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Show 454 RUDIMENTARY ORGANS. CHAP. XIII. of growth, but in order to excrete horny matter, as that the rudimentary nails on the fin of the manatee were formed for this purpose. On my view of descent with modification, the origin of rudimentary organs is simple. We have plenty of cases of rudimentary organs in our domestic produc- · tions,-as the stump of a tail in tailless breeds,-the vestige of an ear in earless breeds,-the reappearance of minute dangling horns in hornless breeds of cattle, more especially, according to Youatt, in young anin1als, -and the state of the whole flowerJ 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 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, a:n organ useful under certain conditions, might become injurious under others, as with the wings ofbeetles 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 chan.ged habits of life, useless or injurious for one purpose, might easily be modified and used for another purpose. ?r an organ might be retained for one alone of Its CHAP 0 XIII. HUDIMENTARY ORGANS. 455 former functions. . An organ '. when render e d use1 e ss, may well be vanable, for Its variations cannot be checked by natural selection. At whatever per· d f 1'.{! di l . IO 0 11e suse or se ection reduces an organ, and this will ge~erally be when the ~eing 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 underst~n~ the greater relativ~ size of rudimentary organs In the embryo, and their lesser relative size in the adult. .But ~ each step of the process of reduction were to be Inhented, not a:t the corresponding age, but at an extrem~ly early peno~ of life (as we have good reason to beheve to be possible) the rudimentary part would tend to be wholly lost, and we should have a case of co~plet~ abortion. The principle, also, of economy, expl~Ined m a former chapter, by which the materials formmg ~ny part or structure, if not useful to the possessor, will ?e saved as far as is possible, will probably oft~n com~ Int~ play; and this will tend to cause the entire obliteration of a rudimentary organ. As the presence of rudimentary organs is thus du~ to the tende~cy in ever~ part of the organisation, which has long existed, to be Inherited-we can understand, on the ?enealogical view of classification, how it is that systematists have found rudimentary parts as useful as, o~ ev~n s?metimes more useful than, parts of high physiOlogica! rmportance. Rudimentary organs may be ~~:pare~ With the letters in a w?rd, still retained in sp~llmg, but become useless In the pronunciation, but WhiC~ serve as a clue in seeking for its derivation. ~n the VIew of de~cent with modification, we may con. ude that the ·existence of organs in a rudimentary Imperfect, and useless condition, or quite aborted, fa |