manner in which they are opened and closed, and in some
accessory details. Now such differences are intelligible, and might even have
been expected, on the supposition that species belonging to distinct families
had slowly become adapted to live more and more out of water, and to
breathe the air. For these species, from belonging to distinct families, would
have differed to a certain extent, and in accordance with the principle that
the nature of each variation depends on two factors; viz., the nature of the
organism and that of the surrounding conditions, their variability assuredly
would not have been exactly the same. Consequently natural selection would
have had different materials or variations to work on, in order to arrive at
the same functional result; and the structures thus acquired would almost
necessarily have differed. On the hypothesis of separate acts of creation the
whole case remains unintelligible. This line of argument seems to have had
great weight in leading Fritz Miiller to accept the views maintained by me
in this volume.

Another distinguished zoologist, the late Professor Claparede, has argued
in the same manner, and has arrived at the same result. He shows that there
are parasitic mites (Acaridae) , belonging to distinct sub-families and families,
which are furnished with hair-claspers. These organs must have been inde-
pendently developed, as they could not have been inherited from a common
progenitor; and in the several groups they are formed by the modification of
the fore legs, of the hind legs, of the maxillae or lips, and of appendages on
the under side of the hind part of the body.



DIFFICULTIES OF THE THEORY 129

In the foregoing cases we see the same end gained and the same function
performed, in beings not at all or only remotely allied, by organs in appear-
ance, though not in development, closely similar. On the other hand, it is a
common rule throughout nature that the same end should be gained, even
sometimes in the case of closely related beings, by the most diversified means.
How differently constructed is the feathered wing of a bird and the 'mem-
brane-covered wing of a bat; and still more so the four wings of a butterfly,
the two wings of a fly, and the two wings with the elytra of a beetle. Bivalve
shells are made to open and shut, but on what a number of patterns is ,the
hinge constructed, from the long row of neatly interlocking teeth in a Nuci jla
to the simple ligament of a Mussel! Seeds are disseminated by their minuce-
ness, by their capsule being converted into a light balloon-like envelope, by
being embedded in pulp or flesh, formed of the most diverse parts, and
rendered nutritious, as well as conspicuously colored, so as to attract and be
devoured by birds, by having hooks and grapnels of many kinds and serrated
awns, so as to adhere to the fur of quadrupeds, and by being furnished with
wings and plumes, as different in shape as they are elegant in structure, so
as to be wafted by every breeze. I will give one other instance : for this sub-
ject of the same end being gained by the most diversified means well deserves
attention. Some authors maintain that organic beings have been formed in
many ways for the sake of mere variety, almost like toys in a shop, but such
a view of nature is incredible. With plants having separated sexes, and with
those in which, though hermaphrodites, the pollen does not spontaneously
fall on the stigma, some aid is necessary for their fertilization. With several
kinds this is effected by the pollen-grains, which are light and incoherent,
being blown by the wind through mere chance on to the stigma; and this is
the simplest plan which can well be conceived. An almost equally simple,
though very diff'erent plan occurs in many plants in which a symmetrical
flower secretes a few drops of nectar, and is consequently visited by insects;
and these carry the pollen from the anthers to the stigma.

From this simple stage we may pass through an inexhaustible number of
contrivances, all for the same purpose and effected in essentially the same
manner, but entailing changes in every part of the flower. The nectar may be
stored in variously shaped receptacles, with the stamens and pistils modified
in many ways, sometimes forming trap-like contrivances, and sometimes
capable of neatly adapted movements through irritability or elasticity. From
such structures we may advance till we come to such a case of extraordinary
adaptation as that lately described by Dr. Criiger in the Coryanthes. This
orchid has part of its labellum or lower lip hollowed out into a great bucket,
into which drops of almost pure water continually fall from two secreting
horns which stand above it; and when the bucket is half -full, the water over-
flows by a spout on one side. The basal part of the labellum stands over the
bucket, and is itself hollowed out into a sort of chamber with two lateral
entrances; within this chamber there are curious fleshy ridges. The most in-
genious man, if he had not witnessed what takes place, could never have
imagined what purpose all these parts serve. But Dr. Criiger saw crowds of



