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