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|04 - Natural Selection||04-09 - Circumstances favourable for the production of new forms through Natural Selection||120||
That natural selection generally acts with extreme slowness I fully admit. It can act only when there are places in the natural polity of a district which can be better occupied by the modification of some of its existing inhabitants.
The occurrence of such places will often depend on physical changes, which generally take place very slowly, and on the immigration of better adapted forms being prevented.
As some few of the old inhabitants become modified, the mutual relations of others will often be disturbed; and this will create new places, ready to be filled up by better adapted forms, but all this will take place very slowly.
Although the individuals of the same species differ in some slight degree from each other, it would often be long before differences of the right nature in various parts of the organisation might occur.
The result would often be greatly retarded by free intercrossing.
Many will exclaim that these several causes are amply sufficient to neutralise the power of natural selection.
I do not believe so.
But I do believe that natural selection will generally act very slowly, only at long intervals of time, and only on a few of the inhabitants of the same region.
I further believe that these slow, intermittent results accord well with what geology tells us of the rate and manner at
which the inhabitants of the world have changed.
Slow though the process of selection may be, if feeble man can do much by artificial selection, I can see no limit to the amount of change, to the beauty and complexity of the coadaptations between all organic beings, one with another and with their physical conditions of life, which may have been effected in the long course of time through nature's power of selection, that is by the survival of the fittest.
|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.
|04 - Natural Selection||04-10 - Extinction caused by Natural Selection||20||
We have seen that the species which are most numerous in individuals have the best chance of producing favourable variations within any given period.
We have evidence of this, in the facts stated in the second chapter showing that it is the common and diffused or dominant species which offer the greatest number of recorded varieties.
Hence, rare species will be less quickly modified or improved within any given period; they will consequently be beaten in the race for life by the modified and improved descendants of the commoner species.
|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.
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."