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Export to Excel select * from OriginOfSpecies where subject = '04 - Natural Selection' order by description limit 84, 4 (Page 22: Row)
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04 - Natural Selection 04-10 - Extinction caused by Natural Selection 10 This subject will he more fully discussed in our chapter on Geology; but it must here be alluded to from being intimately connected with natural selection.

Natural selection acts solely through the preservation of variations in some way advantageous, which consequently endure.

Owing to the high geometrical rate of increase of all organic beings, each area is already fully stocked with inhabitants; and it follows from this, that as the favoured forms increase in number, so, generally, will the less favoured decrease and become rare.

Rarity, as geology tells us, is the precursor to extinction.

We can see that any form which is represented by few individuals will run a good chance of utter extinction, during great fluctuations in the nature of the seasons, or from a temporary increase in the number of its enemies.

But we may go further than this; for, as new forms are produced, unless we admit that specific forms can go on indefinitely increasing in number, many old forms must become extinct.

That the number of specific forms has not indefinitely increased, geology plainly tells us; and we shall presently attempt to show why it is that the number of species throughout the world has not become immeasurably great.

Dodo
Dodo
04 - Natural Selection 04-09 - Circumstances favourable for the production of new forms through Natural Selection 90 Throughout a great and open area, not only will there be a better chance of favourable variations, arising from the large number of individuals of the same species there supported, but the conditions of life are much more complex from the large number of already existing species; and if some of these many species become modified and improved, others will have to be improved in a corresponding degree, or they will be exterminated.

Each new form, also, as soon as it has been much improved, will be able to spread over the open and continuous area, and will thus come into competition with many other forms.

Moreover, great areas, though now continuous, will often, owing to former oscillations of level, have existed in a broken condition; so that the good effects of isolation will generally, to a certain extent, have concurred.

Finally, I conclude that, although small isolated areas have been in some respects highly favourable for the production of new species, yet that the course of modification will generally have been more rapid on large areas; and what is more important, that the new forms produced on large areas, which already have been victorious over many competitors, will be those that will spread most widely, and will give rise to the greatest number of new varieties and species.

They will thus play a more important part in the changing history of the organic world. In accordance with this view, we can, perhaps, understand some facts which will be again alluded to in our chapter on Geographical Distribution; for instance, the fact of the productions of the smaller continent of Australia now yielding before those of the larger Europaeo-Asiatic area.

Australia
Australia

Europe
Europe

Asia
Asia


Thus, also, it is that continental productions have everywherebecome so largely naturalised on islands.

On a small island, the race for life will have been less severe, and there will have been less modification and less extermination.
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04 - Natural Selection 04-11 - Divergence of Character 220 Thus it is, as I believe, that two or more genera are produced by descent with modification, from two or more species of the same genus.

And the two or more parent-species are supposed to be descended from some one species of an earlier genus.

In our diagram, this is indicated by the broken lines, beneath the capital letters, converging in sub-branches downwards towards a single point; this point represents a species, the supposed progenitor of our several new sub-genera and genera.
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04 - Natural Selection 04-05 - Sexual Selection 30 Thus it is, as I believe, that when the males and females of any animal have the same general habits of life, but differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection: that is, by individual males having had, in successive generations, some slight advantage over other males, in their weapons, means of defence, or charms, which they have transmitted to their male offspring alone.

Yet, I would not wish to attribute all sexual differences to this agency: for we see in our domestic animals peculiarities arising and becoming attached to the male sex, which apparently have not been augmented through selection by man.

The tuft of hair on the breast of the wild turkey-cock cannot be of any use, and it is doubtful whether it can be ornamental in the eyes of the female bird; indeed, had the tuft appeared under domestication, it would have been called a monstrosity.

Turkey Cock
Turkey Cock