SUPPLEMENT. 1241 



practice, the attempt will not be made. It may be doubted 

 if ordinary dredging ever fails to leave some thousands of 



.> *^J o 



oysters, great and small, on a bed of any extent. 



Thus, if we admit, for the sake of argument, than an 

 oyster bed may be exhausted by ordinary dredging, the 

 reason why the oysters vanish is not obvious. For sup- 

 posing only a thousand oyster left, they ought to suffice to 

 restore the bed by degrees. I am aware that it is said that 

 in the meanwhile, the enemies and competitors of the 

 oyster have got the upper hand, that the ground has been 

 spoiled by accumulation of mud and so on. But this reason- 

 ing leaves out of sight the fact that the oysters have not 

 been there from all eternity. There was a time when there 

 were no oysters on the ground, and when the oyster larvse 

 immigrated, fixed themselves there, increased and multi- 

 plied, in spite of all obstacles. Why should they not do so 



again ? 



The question is further complicated by the considera- 

 tion that it is by no means certain whether the population 

 of a given oyster bed is kept up by the progeny of its own 

 oysters or by immigrants. As I have pointed out, it is 

 ascertained that the larvae, even under very unfavourable 

 circumstances, may swim about for a week ; and it has 

 been estimated that they are ordinarily locomotive for two 

 or three times that period. Even if we suppose the average 

 period of freedom to be not more than three days, the 

 chance that an oyster larva will eventually settle within a 

 mile of the spot at which it was hatched, in any estuary or 

 in the open sea, must be very small. For, in an estuary, 

 and almost always in the sea, one of the two alternating 

 currents of water is dominant, and a floating body will 

 drift, on the whole, in that direction, often many miles in 

 the course of a day. 



