SEA-FISHERIES LABORATORY. 167 



Some of the other common organisms outside the above 

 main groups show equally contradictory evidence. The pelagic 

 worm, Sagitta bipnnctaia, is present in nearly every haul in 

 numbers varying from one to twenty-seven, but in some 

 series one or two specimens are present in every haul, while in 

 another case successive hauls in one series varied from one to 

 eleven. Other similar examples might be given from some of 

 the larval forms present. The impression one receives from 

 an inspection of the lists and numbers as they stand is that 

 if on each occasion one haul only in place of four or six had 

 been taken and one had used the results of that haul to estimate 

 the abundance of any one organism or group of organisms in 

 that sea-area one might have arrived at conclusions about 

 50 per cent, (or in some cases a great deal more) wrong in either 

 direction. 



Is such a vague result of any value as a basis for calculations 

 as to the population of the sea ? And is it possible that such 

 irregular numbers are compatible with the hypothesis of an 

 even distribution of the plankton throughout a sea- area of 

 constant character ? The answer to such questions depends 

 to some extent upon the possible range of error under the 

 conditions of the experiment, and upon the possibility of allow- 

 ing for that experimental error, and of reducing it by more 

 refined methods of collecting. It may be justifiable to claim, 

 as the result of a good deal of work at sea, that the conditions 

 of these experimental hauls at Port Erin were as free from 

 liability to error as any similar vertical hauls from a boat in the 

 open sea are likely to be. The occasions were chosen so that 

 the weather and state of the tide might be favourable, and every 

 care was taken to have the conditions of the successive hauls 

 in each series as much alike as possible. I feel confident that 

 the possibility of error in the collecting was reduced to a 

 minimum. There is also the possibility of error in the micro- 

 scopic examination and estimation of the contents of the 



