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|OriginOfSpecies - 475 Rows|
|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||01-13 - Summary||20||
With plants which are temporarily propagated by cuttings, buds, &c., the importance of crossing is immense; for the cultivator may here disregard the extreme variability both of hybrids and of mongrels, and the sterility of hybrids; but plants not propagated by seed are of little importance to us, for their endurance is only temporary.
Over all these causes of Change, the accumulative action of Selection, whether applied methodically and quickly, or unconsciously and slowly but more efficiently, seems to have been the predominant Power.
|02 - Variations Under Nature||02-01 - Variability||10||
BEFORE applying the principles arrived at in the last chapter to organic beings in a state of nature, we must briefly discuss whether these latter are subject to any variation.
To treat this subject properly, a long catalogue of dry facts ought to be given; but these shall reserve for a future work.
Nor shall I here discuss the various definitions which have been given of the term species.
No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species.
Generally the term includes the unknown element of a distant act of creation. The term "variety" is almost equally difficult to define; but here community of descent is almost universally implied, though it can rarely be proved.
We have also what are called monstrosities; but they graduate into varieties.
By a monstrosity I presume is meant some considerable deviation of structure, generally injurious, or not useful to the species.
Some authors use the term "variation" in a technical sense, as implying a modification directly due to the physical conditions of life; and "variations" in this sense are supposed not to be inherited; but who can say that the dwarfed condition of shells in the brackish waters of the Baltic, or dwarfed plants on Alpine summits, or the thicker fur of an animal from far northwards, would not in some cases be inherited for at least a few generations?
And in this case I presume that the form would be called a variety.
|02 - Variations Under Nature||02-01 - Variability||20||
It may be doubted whether sudden and considerable deviations of structure such as we occasionally see in our domestic productions, more especially with plants, are ever permanently propagated in a state of nature.
Almost every part of every organic being is so beautifully related to its complex conditions of life that it seems as improbable that any part should have been suddenly produced perfect, as that a complex machine should have been invented by man in a perfect state.
Under domestication mmonstrosities sometimes occur which resemble normal structures in widely different animals.
Thus pigs have occasionally been born with a sort of proboscis, and if any wild species of the same genus had naturally possessed a proboscis, it might have been argued that this had appeared as a monstrosity; but I have as yet failed to find, after diligent search, cases of monstrosities resembling normal structures in nearly allied forms, and these alone bear on the question.
If monstrous forms of this kind ever do appear in a state of nature and are capable of reproduction (which is not always the case), as they occur rarely and singularly, their preservation would depend on unusually favourable circumstances.
They would, also, during the first and succeeding generations cross with the ordinary form, and thus their abnormal character would almost inevitably be lost.
But I shall have to return in a future chapter to the reservation
and perpetuation of single or occasional variations.