IX THE HISTORY AND SCOPE OF ZOOLOGY 333 



of analogous structures holds good at the present day. 

 His homologous structures are now spoken of as 

 " homogenetic " structures, the idea of community of 

 representation in an archetype giving place to com- 

 munity of derivation from a single representative 

 structure present in a common ancestor. Darwinian 

 Morphology has further rendered necessary the intro- 

 duction of the terms "homoplasy" and "homo- 

 plastic " 1 to express that close agreement in form 

 which may be attained in the course of evolutional 

 changes by organs or parts in two animals which 

 have been subjected to similar moulding conditions of 

 the environment, but have no genetic community of 

 origin, to account for their close similarity in form and 

 structure. 



The classification adopted by Owen in his lectures 

 (1855) does not adequately illustrate the progress of 

 zoological knowledge between Cuvier's death and that 

 date, but, such as it is, it is worth citing here. 



Province : Vertebrata (Myelencephala, Owen). 



Classes : MAMMALIA, AVES, KEPTILIA, PISCES. 

 Province: Articulata. 



Classes : ARACHNIDA, INSECTA (including Sub-Classes Myria- 

 poda, ffexapoda), CRUSTACEA (including Sub-Classes Ento- 

 mostraca, Malacostraca), EPIZOA (Epizootic Crustacea), AN- 

 NELLATA (Chaetopods and Leeches), CIKRIPEDIA. 

 Province: Mollusca. 



Classes : CEPHALOPODA, GASTEROPODA, PTEROPODA, LAMELLI- 



BRANCHIATA, BRACHIOPODA, TUNICATA. 



1 See Lankester, " On the Use of the Term Homology in Modern 

 Zoology and the Distinction between Homogenetic and Homoplastic 

 Agreements," Ann. and Mag. Nat. Hist. 1870. 



