Comparison with 1872 |
|
Organs, originally formed by the aid of natural selection, Organs, originally formed by the aid of natural selection, 1872 |
An organ, 1859 1860 1861 1866 1869 |
when
rendered
useless useless 1872 | useless, 1859 1860 1861 1866 1869 |
may well be variable, for
their their 1872 | its 1859 1860 1861 1866 1869 |
variations
can no can no 1869 1872 | cannot 1859 1860 1861 1866 |
longer be longer be 1869 1872 | be 1859 1860 1861 1866 |
checked by natural selection. All this agrees well with what we see under nature.
Moreover, at Moreover, at 1872 | At 1859 1860 1861 1866 1869 |
whatever period of life
either disuse either disuse 1869 1872 | disuse 1859 1860 1861 1866 |
or selection reduces an organ, and this will generally be when the being has come to maturity and
has to exert has to exert 1869 1872 |
to 1859 1860 1861 1866 |
its full powers of action, the principle of inheritance at corresponding ages will
tend to reproduce tend to reproduce 1872 |
reproduce 1859 1860 1861 1866 1869 |
the organ in its reduced state at the same
mature age, but mature age, but 1869 1872 |
age, and consequently 1859 1860 1861 |
age, but 1866 |
will seldom affect
....... 1869 1872 | or reduce 1859 1860 1861 1866 |
it in the embryo. Thus we can understand the greater
size of rudimentary organs in the embryo relatively to the adjoining parts, size of rudimentary organs in the embryo relatively to the adjoining parts, 1872 |
relative size of rudimentary organs in the embryo, 1859 1860 1861 1866 |
size of rudimentary organs in the embryo relatively to its other parts, 1869 |
and their lesser relative size in the adult. 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. ↑2 blocks not present in 1872; present in 1859 1860 1861 1866 1869 | But if each step of the process of reduction were to be inherited, not at the
corresponding age, but at an extremely
early period of life
(as we have good reason to believe
to be possible) the rudimentary part would tend to be
wholly lost, and we should have a case of complete abortion.
The principle, also, of economy,
explained in
a former chapter, by which the materials forming any part
or structure,
if not useful to the possessor, will be saved as far as is possible, will probably often
come into play;
and this will tend to cause
the entire obliteration of a rudimentary organ.
|
|
|
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. |
|
Finally, as Finally, as 1872 |
As the presence of 1859 1860 1861 1866 1869 |
rudimentary
organs, organs, 1872 | organs 1859 1860 1861 1866 1869 |
by whatever steps they may have been degraded into their present useless condition, are the record of a former state of things, and have been retained solely through the power of by whatever steps they may have been degraded into their present useless condition, are the record of a former state of things, and have been retained solely through the power of 1872 |
is thus due to the tendency in every part of the organisation, which has long existed, to be 1859 1860 1861 1866 1869 |
inheritance,— inheritance,— 1872 | inherited— 1859 1860 1861 1866 1869 |
we can understand, on the genealogical view of classification, how it is that
systematists, systematists, 1872 | systematists 1859 1860 1861 1866 1869 |
in placing organisms in their proper places in the natural system, have often in placing organisms in their proper places in the natural system, have often 1872 |
have 1859 1860 1861 1866 1869 |
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
....... 1869 1872 | in seeking 1859 1860 1861 1866 |
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
|