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|OriginOfSpecies - 475 Rows|
|06 - Difficutiles in Theory||06-09 - Cases of Difficulty||10||
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.
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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.
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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
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
Another distinguished zoologist, the late Professor Claparide,
has argued in the same manner, and has arrived at the same
He shows that there are parasitic mites (Acaridae), belonging
to distinct sub-families and families, which are furnished with
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.
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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.