INVESTIGATION OF THE PLANKTON OF THE IRISH SEA. 165 
entire preserved catch with the eye or a hand-lens, and counted directly. 
Sampling and estimation are not applied to the macroplankton, and yet the 
variation is as great there as in the case of the estimated microplankton. 
The experimental error to be expected in the case of the three chief groups 
of organisms, and also in the case of a typical species of each, has been 
calculated, by means of a formula for obtaining the probable error, with 
the following results. 
The total number of diatoms on April 3rd varied in the six hauls from 
3880 to 10,020, the mean being 8055. Two of the hauls are below the mean 
and four above. The smallest haul is 52 per cent. below the mean and the 
largest haul is 24 per cent. above. The question is—Do these variations in 
the catch come within the limits of the probable error of the experiment ? 
If we assume that the estimation of the number of diatoms in each haul is 
correct, then the possible errors are those inseparable from all such collecting 
at sea—slight movements of the boat, unknown currents in the water, irregu- 
larities in the verticality of the line, &e. In this case of the diatoms on 
April 3rd the “probable error” is found to be=1458, and the “ range ” is 
the mean + the probable error—that is, from 6600 to 9900. Comparing this 
range with the estimated results of the hauls, we find that three of the 
series are within the range and three are outside it, and two of the latter 
(3880 and 10,020) are very considerably beyond the limits of the probable 
error of the experiment. 
The diatoms of the other hauls give much the same result when treated in 
the same manner—that is, roughly 50 per cent., or rather more of the observed 
variation in the catches is not covered by the calculated range of error of 
the experiment. 
A series of detailed tables are given in the full report * from which the 
above is summarised, in which each of the principal groups of the plankton 
and also three prominent organisms—the diatom Coscinodiscus radiatus, the 
dinoflagellate Ceratium tripos, and the copepod Pseudocalanus elongatus—are 
shown for all seven series of hauls treated as in the case of the diatoms of 
April 3rd discussed above, and giving in each case the figures necessary to 
make a comparison between the range of variation in the catches and the 
calculated range of error. These tables show that in each case a large pro- 
portion—from 50 per cent. to 22 out of 34—of the observed variations are 
outside the range of error of the experiment. 
To the question, What light does a series of, say, six successive hauls throw 
upon the validity of a single haul (say, the first of the series) ?—the answer 
seems to be that as regards mere size (volume) and general nature (such as 
phytoplankton, zooplankton, or mixed) of the catch the series confirms the 
representative character of the single haul in a general way and within 
limits. 
* Trans. Biol. Soc. L’pool, vol. xxxv, (oc, cit). 
