M Database Inspector (cheetah)
|Not logged in. Login|
|OriginOfSpecies - 475 Rows|
|04 - Natural Selection||04-08 - On the Intercrossing of Individuals||30||
On the belief that this is a law of nature, we can, I think, understand several large classes of facts, such as the following, which on any other view are inexplicable.
Every hybridizer knows how unfavourable exposure to wet is to the fertilisation of a flower, yet what a multitude of flowers have their anthers and stigmas fully exposed to the weather!
If an occasional cross be indispensable, notwithstanding that the plant's own anthers and pistil stand so near each other as almost to insure self-fertilisation, the fullest freedom for the entrance of pollen from another individual will explain the above state of exposure of the organs.
Many flowers, on the other hand, have their organs of fructification closely enclosed, as in the great papilionaceous or pea-family; but these almost invariably present beautiful and curious adaptations in relation to the visits of insects.
So necessary are the visits of bees to many papilionaceous flowers, that their fertility is greatly diminished if these visits be prevented.
Now, it is scarcely possible for insects to fly from flower and flower, and not to carry pollen from one to the other, to the great good of the plant.
Insects act like a camel-hair pencil, and it is sufficient to ensure fertilisation, just to touch with the same brush the anthers of one flower and then the stigma of another; but it must not be supposed that bees would thus produce a multitude of hybrids between distinct species; for if a plant's own pollen and that from another species are placed on the same stigma, the former is so prepotent that it invariably and completely destroys, as has been shown by
Gartner, the influence of the foreign pollen.
|05 - Laws of Variation||05-10 - Secondary Sexual Characters Variable||30||
Finally, then, I conclude that the greater variability of specific characters, or those which distinguish species from species, than of generic characters, or those which are possessed by all the species;- that the frequent extreme variability of an part which is developed in a species in an extraordinary manner in comparison with the same part in its congeners; and the slight degree of variability in a part, however extraordinarily it may be developed, if it be common to a whole group of species;- that the great variability of secondary sexual characters, and their great difference in closely allied species;- that secondary sexual and ordinary specific differences are generally displayed in the same parts of the organisation,- are all principles closely connected together.
All being mainly due to the species of the same group being the descendants of common progenitor, from whom they have inherited much in common,- to parts which have recently and largely varied being more likely still to go on varying than parts which have long been inherited and have not varied,- to natural selection having more or less completely, according to the lapse of time, overmastered the tendency to reversion and to further variability,- to sexual selection being less rigid than ordinary selection,- and to variations in the same parts having been accumulated by natural and sexual selection, and having been thus adapted for secondary sexual, and for ordinary purposes.
|03 - Struggle for Existence||03-11 - The Relation of Organism to Organism the Most Important of All Relations||30||
Look at a plant in the midst of its range, why does it not double or quadruple its numbers?
We know that it can perfectly well withstand a little more heat or cold, dampness or dryness, for elsewhere it ranges into slightly hotter or colder, damper or drier districts. In this case we can clearly see that if we wish in imagination to give the plant the power of increasing in number, we should have to give it some advantage over its competitors, or over the animals which prey on it.
On the confines of its geographical range, a change of constitution with respect to climate would clearly be an advantage to our plant; but we have reason to believe that only a few plants or animals range so far, that they are destroyed exclusively by the rigour of the climate.
Not until we reach the extreme confines of life, in the Arctic regions or on the borders of an utter desert, will competition cease.
The land may be extremely cold or dry, yet there will be competition between some few species, or between the individuals of the same species, for the warmest or dampest spots.
|04 - Natural Selection||04-01 - Natural Selection||30||
Several writers have misapprehended or objected to the term Natural Selection.
Some have even imagined that natural selection induces variability, whereas it implies only the preservation of such variations as arise and are beneficial to the being under its conditions of life.
No one objects to agriculturists speaking of the potent effects of man's selection; and in this case the individual differences given by nature, which man for some object selects, must of necessity first occur.
Others have objected that the term selection implies conscious choice in the animals which become modified; and it has even been urged that, as plants have no volition, natural selection is not applicable to them!
In the literal sense of the word, no doubt, natural selection is a false term; but who ever objected to chemists speaking of the elective affinities of the various elements?- and yet an acid cannot strictly be said to elect the base with which it in preference combines.
It has been said that I speak of natural selection as an active power or Deity; but who objects to an author speaking of the attraction of gravity as ruling the movements of the planets?
Every one knows what is meant and is implied by such metaphorical expressions; and they are almost necessary for brevity.
So again it is difficult to avoid personifying the word Nature; but I mean by Nature, only the aggregate action and product of many natural laws, and by laws the sequence of events as ascertained by us. With a little familiarity such superficial objections will be forgotten.