90 THE MOLLUSCA SECRETE LIME. 



deprived of nourishment." Tlie proofs are evidently defec- 

 tive, for we admit that when the foreign or exterior supply 

 of lime is abundant the secretion of it will be proportional ; 

 and when the system is weakened by a deficiency of food, it 

 seems unreasonable to demand of it the full performance of 

 a function which it could easily perform when in health and 

 vigour. It is, moreover, notorious that the testaceous mol- 

 lusca are to be found in countries where there is no lime- 

 stone ; and that lime is not necessary to the edible snail, in 

 order to the perfect formation of the operculum, has been 

 proved by Mr. Bell, — many snails in his possession having 

 formed that part, though during the whole summer they had 

 no access to any preparation of that earth. * Let me also 

 remind you that the foetus of every testaceous mollusk, while 

 still within the egg, is covered with a shell like its parent; 

 and in the larger species this shell will have attained a con- 

 siderable bulk and consistency ere the time of its exclusion 

 arrives. Now whence has this foetus obtained the calcareous 

 constituents of its shell, if we will not allow of its formation 

 by the foetus itself ? The latter has tasted no food, drunk 

 no impregnated water, no lime is to be detected in the jelly 

 which immediately envelops it, and the access of water is 

 carefully excluded by certain contrivances hereafter to be 

 described. I am not going from this to assert that any or- 

 ganic agent has the power either of creating material ele- 

 ments, or of changing one such element into another ; -f but 

 surely it may not be venturing too far in conjecture to ima- 

 gine that lime or calcium may prove to be other than an 

 elementary body. " It is now ascertained," says Mr. Bake- 

 well, " that lime and the other earths are compounds of 

 oxygen united with metallic bases ; and the brilliant dis- 

 coveries of Sir H. Davy respecting the metallic nature of 

 ammonia, would lead to the conclusion, that the metallic 

 bases of all the alkalies and alkaline earths, which have 

 many properties in common, may, like ammonia, be com- 

 pounds of hydrogen and azote, but differently combined. 

 Now it is well known that hydrogen and azote, which exist 

 as elementary constituent parts of almost all animal sub- 

 stances, may be derived from water and the atmosphere ; and 

 should the compound nature of the metallic bases of the 

 earths be ascertained, the formation of lime by animal secre- 

 tion will admit of an easy explanation." J 



Were it proved that all limestone is an animal production, 



* Zool. Journ. i. 96—97. t Piout's Bridgcw. Treat. 431. 



t Introd. to Geology, 110. 



