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.