Homology

Homology Ho*mol"o*gy, n. [Gr. ? agreement. See {Homologous}.] 1. The quality of being homologous; correspondence; relation; as, the homologyof similar polygons. [1913 Webster]

2. (Biol.) Correspondence or relation in type of structure in contradistinction to similarity of function; as, the relation in structure between the leg and arm of a man; or that between the arm of a man, the fore leg of a horse, the wing of a bird, and the fin of a fish, all these organs being modifications of one type of structure. [1913 Webster]

Note: Homology indicates genetic relationship, and according to Haeckel special homology should be defined in terms of identity of embryonic origin. See {Homotypy}, and {Homogeny}. [1913 Webster]

3. (Chem.) The correspondence or resemblance of substances belonging to the same type or series; a similarity of composition varying by a small, regular difference, and usually attended by a regular variation in physical properties; as, there is an homology between methane, {CH4}, ethane, {C2H6}, propane, {C3H8}, etc., all members of the paraffin series. In an extended sense, the term is applied to the relation between chemical elements of the same group; as, chlorine, bromine, and iodine are said to be in homology with each other. Cf. {Heterology}. [1913 Webster]

{General homology} (Biol.), the higher relation which a series of parts, or a single part, bears to the fundamental or general type on which the group is constituted. --Owen.

{Serial homology} (Biol.), representative or repetitive relation in the segments of the same organism, -- as in the lobster, where the parts follow each other in a straight line or series. --Owen. See {Homotypy}.

{Special homology} (Biol.), the correspondence of a part or organ with those of a different animal, as determined by relative position and connection. --Owen. [1913 Webster]