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Export to Excel select * from OriginOfSpecies where subject = '04 - Natural Selection' order by subject, title, ordinal limit 32, 4 (Page 9: Row)
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04 - Natural Selection 04-07 - Illustrations of the Action of Natural Selection: 100 I am well aware that this doctrine of natural selection, exemplified in the above imaginary instances, is open to the same objections which were first urged against Sir Charles Lyell's noble views on "the modern changes of the earth, as illustrative of geology"; but we now seldom hear the agencies which we see still at work, spoken of as trifling or insignificant, when used in explaining the excavation of the deepest valleys or the formation of long lines of inland cliffs.

Natural selection acts only by the preservation and accumulation of small inherited modifications, each profitable to the preserved being; and as modern geology has almost banished such views as the excavation of a great valley by a single diluvial wave, so will natural selection banish the belief of the continued creation of new organic beings, or of any great and sudden modification in their structure.

Grand Canyon
Grand Canyon
04 - Natural Selection 04-08 - On the Intercrossing of Individuals 10 I must here introduce a short digression. In the case of animals and plants with separated sexes, it is of course obvious that two individuals must always (with the exception of the curious and not well-understood cases of parthenogenesis) unite for each birth; but in the case of hermaphrodites this is far from obvious.

snail
snail


Nevertheless there is reason to believe that with all hermaphrodites two individuals, either occasionally or habitually, concur for the reproduction of their kind.

This view was long ago doubtfully suggested by Sprengel, Knight and Kolreuter. We shall presently see its importance; but I must here treat the subject with extreme brevity, though I have the materials prepared for an ample discussion.

All vertebrate animals, all insects, and some other large groups of animals, pair for each birth. Modern research has much diminished the number of supposed hermaphrodites, and of real hermaphrodites a large number pair; that is, two individuals regularly unite for reproduction, which is all that concerns us.

But still there are many hermaphrodite animals which certainly do not habitually pair, and a vast majority of plants are hermaphrodites.

What reason, it may be asked, is there for supposing in these cases that two individuals ever concur in reproduction?

As it is impossible here to enter on details, I must trust to some general considerations alone.
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 04-08 - On the Intercrossing of Individuals 30 On the belief that this is a law of nature, we can, I think, understand several large classes of facts, such as the following, which on any other view are inexplicable.

Every hybridizer knows how unfavourable exposure to wet is to the fertilisation of a flower, yet what a multitude of flowers have their anthers and stigmas fully exposed to the weather!

If an occasional cross be indispensable, notwithstanding that the plant's own anthers and pistil stand so near each other as almost to insure self-fertilisation, the fullest freedom for the entrance of pollen from another individual will explain the above state of exposure of the organs.

pistil
pistil


Many flowers, on the other hand, have their organs of fructification closely enclosed, as in the great papilionaceous or pea-family; but these almost invariably present beautiful and curious adaptations in relation to the visits of insects.

So necessary are the visits of bees to many papilionaceous flowers, that their fertility is greatly diminished if these visits be prevented.

Now, it is scarcely possible for insects to fly from flower and flower, and not to carry pollen from one to the other, to the great good of the plant.

Insects act like a camel-hair pencil, and it is sufficient to ensure fertilisation, just to touch with the same brush the anthers of one flower and then the stigma of another; but it must not be supposed that bees would thus produce a multitude of hybrids between distinct species; for if a plant's own pollen and that from another species are placed on the same stigma, the former is so prepotent that it invariably and completely destroys, as has been shown by
Gartner, the influence of the foreign pollen.

pollen
pollen