THE MOLLUSCA SECRETE LIME. 89 



shore. Such, to adopt the simile of the poet, is the rotation 

 of the unwearied wheel which Nature rides upon ! 



I am not, however, one of that class who unhesitatingly 

 maintain that from such and similar sources the mollusca 

 derive the whole of the calcareous materials of their shells. 

 By the mechanical operation of the waters upon limestone 

 rocks, and by the chemical action of carbonic acid, a large 

 portion of lime in a state of very minute division, or in solu- 

 tion, is carried by rivers into lakes and seas, a portion which 

 is greatly increased there by the lithophagous mollusca, by 

 the action of the tides, and by the trituration of dead shells 

 and coral. Hence the source, many say, of the lime which 

 the mollusca require : it is imbibed with their food and with 

 their drink, in small quantities at a time it is true, but suffi- 

 cient to supply materials for the very slow and imperceptible 

 excretion of the shell. Now I doubt this. The analysis of 

 sea-water gives us the exact quantity of lime supplied by 

 these extrinsic agencies ; and in a pint from the Firth of 

 Forth, Dr. Murray finds only five grains and a fraction of 

 muriate of lime,* a proportion which appears to me too 

 small for the necessary demands of the hosts of animals in 

 whose composition lime enters as an ingredient. I am there- 

 fore inclined to believe, for it becomes us to speak warily on 

 such a question, and the opinion is very unfashionable, that 

 the testaceous mollusca, in common with other animals, have 

 really the power, not merely of separating lime from its com- 

 binations and mixtures, but can produce it by the powers 

 of vitality, from elements hitherto considered as simple.f 

 There is a natural repugnance to the admission of an hypo- 

 thesis like this, which assumes the existence of almost a crea- 

 tive power in animal bodies, and its necessity has been denied. 

 Shells, they tell us truly, are thicker and coarser in the 

 lakes and ponds of a limestone district than of districts whose 

 rocks belong to other formations ; and land-shells are much 

 more abundant in the former than in the latter, and whence 

 this difference in the quality of the shell, and the productive- 

 ness of the race, but from the difference in the supply of this 

 material ? Where it is procured in scantiness, there the 

 animals are few in numbers, and the shell is thin and clear. 

 Following the same line of argument they have asserted that 

 the edible snail, which, under ordinary circumstances, forms 

 a calcareous operculum previously to hybernation, is unable 

 to make anything more than a membranous substitute " when 



* Syst. of Chemistry, iii. 696. 



t Dr. Dmmmond entertains the same opinion. Letters to a Young 

 Naturalist, p. 211. 



