CHAPTER VI - DIFFICULTIES OF THE THEORY
Special Difficulties of the Theory Of Natural Selection
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
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 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 electrical discharge; and, as Dr.
Radcliffe insists, "in the electrical apparatus of the torpedo
during rest, there would seem be a charge in every respect like that
which is met with in muscle and nerve during rest, and the discharge
of the torpedo, instead of being peculiar, may be only another form of
the discharge which depends 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 possible by which these organs
might have been gradually developed.
These organs appear at first to offer another and far more serious
difficulty; 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 modified 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 considered as
homologous, but only as analogous in function. Consequently there is
no reason to suppose that they have been inherited from a common
progenitor; 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 Asclepias,-
genera almost as remote as is possible amongst flowering plants; but
here again the parts are not homologous. In all cases of beings, far
removed from each other in the scale of organisation, 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 between 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 admirable
memoir on these organs in the Cephalopoda. It is impossible for me
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
invention, so in the several foregoing cases it appears that natural
selection, working for the good of each being, and taking advantage of
all favourable variations, has produced similar organs, as far as
function is concerned, in distinct organic beings, which owe none of
their structure in common to inheritance from a common progenitor.
Fritz Muller, 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 apparatus and fitted to live out of the
water. In two of these families, which were more especially examined
by Muller and which are nearly related to each other, the species
agree most closely in all important characters; namely, in their sense
organs, circulating system, 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 belonging to both families which live on the land, the
equally important air-breathing apparatus would have been the same;
for why should this one apparatus, given for the same purpose, have
been made to differ, whilst all the other important organs were
closely similar or rather identical?
Fritz Muller argues that this close similarity in so many points
of structure 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 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 Muller to accept the views maintained by me in this
volume.
Another distinguished zoologist, the late Professor Claparide, 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 independently 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.
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 appearance, 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 membrane-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 Nucula to the simple ligament of a Mussel! Seeds are disseminated
by their minuteness,- 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 coloured, so as to attract and be devoured by birds,- by
having hooks and grapnels of many kinds and serrated arms, 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 subject 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 fertilisation. 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 different, 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. Cruger 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 overflows 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 ingenious man, if he
had not witnessed what takes place, could never have imagined what
purpose all these parts serve. But Dr. Cruger saw crowds of 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 doing this they frequently pushed each other into the
bucket, and their wings being thus wetted they could not fly away, but
were compelled to crawl out through the passage formed by the spout or
overflow. Dr. Cruger saw a "continual 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 forcing its way out,
first rubs its back against the viscid stigma and then against the
viscid glands of the pollen-masses. The pollen-masses are thus glued
to the back of the be which first happens to crawl out through the
passage of a lately expanded flower, and are thus carried away. Dr.
Cruger 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 fertilised. 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 them 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 Coryanthes, in order to gnaw the labellum; in doing this they
inevitably touch a long, tapering, sensitive 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, like 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 retaining the pollen, fertilisation is effected.
How, it may be asked, in the foregoing and in innumerable other
instances, can we understand the graduated scale of complexity and the
multifarious 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 sum of many inherited changes, through which the species
has passed during its successive adaptations to changed habits and
conditions of life.
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, towards 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 appear 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 independent beings, each supposed to have
been separately created for its 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.
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