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|01 - Variations Under Domestication||01-11 - Unknown Origin of our Domestic Productions||40||
Nor let it be thought that some great deviation of structure would be necessary to catch the fancier's eye: he perceives extremely small differences, and it is in human nature to value any novelty, however slight, in one's own possession.
Nor must the value which would formerly have been set on any slight differences in the individuals of the same species be judged of by the value which is now set on them, after several breeds have fairly been established.
I is known that with pigeons many slight variations now occasionally appear, but these are rejected as faults or deviations from the standard of perfection in each breed.
The common goose has not given rise to any marked varieties; hence the Toulouse and the common breed, which differ only in colour, that most fleeting of characters,have lately been exhibited as distinct at our poultry shows.
|01 - Variations Under Domestication||01-12 - Circumstances favourable to Man's Power of Selection||30||
But the goose, under the conditions to which it is exposed when domesticated seems to have a singularly inflexible organisation, though it has varied to a slight extent, as I have elsewhere described.
Some authors have maintained that the amount of variation in our domestic productions is soon reached, and can never afterwards be exceeded.
It would be somewhat rash to assert that the limit has been attained in any one case; for almost all our animals and plants have been greatly improved in many ways within a recent period; and this implies variation.
It would be equally rash to assert that characters now increased to their utmost limit, could not, after remaining fixed for many centuries, again vary under new conditions of life.
No doubt, as Mr. Wallace has remarked with much truth, a limit will be at last reached. For instance, there must be a limit to the fleetness of any terrestrial animal, as this will be determined by the friction to be overcome, the weight of body to be carried, and the power of contraction in the muscular
But what concerns us is that the domestic varieties of the same species differ from each other in almost every character, which man has attended to and selected, more than do the distinct species of the same genera.
Isidore Geoffroy St-Hilaire has proved this in regard to size, and so it is with colour and probably with the length of hair.
With respect to fleetness, which depends on many bodily characters, Eclipse was far fleeter, and a dray-horse is incomparably stronger than any two natural species belonging to the same genus.
So with plants, the seeds of the different varieties of the bean or maize probably differ more in size, than do the seeds of the distinct species in any one genus in the same two families. The same remark holds good in regard to the fruit of the several varieties of the plum, and still more strongly with the melon, as well as in many other analogous cases.
|01 - Variations Under Domestication||01-13 - Summary||10||
To sum up on the origin of our domestic races of animals and plants.
Changed conditions of life are of the highest importance in causing variability, both by acting directly on the organisation, and indirectly by affecting the reproductive system.
It is not probable that variability is an inherent and necessary contingent, under all circumstances.
The greater or less force of inheritance and reversion, determine whether variations shall endure.
Variability is governed by many unknown laws, of which correlated growth is probably the most important.
Something, but how much we do not know, may be attributed to the definite action of the conditions of life. Some, perhaps a great, effect may be attributed to the increased use or disuse of parts.
The final result is thus rendered infinitely complex.
In some cases the intercrossing of aboriginally distinct species appears to have played an important part in the origin of our breeds.
When several breeds have once been formed in any country, their occasional intercrossing, with the aid of selection, has, no doubt, largely aided in the formation of new sub-breeds; but the importance of crossing has been much exaggerated, both in regard to animals and to those plants which are propagated by seed.
|01 - Variations Under Domestication||01-01 - Causes of Variability||10||
WHEN we compare the individuals of the same variety or sub-variety of our older cultivated plants and animals, one of the first points which strikes us is, that they generally differ more from each other than do the individuals of any one species or variety in a state of nature.
And if we reflect on the vast diversity of the plants and animals which have been cultivated, and which have varied during all ages under the most different climates and treatment, we are driven to conclude that this great variability is due to our domestic productions having been raised under conditions of life not so uniform as, and somewhat different from, those to which the parent species had been exposed under nature.
There is, also, some probability in the view propounded by Andrew Knight, that this variability may be partly connected with excess of food.
It seems clear that organic beings must be exposed during several generations to new conditions to cause any great amount of variation; and that, when the organization has once begun to vary, it generally continues varying for many generations.
No case is on record of a variable organism ceasing to vary under cultivation.
Our oldest cultivated plants, such as wheat, still yield new varieties: our oldest, domesticated animals are still capable of rapid improvement or modification.