PLANKTON 247 



several months in the spring, summer and autumn of 1920. 

 They consisted of seven series of four to six successive 

 (that is, as nearly as possible simultaneous) vertical hauls 

 taken with the " Nansen " net of No. 20 silk.^ The 

 *'Nansen " net is shown in Fig. 16, on p. 233, and, attached 

 to the Lucas sounding machine, at Plate XVIII, Fig. 2. 



An apparent uniformity in the successive catches of each 

 series was obvious at the time of collecting. It seemed to 

 the eye to be the same catch that was emptied from the 

 Nansen-bucket into the bottle of formaline time after time 

 throughout a series. And this apparent uniformity of 

 volume was in most cases confirmed by the subsequent 

 measurements in the laboratory — for example, the six 

 successive hauls from 8 fathoms on April 3 all measure 0*2 

 c.c, four out of five of those from 20 fathoms on April 6 are 

 0*6 CO., and all four on August 7 from 20 fathoms measure 

 0'5 c.c. The remaining four series show some variation, 

 but the percentage deviation from the average of each 

 series is in no case great (see table, p. 248). 



If, however, we make a microscopic investigation of the 

 catches, we find that even in the same series, similar volumes 

 of the plankton may be made up rather differently, and 

 may in some cases show surprising differences in the numbers 

 of a species in successive hauls, such as 10 and 100, 40 

 and 800, 4,000 and 18,000. Notwithstanding, then, some 

 appearance of similarity between the hauls of a series, 

 there is a considerable percentage deviation in the case of 

 some hauls from the average of their series^not infrequently 

 about plus or minus 50 per cent., and in several cases about 

 70, and in one case plus 129. The following table gives 

 the percentage deviations in the case of the volumes of the 

 catches, and also of the counted or estimated numbers of 



1 For full details as to the conditions of the experiment, and the 

 methods of obtaining the results here given, see " Variation in 

 Successive Vertical Plankton Hauls at Port Erin," Trans, Biol. Soc, 

 L'pool, vol. XXXV, p. 161, 1921. 



