Evolutionary developmental biology - How does evolutionary change in an organ occur?
The theory of recapitulation - otherwise known as the Biogenetic Law or Haeckel's law - is easily stated: in a phrase,
“ontogeny recapitulates phylogeny”.
In other words, it is the partly or wholly erroneous theory that an individual, during its development, passes through a series of stages corresponding to its successive evolutionary ancestors. An individual thus develops by 'climbing up its family tree'.
Recapitulation is noticeably common.
This theory is based upon transitory appearance of structures resembling gill slits in the development of humans, and other mammals. Mammals evolved from an ancestral fish stage and their embryonic gill slits recapitulate the piscine ancestry.
However, there are many examples of specialized larval forms that are not recapitulated ancestral stages (butterflies are a good example; the lifecycle of this swallowtail butterfly opposite shows no indication of passing through phylogenetic stages).