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|04 - Natural Selection||04-01 - Natural Selection||10||
How will the struggle for existence, briefly discussed in the last chapter, act in regard to variation?
Can the principle of selection, which we have seen is so potent in the hands of man, apply under nature?
I think we shall see that it can act most efficiently.
Let the endless number of slight variations and individual differences occurring in our domestic productions, and, in a lesser degree, in those under nature, be borne in mind; as well as the strength of the hereditary tendency.
Under domestication, it may be truly said that the whole organisation becomes in some degree plastic.
But the variability, which we almost universally meet with in our domestic productions, is not directly produced, as Hooker and Asa Gray have well remarked, by man; he can neither originate varieties, nor prevent their occurrence; he can preserve and accumulate such as do occur.
Unintentionally he exposes organic beings to new and changing conditions of life, and variability ensues; but similar changes of conditions might and do occur under nature.
Let it also be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life; and consequently what infinitely varied diversities of structure might be of use to each being under changing conditions of life.
Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should occur in the course of many successive generations?
If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind?
On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed.
This preservation of favourable individual differences and variations, and the destruction of those which are injurious, I have called Natural Selection.
|04 - Natural Selection||04-09 - Circumstances favourable for the production of new forms through Natural Selection||100||
Hence, we can understand how it is that the flora of Madeira, according to Oswald Heer, resembles to a certain extent the extinct tertiary flora of Europe. All fresh-water basins, taken together, make a small area compared with that of the sea or
of the land.
Consequently, the competition between fresh-water productions will have been less severe than elsewhere; new forms will have been then more slowly produced, and old forms more slowly exterminated.
And it is in fresh-water basins that we find seven genera of Ganoid fishes, remnants of a once preponderant order: and in fresh water we find some of the most anomalous forms now known in the world as the Ornithorhynchus and Lepidosiren which, like fossils, connect to a certain extent orders at present widely sundered in the natural scale.
These anomalous forms may be called living fossils; they have endured to the present day, from having inhabited a confined area, and from having been exposed to less varied, and therefore less severe, competition.
|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||04-12 - On the Degree to which Organisation tends to advance||100||
Finally, I believe that many lowly organised forms now exist throughout the world, from various causes. In some cases variations or individual differences of a favourable nature may never have arisen for natural selection to act on and accumulate. In no case, probably, has time sufficed for the utmost possible amount of development. In some few cases there has been what we must call retrogression of organisation.
But the main cause lies in the fact that under very simple conditions of life a high organisation would be of no service,- possibly would be of actual disservice, as being of a more delicate nature, and more liable to be put out of order and injured.
Looking to the first dawn of life, when all organic beings, as we may believe, presented the simplest structure, how, it has been asked, could the first steps in the advancement or differentiation of parts have arisen?
Mr. Herbert Spencer would probably answer that, as soon as simple unicellular organism came by growth or division to be compounded of several cells, or became attached to any supporting surface, his law "that homologous units of any order become differentiated in proportion as their relations to incident forces" would come into action. But as we have no facts to guide us, speculation on the subject is almost useless.