HARDWICKE'S SCIENCE-GOSSIP. 



241 



THE COLOURING AND BANDING IN LAND AND 



FRESHWATER SHELLS. 



By C. CLARE FRYER. 



R. J. W. WIL- 

 LIAMS' attempt 

 to solve the prob- 

 lem of the origin 

 : of the colours in 

 land and fresh- 

 water shells seems 

 to us just a little 

 premature. Any 

 theory dealing 

 with their origin 

 and evolution must 

 rest on a good 

 understanding of 

 the relationship 

 between the shell 

 and its environ- 

 ment, the meaning 

 and uses of the 

 colours, their che- 

 mical composition 

 and physiological development, as well as their 

 variations, and in all these departments much work 

 remains to be done before we can formulate a good 

 theory as to the origin of colour. 



It is a pity, too, seeing how little has been written 

 on the subject by conchologists, that Mr. J. \V. 

 Williams does not give the views of those from whom 

 he differs, they would have formed a fitting preface 

 to his article and have enabled us to duly appreciate 

 his original theories. 



With the qualification which I have italicised, the 

 statement that "the evolution of colours and bands 

 has in the main differentiated along a line from the 

 simple to the complex," is probably more in accord- 

 ance with the teachings of nature ; but so generally 

 does Mr. Williams apply his dictum without the 

 qualification, as to create the impression that all the 

 species have evolved along a line, i.e., uninterruptedly 

 and undeviating, in one direction from the simple 

 horn-coloured progenitor to the complex, banded 

 No. 311— November 1890. 



species of to-day, and if this is what he intends to 

 convey, it certainly does not accord with the views of 

 evolution as laid down by Darwin, Wallace, and 

 Spencer, and is, as I hope to show presently, hard to 

 reconcile with the facts ; but to proceed with these 

 in their order : — 



(1.) The colour of the chitinous plug in the 

 trochosphere stage is due to the very constitution of 

 the conchiolin, not chitin, of which it is composed, 

 and not to pigment cells. 



(2 and 5.) The nucleus of the secondary shell is 

 perhaps generally, but certainly not always, white, 

 whitish, or horn-coloured ; for instance, it is black in 

 Helix virgata, and coloured in several foreign 

 members of the group (H. syriaca, H. maritima, 

 etc.). 



(3.) That the majority of the Limnea are horn- 

 coloured and bandless is true, but the Chilinia are 

 marked with spots and bands, while the colours 

 developed in the Paludinid?e, Neritidse, Cerithiadse, 

 etc., present real difficulties to such a view : but 

 perhaps these would all disappear if Mr. Williams 

 would explain what he understands by "environ- 

 mental conditions," also why they should be less in 

 freshwater than on land. 



(4.) It is difficult to see how we may legitimately 

 consider white to be "an advance in colour," since 

 white is not necessarily a pigmentary colour at all, 

 but may be due to the molecular structure of the 

 surface, and at the most can only be akin to 

 pigment, " differing widely from it in optical pro- 

 perties in that no absorption takes place." * 

 Moreover, are we not involving a great deal by 

 selecting an embryological variation of such all 

 pervading occurrence among the distinct classes of 

 animals, and of a nature so little understood, as 

 albinism, for the first advance in shell colour? If 

 we admit that the primitive mollusc was an albino,, 

 must we not admit the same of all classes of animals 



* "The Colours of Animals," by H. E. Poulton, p. 187. 



