iNCisuKA (scissurella) lytteltonensis. 37 



always present to my mind — at an identical conclusion is to 

 give unequivocal support to the validity of the arguments by 

 which it was sustained. In the essay in question Lankester 

 showed that the term homology, which really belonged to 

 the platonic school of the natural philosophers of the end of 

 the eighteenth and the beginning of the nineteenth century, 

 acquired a new connotation after the publication of the 'Origin 

 of Species.' But this new connotation was indefinite. On 

 the one hand structures were said to be homologous which 

 ''are genetically related, in so far as they have a single 

 representative in a conmion ancestor.^' For this kind of 

 homology Lankester proposed to substitute the term " homo- 

 geny." On the other hand, various organs were described as 

 homologous which could not possibly be included under the 

 idea of homogeny, because, over and above general resem- 

 blances such as might be referred to inheritance from a 

 common ancestor, they exhibited a number of detailed 

 resemblances such as could not possibly be supposed to have 

 been represented, in like detail, in a generalised ancestral 

 form. Therefore, Lankester pointed out, there must be a 

 second quantity covered by the term homology, and he 

 described it in the following words: "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 some- 

 times homogenetic, the resulting correspondences called forth 

 in the several parts of the two organisms will be nearly or 

 exactly alike. ... I propose to call this kind of agree- 

 ment homoplasis or homoplasy. . . . What exactly 

 is to be ascribed to homogeny and what to homoplasy in the 

 relations of a series of structures is a matter for careful con- 

 sideration." Somewhat further on in the essay homoplasy is 

 defined as " depending on a common action of evoking causes 

 or moulding environment on homogenous (= homogenetic) 



