On the view of descent with modification, the origin of rudimentary organs
is comparatively simple; and we can understand to a large extent the laws
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.
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