CHAPTER XIV. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY -- EMBRYOLOGY -- RUDIMENTARY ORGANS.
6. RUDIMENTARY, ATROPHIED, AND ABORTED ORGANS. (continued)
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 will seldom affect 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.