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Export to Excel select * from OriginOfSpecies where subject = '04 - Natural Selection' order by title limit 44, 4 (Page 12: Row)
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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.
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04 - Natural Selection 04-09 - Circumstances favourable for the production of new forms through Natural Selection 30 But we shall see in the sixth chapter that intermediate varieties, inhabiting intermediate districts, will in the long run generally be supplanted by one of the adjoining varieties.

Intercrossing will chiefly affect those animals which unite for each birth and wander much, and which do not breed at a very quick rate.

Hence with animals of this nature, for instance, birds, varieties will generally be confined to separated countries; and this I find to be the case.

With hermaphrodite organisms which cross only ccasionally, and likewise with animals which unite for each birth, but which wander little and can increase at a rapid rate, a new
and improved variety might be quickly formed on any one spot, and might there maintain itself in a body and afterwards spread, so that the individuals of the new variety would chiefly cross together.

On this principle, nurserymen always prefer saving seed from a large body of plants, as the chance of intercrossing is thus lessened.

snail
snail
04 - Natural Selection 04-09 - Circumstances favourable for the production of new forms through Natural Selection 40 Even with animals which unite for each birth, and which do not propagate rapidly, we must not assume that free intercrossing would always eliminate the effects of natural selection; for I can bring forward a considerable body of facts showing that within the same area, two varieties of the same animal may long remain distinct, from haunting different stations, from breeding at slightly different seasons, or from the individuals of each variety preferring to pair together.
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04 - Natural Selection 04-09 - Circumstances favourable for the production of new forms through Natural Selection 50 Intercrossing plays a very important part in nature by keeping the individuals of the same species, or of the same variety, true and uniform in character. It will obviously thus act far more efficiently with those animals which unite for each birth; but, as already stated, we have reason to believe that occasional intercrosses take place with all animals and plants.

Even if these take place only at long intervals of time, the young thus produced will gain so much in vigour and fertility over the offspring from long-continued self-fertilisation, that they will have a better chance of surviving and propagating their kind; and thus in the long run the influence of crosses, even at rare intervals, will be great.

With respect to organic beings extremely low in the scale, which do not propagate sexually, nor conjugate, and which cannot possibly intercross, uniformity of character can be retained by them under the same conditions of life, only through the principle of inheritance, and through natural selection which will destroy any
individuals departing from the proper type.

amoeba
amoeba


If the conditions of life change and the form undergoes modification, uniformity of character can be given to the modified offspring, solely by natural selection preserving similar favourable variations.