M Database Inspector (cheetah)
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|OriginOfSpecies - 475 Rows|
|04 - Natural Selection||04-10 - Extinction caused by Natural Selection||30||
From these several considerations I think it inevitably follows, that as new species in the course of time are formed through natural selection, others will become rarer and rarer, and finally extinct.
The forms which stand in closest competition with those undergoing modification and improvement will naturally suffer most.
And we have seen in the chapter on the Struggle for Existence that it is the most closely-allied forms,- varieties of the same species, and species of the same genus or of related genera,- which, from having nearly the same structure, constitution, and habits, generally come into the severest competition with each other; consequently, each new variety or species, during the progress of its formation, will generally press hardest on its nearest kindred, and tend to exterminate them.
We see the same process of extermination amongst our domesticated productions, through the selection of improved forms by man.
Many curious instances could be given showing how quickly new breeds of cattle, sheep, and other animals, and varieties of flowers, take the place of older and inferior kinds.
In Yorkshire, it is historically known that the ancient black cattle were displaced by the long-horns, and that these "were swept away by the shorthorns" (I quote the words of an agricultural writer) "as if by some murderous pestilence."
|04 - Natural Selection||04-11 - Divergence of Character||10||
The principle, which I have designated by this term, is of high importance, and explains, as I believe, several important facts.
In the first place, varieties, even strongly-marked ones, though having somewhat of the character of species- as is shown by the hopeless doubts in many cases how to rank them- yet certainly differ far less from each other than do good and distinct species.
Nevertheless, according to my view, varieties are species in the process of formation, or are, as I have called them, incipient species.
How, then, does the lesser difference between varieties become augmented into the greater difference between species?
That this does habitually happen, we must infer from most of the innumerable species throughout nature presenting well-marked differences; whereas varieties, the supposed prototypes and parents of future well-marked species, present slight and ill-defined differences.
Mere chance, as we may call it, might cause one variety to differ in some character from its parents, and the offspring of this variety again to differ from its parent in the very same character and in a greater degree; but this alone would never account for so habitual and large a degree of difference as that between the species of the same genus.
|04 - Natural Selection||04-11 - Divergence of Character||20||
As has always been my practice, I have sought light on this head from our domestic productions.
We shall here find something analogous.
It will be admitted that the production of races so different as short-horn and Hereford cattle, race and cart horses, the several breeds of pigeons, &c., could never have been effected by the mere chance accumulation of similar variations during many successive generations.
In practice, a fancier is, for instance, struck by a pigeon having a slightly shorter beak; another fancier is struck by a pigeon having a rather longer beak; and on the acknowledged principle that "fanciers do not and will not admire a medium standard, but like extremes," they both go on (as has actually occurred with the sub-breeds of the tumbler-pigeon) choosing and breeding from birds with longer and longer beaks, or with shorter and shorter beaks.
Again, we may suppose that at an early period of history, the men of one nation or district required swifter horses, whilst those of another required stronger and bulkier horses.
The early differences would be very slight; but, in the course of time from the continued selection of swifter horses in the one case, and of stronger ones in the other, the differences would become greater, and would be noted as forming two sub-breeds.
Ultimately, after the lapse of centuries, these sub-breeds would become converted into two well-established and distinct breeds.
As the differences became greater, the inferior animals with intermediate characters, being neither swift nor very strong, would not have been used for breeding, and will thus have tended to disappear.
Here, then, we see in man's productions the action of what may be called the principle of divergence, causing differences, at first barely appreciable, steadily to increase, and the breeds to diverge in character, both from each other and from their common parent.
|04 - Natural Selection||04-11 - Divergence of Character||260||
We have seen that in each country it is the species belonging to the larger genera which oftenest present varieties or incipient species.
This, indeed, might have been expected; for, as natural selection acts through one form having some advantage over other forms in the struggle for existence, it will chiefly act on those which already have some advantage; and the largeness of any group shows that its species have inherited from a common ancestor some advantage in common.
Hence, the struggle for the production of new and modified descendants will mainly lie between the larger groups which are all trying to increase in number.
One large group will slowly conquer another large group, reduce its numbers, and thus lessen its chance of further variation and improvement.
Within the same large group, the later and more highly perfected sub-groups, from branching out and seizing on many new places in the polity of Nature, will constantly tend to supplant and destroy the earlier and less improved sub-groups. Small and broken groups and sub-groups will finally disappear.
Looking to the future, we can predict that the groups of organic beings which are now large and triumphant, and which are least broken up, that is, which have as yet suffered least extinction, will, for a long period, continue to increase. But which groups will ultimately prevail, no man can predict; for we know that many groups formerly most extensively developed, have now become extinct.
Looking still more remotely to the future, we may predict that, owing to the continued and steady increase of the larger groups, a multitude of smaller groups will become utterly extinct, and leave no modified descendants; and consequently that, of the species living at any one period, extremely few will transmit descendants to a remote futurity.
I shall have to return to this subject in the chapter on Classification, but I may add that as, according to this view, extremely few of the more ancient species have transmitted descendants to the present day, and, as all the descendants of the same species form a class, we can understand how it is that there exist so few classes in each main division of the animal and vegetable kingdoms.
Although few of the most ancient species have left modified descendants' yet, at remote geological periods, the earth may have been almost as well peopled with species of many genera, families, orders, and classes, as at the present time.