On the Origin of Species · ¶680

It is an important fact that rudimentary organs, such as teeth in the upper jaws of whales and ruminants, can often be detected in the embryo, but afterwards wholly disappear. It is also, I believe, a universal rule, that a rudimentary part or organ is of greater size relatively to the adjoining parts in the embryo, than in the adult; so that the organ at this early age is less rudimentary, or even cannot be said to be in any degree rudimentary. Hence, also, a rudimentary organ in the adult, is often said to have retained its embryonic condition.

Charles Darwin · John Murray, London, 1859, first edition · Gutenberg #1228; original diagram from Darwin Online F373
Charles Darwin · John Murray, London, 1859, first edition · Gutenberg #1228; original diagram from Darwin Online F373
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