130 THE ORIGIN OF SPECIES

large humble-bees visiting the gigantic flowers of this orchid, not in order to
suck nectar, but to gnaw off the ridges within the chamber above the bucket;
in doin^■ this they frequently pushed each other into the bucket, and their
wings yeing thus wetted they could not fly away, but were compelled to crawl
out through the passage formed by the spout or overflow. Dr. Griiger saw a
"coninual procession" of bees thus crawling out of their involuntary bath.
The passage is narrow, and is roofed over by the column, so that a bee, in
iorcjn^ its way out, first rubs its back against the viscid stigma and then
agKifist the viscid glands of the pollen-masses. The pollen-masses are thus
§ted to the back of the bee which first happens to crawl out through the
pli-sage of a lately expanded flower, and are thus carried away. Dr. Griiger
sent me a flower in spirits of wine, with a bee which he had killed before it
had quite crawled out, with a pollen-mass still fastened to its back. When the
bee, thus provided, flies to another flower, or to the same flower a second
time, and is pushed by its comrades into the bucket and then crawls out by
the passage, the pollen-mass necessarily comes first into contact with the
viscid stigma, and adheres to it, and the flower is fertilized. Now, at last we
see the full use of every part of the flower, of the water-secreting horns of
the bucket half -full of water, which prevents the bees from flying away, and
forces theni to crawl out through the spout, and rub against the properly
placed viscid pollen-masses and the viscid stigma.

The construction of the flower in another closely allied orchid, namely, the
Catasetum, is widely different, though serving the same end; and is equally
curious. Bees visit these flowers, like those of the Goryanthes, in order to
gnaw the labellum; in doing this they inevitably touch a long, tapering, sen-
sitive projection, or, as I have called it, the antenna. This antenna, when
touched, transmits a sensation or vibration to a certain membrane which is
instantly ruptured; this sets free a spring by which the pollen-mass is shot
forth Hke an arrow, in the right direction, and adheres by its viscid extremity
to the back of the bee. The pollen-mass of the male plant (for the sexes are
separate in this orchid) is thus carried to the flower of the female plant, where
it is brought into contact with the stigma, which is viscid enough to break
certain elastic threads, and retain the pollen, thus effecting fertilization.

How, it may be asked, in the foregoing and in innumerable other in-
stances, can we understand the graduated scale of complexity and the multi-
farious means for gaining the same end. The answer, no doubt, is, as already
remarked, that when two forms vary, which already differ from each other
in some slight degree, the variability will not be of the same exact nature, and
consequently the results obtained through natural selection for the same
general purpose will not be the same. We should also bear in mind that every
highly developed organism has passed through many changes; and that each .
modified structure tends to be inherited, so that each modification will not
readily be quite lost, but may be again and again further altered. Hence, the
structure of each part of each species, for whatever purpose it may serve, is
the suni of many inherited changes, through which the species has passed
during its successive adaptations to changed habits and conditions of life.



DIFFICULTIES OF THE THEORY 131

Finally then, although in many cases it is most difficult even to conjecture
by what transitions organs have arrived at their present state ; yet, considering
how small the proportion of living and known forms is to the extinct and
unknown, I have been astonished how rarely an organ can be named, toward
which no transitional grade is known to lead. It certainly is true, that new
organs appearing as if created for some special purpose rarely or never ap-
pear in any being; as indeed is shown by that old, but somewhat exaggerated,
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 there be so much variety
and so little real novelty? Why should all the parts and organs of many in-
dependent beings, each supposed to have been separately created for its own
proper place in nature, be so commonly linked together by graduated steps?
Why should not Nature take a sudden leap from structure to structure? On
the theory of natural selection, we can clearly understand why she should
not; for natural selection acts only by taking advantage of slight successive
variations; she can never take a great and sudden leap, but must advance
by short and sure, though slow steps.

ORGANS OF LITTLE APPARENT IMPORTANCE, AS AFFECTED BY
NATURAL SELECTION

As natural selection acts by life and death, by the survival of the fittest,
and by the destruction of the less well-fitted individuals, I have sometimes
felt great difficulty in understanding the origin or formation of parts of little
importance; almost as great, though of a very different kind, as in the case
of the most perfect and complex organs.

In the first place, we are much too ignorant in regard to the whole econ-
omy 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 very triffing
characters, such as the down on fruit and the color of its flesh, the color of
the skin and hair of quadrupeds, which, from being correlated with consti-
tutional differences, or from determining the attacks of insects might as-
suredly 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 modifi-
cations, each better and better fitted, for so trifling an object as to drive
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 depend 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 ffies, but they are incessantly
harassed and their strength reduced, so that they are more subject to disease.



132 THE ORIGIN OF SPECIES

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 per-
fected at a former period, have been transmitted to existing species in nearly
the same state, although now of very slight use; but any actually injurious
deviations in their structure would of course 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 in the case of the dog, though the aid in this latter
respect must be slight, for the hare, with hardly any tail, can double still
more quickly.

In the second place, we may easily err in attributing importance to char-
acters, and in believing that they have been developed through natural
selection. We must by no means overlook the effects of the definite action
of changed conditions of life, of so-called spontaneous variations, which
seem to depend in a quite subordinate degree on the nature of the condi-
tions, of the tendency to reversion to long-lost characters, of the complex
laws of growth, such as of correlation, comprehension, of the pressure of
one part on another, etc., and finally of sexual selection, by which characters
of use to one sex are often gained and then transmitted more or less per-
fectly to the other sex, though of no use to the sex. But structures thus
indirectly gained, although at first of no advantage to a species, may sub-
sequently have been taken advantage of by its modified descendants, under
new conditions of life and newly acquired habits.

