Special homology, on the other hand, relates to resemblances between organs in different animals. The interesting thing is that Gegenbaur defines it genetically. Special homology is the name we give "to the relations which obtain between two organs which have had a common origin, and which have also a common embryonic history" (_Elements_, p. 64). This is his definition; but, in practice, Gegenbaur establishes homologies by comparison just as the older anatomists did, and infers common descent from homology, not homology from common descent. "Special homology," he continues, "must be again separated into sub-divisions, according as the organs dealt with are essentially unchanged in their morphological characters, or are altered by the addition or removal of parts" (p. 65). In the former case the homology is said to be "complete," in the latter "incomplete." Thus the bones of the upper arm are completely homologous throughout all vertebrate classes from Amphibia upwards, while the heart of a fish is incompletely homologous with the heart of a mammal. Independently of Gegenbaur, Sir E. Ray Lankester proposed in 1870 a genetic definition of homology.[385] He proposed, indeed, to do away with the term homology altogether, on the ground that it included many resemblances which were obviously not due to common descent--as, for instance, the resemblance of metameres. So, too, organs which were homologous in the ordinary sense, as the heart of birds and mammals, might have arisen separately in evolution. He proposed, therefore, that "structures which are genetically related, in so far as they have a single representative in a common ancestor," should be called _homogenous_(p. 36). All other resemblances were to be called _homoplastic_. "Homoplasy includes all cases of close resemblance of form which are not traceable to homogeny, all details of agreement not homogenous, in structures which are broadly homogenous, as well as in structures having no genetic affinity" (p. 41). Serial homology, for instance, was a case of homoplasy. The term "analogy" was to be retained for cases of functional resemblance, whether homogenetic or not. The attempt was an interesting one, but most morphologists wisely adhered to the old concept of homology, in spite of Lankester's declaration that this belonged to an older "Platonic" philosophy, and ought to be superseded by a term more consonant with the new philosophy of evolution. [366] _Generelle Morphologie der Organismen. Allgemeine Grundzuege der organischen Formenwissenschaft, mechanisch begruendet durch die von Ch. Darwin reformierte Descendenztheorie_. Berlin, 1866. Reprinted in part as _Prinzipien der generellen Morphologie der Organismen_. Berlin, 1906. [367] He mentions as his predecessors in this field, Bronn, J. Mueller, Burmeister, and G. Jaeger.