266 ERNST HAECKKL AND CARL GEGENBAUR 



with the ancestral form, which remains essentially hypo- 

 thetical" (p. 75). 



The facts of development, Gegenbaur goes on to say, 

 help us out greatly in our search for ancestral forms, for 

 the early stages in the ontogeny of a highly organised animal 

 give us some idea of the organisation of its original ancestor. 

 Characters common to the early ontogeny of all the members 

 of a large group are particularly important in this respect 

 (cf. von Baer's law). 



Gegenbaur distinguishes homologous or morphologically 

 equivalent structures from such as are analogous or physio- 

 logically eqivalent, just as did Owen and the older anatomists. 

 Like von Baer he recognises homologies, as a rule, only 

 within the type. 



He contributed, however, to the common stock a useful 

 analysis of the concept of homology, and established certain 

 classes and degrees of it. He distinguished first between 

 general and special homology, in quite a different sense 

 from Owen. 



General homology. in Gegenbaur's sense, relates to resem- 

 blances of organs within the organism, and includes four 

 kinds of resemblance, homotypy, homodynamv, homonomy 

 and homonymy. Right and left organs are homotypic, 

 metameric organs are homodynamic ; homonomy is the rela- 

 tion exemplified by fin-rays or fingers, which are arranged 

 with reference to a transverse axis of the body; homonymy 

 is a sort of metamerism in secondary parts (not the main 

 axis) of the body, and is shown by the various divisions of 

 the appendages (Grundzitgc, p. 80). 



Special homology, on the other hand, relates to re- 

 semblances 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" 

 (fc/t-iin-Hfs, 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 separ- 



