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.
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| sheep |
We have seen how the colour of the hogs, which feed on the "paint-root" in Virginia, determines whether they shall live or die.
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| pig |
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| hog |
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.
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| peach |
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| nectarine |
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| Curculio |
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| plum |
