M Database Inspector (cheetah)
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|OriginOfSpecies - 475 Rows|
|06 - Difficutiles in Theory||06-11 - Organs of Small Importance||20||
In the first place, we are much too ignorant in regard to the whole economy of any one organic being, to say what slight modifications would be of importance or not. In a former chapter I have given instances of very trifling characters, such as the down on fruit and the colour of its flesh, the colour of the skin and hair of quadrupeds, which, from being correlated with constitutional differences or from determining the attacks of insects, might assuredly be acted on by natural selection.
The tail of the giraffe looks like an artificially constructed fly-flapper; and it seems at first incredible that this could have been adapted for its present purpose by successive slight modifications, each better and better fitted, for so trifling an object as to drive away flies; yet we should pause before being too positive even in this case, for we know that the distribution and existence of cattle and other animals in South America absolutely depend on their power of resisting the attacks of insects: so that individuals which could by any means defend themselves from these small enemies, would be able to range into new pastures and thus gain a great
It is not that the larger quadrupeds are actually destroyed (except in some rare cases) by flies, but they are incessantly harassed and their strength reduced, so that they are more subject to disease, or not so well enabled in a coming dearth to search for food, or to escape from beasts of prey.
|04 - Natural Selection||04-06 - On the generality of Intercross Between Individuals of the Same Species||20||
In order to make it clear how, as I believe, natural selection
acts, I must beg permission to give one or two imaginary
Let us take the case of a wolf, which preys on various
animals, securing some by craft, some by strength, and
some by fleetness; and let us suppose that the fleetest prey,
a deer for instance, had from any change in the country
increased in numbers, or that other prey had decreased in
numbers, during that season of the year when the wolf was
hardest pressed for food.
Under such circumstances the swiftest and slimmest
wolves would have the best chance of surviving and so
be preserved or selected,- provided always that they
retained strength to master their prey at this or some
other period of the year, when they were compelled to
prey on other animals.
I can see no more reason to doubt that this would be the
result, than that man should be able to improve the fleetness
of his greyhounds by careful and methodical selection, or by
that kind of unconscious selection which follows from each
man trying to keep the best dogs without any thought of
modifying the breed.
|04 - Natural Selection||04-07 - Illustrations of the Action of Natural Selection:||20||
I may add, that, according to Mr. Pierce, there are two varieties of the wolf inhabiting the Catskill Mountains, in the United States, one with a light greyhound-like form, which pursues deer, and the other more bulky, with shorter legs, which more frequently attacks the shepherd's flocks.
|04 - Natural Selection||04-09 - Circumstances favourable for the production of new forms through Natural Selection||20||
Unless favourable variations be inherited by some at least of the offspring, nothing can be effected by natural selection.
The tendency to reversion may often check or prevent the work; but as this tendency has not prevented man from forming by selection numerous domestic races, why should it prevail against natural selection?
In the case of methodical selection, a breeder selects for some definite object, and if the individuals be allowed freely to intercross, his work will completely fail.
But when many men, without intending to alter the breed, have a nearly common standard of perfection, and all try to procure and breed from the best animals, improvement surely but slowly follows from this unconscious process of selection, notwithstanding that there is no separation of selected individuals.
Thus it will be under nature; for within a confined area, with some place in the natural polity not perfectly occupied, all the individuals varying in the right direction, though in different degrees, will tend to be preserved.
But if the area be large, its several districts will almost certainly present different conditions of life; and then, if the same species undergoes modification in different districts, the newly-formed varieties will intercross on the confines of each.