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|04 - Natural Selection||04-01 - Natural Selection||10||
How will the struggle for existence, briefly discussed in the last chapter, act in regard to variation?
Can the principle of selection, which we have seen is so potent in the hands of man, apply under nature?
I think we shall see that it can act most efficiently.
Let the endless number of slight variations and individual differences occurring in our domestic productions, and, in a lesser degree, in those under nature, be borne in mind; as well as the strength of the hereditary tendency.
Under domestication, it may be truly said that the whole organisation becomes in some degree plastic.
But the variability, which we almost universally meet with in our domestic productions, is not directly produced, as Hooker and Asa Gray have well remarked, by man; he can neither originate varieties, nor prevent their occurrence; he can preserve and accumulate such as do occur.
Unintentionally he exposes organic beings to new and changing conditions of life, and variability ensues; but similar changes of conditions might and do occur under nature.
Let it also be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life; and consequently what infinitely varied diversities of structure might be of use to each being under changing conditions of life.
Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should occur in the course of many successive generations?
If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind?
On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed.
This preservation of favourable individual differences and variations, and the destruction of those which are injurious, I have called Natural Selection.
|04 - Natural Selection||04-02 - Its Power Compared with Man's Selection||10||
As man can produce a great result with his domestic animals and plants by adding up in any given direction individual differences, so could natural selection, but far more easily from having incomparably longer time for action.
Nor do I believe that any great physical change, as of climate, or any unusual degree of isolation to check immigration, is necessary in order that new and unoccupied places should be left, for natural selection to fill up by improving some of the varying inhabitants.
For as all the inhabitants of each country are struggling together with nicely balanced forces, extremely slight modifications in the structure or habits of one species would often give it an advantage over others; and still further modifications of the same kind would often still further increase the advantage, as long as the species continued under the same conditions of life and profited by similar
means of subsistence and defence.
No country can be named in which all the native inhabitants are now so perfectly adapted to each other and to the physical conditions under which they live, that none of them could be still better adapted or improved; for in all countries, the natives have been so far conquered by naturalised productions, that they have allowed some foreigners to take firm possession of the land.
And as foreigners have thus in every country beaten some of the natives, we may safely conclude that the natives might have been modified with advantage, so as to have better resisted the intruders.
|04 - Natural Selection||04-03 - Its Power on Characters of Trifling Importance||10||
It may metaphorically be said that natural selection is daily and hourly scrutinising, throughout the world, the slightest variations; rejecting those that are bad, preserving and adding up all that are good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life.
We see nothing of these slow changes in progress, until the hand of time has marked the lapse of ages, and then so imperfect is our view into long-past geological ages, that we see only that the forms of life are now different from what they formerly were.
In order that any great amount of modification should be effected in a species, a variety when once formed must again, perhaps after a long interval of time, vary or present individual differences of the same favourable nature as before; and these must be again preserved, and so onwards step by step.
Seeing that individual differences of the same kind perpetually recur, this can hardly be considered as an unwarrantable assumption.
But whether it is true, we can judge only by seeing how far the hypothesis accords with and explains the general phenomena of nature. On the other hand, the ordinary belief that the amount of possible variation is a strictly limited quantity is likewise a simple assumption.
Although natural selection can act only through and for the good of each being, yet characters and structures, which we are apt to consider as of very trifling importance, may thus be acted on.
When we see leaf-eating insects green and bark-feeders mottled-grey; the alpine ptarmigan white in winter, the red grouse the colour of heather, we must believe that these tints are of service to these birds and insects in preserving them from danger.
Grouse, if not destroyed at some period of their lives, would increase in countless numbers; they are known to suffer largely from birds of prey; and hawks are guided by eyesight to their prey- so much so, that on parts of the Continent persons are warned not to keep white pigeons, as being the most liable to destruction.
Hence natural selection might be effective in giving the proper colour to each kind of grouse, and in keeping that colour, when once acquired, true and constant.
Nor ought we to think that the occasional destruction of an animal of any particular colour would produce little effect: we should remember how essential it is in a flock of white sheep to destroy a lamb with the faintest trace of black.
We have seen how the colour of the hogs, which feed on the "paint-root" in Virginia, determines whether they shall live or die.
In plants, the down on the fruit and the colour of the
flesh are considered by botanists as characters of the most
trifling importance: yet we hear from an excellent
horticulturist, Downing, that in the United States,
smooth-skinned fruits suffer far more from a beetle, a
Curculio, than those with down; that purple plums suffer far
more from a certain disease than yellow plums; whereas
another disease attacks yellow-fleshed peaches far more
than those with other coloured flesh. If, with all the aids of art,
these slight differences make a great difference in cultivating
the several varieties, assuredly, in a state of nature, where
the trees would have to struggle with other trees, and with a
host of enemies, such differences would effectually settle
which variety, whether a smooth or downy, a yellow or purple
fleshed fruit, should succeed.
|04 - Natural Selection||04-04 - Its Power at All Ages and on Both Sexes||10||
As we see that those variations which, under domestication, appear at any particular period of life, tend to reappear in the offspring at the same period;- for instance, in the shape, size, and flavour of the seeds of the many varieties of our culinary and agricultural plants; in the caterpillar and cocoon stages of the varieties of the silk-worm; in the eggs of poultry, and in the colour of the down of their chickens; in the horns of our sheep and cattle when nearly adult;- so in a state of nature natural selection will be enabled to act on and modify organic beings at any age, by the accumulation of variations profitable at that age, and by their inheritance at a corresponding age.
If it profit a plant to have its seeds more and more widely disseminated by the wind, I can see no greater difficulty in this being effected through natural selection, than in the cotton-planter increasing and improving by selection the down in the pods on his cotton-trees.
Natural selection may modify and adapt the larva of an insect to a score of contingencies, wholly different from those which concern the mature insect; and these modifications may affect, through correlation, the structure of the adult.
So, conversely, modifications in the adult may affect the structure of the larva; but in all cases natural selection will ensure that they shall not be injurious: for if they were so, the species would become extinct.