LABORATORY METHODS 2? 



a particular diatom recorded for a station we must recognize that it is possible that the 

 figure may actually be 15,000 or 45,000. At first sight this may appear to be a very large 

 degree of error, but when the variation in numbers of any diatom round South Georgia 

 is seen, it will be realized that this possible source of error is of little significance. For 

 example Corethron valdiviae is recorded as occurring in the following numbers at six 

 different stations : 



10,866,000 (the largest number recorded) 



1,004,000 



562,000 



61,000 



5,000 



200 (the smallest number recorded) 



We are in this work concerning ourselves only with the big differences ; the very nature 

 of the distribution of the plankton we are studying and the necessary limitations to our 

 methods in the field will not allow us to attempt the establishment of small differences. 

 When we are comparing one region represented by 5000 Corethron with another re- 

 presented by 562,000, what does it matter if that 5000 is really 7500 or 2500, or again if 

 the 562,000 is really 281,000 or 743,000? In the very unlikely event of both the figures 

 which are being compared erring in opposite directions to the exceptional amount of 

 50 per cent the difference in the resulting comparisons is very small: e.g. 2500 is 

 0-3 per cent of 743,000, but also 7500 is only 2-6 per cent of 281,000. 



Similarly, when we compare an area represented by 61,000 with another represented 

 by 10,866,000, what does it matter if the former is 90,500 or 30,500 or the latter 5,433,000 

 or 16,299,000? The smaller value of the former can be expressed as 0-2 per cent of the 

 larger value of the latter, whilst the larger value of the former is but 1 -8 per cent of the 

 smaller value of the latter. 



When very small numbers are recorded in the analysis their actual value should not 

 be regarded as significant — they merely indicate the presence of the organism in small 

 numbers. For example, if the fraction of the sample examined is 1/600, and only one of 

 a particular organism is taken up in this fraction by the pipette, it will figure as 600. 



Analysis of N 70 V samples 



Firstly the volume of each sample was measured by methods similar to those just 

 described for the N 50 V samples ; but volume, as with the N 50 V samples, gives only 

 a very rough indication of the relative bulk of the different samples on account of the 

 varying settling and packing properties of the different organisms. 



Next all the larger organisms were picked out and recorded : Medusae, Siphonophora, 

 the larger Chaetognatha, the larger Calanoid Copepoda, Mysidae, Amphipoda, Euphau- 

 siacea, Pteropoda, etc. These are organisms which on account of their size are not likely 

 to be fairly sampled by the stempel pipette, and they are picked out as well as recorded 

 at this stage, since they tend to entangle the smaller organisms and so encumber analysis 

 of the fraction of the sample later examined. As a general rule Calanoid Copepoda of 



