BioConcepts

missing linkmissing link (ger.)

• Something lacking to complete a series or to form an intermediate between two things, esp. in an evolutionary process; (Physical Anthropol.) a hypothetical animal assumed to be an evolutionary link between man and the anthropoid apes, esp. as sought by early evolutionary biologists. (OED 2011)
  category phylogenesis

  1844

  The internal structure of the stem, and the character of the seed-vessels, shew them [scil. the plants of the club-moss family (Lycopodiacae)] to have been a link between single-lobed and double-lobed plants, a fact worthy of note, as it favours the idea that, in vegetable, as well as animal creation, a progress has been observed, in conformity with advancing conditions. It is also curious to find a missing link of so much importance in a genus of plants which has long ceased to have a living place upon earth.

  [Chambers, L.] (1844). Vestiges of the Natural History of Creation: 86-7.

  1851

  A break in the chain implying no doubt many missing links in the series of geological monuments which we may some day be able to supply.

  Lyell, C. (1851). Elements of Geology: 220 (acc. to OED 1989).