36 GILBERT C. BOURNE. 



the affinities of animals from morphological evidence, namely, 

 that a similar environment and similar habits of life reacting 

 on a similar organisation may often produce very similar struc- 

 tural results. Not, however, identical, for however similar the 

 results may appear at first sight in all cases of convergence 

 a close analysis will always disclose differences which exclude 

 the idea of direct descent of the animals in question. This 

 instance is particularly instructive ; the Haliotidae, Scissu- 

 rellidae and Fissurellidae have all inherited the same structure 

 from a presumably Pleurotomariid ancestor, viz. the slit in 

 the mantle and the corresponding labral incision in the shell. 

 It has been variously modified, and similar modifications are 

 displayed independently by different groups, the similarity of 

 the evolutionary series being, as far as one can judge, 

 correlated with the adoption of similar habits. 



Addendum. 



It is long since I first read the short but profound essay 

 of Sir Ray Lankester (9) '^ On the Use of the term Homology 

 in Modern Zoology, and the Distinction between Homogenetic 

 and Homoplastic Agreements." On referring again to this 

 essay, I find that the conclusions arrived at in the foregoing 

 paragraph, as also similar conclusions arrived at after a 

 detailed study of various members of the Neritidge (1), are 

 unconsciously expressed in nearly the same words that he 

 used forty years ago. I have to beg Sir Ray Lankester's 

 pardon for not making specific reference to his essay in my 

 former paper. But I find a certain satisfaction in not having 

 had the form of his argument clearly in my mind while I was 

 working to the same conclusion from evidence gathered from 

 the study of the probable lines of descent of animals belong- 

 ing to a different class to that which he used to illustrate his 

 original thesis. Had I consciously set out to prove, or even 

 to disprove, his contention, I could not have avoided a certain 

 amount of bias. To have arrived unconsciously — or sub- 

 consciously, for the idea of homoplasy inculcated by him was 



