1902.] NATURAL SCIENCES OF PHILADELPHIA. 209 



correspoudeucs of one part in organism A with a similar part in 

 organism B is a special homology, when interpreted as a proof of 

 genetic affinity, A special homology is then our interpretation of 

 the similai'ity of the parts of two organisms as denoting homophyly, 

 or likeness of genesis. 



Various degrees of special homologies have been distinguished. 

 Thus Gegenbaur:^ complete homolocjy, when the organ in position 

 and connection is retained unaltered ; defective, when a part oi'igin- 

 ally present has been lost ; augmentative, wdaen a part not originally 

 present is added ; imitatory, as when one metamere produces a struc- 

 ture characteristic of a metamere further anterior or posterior (the 

 three last being classed by Gegenbaur as i)icomplete homologies). 

 Frequently also the distinction is made between adult and embryonic 

 (or larval) homologies, a distinction based upon the stage of the 

 iudi\'idual in which the homology is determined. Another grouping 

 of homologies, due mainly to the study of cell lineage, is actual 

 homology, when at the stage considered the relation is found ; pros- 

 pective, as when two blast omeres (of different organisms) are con- 

 sidered homologous because they give rise to similar organs in the 

 adults ; and retrospective, as when two adult parts (of different 

 organisms) are considered homologous because they are derived from 

 corresponding cells or. regions of the embryos (or larvse) of these 

 organisms. Still further classifications of homologies are possible, 

 but these are sufficient for our present purpose. 



Under the idea of "homology" we shall then understand 

 ' ' homophyly, ' ' and restrict in Avhat is to follow the term * ' homol- 

 ogy " to the idea of " special homology," unless otherwise specified. 

 In the determination of homologies we may proceed with the adult 

 structure (the anatomical method), or with the embryonic or larval 

 structure (the embr3'ological method). And we shall have to 

 decide, if possible and without bias, which of these methods is the 

 correct one : the one of Owen which lays stress upon the adult rela- 

 tions of structure, or the one which lays primary importance upon 

 ontogenetic similarities, and has been defined by Darwin'' as follows : 

 " Homology. — That relation between parts which results from their 

 development from corresponding embryonic parts, cither in different 

 animals .... or in the same individual." 



^ Vergleichende Anatomie der Wirbelthiere, I, 1898, 

 " Orirjin of Species, Glossary 



14 