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 color was a beautiful adaptation to conceal this tree-
frequenting bird from its enemies; and consequently that it was a character
of importance and had been acquired through natural selection; as it is,
the color is probably in chief part due to sexual selection. A trailing palm
in the Malay Archipelago climbs the loftiest trees by the aid of exquisitely
constructed hooks clustered around the ends of the branches, and this con-
trivance, no doubt, is of the highest service to the plant; but as we see
nearly similar hooks on many trees which are not climbers, and which, as
there is reason to believe from the distribution of the thorn-bearing species
in Africa and South America, serve as a defence against browsing quad-
rupeds, so the spikes on the palm may at first have been developed for this
object, and subsequently have been improved and taken advantage of by
the plant as it underwent further modification and became a climber. The
naked skin on the head of a vulture is generally considered as a direct
adaptation for wallowing in putridity; and so it may be, or it may possibly



DIFFICULTIES OF THE THEORY 133

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 cause of each slight variation or
individual difference; and we are immediately made conscious of this by
reflecting on the differences between the breeds of our domesticated animals
in different countries, more especially in the less civilized countries, where
there has been but little methodical selection. Animals kept by savages in
different countries often have to struggle for their own subsistence, and are
exposed to a certain extent to natural selection, and individuals with slightly
different constitutions would succeed best under different climates. With
cattle susceptibility to the attacks of flies is correlated with color, as is the
liability to be poisoned by certain plants; so that even color would be thus
subjected to the action of natural selection. Some 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 the head
would probably be affected. The shape, also, of the pelvis might affect by
pressure the shape of certain parts of the young in the womb. The laborious
breathing necessary in high regions tends, as we have good reason to believe,
to increase the size of the chest; and again correlation would come into
play. The effects of lessened exercise, together with abundant food, on the
whole organization is probably still more important; and this, as H. von
Nathusius has lately shown in his excellent Treatise, is apparently one chief
cause of the great modification which the breeds of swine have undergone.
But we are far too ignorant to speculate on the relative importance of the
several known and unknown causes of variation; and I have made these
remarks only to show that, if we are unable to account for the characteristic
differences of our several domestic breeds, which nevertheless are generally
admitted to have arisen through ordinary generation from one or a few
parent-stocks, we ought not to lay too much stress on our ignorance of the
precise cause of the slight analogous differences between true species.

UTILITARIAN DOCTRINE, HOW FAR TRUE: BEAUTY, HOW ACQUIRED

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



134 THE ORIGIN OF SPECIES

that many structures have been created for the sake of beauty, to delight
man or the Creator (but this latter point is beyond the scope of scientific
discussion) , or for the sake of mere variety, a view already discussed. Such
doctrines, if true, would be absolutely fatal to my theory. I fully admit that
many structures are now of no direct use to their possessors, and many
never have been of any use to their progenitors; but this does not prove
that they were formed solely for beauty or variety. No doubt the definite
action of changed conditions, and the various causes of modifications, lately
specified, have all produced an effect, probably a great effect, independ-
ently of any advantage thus gained. But a still more important considera-
tion is that the chief part of the organization of every living creature is due
to inheritance; and consequently, though each being assuredly is well fitted
for its place in nature, many structures have now no very close and direct
relation to present habits of life. 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 similar 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 webbed feet no doubt were as
useful to the progenitor of the upland goose and of the frigate-bird, as
they now are to the most aquatic of living birds. So we may believe that
the progenitor of the seal did not possess 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, were originally
developed, on the principle of utility, probably through the reduction of
more numerous bones in the fin of some ancient fish-like progenitor of the
whole class. It is scarcely possible to decide how much allowance ought to
be made for such causes of change, as the definite action of external condi-
tions, so-called spontaneous variations, and the complex laws of growth;
but with these important exceptions, we may conclude that the structure
of every living creature either now is, or was formerly, of some direct or
indirect use to its possessor.

With respect to the belief that organic beings have been created beauti-
ful for the delight of man — a belief which it has been pronounced is sub-
versive of my whole theory — I may first remark that the sense of beauty
obviously depends on the nature of the mind, irrespective of any real
quality in the admired object; and that the idea of what is beautiful, is not
innate or unalterable. We see this, for instance, in the men of different
races admiring an entirely different standard of beauty in their women. If
beautiful objects had been created solely for man's gratification, it ought to



Online LibraryCharles DarwinThe origin of species → online text (page 18 of 50)