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|OriginOfSpecies - 475 Rows|
|13 - Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Or||13-03 - Rules and difficulties in classification, explained on the theory of descent with modification||20||
These resemblances, though so intimately connected with the whole life of the being, are ranked as merely `adaptive or analogical characters;' but to the consideration of these resemblances we shall have to recur.
It may even be given as a general rule, that the less any part of the organisation is concerned with special habits, the more important it becomes for classification.
As an instance: Owen, in speaking of the dugong, says, `The generative organs being those which are most remotely related to the habits and food of an animal, I have always regarded as affording very clear indications of its true affinities.
We are least likely in the modifications of these organs to mistake a merely adaptive for an essential character.' So with plants, how remarkable it is that the organs of vegetation, on which their whole life depends, are of little signification, excepting in the first main divisions; whereas the organs of reproduction, with their product the seed, are of paramount importance!
We must not, therefore, in classifying, trust to resemblances in parts of the organisation, however important they may be for the welfare of the being in relation to the outer world.
|06 - Difficutiles in Theory||06-09 - Cases of Difficulty||20||
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.
|14 - Recapitulation and Conclusion||14-02 - Recapitulation of the general and special circumstances in its favour||20||
There is no obvious reason why the principles which have acted so efficiently under domestication should not have acted under nature.
In the preservation of favoured individuals and races, during the constantly-recurrent Struggle for Existence, we see the most powerful and ever-acting means of selection.
The struggle for existence inevitably follows from the high geometrical ratio of increase which is common to all organic beings.
This high rate of increase is proved by calculation, by the effects of a succession of peculiar seasons, and by the results of naturalisation, as explained in the third chapter.
More individuals are born than can possibly survive.
A grain in the balance will determine which individual shall live and which shall die, -- which variety or species shall increase in number, and which shall decrease, or finally become extinct.
As the individuals of the same species come in all respects into the closest competition with each other, the struggle will generally be most severe between them; it will be almost equally severe between the varieties of the same species, and next in severity between the species of the same genus.
But the struggle will often be very severe between beings most remote in the scale of nature.
The slightest advantage in one being, at any age or during any season, over those with which it comes into competition, or better adaptation in however slight a degree to the surrounding physical conditions, will turn the balance.
With animals having separated sexes there will in most cases be a struggle between the males for possession of the females.
The most vigorous individuals, or those which have most successfully struggled with their conditions of life, will generally leave most progeny.
But success will often depend on having special weapons or means of defence, or on the charms of the males; and the slightest advantage will lead to victory.
|03 - Struggle for Existence||03-03 - Geometrical Ratio of Increase||20||
There is no exception to the rule that every organic being naturally increases at so high a rate, that, if not destroyed, the earth would soon be covered by the progeny of a single pair.
Even slow-breeding man has doubled in twenty-five years, and at this rate, in less than a thousand years, there would literally not be standing-room for his progeny.
Linnaeus has calculated that if an annual plant produced only two seeds- and there is no plant so unproductive as this- and their seedlings next year produced two, and so on, then in twenty years there should be a million plants.
The elephant is reckoned the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase; it will be safest to assume that it begins breeding when thirty years old, and goes on breeding till ninety years old, bringing forth six young in the interval, and surviving till one hundred years old; if this be so, after a period of from 740 to 750 years there would be nearly nineteen million elephants alive, descended from the first pair.