In Chapter XIII ("Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs") of The Origin of Species, Charles Darwin explores how natural it is to see harmless but unimportant bodily features persisting under an evolutionary process, and how unsatisfactory other explanations are for such irrelevant-to-survival organs:

I have now given the leading facts with respect to rudimentary organs. In reflecting on them, every one must be struck with astonishment: for the same reasoning power which tells us plainly that most parts and organs are exquisitely adapted for certain purposes, tells us with equal plainness that these rudimentary or atrophied organs, are imperfect and useless. In works on natural history rudimentary organs are generally said to have been created "for the sake of symmetry," or in order "to complete the scheme of nature;" but this seems to me no explanation, merely a restatement of the fact. Would it be thought sufficient to say that because planets revolve in elliptic courses round the sun, satellites follow the same course round the planets, for the sake of symmetry, and to complete the scheme of nature? An eminent physiologist accounts for the presence of rudimentary organs, by supposing that they serve to excrete matter in excess, or injurious to the system; but can we suppose that the minute papilla, which often represents the pistil in male flowers, and which is formed merely of cellular tissue, can thus act? Can we suppose that the formation of rudimentary teeth which are subsequently absorbed, can be of any service to the rapidly growing embryonic calf by the excretion of precious phosphate of lime? When a man's fingers have been amputated, imperfect nails sometimes appear on the stumps: I could as soon believe that these vestiges of nails have appeared, not from unknown laws 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.

... and, a few pages later, via a linguistic metaphor:

As the presence of rudimentary organs is thus due to the tendency in every part of the organisation, which has long existed, to be inherited — we can understand, on the genealogical view of classification, how it is that systematists have found rudimentary parts as useful as, or even sometimes more useful than, parts of high physiological importance. Rudimentary organs may be compared with the letters in a word, still retained in the spelling, but become useless in the pronunciation, but which serve as a clue in seeking for its derivation. On the view of descent with modification, we may conclude that the existence of organs in a rudimentary, imperfect, and useless condition, or quite aborted, far from presenting a strange difficulty, as they assuredly do on the ordinary doctrine of creation, might even have been anticipated, and can be accounted for by the laws of inheritance.

This discussion was presaged succinctly in Darwin's notebooks of 1837-38, two decades before The Origin of Species was published, in which he notes:

When one sees nipple on man's breast, one does not say some use, but sex not having been determined — so with useless wings under elytra of beetles — born from beetles with wings, and modified — if simple creation merely, would have been born without them.

TopicScience - TopicLiterature - 2006-12-20

(correlates: DarwinOnTaxonomy, ExcrementalTypo, AirFlow, ...)