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.
|
| 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.
|
| possible), the rudimentary part would tend to be 1860 1861 1866 |
| possible) the rudimentary part would tend to be 1859 |
| OMIT 1869 |
| 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.
|
|
| As the presence of
rudimentary organs
is thus due to the tendency in every part of the organisation, which has long existed, to be
inherited—
we can understand, on the genealogical view of classification, how it is that systematists
have
found rudimentary parts as useful as, or even sometimes more useful than, parts of high physiological importance. Rudimentary organs may be compared with the letters in a word, still retained in the spelling, but become useless in the pronunciation, but which serve as a clue in seeking
for its derivation. On the view of descent with
modification, we may conclude that the existence of organs in a rudimentary, imperfect, and useless condition, or quite aborted, far
|
