governing their imperfect development. We have plenty of cases
of rudimentary organs in our domestic productions, as the stump of a
tail in tailless breeds, the vestige of an ear in earless breeds of
sheep--the reappearance of minute dangling horns in hornless breeds of
cattle, more especially, according to Youatt, in young animals--and the
state of the whole flower in the cauliflower. We often see rudiments of
various parts in monsters; but I doubt whether any of these cases throw
light on the origin of rudimentary organs in a state of nature, further
than by showing that rudiments can be produced; for the balance of
evidence clearly indicates that species under nature do not undergo
great and abrupt changes. But we learn from the study of our domestic
productions that the disuse of parts leads to their reduced size; and
that the result is inherited.

It appears probable that disuse has been the main agent in 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 small
stages, is within the power of natural selection; so that an organ
rendered, through changed habits of life, useless or injurious for one
purpose, might be modified and used for another purpose. An organ
might, also, be retained for one alone of its former functions. Organs,
originally formed by the aid of natural selection, when rendered useless
may well be variable, for their variations 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 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 wi