M Database Inspector (cheetah)
Not logged in. Login


105 rows, page 7 of 27 (4/p)
1 2 3 4 5 6 7 8 9 10 11 12 13 20 27

Export to Excel select * from OriginOfSpecies where subject = '04 - Natural Selection' order by description limit 24, 4 (Page 7: Row)
subject
title
ordinal
description Desending Order (top row is first)
04 - Natural Selection 04-09 - Circumstances favourable for the production of new forms through Natural Selection 40 Even with animals which unite for each birth, and which do not propagate rapidly, we must not assume that free intercrossing would always eliminate the effects of natural selection; for I can bring forward a considerable body of facts showing that within the same area, two varieties of the same animal may long remain distinct, from haunting different stations, from breeding at slightly different seasons, or from the individuals of each variety preferring to pair together.
Full Size
04 - Natural Selection 04-12 - On the Degree to which Organisation tends to advance 100 Finally, I believe that many lowly organised forms now exist throughout the world, from various causes. In some cases variations or individual differences of a favourable nature may never have arisen for natural selection to act on and accumulate. In no case, probably, has time sufficed for the utmost possible amount of development. In some few cases there has been what we must call retrogression of organisation.

But the main cause lies in the fact that under very simple conditions of life a high organisation would be of no service,- possibly would be of actual disservice, as being of a more delicate nature, and more liable to be put out of order and injured.

Looking to the first dawn of life, when all organic beings, as we may believe, presented the simplest structure, how, it has been asked, could the first steps in the advancement or differentiation of parts have arisen?

Mr. Herbert Spencer would probably answer that, as soon as simple unicellular organism came by growth or division to be compounded of several cells, or became attached to any supporting surface, his law "that homologous units of any order become differentiated in proportion as their relations to incident forces" would come into action. But as we have no facts to guide us, speculation on the subject is almost useless.
04 - Natural Selection 04-10 - Extinction caused by Natural Selection 30 From these several considerations I think it inevitably follows, that as new species in the course of time are formed through natural selection, others will become rarer and rarer, and finally extinct.

The forms which stand in closest competition with those undergoing modification and improvement will naturally suffer most.

And we have seen in the chapter on the Struggle for Existence that it is the most closely-allied forms,- varieties of the same species, and species of the same genus or of related genera,- which, from having nearly the same structure, constitution, and habits, generally come into the severest competition with each other; consequently, each new variety or species, during the progress of its formation, will generally press hardest on its nearest kindred, and tend to exterminate them.

We see the same process of extermination amongst our domesticated productions, through the selection of improved forms by man.

Many curious instances could be given showing how quickly new breeds of cattle, sheep, and other animals, and varieties of flowers, take the place of older and inferior kinds.

cattle
cattle

sheep
sheep


In Yorkshire, it is historically known that the ancient black cattle were displaced by the long-horns, and that these "were swept away by the shorthorns" (I quote the words of an agricultural writer) "as if by some murderous pestilence."
04 - Natural Selection 04-09 - Circumstances favourable for the production of new forms through Natural Selection 100 Hence, we can understand how it is that the flora of Madeira, according to Oswald Heer, resembles to a certain extent the extinct tertiary flora of Europe. All fresh-water basins, taken together, make a small area compared with that of the sea or
of the land.

Madeira
Madeira

Europe
Europe


Consequently, the competition between fresh-water productions will have been less severe than elsewhere; new forms will have been then more slowly produced, and old forms more slowly exterminated.

And it is in fresh-water basins that we find seven genera of Ganoid fishes, remnants of a once preponderant order: and in fresh water we find some of the most anomalous forms now known in the world as the Ornithorhynchus and Lepidosiren which, like fossils, connect to a certain extent orders at present widely sundered in the natural scale.

Ornithorhynchus
Ornithorhynchus

lepidosiren
lepidosiren


These anomalous forms may be called living fossils; they have endured to the present day, from having inhabited a confined area, and from having been exposed to less varied, and therefore less severe, competition.