438 PROFESSOR LANKESTER. 



parasitic and sub-parasitic animals, that there has been a 

 reversal of the stream of development, and that these forms 

 are the result, not of progressive adaptation, but of retro- 

 gressive adaptation or dege?ieration. 



In regard to both assumptions, the qualification has been 

 admitted grudgingly and insufficiently up to the present 

 time. It is not, as a rule, sufficiently conceded that homo- 

 plasy is as much a vera causa of structural likeness as 

 homogeny, and that, whilst we pursue the logical method of 

 assuming (lo begin with) a uniform cause — namely, homo- 

 geny — in order to account for structural likeness, yet we 

 should be on the alert whenever difficulty arises in the 

 consequences deduced in a particular case from the employ- 

 ment of homogeny, to test at once the applicability of 

 homoplasy. I# 



Thus Mr, Jhering, appropriating to himself the doctrine 

 and the term " homoplasy," has suggested that, in the group 

 of the MoUusca (as usually recognised by naturalists), are 

 included two homoplastic groups of totally different origin. 

 There is nothing improbable in this application of the doc- 

 trine of homoplasy, and indeed I am inclined to think that 

 some such application of it to the group of the Arthropoda 

 is the only escape from the difficulties which the hypothesis 

 of homogeny presents in regard to that group. With regard, 

 however, to the Mollusca, Mr. Jhering appears to have been 

 singularly unfortunate in the skill which he has displayed 

 in using borrowed tools. The two homoplastic groups which 

 Mr. Jhering fancies he has detected under the common type 

 Mollusca have each of them, according to him, independentlg 

 developed that very remarkable apparatus, the lingual ribbon^ 

 with its cushions and muscles. Such an assumption is entirely 

 devoid of justification. We see in Mr. Jhering's case an in- 

 teresting exhibition of the necessity for ascertaining and re- 

 specting the limits within which homoplasy may reasonably 

 be assumed as a possible cause of structural identity in a com- 

 parison of organisms. We know of no case in which there 

 is any ground for inferring that homoplasy — that is, inde- 

 pendent adaptation — has produced two structures so complex 

 and varied in detail and yet so absolutely the same in all 

 respects as are the lingual apparatus of Chiton, on the one 

 hand, and of the normal Gastropoda, on the other. The 

 assumption, without any collateral evidence in its favour, 

 of such potency for homoplasy, is a violation of common 

 sense, sufficiently reckless, even without the employ- 

 ment of folio paper, to give to its perpetrator a temporary 

 notoriety. 



