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structure and function, 1869 1872
function, 1859 1860 1861 1866

Organs, originally formed by the aid of natural selection, 1872
An organ, 1859 1860 1861 1866 1869

when 1859 1860 1861 1866 1872
originally formed by the aid of natural selection, when 1869

has to exert 1869 1872
to 1859 1860 1861 1866

tend to reproduce 1872
reproduce 1859 1860 1861 1866 1869

mature age, but 1869 1872
age, and consequently 1859 1860 1861
age, but 1866

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

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.

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.
Any change in structure and function, which can be effected by
is within the power of natural selection; so that an organ rendered,
changed habits of life, useless or injurious for one purpose, might
be modified and used for another purpose.
Or an
also, be
retained for one alone of its former functions. Organs, originally formed by the aid of natural selection, when rendered
may well be variable, for
can no
longer be
checked by natural selection. All this agrees well with what we see under nature.
Moreover, at
whatever period of life
either disuse
or selection reduces an organ, and this will generally be when the being has come to maturity and has to exert its full powers of action, the principle of inheritance at corresponding ages will tend to reproduce the organ in its reduced state at the same mature age, but will seldom affect
or reduce
it in the embryo. Thus we can understand the greater size of rudimentary organs in the embryo relatively to the adjoining parts, 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.
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