M Database Inspector (cheetah)
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|01 - Variations Under Domestication||01-12 - Circumstances favourable to Man's Power of Selection||10||
I will now say a few words on the circumstances, favourable, or the reverse, to man's power of selection.
A high degree of variability is obviously favourable, as freely giving the materials for selection to work on; not that mere individual differences are not amply sufficient, with extreme care, to allow of the accumulation of a large amount of modification in almost any desired direction.
But as variations manifestly useful or pleasing to man appear only occasionally, the chance of their appearance will be much increased by a large number of individuals being kept Hence, number is of the highest importance for success.
On this principle Marshall formerly remarked, with respect to the sheep of parts of Yorkshire, "as they generally belong to poor people, and are mostly in small lots, they never can be improved." On the other hand, nurserymen, from keeping large stocks of the same plant, are generally far more successful than amateurs in raising new and valuable varieties.
A large number of individuals of an animal or plant can be reared only where the conditions for its propagation are favourable. When the individuals are scanty, all will be allowed to breed, whatever their quality may be, and this will effectually prevent selection.
But probably the most important element is that the animal or plant should be so highly valued by man, that the closest attention is paid to even the slightest deviations in its qualities or structure.
Unless such attention be paid nothing can be effected.
I have seen it gravely remarked, that it was most fortunate that the strawberry began to vary just when gardeners began to attend to this plant.
No doubt the strawberry had always varied since it was cultivated, but the slightest varieties had been neglected.
As soon, however, as gardeners picked out individual plants with slightly larger, earlier, or better fruit, and raised seedlings from them, and again picked out the best seedlings and bred from them, then (with some aid by crossing distinct species) those many admirable varieties of the strawberry were raised which have appeared during the last half-century.
|06 - Difficutiles in Theory||06-05 - Diversified Habits in the Same Species||10||
I will now give two or three instances both of diversified and of changed habits in the individuals of the same species.
In either case it would be easy for natural selection to adapt the structure of the animal to its changed habits, or exclusively to one of its several habits.
It is, however, difficult to decide, and immaterial for us, whether habits generally change first and structure afterwards; or whether slight modifications of structure lead to changed habits; both probably often occurring almost simultaneously.
Of cases of changed habits it will suffice merely to allude to that of the many British insects which now feed on exotic plants, or exclusively on artificial substances.
Of diversified habits innumerable instances could be given: I have often watched a tyrant flycatcher (Saurophagus sulphuratus) in South America, hovering over one spot and then proceeding to another, like a kestrel, and at other times standing stationary on the margin of water, and then dashing into it like a kingfisher at a fish.
In our own country the larger titmouse (Parus major) may be seen climbing branches, almost like a creeper; it sometimes, like a shrike, kills small birds by blows on the head; and I have many times seen and heard it hammering the seeds of the yew on a branch, and thus breaking them like a nuthatch.
In North America the black bear was seen by Hearne swimming for hours with widely open mouth, thus catching, almost like a whale, insects in the water.
|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.
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.
|05 - Laws of Variation||05-04 - Correlation of Growth||10||
I mean by this expression that the whole organisation is so tied together during its growth and development, that when slight variations in any one part occur, and are accumulated through natural selection, other parts become modified.
This is a very important subject, most imperfectly understood, and no doubt wholly different classes of facts may be here easily confounded together.
We shall presently see that simple inheritance often gives the false appearance of correlation.
One of the most obvious real cases is, that variations of structure arising in the young or larvae naturally tend to affect the structure of the mature animal.
The several parts of the body which are homologous, and which, at an early embryonic period, are identical in structure, and which are necessarily exposed to similar conditions, seem eminently liable to vary in a like manner: we see this in the right and left sides of the body varying in the same manner; in the front and hind legs, and even in the jaws and limbs, varying together, for the lower jaw is believed by some anatomists to be homologous with the limbs.
These tendencies, I do not doubt, may be mastered more or less completely by natural selection; thus a family of stags once existed with an antler only on one side; and if this had been of any great use to the breed, it might probably have been rendered permanent by selection.