HARDWICKE'S SCIENCE-GOSSIP. 



121 



THE SHELL-COLOURING OF NON-MARINE MOLLUSCA. 



By WILFRED MARK WEBB, F.L.S. 



HE theory brought 

 forward by Mr. 

 J. W. Williams* 

 with regard to the 

 primitive colouring 

 of non-marine shells 

 has already been 

 discussed at length 

 by Mr. S. Facet 

 and Mr. C. Clare 

 Fryer, % but the 

 latter writer has 

 brought forward 

 some very interest- 

 ing points, and Mr. 

 Williams's § reply 

 to the former critic 

 calls for further re- 

 marks. 



Mr. Pace gives 

 an example of the want of clearness which he finds 

 in Mr. Williams's writing ; it can, however, be 

 said, in favour of the latter, that authors even less 

 lucid were quoted from, by the present writer in 

 "The Universal Review." || The meaning of the 

 passage in question is, that "evolution of all kinds" 

 has been from the simple to the complex. It is 

 only necessary to bring forward the case of parasites 

 in order to show the fallacy of such a statement, for, 

 in many instances, although they had arrived at a 

 stage of considerable differentiation before contracting 

 their peculiar habits, they have since evolved in a 

 downward direction. Mr. Williams gives it to be 

 understood, in the concluding paragraph of his article, 

 that he has studied Darwin's works, but in "The 



• "The Colouring and Banding in Land and Freshwater 

 Shells," Science-Gossip, August, 1890, p. 178. 



t "The Colouring and Banding of Freshwater Shells," 

 Science-Gossip, October, 1890, p. 233. 



X "The Colouring and Banding in Land and Freshwater 

 Shells," Science-Gossip, November, 1890, p. 241. 



C Ibid. Science-Gossip, December, 1890, p. 274. 



11 "The Zoology of the Magazines," "Universal Review," 

 October, 1890, p. 2^0. 



No. 318.— June 1891. 



Origin of Species "* considerable space is given to dis- 

 cussing evolution "from the complex to the simple," 

 and the following words occur : — " for natural selec- 

 tion, or the survival of the fittest, does not necessarily 

 include progressive development." 



To turn now to the various paragraphs in which 

 the facts supposed by Mr. Williams, to support his 

 theory, are set forth : 



(i.) With regard to the constitution of the primary 

 shell : — either Mr. Pace has some authority, with 

 which the writer is unacquainted, for saying that it 

 consists of conchyclin, or he has taken for granted 

 that the tissue which forms this substance in the 

 second instance, did so in the first. Has anybody 

 analysed the primary shell ? By-the-bye, the passage 

 quoted from Balfour's ' Embryology ' in Mr. Williams's 

 reply occurs on page 189 (not on page 229 as stated). 

 But in quibbling over chitin and conchyclin Mr. 

 Williams misses the crucial point of the criticism, to 

 wit, that as the substance of the primary shell is 

 " naturally horn-coloured" it can have no bearing, 

 in the matter of colouring, on the calcareous pigmen- 

 ted secondary shell, to the animal part of which it 

 alone can possibly be compared. 



(2). Even, allowing for a moment, that no pigment 

 occurs in the very young secondary shell, one would 

 not expect to find significant bands developed before 

 the animal is free-living or while it is so small that 

 shell-markings would apparently make no difference 

 to it, 



(3.) In his first article on the subject under con- 

 sideration, Mr. Wilhams evidently meant to say that 

 the majority of fresh-water Pulmonates were " horn- 

 coloured and bandless." With regard to his reply, it 

 must be said that it is not the part of a sane man to 

 require from a questioner the "proofs of his state- 

 ment." It would appear, that all Mr. Pace meant by 

 asking how "environmental conditions" could be 

 "less in water than on land" was to point out that 

 the application of the adjective "less" to such a 



* "The Ori^n of Species," by Charles Darwin, F.R.S., 

 6th edition, p. 98. 



