CH. VIIl] A CENSUS OF THE SEA 163 



probable vabie, and so to assign limits of error to our estimated 

 average values. 



Such a scheme of investigation is perhaps a Utopian one, and 

 in the meantime one must take the results as they are. Their 

 perfection is of course highly desirable, but until this is attained 

 one must not be precluded from extracting what value he can from 

 admittedly imperfect data. 



Diatoms. These are the most abundant organisms in the plank- 

 ton ^ The average catches which I have quoted shew that there 

 were, in February 1903, over one million of Chaetoceros, and about 

 ^ million of Coscinodiscus and Biddulphia underneath each square 

 metre of surface of considerable areas of the North Sea on the 

 continental side. Now the Chaetoceros, and some of the smaller 

 Coscinodiscus species, were probably underestimated: and other 

 genera of diatoms were also present. But enormous as these 

 numbers are, they do not represent the abundance in which the 

 diatoms appeared at times in Northern seas. Brandt- quotes a 

 haul made in the Bay of Kiel in which there were 3173 millions of 

 diatoms (chiefly Chaetoceros). This mass of life was taken by a 

 net which was lowered to twenty metres beneath the surface and 

 then drawn up again. The area of its mouth was 01 square 

 metre. Therefore it fished through 2 cub. metres of sea, but all the 

 water that entered the net did not again pass out through the 

 pores of the silk. It was calculated that only 1 J cub. metres did so. 

 Also it was estimated that the silk net only retained one-third of 

 the diatoms that were contained in the Avater. That is 1|- cub. 

 metres of water from the Bay of Kiel contained about 9000 

 millions of diatoms, or 6000 to each cub. centimetre. But a cub. 

 centimetre contains about 30 drops and therefore everij drop of sea 

 water from this 2)ctrt of Kiel Bay contained some 200 diatoms. 

 This was, of course, an exceptionally large haul. Hensen puts the 

 average number of diatoms present in the West Baltic at about 457 

 millions per cub. metre, or about 457 per cub. centimetre. This 

 estimate applies to the period of maximum abundance, and to the 



^ Except the bacteria, of course. 



- " Beitrage zur Kenntn. chem. Zusamm. Planktons," Wiss. Meeres. Kiel 

 Komm. Bd. in. Abth. Kiel, 1898. 



11—2 



