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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 Balanus 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 A4alanus 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 those species which 
have a membranous basis. I put this classification in the form of a table, which may make 
the comparison easier: 
Z, Basis membranous (Sectzon £) 
IT. Basis calcareous (Sections A, B, C, D and F) 
z. Shell elongated, basis boat-shaped (Sectzon £) 
2. Shell not elongated, basis not boat-shaped (Sections A, C, D and F) 
a. Parietes not permeated (Sectzon F) 
6. Parietes permeated (Sections A, C and D) 
g. Basis not permeated (Sectzon D) 
(i. Basis permeated (Sections A and C) 
* Radii not permeated (Sectzon C) 
*™ Radii permeated (Section A) 
We see than, that to refer a species of the genus Ba/anus 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 sufficient. 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. declivis, B. navicula, B. 
terebratus 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 Darwiy’s 
classification of the genus a/anus the division into sections rests on the structure of the shell, 
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