40 GILBERT C. BOUENE. 



ontogeny of a Fissurellid and it involves a homogenetic 

 character, therefore it also is due to liomoplasy. On the 

 same reasoning the resemblances in the shell, foot, and mantle 

 of more distantly related forms, the Patellidas, Septaria, the 

 Capulidae, and Siphonariidae are homoplastic. But should the 

 pallial branchise of a Patella and the gill of a Siphonaria, be 

 attributed to parallelism or homoplasy ? They are certainly 

 not genetically derived from the typical moUuscan ctenidium, 

 and to this extent are deficient in the element of homology 

 which Osborn says should always be associated with homology. 

 On the other hand they are vascular outgrowths of the mantle, 

 which is assuredly a homogenetic structui-e in all the forms in 

 question, and therefore there is an element, though a more 

 remote element, of homology. In this case it is siujply a 

 question of the importance attached to the degree of homo- 

 logy whether these structm-es should be ascribed to parallel 

 or homoplastic development. But Lankester's term, homo- 

 plasy, as originally defined, covers all the cases. It appears 

 to me thatj while there is a contrast between homoplasy and 

 convergence, there is no such contrast between homoplasy 

 and parallelism, and that for the sake of clarity the last term 

 should be abandoned, homoplasy being retained in the sense 

 originally defined by Lankester. It has the priority over 

 Fiirbringer's term homomorphy, which, as Osborn points 

 out, has the same connotation ; and it has the advantage of 

 indicating a resemblance due to the moulding influence of 

 environment, whereas homomorphy only calls attention to 

 similarity of form. 



In the latter half of his essay Osborn raises a most interest- 

 ing question, which has presented itself with various degrees 

 of insistence to workers in various groups of the animal 

 kingdom. Drawing his evidence from palaeontological as 

 well as recent types, he points out that the accessory cusps in 

 the molar teeth of Mammalia arise in the same order and with 

 the same relations to the primary cusps in groups which can 

 be proved to have diverged widely from one another before 

 any complication of the tooth pattern arose. Here^ then, are 



