islands; and in this case natural selection would continue slowly to reduce the organ, until it was rendered harmless and rudimentary. ↑| 3 blocks not present in 1859 1860 1861 1866 1869; present in 1872 | | It appears probable that disuse has been the main agent in rendering organs rudimentary.
It would at first lead by slow steps to the more and more complete reduction of a part, until at last it became rudimentary,— as in the case of the eyes of animals inhabiting dark caverns, and of the wings of birds inhabiting oceanic islands, which have seldom been forced by beasts of prey to take flight, and have ultimately lost the power of flying.
Again, an organ, useful under certain conditions, might become injurious under others, as with the wings of beetles living on small and exposed islands; and in this case natural selection will have aided in reducing the organ, until it was rendered harmless and rudimentary.
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Any change in function,
which can be effected by insensibly
small steps,
is within the power of natural selection; so that an organ rendered, during
changed habits of life, useless or injurious for one purpose, might easily
be modified and used for another purpose. Or an
organ might
be
retained for one alone of its former functions. An organ,
when
rendered useless,
may well be variable, for its
variations cannot
be
checked by natural selection. ↑| 1 blocks not present in 1859 1860 1861 1866 1869; present in 1872 | | All this agrees well with what we see under nature.
|
| age, and consequently 1859 1860 1861 |
| age, but 1866 |
| mature age, but 1869 1872 |
| 1 blocks not present in 1859 1860 1861 1866 1869; present in 1872 | | If, for instance, the digit of an adult animal was used less and less during many generations, owing to some change of habits, or if an organ or gland was less and less functionally exercised, we may infer that it would become reduced in size in the adult descendants of this animal, but would retain nearly its original standard of development in the embryo.
|
| economy, 1859 1860 1861 |
| economy 1866 |
| the economy of organisation, 1869 |
| explained in 1859 1860 1861 1869 |
| in organisation, explained in 1866 |
| 7 blocks not present in 1859 1860 1861 1866 1869; present in 1872 | | There remains, however, this difficulty.
After an organ has ceased being used, and has become in consequence much reduced, how can it be still further reduced in size until the merest vestige is left; and how can it be finally quite obliterated?
It is scarcely possible that disuse can go on producing any further effect after the organ has once been rendered functionless.
Some additional explanation is here requisite which I cannot give.
If, for instance, it could be proved that every part of the organisation tends to vary in a greater degree towards diminution than towards augmentation of size, then we should be able to understand how an organ which has become useless would be rendered, independently of the effects of disuse, rudimentary and would at last be wholly suppressed; for the variations towards diminished size would no longer be checked by natural selection.
The principle of the economy of growth, explained in a former chapter, by which the materials forming any part, if not useful to the possessor, are saved as far as is possible, will perhaps come into play in rendering a useless part rudimentary.
But this principle will almost necessarily be confined to the earlier stages of the process of reduction; for we cannot suppose that a minute papilla, for instance, representing in a male flower the pistil of the female flower, and formed merely of cellular tissue, could be further reduced or absorbed for the sake of economising nutriment.
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