M Database Inspector (cheetah)
Not logged in. Login


105 rows, page 13 of 27 (4/p)
1 7 8 9 10 11 12 13 14 15 16 17 18 19 27

Export to Excel select * from OriginOfSpecies where subject = '04 - Natural Selection' order by description limit 48, 4 (Page 13: Row)
subject
title
ordinal
description Desending Order (top row is first)
04 - Natural Selection 04-11 - Divergence of Character 240 In the diagram, each horizontal line has hitherto been supposed to represent a thousand generations, but each may represent a million or more generations; it may also represent a section of the successive strata of the earth's crust including extinct remains. We shall, when we come to our chapter on Geology, have to refer again to this subject, and I think we shall then see that the diagram throws light on the affinities of extinct beings, which, though generally belonging to the same orders, families, or genera, with those now living, yet are often, in some degree, intermediate in character between existing groups; and we can understand this fact, for the extinct species lived at various remote epochs when the branching lines of descent had diverged less.
04 - Natural Selection 04-08 - On the Intercrossing of Individuals 20 In the first place, I have collected so large a body of facts, and made so many experiments, showing, in accordance with the almost universal belief of breeders, that with animals and plants a cross between different varieties, or between individuals of the same variety but of another strain, gives vigour and fertility to the offspring; and on the other hand, that close interbreeding diminishes vigour and fertility; that these facts alone incline me to believe that it is a general law of
nature that no organic being fertilises itself for a perpetuity of generations; but that a cross with another individual is occasionally- perhaps at long intervals of time- indispensable.
04 - Natural Selection 04-05 - Sexual Selection 10 Inasmuch as peculiarities often appear under domestication in one sex and become hereditarily attached to that sex, so no doubt it will be under nature.

Thus it is rendered possible for the two sexes to be modified through natural selection in relation to different habits of life, as is sometimes the case; or for one sex to be modified in relation to the other sex, as commonly occurs.

This leads me to say a few words on what I have called Sexual Selection. This form of selection depends, not on a struggle for existence in relation to other organic beings or to external conditions, but on a struggle between the individuals of one sex, generally the males, for the possession of the other sex.

The result is not death to the unsuccessful competitor, but few or no offspring. Sexual selection is, therefore, less rigorous than natural selection.

Generally, the most vigorous males, those which are best fitted for their places in nature, will leave most progeny.

But in many cases, victory depends not so much on general vigor, as on having special weapons, confined to the male sex.

A hornless stag or spurless cock would have a poor chance of leaving numerous offspring.

stag
stag

cock
cock

spurs
spurs


Sexual selection, by always allowing the victor to breed, might surely give indomitable courage, length to the spur, and strength to the wing to strike in the spurred leg, in nearly the same manner as does the brutal cockfighter by the careful selection of his best cocks.

cockfight
cockfight


How low in the scale of nature the law of battle descends, I know not; male alligators have been described as fighting, bellowing, and whirling round, like Indians in a war-dance, for the possession of the females; male salmons have been observed fighting all day long; male stagbeetles sometimes bear wounds from the huge mandibles of other males; the males of certain hymenopterous insects have been frequently seen by that inimitable observer M. Fabre, fighting for a particular female who sits by, an apparently unconcerned beholder of the struggle, and then retires with the conqueror.

alligator
alligator

crocodile
crocodile

Stag Beetle
Stag Beetle

Indian
Indian


The war is, perhaps, severest between the males of polygamous animals, and these seem oftenest provided with special weapons.

The males of carnivorous animals are already well armed; though to them and to others, special means of defence may be given through means of sexual selection, as the mane of the lion, and the hooked jaw to the male salmon; for the shield may be as important for victory, as the sword or spear.

mane
mane

salmon
salmon

sword
sword

spear
spear
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.