18 prksident's address. 



Schiitt estimates the mean variation of the plankton at about 16 per 

 cent, above or below. This does not seem to me to indicate the 

 uniformity that might be expected in this ' halistatic ' area occupying 

 the centre of the North Atlantic Gulf Stream circulation. Hensen 

 also made almost simultaneous hauls with the same net in quick 

 succession to test the amount of variation, and found that the average 

 error was about 13 per cent. 



As so much depends in all work at sea upon the weather, the con- 

 ditions under which the ship is working, and the care taken in the 

 experiment, with the view of getting further evidence under known 

 conditions I carried out some similar experiments at Port Erin on four 

 occasions during last April and on a further occasion a month later, 

 choosing favourable weather and conditions of tide and wind, so as 

 to be able to maintain an approximate position. On each of four days 

 in April the Nansen net, with No. 20 silk, was hauled six times from 

 the same depth fon two occasions 8 fathoms and on two occasions 

 20 fathoms)', the hauls being taken in rapid succession and the catches 

 being emptied from the net into bottles of 5 per cent, formaline, in 

 which they remained until examined microscopically. 



The results were of interest, for although they showed considerable 

 uniformity in the amount of the catch — for example, six successive 

 hauls from 8 fathoms being all of them 0-2 c.c. and four out of five 

 from 20 fathoms being 0"6 c.c. — the volume was made up rather 

 differently in the successive hauls. The same organisms are present 

 for the most part in each haul, and the chief groups of organisms are 

 present in much the same proportion. For example, in a series where 

 the Copepoda average about 100 the Dinoflagellates average about 300 

 and the Diatoms about 8000, but the percentage deviation of indi- 

 vidual hauls from the average may be as much as plus or viinus 50. 

 The numbers for each organism (about 40) in each of the twenty-six 

 hauls have been worked out, and the details will be published elsewhere, 

 but the conclusion I come to is that if on each occasion one haul only, 

 in place of six, had been taken, and if one had used tbat haul to 

 estimate the abundance of any one organism in that sea-area, one 

 might have been about 60 per cent, wrong in either direction. 



Successive improvements and additions to Hensen 's methods in 

 collecting plankton have been made by Lohmann, Apstein, Gran, and 

 others, such as pumping up water of different layers through a hose- 

 pipe and filtering it through felt, filter-paper, and other materials 

 which retain much of the micro-plankton that escapes through the 

 meshes of the finest silk. Use has even been made of the extraordinarily 

 minute and beautifully regular natural filter spun by the pelagic animal 

 Appendinilaria for the capture of its own food. This giid-like trap. 



