M Database Inspector (cheetah)
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|OriginOfSpecies - 475 Rows|
|14 - Recapitulation and Conclusion||14-03 - Causes of the general belief in the immutability of species||20||
Glancing at instincts, marvellous as some are, they offer no greater difficulty than does corporeal structure on the theory of the natural selection of successive, slight, but profitable modifications.
We can thus understand why nature moves by graduated steps in endowing different animals of the same class with their several instincts.
I have attempted to show how much light the principle of gradation throws on the admirable architectural powers of the hive-bee.
Habit no doubt sometimes comes into play in modifying instincts; but it certainly is not indispensable, as we see, in the case of neuter insects, which leave no progeny to inherit the effects of long-continued habit.
|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.