INTRODUCTION. XXV 



tailed, it will be proper to add another, which, 

 Bruguiere alledges, has not only a powerful in- 

 fluence on the brilliancy of their colours, but on the 

 diversity of their tints, and which is wholly inde- 

 pendent of the physical structure of the animal. 

 This cause, though at first it appears too far removed 

 to produce such marked effects, is no other than the 

 influence of light, combined, perhaps, with that of 

 heat. 



Two individuals, says Bruguiere, of the same 

 species, one found either in the northern ocean, or 

 the Mediterranean, and the other in the seas of hot 

 climates, present different tints, and always a vi- 

 vacity of colour, decidedly in favour of the spe- 

 cimen from the torrid zone. Their shells, though 

 of a similar shape, constantly differ in their colours, 

 whence originate all those varieties, which some 

 conchologists have considered as distinct species, 

 although it is very probable, nay almost certain, 

 that these differences depend merely on the action 

 of climate, of nourishment, or of particular cir- 

 cumstances, not as yet sufficiently understood. 



The difference of temperature, where these in- 

 dividuals are supposed to live, would seem at first 

 to be the principal cause of their difference of colour, 

 if we were not certain that shells naturally coloured, 

 either in the seas of the torrid zone, or those of the 

 temperate, acquire more or less intensity in their 

 vol. i. c 



