islands; and in this case natural 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
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