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numerous and even very interesting species were unknown in Darwin's time, the collection he 

 used was indeed to a high degree representative. Moreover, his knowledge of the subject was 

 an extremely thorough one : with much better methods than were known in his time, some of 

 the later workers may have been able to penetrate further into details of anatomy or embryo- 

 logy, but none of them have surpassed or even equalled him in sharpness of observation, and 

 in acuteness of separating different forms and uniting what belonged together. It must have 

 been extremely difficult for Darwin to find out how the numerous species of Ba/anus were to be 

 classified in natural groups; that these groups in the main are still used, nearly 60 years after 

 the publication of Darwin's Monograph, certainly proves that in several respects his classification 

 was founded on a reasonable basis. 



Darwin, when separating the genus Balamis with its numerous species into sections, 

 exclusively made use of the structure of the shell namely, the parietes, the radii, and the 

 basis. The barnacles of his Section A have the parietes, basis, and radii permeated by pores ; 

 those of Section D have the parietes permeated, the basis and radii not permeated; those of 

 Section F have the parietes and radii not permeated, the basis sometimes permeated, and 

 sometimes not. His Section B embraces those species in which the basis and parietes are 

 sometimes permeated and sometimes not, the radii not permeated, but with an elongated shell 

 and a boat-shaped basis ; and, finally, his Section E was instituted for thosë species which 

 have a membranous basis. I put this classification in the form of a table, which may make 

 the comparison easier : 



ƒ. Basis membranous (Section E) 

 II. Basis calcareous (Sections A, B, C, D and F) 



/. Shell elongated, basis boat-shaped [Section B) 



2. Shell not elongated, basis not boat-shaped (Sections A, C, D and F) 



a. Parietes not permeated [Section F) 



b. Parietes permeated (Sections A, C and D) 



k. Basis not permeated (Section D) 

 p. Basis permeated (Sections A and C) 



* Radii not permeated (Section C) 

 ** Radii permeated (Section A) 



We see than, that to refer a species of the genus Balanus to one of these sections, a 

 careful study of the structure of the different parts of the shell is necessary, but this structure 

 was at the same time thought sufficiënt. Although Darwin carefully investigated the structure 

 of the animal's body, and especially the parts of the mouth and the cirri, he seems to have 

 been impressed more by the general agreement of that structure in the different species, than 

 by the differences shown. Of several and very interesting species (B. dcclivis, B. navicula, B. 

 terebrahis etc.) the animal's body was unknown to him, and this circumstance may have 

 contributed to his not using these parts for purposes of classification. The fact is that in Darwin's 

 classification of the genus Balamis the division into sections rests on the structure of the shell, 



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