HOMOLOGY (from Gk. op.oXey/a, homologia, conformity, from 60No-yos, bon °logos, harmoniz ing, from 6,u6s, homos, same + -X0-yla, -login, account, from Xl-yco, legein, to say). (I) In biology, a term used to indicate structural co•re spondence based on blood-relationship, as op posed to analogy, which is applied to functional resemblance, such as that between the wings of a bird and those of a butterfly. Different sorts of homology have been recognized. There is the homology of the different parts of the body: namely, serial homology, as of the front and hind paired fins of fishes, or the arms and legs of man; antimerie homology. as of the petals of a flower or the right and left sides of the body. There is homology of corresponding or gans in different individuals. This is most cer tain in individuals of the same species; some what less certain between genera and families: often doubtful between orders and classes; usu ally speculative between phyla. The criteria of homology are: (a) The criterion of connections.
A part having similar relations in two species. and making, similar connections is homologous in the two cases. (b) The criterion of structure. Homologous parts have a fundamental similarity of structure. (e) The criterion of development. Homologous organs arise from the same germ layer in corresponding parts of the body and develop in the same fashion. Despite these elear-ent criteria, homologies are practically often very difficult of determination, particu larly beyond the limits of a class.
(2) In geometry. figures which are in per spective are often spoken of as homologous. he cause the centre and axis of perspective intro duced by Poneelet were called by Chasles centre and axis of homology. (See the articles CENTRE : The term i. used in another sen,e to signify the relation of reciprocity or duality (q.v.) which exists between geometry and al gebra.