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.

&fist; 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.