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|14 - Recapitulation and Conclusion||14-04 - How far the theory of natural selection may be extended||10||
It may be asked how far I extend the doctrine of the modification of species.
The question is difficult to answer, because the more distinct the forms are which we may consider, by so much the arguments fall away in force.
But some arguments of the greatest weight extend very far.
All the members of whole classes can be connected together by chains of affinities, and all can be classified on the same principle, in groups subordinate to groups.
Fossil remains sometimes tend to fill up very wide intervals between existing orders.
Organs in a rudimentary condition plainly show that an early progenitor had the organ in a fully developed state; and this in some instances necessarily implies an enormous amount of modification in the descendants.
Throughout whole classes various structures are formed on the same pattern, and at an embryonic age the species closely resemble each other.
Therefore I cannot doubt that the theory of descent with modification embraces all the members of the same class.
I believe that animals have descended from at most only four or five progenitors, and plants from an equal or lesser number.
Analogy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype.
But analogy may be a deceitful guide.
Nevertheless all living things have much in common, in their chemical composition, their germinal vesicles, their cellular structure, and their laws of growth and reproduction.
We see this even in so trifling a circumstance as that the same poison often similarly affects plants and animals; or that the poison secreted by the gall-fly produces monstrous growths on the wild rose or oak-tree.
Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed.
|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.
|08 - Hybridism||08-06 - Parallelism between the effects of changed conditions of life and crossing||10||
It may seem fanciful, but I suspect that a similar parallelism extends to an allied yet very different class of facts. It is an old and almost universal belief, founded, I think, on a considerable body of evidence, that slight changes in the conditions of life are beneficial to all living things.
We see this acted on by farmers and gardeners in their frequent exchanges of seed, tubers, &c., from one soil or climate to another, and back again.
During the convalescence of animals, we plainly see that great benefit is derived from almost any change in the habits of life.
Again, both with plants and animals, there is abundant evidence, that a cross between very distinct individuals of the same species, that is between members of different strains or sub-breeds, gives vigour and fertility to the offspring.
I believe, indeed, from the facts alluded to in our fourth chapter, that a certain amount of crossing is indispensable even with hermaphrodites; and that close interbreeding continued during several generations between the nearest relations, especially if these be kept under the same conditions of life, always induces weakness and sterility in the progeny.
Hence it seems that, on the one hand, slight changes in the conditions of life benefit all organic beings, and on the other hand, that slight crosses, that is crosses between the males and females of the same species which have varied and become slightly different, give vigour and fertility to the offspring.
But we have seen that greater changes, or changes of a particular nature, often render organic beings in some degree sterile; and that greater crosses, that is crosses between males and females which have become widely or specifically different, produce hybrids which are generally sterile in some degree.
I cannot persuade myself that this parallelism is an accident or an illusion.
Both series of facts seem to be connected together by some common but unknown bond, which is essentially related to the principle of life.
|05 - Laws of Variation||05-07 - Multiple, Rudimentary, and Lowly-organised Structures are Variable||10||
It seems to be a rule, as remarked by the younger Geoffroy, both with varieties and species, that when any part or organ is repeated many times in the same individual (as the vertebrae in snakes, and the stamens in polyandrous flowers) the number is variable; whereas the same part or organ, when it occurs in lesser numbers, is constant.
The same author as well as some botanists have further remarked that multiple parts are extremely liable to vary in structure. As "vegetable repetition," to use Prof. Owen's expression, is a sign of low organisation, the foregoing statements accord with the common opinion of naturalists, that beings which stand low in the scale of nature are more variable than those which are higher.