l selection would continue slowly to reduce the organ,
until it was rendered harmless and rudimentary.

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. At whatever period of life disuse or
selection reduces an organ, and this will generally be when the being
has come to maturity and to its full powers of action, the principle
of inheritance at corresponding ages will reproduce the organ in its
reduced state at the same age, and consequently will seldom affect or
reduce it in the embryo. Thus we can understand the greater relative
size of rudimentary organs in the embryo, and their lesser relative size
in the adult. 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.

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 from presenting a strange difficulty,
as they assuredly do on the ordinary doctrine of creation, might
even have been anticipated, and can be ac