HOMOLOGY, ANALOGY AND PLASIS 



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simple homogeny is in such cases connected 

 by the term homology, and he proceeded 

 to state the proposition thus: "When 

 identical or nearly similar forces, or 

 environments, act on two or more parts 

 of an organism which are exactly or nearly 

 alike, the resulting modifications of the 

 various parts will be exactly or nearly 

 alike. Further, if, instead of similar 

 parts in the same organism, we suppose 

 the same forces to act on parts in two 

 organisms, which parts are exactly or 

 nearly alike and sometimes homogenetic, 

 the resulting correspondences called forth 

 in the several parts in the two organisms 



will be nearly or exactly alike 



I propose to call this kind of agreement 



homoplasis or homoplasy What is 



put forward here is this: that under the 

 term 'homology', belonging to another 

 philosophy, evolutionists have described 

 and do describe two kinds of agreement — ■ 

 the one, now proposed to be called 

 'homogeny', depending simply on the 

 inheritance of a common part; the other, 

 proposed to be called 'homoplasy, ' depend- 

 ing on a common action of evoking causes 

 or moulding environments on such homo- 

 geneous parts, or on parts which for other 

 reasons offer a likeness of material to 

 begin with." 



Lankester's paper is perhaps the most 

 interesting tour deforce in the history of the 

 subject. We shall take up in order the 

 questions it raises. 



Homogeny. As already indicated, this 

 is simply a distinctive name for embryo- 

 logical homology, so designed as to pre- 

 clude any chance of misapplication or 

 misapprehension. Because of its doubtful 

 antecedents (note his phrase, "belonging 

 to another philosophy"), he drops the 

 word homology altogether. 



Homoplasy. This conception, being a 

 novel one, demands and obtains chief 

 consideration in his paper. It has for us 



a two-fold interest: (1) in respect of the 

 plasis or moulding idea; (z) in respect of 

 the relation of homoplasy to the old 

 analogy. 



Plasis or moulding. Significant is his 

 clear enunciation, at this early date, of the 

 conception that (by whatever imaginable 

 means) forces of environment act upon and 

 mould developing structure. The lan- 

 guage, be it observed, is not that of 

 natural selection; actively determining or 

 causal forces are predicated. Though the 

 experimental procedure is lacking, his 

 speech is essentially the speech of Entwick- 

 lungsmechanik, and to this extent fore- 

 shadows a new outlook. 



Homoplasy and analogy. In view of 

 what has preceded, we naturally ask: 

 "In what regard, if any, does homoplasy 

 differ from the old (structural) analogy?" 

 Lankester did refer to the point in his 

 paper, but elected, for reasons which he 

 does not specify, to differentiate between 

 the two. By confining the application 

 of his term "homoplasy" to organisms 

 or parts of organisms which are nearly 

 or exactly alike, he seems deliberately to 

 have limited its scope. Possibly, too, he 

 was influenced by the feeling that the 

 classical cases of analogy would pre- 

 suppose forces of external environment, 

 whereas in his illustrations of homoplasy 

 the internal environment of the animal 

 is apparently the main moulding influence. 



In later life Lankester has shown a 

 disposition to extend the scope of his 

 homoplasy so as to cover all cases of 

 convergence or parallelism. His original 

 definition involved simultaneous specifica- 

 tion of two separate likenesses, (1) 

 "identical or nearly similar forces of 

 environments," (z) "parts of an organism 

 which are exactly or nearly alike," or 

 "parts in two organisms, which parts are 

 exactly or nearly alike," or "parts which 

 for other reasons show a likeness of 





