(sect, a), balanus tintinnabui.um. 197 



the genus. There are some other species which vary quite as much 

 in external appearance; for instance, B. perforatus ; but B. tintin- 

 nabulum also varies in far more important points, as in the proportions 

 and structure of the opercular valves. The difficulty in determining 

 whether or not the differences are specific, is wonderfully increased by 

 whole groups of individuals varying in exactly the same manner. I 

 have seen three most distinct varieties taken from the bottom of the 

 same vessel, so that I did not at first entertain the least doubt that they 

 were three distinct species. I may mention, as showing the vacillations 

 which I have experienced on this subject, that beginning with the im- 

 pression, that the above three varieties were really distinct species, after 

 going over the several immense collections of specimens placed at my dis- 

 posal, I came to the conclusion that the above three, and several other 

 forms presently to be described, were only varieties ; yet after an interval 

 of some months, having to look at some of these specimens again, I could 

 not but think that I had come to a false conclusion, and so went into all 

 the details again, and satisfied myself that I had followed a right course ; 

 after another interval, I had to repeat the same process, and even now lean 

 never look at a group of the beautifully coloured shells with their small 

 rounded orifices of var. coccopoma attached to the Avicula margaritifera, 

 or again at var. cV Orbignii, with its toothed orifice and white tips to 

 the compartments, without thinking that they must be specifically 

 distinct from the dull-coloured specimens with large entire orifices so 

 common on ships' bottoms ; yet I can produce a full series of inter- 

 mediate forms, and I can further show, in each variety, that the several 

 points of difference by which each is characterised, are variable. I 

 may be permitted to add, in order to show that it has not been from 

 indolence that I have combined so many forms, that I had named and 

 already written out full descriptions of most of the varieties, before 

 determining to sacrifice them. 



Seeing that B. tintinnabulum and amphitrite are the two most 

 variable species in the genus, more especially in the important cha- 

 racters derived from the opercular valves, and knowing that these 

 species are attached so very frequently to ships' bottoms, one is led to 

 suspect that their extreme tendency to vary may be due to their being 

 exposed to varying and peculiar conditions, whilst transported to new 

 and distant localities. It is even just possible, as may be inferred 

 from the facts given in the Introduction (p. 102) in regard to certain 

 monstrous specimens of Bal. balanoides having been apparently im- 

 pregnated by adjoining individuals, that the varieties may interbreed, 

 and so produce numerous intermediate forms. Whether or not this 

 could take place, I am inclined to look at these two species, as in an 

 almost analogous condition with our domestic animals, which give rise 

 to such infinitely numerous varieties. It appears to me probable, that 

 several of the varieties keep true to their peculiarities, as long as they 

 continue to breed in the same locality; but that when their larvee 

 become attached to ships' bottoms, and are thus transported and 

 exposed to new conditions, they give rise to new and ever-varying 

 varieties. I will first give a full description of the more common forms 

 of B. tintinnabulum, which undoubtedly belong to the same species, 



