166 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



self-cleansing is, of course, quite a different one, and is 

 unaffected by any question of irregular sampling, for all the 

 samples were taken and forwarded in the same way, and 

 whatever applies to the original sampling also applies to the 

 sampling after the relaying. But we have to consider the 

 difference in the apparent bacterial contents of two samples 

 of mussels sent from the same area. In summer the population 

 of Barmouth is a holiday one (say about 5,000), whereas in 

 winter it may be reduced to about half that number. In 

 summer the volume of fresh water in the river is small, whereas 

 in the months of January and February it may be considerable 

 on account of rains. The smaller population and the increased 

 amount of flushing of the river may perhaps account for the 

 difference in the apparent pollution, but perhaps the difference 

 may be due, to some extent, to the conditions of sampling. 

 I mention this particular case as illustrating the contention 

 that the actual conditions, as regards pollution, of a particular 

 natural shellfish area, at a particular time, may not be 

 accurately estimated merely from samples collected and 

 transmitted by persons other than those making the analysis. 

 In cases where the results of analysis are important, the 

 collection and further history of the sample prior to analysis 

 is equally important. 



