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|14 - Recapitulation and Conclusion||14-02 - Recapitulation of the general and special circumstances in its favour||10||
Now let us turn to the other side of the argument.
Under domestication we see much variability.
This seems to be mainly due to the reproductive system being eminently susceptible to changes in the conditions of life so that this system, when not rendered impotent, fails to reproduce offspring exactly like the parent-form.
Variability is governed by many complex laws, -- by correlation of growth, by use and disuse, and by the direct action of the physical conditions of life.
There is much difficulty in ascertaining how much modification our domestic productions have undergone; but we may safely infer that the amount has been large, and that modifications can be inherited for long periods.
As long as the conditions of life remain the same, we have reason to believe that a modification, which has already been inherited for many generations, may continue to be inherited for an almost infinite number of generations.
On the other hand we have evidence that variability, when it has once come into play, does not wholly cease; for new varieties are still occasionally produced by our most anciently domesticated productions.
Man does not actually produce variability; he only unintentionally exposes organic beings to new conditions of life, and then nature acts on the organisation, and causes variability.
But man can and does select the variations given to him by nature, and thus accumulate them in any desired manner.
He thus adapts animals and plants for his own benefit or pleasure.
He may do this methodically, or he may do it unconsciously by preserving the individuals most useful to him at the time, without any thought of altering the breed.
It is certain that he can largely influence the character of a breed by selecting, in each successive generation, individual differences so slight as to be quite inappreciable by an uneducated eye.
This process of selection has been the great agency in the production of the most distinct and useful domestic breeds.
That many of the breeds produced by man have to a large extent the character of natural species, is shown by the inextricable doubts whether very many of them are varieties or aboriginal species.
|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.
|14 - Recapitulation and Conclusion||14-02 - Recapitulation of the general and special circumstances in its favour||30||
As geology plainly proclaims that each land has undergone great physical changes, we might have expected that organic beings would have varied under nature, in the same way as they generally have varied under the changed conditions of domestication.
And if there be any variability under nature, it would be an unaccountable fact if natural selection had not come into play.
It has often been asserted, but the assertion is quite incapable of proof, that the amount of variation under nature is a strictly limited quantity.
Man, though acting on external characters alone and often capriciously, can produce within a short period a great result by adding up mere individual differences in his domestic productions; and every one admits that there are at least individual differences in species under nature.
But, besides such differences, all naturalists have admitted the existence of varieties, which they think sufficiently distinct to be worthy of record in systematic works.
No one can draw any clear distinction between individual differences and slight varieties; or between more plainly marked varieties and subspecies, and species.
Let it be observed how naturalists differ in the rank which they assign to the many representative forms in Europe and North America.
|14 - Recapitulation and Conclusion||14-02 - Recapitulation of the general and special circumstances in its favour||40||
If then we have under nature variability and a powerful agent always ready to act and select, why should we doubt that variations in any way useful to beings, under their excessively complex relations of life, would be preserved, accumulated, and inherited?
Why, if man can by patience select variations most useful to himself, should nature fail in selecting variations useful, under changing conditions of life, to her living products?
What limit can be put to this power, acting during long ages and rigidly scrutinising the whole constitution, structure, and habits of each creature, -- favouring the good and rejecting the bad?
I can see no limit to this power, in slowly and beautifully adapting each form to the most complex relations of life.
The theory of natural selection, even if we looked no further than this, seems to me to be in itself probable.