HOMOLOGY. Owen (1804-92) introduced the terms homo logy and analogy into biological literature to imply two differ ent types of similarity of structure in organisms. Homologous structures were such as were capable of being shown by study of a series of forms to be structurally equivalent, though not necessarily functionally similar, to a part or organ of the "arche type." Analogous structures were those which showed physi ological similarity only. With the coming of the evolution theory the archetype of Cuvierian biology became identified with a common ancestor, so that the current definition of homology in biology is that of Dendy (1912) : "homologous organs are such as have the same essential structure which they owe to inherit ance from a common ancestor." In 187o Lankester introduced the term homoplasy to indicate structural resemblance associated with similar physiological activity but derived independently in the evolutionary process, and added the term homogeny to sig nify homology in the sense in which that term is now employed. In the light of modern experimental work on evolution it is doubtful whether the distinction, stated thus, is of so fundamental a character as it then appeared. The important researches of Sturtevant (1924) show that similar bodily modifications have arisen as sports again and again in different species of the fruit fly Drosophila; and that these mutant characters can be referred to changes in the corresponding location on equivalent chromo somes (see CYTOLOGY). There is good reason to believe parallel mutation (appearance of sports) has occurred constantly in the evolutionary process; and if this is so, it is impossible on purely morphological grounds to say when a new structure has arisen independently or has been inherited from a common ancestor. Owen's archetype may be replaced in the future not by the Dar winian concept of a common ancestor, but by a common locus of instability in the chromosome, or a similar physico-chemical pat tern in the hereditary materials. (See ZOOLOGY; EVOLUTION.) (L. T. H.)