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Export to Excel select * from OriginOfSpecies where subject = '04 - Natural Selection' order by description desc limit 20, 4 (Page 6: Row)
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description Desending Order (top row is last)
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 10 This is an extremely intricate subject.

A great amount of variability, under which term individual differences are always included, will evidently be favourable.

A large number of individuals, by giving a better chance within any given period for the appearance of profitable variations, will compensate for a lesser amount of variability in each individual, and is, I believe, a highly important element of success.

Though Nature grants long periods of time for the work of natural selection, she does not grant an indefinite period; for as all organic beings are striving to seize on each place in the economy of nature, if any one species does not become modified and improved in a corresponding degree with its competitors, it will be exterminated.

Cheetah
Cheetah
04 - Natural Selection 04-11 - Divergence of Character 50 The truth of the principle that the greatest amount of life can be supported by great diversification of structure, is seen under many natural circumstances.

In an extremely small area, especially if freely open to immigration, and where the contest between individual and individual must be very severe, we always find great diversity in its inhabitants.

For instance, I found that a piece of turf, three feet by four in size, which had been exposed for many years to exactly the same conditions, supported twenty species of plants, and these belonged to eighteen genera and to eight orders, which shows how much these plants differed from each other.

So it is with the plants and insects on small and uniform islets: also in small ponds of fresh water.

Farmers find that they can raise most food by a rotation of plants belonging to the most different orders: nature follows what may be called a simultaneous rotation.

Most of the animals and plants which live close round any small piece of ground, could live on it (supposing its nature not to be in any way peculiar), and may be said to be striving to the utmost to live there; but, it is seen, that where they come into the closest competition, the advantages of diversification of structure, with the accompanying differences of habit and constitution, determine that the inhabitants, which thus jostle each other most closely, shall, as a general rule, belong to what we call different genera and orders.
04 - Natural Selection 04-12 - On the Degree to which Organisation tends to advance 90 The three lowest orders of mammals, namely, marsupials, edentata, and rodents, co-exist in South America in the same region with numerous monkeys, and probably interfere little with each other.

kangaroo
kangaroo

armadillo
armadillo

rat
rat

monkey
monkey

South America
South America


Although organisation, on the whole, may have advanced and be still advancing throughout the world, yet the scale will always present many degrees of perfection; for the high advancement of certain whole classes, or of certain members of each class, does not at all necessarily lead to the extinction of those groups with which they do not enter into close competition.

In some cases, as we shall hereafter see, lowly organised forms appear to have been preserved to the present day, from inhabiting confined or peculiar stations, where they have been subjected to less severe competition, and where their scanty numbers have retarded the chance of favourable variations arising.