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|OriginOfSpecies - 475 Rows|
|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.
|01 - Variations Under Domestication||01-13 - Summary||20||
With plants which are temporarily propagated by cuttings, buds, &c., the importance of crossing is immense; for the cultivator may here disregard the extreme variability both of hybrids and of mongrels, and the sterility of hybrids; but plants not propagated by seed are of little importance to us, for their endurance is only temporary.
Over all these causes of Change, the accumulative action of Selection, whether applied methodically and quickly, or unconsciously and slowly but more efficiently, seems to have been the predominant Power.
|05 - Laws of Variation||05-06 - False Correlation||20||
Some other correlations are apparently due to the manner in which natural selection can alone act.
For instance, Alph. de Candolle has remarked that winged seeds are never found in fruits which do not open; I should explain this rule by the impossibility of seeds gradually becoming winged through natural selection, unless the capsules were open; for in this case alone could the seeds, which were a little better adapted to be wafted by the wind, gain an advantage over others less well fitted for wide dispersal.
|05 - Laws of Variation||05-13 - Summary||20||
In these remarks we have referred to special parts or organs being still variable, because they have recently varied and thus come to differ; but we have also seen in the second chapter that the same principle applies to the whole individual; for in a district where many species of a genus are found- that is, where there has been much former variation and differentiation, or where the manufactory of new specific forms has been actively at work- in that district and amongst these species, we now find, on an average, most varieties.
Secondary sexual characters are highly variable, and such characters differ much in the species of the same group.
Variability in the same parts of the organisation has generally been taken advantage of in giving secondary sexual differences to the two sexes of the same species, and specific differences to the several species of the same genus.
Any part or organ developed to an extraordinary size or in an extraordinary manner, in comparison with the same part or organ in the allied species, must have gone through an extraordinary amount of modification since the genus arose; and thus we can understand why it should often still be variable in a much higher degree than other parts; for variation is a long-continued and slow process, and natural selection will in such cases not as yet have had time to overcome the tendency to further variability and to reversion to a less modified state.
But when a species with any extraordinarily-developed organ has become the parent of many modified descendants- which on our view must be a very slow process, requiring long lapse of time- in this case, natural selection has succeeded in giving a fixed character to the organ, in however extraordinary a manner it may have been developed.
Species inheriting nearly the same constitution from a common parent, and exposed to similar influences, naturally tend to present analogous variations, or these same species may occasionally revert to some of the characters of their ancient progenitors.
Although new and important modifications may not arise from reversion and analogous variation, such modifications will add to the beautiful and harmonious diversity of nature.
Whatever the cause may be of each slight difference between the offspring and their parents- and a cause for each must exist- we have reason to believe that it is the steady accumulation of beneficial differences which has given rise to all the more important modifications of structure in relation to the habits of each species.