CHECKS TO INCREASE OF MOLLUSCA. 97 



strata ; although we will concede that the gregarious species 

 are fewer, and perhaps less accumulative, than they were in 

 the primaeval seas, which some geologists have imagined 

 were overcharged with carbonate of lime, affording a more 

 abundant supply of matter to the shell-fish, and a consequent 

 greater multiplication of them. But the introduction of 

 Man on the scene has proved a more certain, and a very 

 powerful check to the accumulative results of the present 

 races of molluscans; for the species which are most con- 

 ducive to the process, he dredges up in millions, to be used 

 for food, for bait, for ornament, and for the purpose of con- 

 version into lime and manure. Reflect for a moment, and 

 you will readily admit how widely and surely this check 

 operates. How countless are the loads of oysters alone 

 which are annually dredged up on our coast, and on every 

 habitable shore ! multiply these by some two or three 

 thousands of years; calculate their increase from one gene- 

 ration to another (which would have gone on, in a more 

 than geometrical progression, had they been left undisturbed 

 in their native haunts), and then you may estimate the 

 depth and width of their banks which would have covered 

 the sea's bottom. And this is to take one species only into 

 the calculation ; but, in fact, no mollusk, which occurs in 

 sufficient numbers to alter it materially, is left to its natural 

 spread and increase, for what he cannot use as food or bait, 

 man burns into lime, or strews on his land as manure, 

 a purpose which he has ascertained they answer admirably 

 well, their superiority to common lime depending doubtless 

 on the animal matter which enters into their composition. 



Nor are you to underrate the numbers used in this way, 

 for they are incalculable. In our own land, indeed, where 

 limestone is abundant, and the supply of recent shells pre- 

 carious and limited, we reckon not much upon them* ; but 

 the greater part of the lime used in America, for agricultural 

 and architectural purposes, is made of calcined shells. The 



* In 1662, Mr. Ray saw the people on the Welsh coast " burning cockle- 

 shells, thereof to make lime. The manner thus. They make an hole in 

 the ground, therein they put furze, upon that wood, upon the wood small 

 stone coal, and then a layer of cockle-shells, and so shells and coals, s. s. s., 

 and then put fire to them, these burnt, make excellent lime." Sel. Hem. 

 245. Lister asserts, that in his time, living mussels were extensively used 



Phil. Trans, for 1708, and communicated to the Royal Society by the 

 Archbishop of Dublin. " Marl is not used in the north parts of Ireland; 

 but about the seaside the great manure is shells : towards the eastern part 



H 



