Rudimentary organs and evolutionary reduction

In 1859 Darwin argued that disuse will often reduce an organ that has become useless, and that this makes the meaning of rudimentary organs clear¹. He added a rule about timing. A rudimentary organ is relatively larger in the embryo than in the adult, so at that age it is less rudimentary, or not rudimentary at all². For this reason an adult rudimentary organ is often said to have kept its embryonic condition². Selection and disuse act mostly on the mature animal, so the young organ is left largely untouched¹.

In Insectivorous Plants (1875) Darwin applied the idea to Drosera binata. Some of its leaf structures vary in presence, size and position. He took this as a sign that they are of little service and are tending towards abortion³. He proposed that Drosophyllum probably represents an early progenitor of the genus Drosera⁴. Its tentacles cannot move when excited, yet the plant still captures insects. Drosera binata, on this view, keeps remnants of ancestral characters that most other species have lost⁴. He named the loss of tentacle movement as the most important functional difference between the two genera⁵.

In The Power of Movement in Plants (1880) Darwin described rudimentary leaflets on another plant. As with most rudimentary organs, he wrote, they vary in size, leave their normal position and are often missing⁶. He also noted that the first leaves of seedlings lack them⁶. Rádl (1930) records a similar general claim in the older literature. Organs of which an animal has more than one, rudimentary organs, and organs of simple structure show the most variation⁹.

Metchnikoff (1903) treated rudimentary organs as vestiges of more developed organs that served a useful function in ancestors. He held that their abundance in man is further proof of animal origin⁷. Rádl (1930) gave the idea a different history. In his account Haeckel made great use of it, because a purposeless organ seemed to disprove that nature reveals a definite purpose⁸. Haeckel called this study Dysteleology. Rádl adds that Haeckel forgot De Candolle had already built much of his morphology on the same assumption⁸.