166 SIR W. A. HERDMAN : RESULTS OF CONTINUOUS 
But, if one next proceeds to deal quantitatively with the groups and the 
individual species, it is found that the hauls in a series may differ widely — 
up to fully 50 per cent. of the variations from the mean of the series extend 
beyond the range of error and are therefore not due to possible imperfections 
in the experiment. Thus more than half the differences between the hauls of 
a series remain unaccounted for, and may naturally be interpreted as 
evidence of an unequal distribution of the plankton in closely adjacent areas 
of water or in the same area in successive periods of time. 
Whether the present methods of collecting and of estimating are 
sufficiently accurate to enable us to determine the amount of this inequality 
in the distribution, so as to be able to assign probable upper and lower limits 
to the number of each organism per unit volume of water, may be doubtful, 
but we may hope that improvements in method and accumulation of evidence 
may in time enable us to make some approximation to an estimate of the 
population of various sea-areas. Other more refined methods of collecting 
samples of the microplankton have been recently devised, such as the filtering 
and centrifuging (or other exhaustive examination) of small measured 
quantities of water, or the cultivation of every organism in a very small 
volume of water, These methods have added much to our knowledge of the 
minuter and more elusive forms of the plankton, but the drawback to all of 
them is that they deal with relatively small volumes (one, three, or five litres) 
of the water, and it must remain doubtful whether the same organisms in the 
same quantity would have been present in the next bucketful of water that 
might have been taken from the sea. 
Even if we had no hope of attaining to greater accuracy, our present 
planktonic results are of some value. Although estimates which may be 50 
per cent. wrong in either direction do not justify us in calculating exactly 
the number of organisms or of potential food present per area of sea or 
volume of water, they do give us a useful approximation *. Even if 100 per 
cent. out, doubling or halving the estimated number is a relatively small 
variation compared with the much larger increases and reductions, amounting 
to, it may be, ten thousand times in the case of diatoms, ten to fifty times in 
the dinoflagellates, and five to twenty times in Copepoda, which we find 
between adjacent months—and even greater differences if we take groups of 
months—in a survey of the seasonal variations of the plankton. 
PHYTOPLANKTON IN RELATION TO FisH LARVA. 
It has been stated above that it is only a very small number of kinds of 
“organisms (plants and animals) that make up the bulk of the plankton that 
* As W. E. Allen, of California, says:—“Tow-net catches give no trustworthy indi- 
cation of the relative amounts of phytoplankton in two different locations or in the same 
location at two different times, although they may have a high suggestive value " 
(‘ Ecology, ii, July 1921, p. 216). 
