LANKESTER ON HOMOGENY 267 



ated 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 pro- 

 posed in 1870 a genetic definition of homology. 1 He pro- 

 posed, 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 liomogenous (p. 36). All 

 other resemblances were to be called homoplastic. " Homo- 

 plasy 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 morpholo- 

 gists 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 older " Platonic " philosophy, and ought to be superseded 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. 



1 " On the use of the term Homology in Modern Zoology, and the 

 distinction between Homogenetic and Homoplastic agreements," Ann. 

 Mag. Nat. Hist. (4), vi., pp. 35-43, 1870. 



