360 THE ZOOLOGIST. 



namely, the revelation of one or other of the many agencies in their 

 growth and structure. 



Since there are all sorts and conditions of resemblances we 

 require technical terms. Of these there is abundance, and it is with 

 reluctance that I propose adding to them. I do so because un- 

 fortunately some terms are undefined, perhaps not definable ; others 

 have not ' caught on,' or they suffer from that mischievous law of 

 priority in nomenclature. 



The terms concerning morphological homologies date from Owen ; 

 Gegenbaur and Haeckel re-arranged them slightly. Lankester, in 

 1870, introduced the terms homogenous, meaning alike born, and 

 homoplastic or alike moulded. Mivart rightly found fault with the 

 detailed definition and the subdivisions of Homoplasy, and very 

 logically invented dozens of new terms, few of which, if any, have 

 survived. It is not necessary to survey the ensuing literature. For 

 expressing the same phenomenon we have now the choice between 

 Homoplasy, Homomorphy, Isomorphy, Heterophyletic Convergence, 

 Parallelism, &c. After various papers by Osborn, who has gone very 

 fully into these questions, and Willey's ' Parallelism,' Abel, in his 

 fascinating ' Grundziige der Palaeobiologie,' has striven to show by 

 numerous examples that the resemblances or ' adaptive formations ' 

 are cases of parallelism if they depend upon the same function of 

 homologous organs, and convergences if brought about by the same 

 function of non-homologous organs. 



I suggest an elastic terminology for the various resemblances 

 indicative of the degree of homology of the respective organs, the 

 degree of affinity of their owners, and lastly the degree of the 

 structural likeness attained, 



Homogeny. — The structural feature is invented once and is 

 transmitted, without a break, to the descendants, in which it remains 

 unaltered, or it changes by mutation or by divergence, neither of 

 which changes can bring the ultimate results nearer to each other. 

 Nor can their owners become more like each other, since the respec- 

 tive character made its first appearance either in one individual, or, 

 more probably, in many of one and the same homogenous com- 

 munity. 



Homoplasy. — The feature or character is invented more than 

 once, and independently. This phenomenon excludes absolute 

 identity ; it implies some unlikeness due to some difference in the 

 material, and there is further the chance of the two or more inven- 

 tions, and therefore also of their owners, becoming more like each 

 other than they were before. 



(To be continued.) 



