108 ■ THE COLOURS OF ANIMALS 



cision and rapidity. The resemblance between 

 certain individuals of Ovulum and the rose-coloured 

 coral, and between other individuals and the yellow 

 coral (see pp. 70-71), is probably due to the existence 

 of a power of adjustment ; but this suggestion needs 

 experimental verification. A fact mentioned by Morse 

 is even more convincing : he states that individuals 

 of the same molluscan species occupying different 

 stations are differently coloured, and he quotes from 

 Dr. A. A. Gould the observation that the colour of all 

 the shells found in the sandy harbour of Provincetown 

 is remarkably light. ^ There is no evidence as to 

 whether the change in colour, if produced at all, takes 

 place rapidly or slowly ; but the latter is the more 

 probable in these animals. 



Professor Stewart found four or five bright red 

 individuals of the Nudibranchiate mollusc Archidoris 

 tuberculata in a mass of bright red sponge {Hymenia- 

 cidon sanguinea) upon which they were feeding.^ 

 The colour was very different from that of individuals 

 taken upon another sponge (Halichondria) . The ob- 

 servation strongly suggests the existence of a power of 

 Variable Protective Kesemblance, although it is possible 

 that the colour of the food may be made use of. 



It is very likely that Variable Eesemblance will 

 be found to occur far more generally than has been 

 hitherto supposed. 



* Proc. Boston Soc. Nat. Hist. vol. xiv. April 5, 1871. 

 - W. Gaxstang, Lc. p. 177. 



