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|04 - Natural Selection||04-04 - Its Power at All Ages and on Both Sexes||20||
Natural selection will modify the structure of the young in relation to the parent, and of the parent in relation to the young.
In social animals it will adapt the structure of each individual for the benefit of the whole community, if the community profits by the selected change.
What natural selection cannot do, is to modify the structure of one species, without giving it any advantage, for the good of another species; and though statements to this effect may be found in works of natural history, I cannot find one case which will bear investigation.
A structure used only once in an animal's life, if of high importance to it, might be modified to any extent by natural selection; for instance, the great jaws possessed by certain insects, used exclusively for opening the cocoon- or the hard tip to the beak of unhatched birds, used for breaking the egg.
It has been asserted, that of the best short-beaked tumbler-pigeons a greater number perish in the egg than are able to get out of it; so that fanciers assist in the act of hatching.
Now if nature had to make the beak of a full-grown pigeon very short for the bird's own advantage, the process of modification would be very slow, and there would be simultaneously the most rigorous selection of all the young birds within the egg, which had the most powerful and hardest beaks, for all with weak beaks would inevitably perish; or, more delicate and more easily broken shells might be selected, the thickness of the shell being known to vary like every other structure.
It may be well here to remark that with all beings there must be much fortuitous destruction, which can have little or no influence on the course of natural selection.
For instance a vast number of eggs or seeds are annually devoured, and these could be modified through natural selection only if they varied in some manner which protected them from their enemies.
Yet many of these eggs or seeds would perhaps, if not destroyed, have yielded individuals better adapted to their conditions of life than any of these which happened to survive.
So again a vast number of mature animals and plants, whether or not they be the best adapted to their conditions, must be annually destroyed by accidental causes, which would not be in the least degree mitigated by certain changes of structure or constitution which would in other ways be beneficial to the species.
But let the destruction of the adults be ever so heavy, if the number which can exist in any district be not wholly kept down by such causes,- or again let the destruction of eggs or seeds be so great that only a hundredth or a thousandth part are developed,- yet of those which do survive, the best adapted individuals, supposing that there is any variability in favourable direction, will tend to propagate their kind in larger numbers than the less well adapted.
If the numbers be wholly kept down by the causes just indicated, as will often have been the case, natural selection will be powerless in certain beneficial directions; but this is no valid objection to its efficiency at other times and in other ways; for we are far from having any reason to suppose that many species ever undergo modification and improvement at the same time in the same area.
|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.
|04 - Natural Selection||04-12 - On the Degree to which Organisation tends to advance||60||
Science has not as yet proved the truth of this belief, whatever the future may reveal.
On our theory the continued existence of lowly organisms offers no difficulty; for natural selection, or the survival of the fittest, does not necessarily include progressive development- it only takes advantage of such variations as arise and are beneficial to each creature under its complex relations of life.
And it may be asked what advantage, as far as we can see, would it be to an infusorian animalcule- to an intestinal
worm- or even to an earthworm, to be highly organised.
If it were no advantage, these forms would be left, by natural selection, unimproved or but little improved, and might remain for indefinite ages in their present lowly condition.
And geology tells us that some of the lowest forms, as the infusoria and rhizopods, have remained for an enormous period in nearly their present state.
|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.