At the beginning of the 19th century the term analogy was still synonymous with similarity, as for example in the case of Georges Cuvier. Exact criteria for determining analogy are first found in the work of Etienne Geoffroy Saint-Hilaire. Thereupon the English scientists Sharp MacLeay, William Swainson, John Obadiah Westwood and Edwin Strickland distinguished between analogy as correspondence between certain parts of the organism, i. e. only superficial resemblance, and affinity as an essential similarity in some remarkable aspects of form. Relying on these theories Richard Owen developed his theory of analogy ("a part which has the same function as another") and homology ("the same organ in different animals under every variety of form and function"). The criteria to distinguish between these two terms had to be modified and specified when the theory of evolution was developed by Charles Darwin. In the work of Thomas Henry Huxley, Ernst Haeckel and Carl Gegenbaur the modern biological term of analogy was developed, but at the same time it lost much of its importance and homology as a criterion for natural affinity became the central objective of further biological research.