Originally termed a homotypy by Ernst H. Haeckel (1834-1919) that great coiner of neologisms, it refers to similarities in repetitive or serial structures within the same organism. Examples include the arms and legs of humans or among the branches and leaves of a tree. A more complex example is the jointed appendages of anthropoids. In the crayfish, there are 19 pairs of appendages that have the same pattern, but serve different functions (e.g., chewing, egg carrying, food handling, locomotion, mating, and swimming). The concept has also been applied tentatively to the study of ontogenetic development within species in terms of whether structure-function relationships remain the same or change.
See Comparative method, Homology, Laws of nature, Structure-function relationships