TAO EVENING DISCOURSES. 
the bottom to the surface, and having their constants determined so that 
it is known what volume of water passes through the net under certain 
conditions, and yields a certain quantity of Plankton. [Examples of the 
nets and the methods of working shown. | 
Now if this constancy of distribution postulated by Hensen could be 
relied upon over considerable areas of the sea, far-reaching conclusions, 
having important bearings upon fisheries questions, might be arrived at ; 
and such have, in fact, been put forward by the Kiel Planktologists and 
their followers—such as the calculation by Hensen and Apstein that the 
North Sea in the spring of 1895 contained at least 157 billions of the eggs 
and larve of certain edible fish; and from this figure and the average 
numbers of eggs produced by the fish, their further computation of the 
total number of the mature fish population which produced the eggs—a 
grand conclusion, but one based upon only 158 samples, taken in the pro- 
portion of one square metre sampled for each 5,465,968 square metres of 
sea. Or, again, Hensen’s estimation, from 120 samples, of the number of 
certain kinds of fish eggs in a part of the West Baltic; from which, by 
comparing with the number! of such eggs that would normally be pro- 
duced by the fish captured in that area, he arrived at the conclusion that 
the fisherman catches about one-fourth of the total fish population— 
possibly a correct approximation, though differing considerably from esti- 
mates that have been made for the North Sea. 
Such generalisations are most attractive, and if it can be established 
that they are based upon sufficiently reliable data, their practical utility 
to man in connection with sea-fishery legislation may be very great. But 
the comparatively small number of the samples, and the observed irregu- 
larity in the distribution of the Plankton (containing, for example, the 
fish eggs) over wide areas, such as the North Sea, leave the impression 
that further observations are required before such conclusions can be 
accepted as established. 
Of the criticisms that have appeared in Germany, in the United 
States and elsewhere, the two most fundamental are: (1) That the samples 
are inadequate; and (2) that there is no such constancy and regularity 
in distribution as Hensen and some others have supposed. It has been 
shown by Kofoid, by Lohmann, and by others that there are imperfee: 
tions in the methods which were not at first realised, and that under 
some circumstances anything from 50 to 98 per cent. of the more minute 
organisms of the Plankton may escape capture by the finest silk quanti- 
tative nets. The mesh of the silk is 535th inch across, but many of the 
organisms are only gg5 pth inch in diameter, and so can readily escape. 
[Examples shown. | 
Other methods have been devised to supplement the Hensen nets, such 
as the filtering of water pumped up through hose-pipes let down to known 
depths, and also the microscopic examination in the laboratory of the 
centrifuged contents of comparatively small samples of water obtained by 
means of closing water-bottles from various zones in the ocean. But even 
if deficiencies in the nets be thus made good by supplementary methods, 
and be allowed for in the calculations, there still remains the second and 
more fundamental source of error, namely, unequal distribution of the 
organisms in the water; and in regard to this a large amount of evidence 
has now been accumulated, since the time when Darwin, during the voyage 
of the Beagle, on March 18, 1832, noticed off the coast of South 
America vast tracts of water discoloured by the minute floating Alga, 
Trichodesmium erythreum, which is said to have given its name to the 
Red Sea, and which Captain Cook’s sailors in the previous century called 
‘sea-sawdust.’ Many other naturalists since have seen the same pheno- 
1 It is probable that too high a figure was taken for this. 
