origin of rudimentary organs in a state of
nature, {455} further than by showing that rudiments can be produced; for I
doubt whether species under nature ever undergo abrupt changes. I believe
that disuse has been the main agency; that it has led in successive
generations to the gradual reduction of various organs, until they have
become 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 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 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 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 {456} 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 lo