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|OriginOfSpecies - 475 Rows|
|06 - Difficutiles in Theory||06-13 - Summary: The Law of Unity of Type and of the Conditions of Existence Embraced by the Theory of Natural Selection||10||
We have in this chapter discussed some of the difficulties and objections which may be urged against the theory.
Many of them are serious; but I think that in the discussion light has been thrown on several facts, which on the belief of independent acts of creation are utterly obscure.
We have seen that species at any one period are not indefinitely variable, and are not linked together by a multitude of intermediate gradations, partly because the process of natural selection is always very slow, and at any one time acts only on a few forms; and partly because the very process of natural selection implies the continual supplanting and extinction of preceding and intermediate gradations.
Closely allied species, now living on a continuous area, must often have been formed when the area was not continuous, and when the conditions of life did not insensibly graduate away from one part to another.
When two varieties are formed in two districts of a continuous area, an intermediate variety will often be formed, fitted for an intermediate zone; but from reasons assigned, the intermediate variety will usually exist in lesser numbers than the two forms which it connects; consequently the two latter, during the course of further modification, from existing in greater numbers, will have a great advantage over the less numerous intermediate variety, and will thus generally succeed in supplanting and exterminating it.
|01 - Variations Under Domestication||01-13 - Summary||10||
To sum up on the origin of our domestic races of animals and plants.
Changed conditions of life are of the highest importance in causing variability, both by acting directly on the organisation, and indirectly by affecting the reproductive system.
It is not probable that variability is an inherent and necessary contingent, under all circumstances.
The greater or less force of inheritance and reversion, determine whether variations shall endure.
Variability is governed by many unknown laws, of which correlated growth is probably the most important.
Something, but how much we do not know, may be attributed to the definite action of the conditions of life. Some, perhaps a great, effect may be attributed to the increased use or disuse of parts.
The final result is thus rendered infinitely complex.
In some cases the intercrossing of aboriginally distinct species appears to have played an important part in the origin of our breeds.
When several breeds have once been formed in any country, their occasional intercrossing, with the aid of selection, has, no doubt, largely aided in the formation of new sub-breeds; but the importance of crossing has been much exaggerated, both in regard to animals and to those plants which are propagated by seed.
|03 - Struggle for Existence||03-05 - Nature of the Checks to Increase||10||
In looking at Nature, it is most necessary to keep the foregoing considerations always in mind- never to forget that every single organic being may be said to be striving to the utmost to increase in numbers; that each lives by a struggle at some period of its life; that heavy destruction inevitably falls either on the young or old, during each generation or at recurrent intervals. Lighten any cheek, mitigate the destruction ever so little, and the number of the species will
almost instantaneously increase to any amount.
|03 - Struggle for Existence||03-09 - Complex Relations of all Animals and Plants Throughout Nature||10||
Many cases are on record showing how complex and unexpected are the checks and relations between organic beings, which have to struggle together in the same country.
I will give only a single instance, which, though a simple one, interested me.
In Staffordshire, on the estate of a relation, where I had ample means of investigation, there was a large and extremely barren heath, which had never been touched by the hand of man; but several hundred acres of exactly the same nature had been enclosed twenty-five years previously and planted with Scotch fir.
The change in the native vegetation of the planted part of the heath was most remarkable, more than is generally seen in passing from one quite different soil to another: not only the proportional numbers of the heath-plants were wholly changed, but twelve species of plants (not counting grasses and carices) flourished in the plantations, which could not be found on the heath.
The effect on the insects must have been still greater, for six insectivorous birds were very common in the plantations, which were not to be seen on the heath; and the heath was frequented by two or three distinct insectivorous birds.
Here we see how potent has been the effect of the introduction of a single tree, nothing whatever else having been done, with the exception of the land having been enclosed, so that cattle could not enter.
But how important an element enclosure is, I plainly saw near Farnham, in Surrey. Here there are extensive heaths, with a few clumps of old Scotch firs on the distant hilltops: within the last ten years large spaces have been enclosed, and self-sown firs are now springing up in multitudes, so close together that all cannot live.
When I ascertained that these young trees had not been sown or planted, I was so much surprised at their numbers that I went to several points of view, whence I could examine hundreds of acres of the unenclosed heath, and literally I could not see a single Scotch fir, except the old planted clumps.
But on looking closely between the stems of the heath, I found a multitude of seedlings and little trees which had been perpetually browsed down by the cattle. In one square yard, at a point some hundred yards distant from one of the old clumps, I counted thirty-two little trees; and one of them, with twenty-six rings of growth, had, during many years, tried to raise its head above the stems of the heath, and had failed.
No wonder that, as soon as the land was enclosed, it became thickly clothed with vigorously growing young firs.
Yet the heath was so extremely barren and so extensive that no one would ever have imagined that cattle would have so closely and effectually searched it for food.