742 EVENING DISCOURSES. 
strong tidal and other local currents, with rapid changes of conditions, 
and where the Plankton is largely influenced by its proximity to land. 
(2) There are considerable sea areas, such as the centre of the North Sea 
and the centre of the Irish Sea, where the Plankton is removed from coastal 
conditions, but is influenced by various factors which cause great irregularity 
in its distribution. These are the localities! of the greatest economic 
importance to man, and to which attention should especially be directed. 
(3) There are large oceanic areas in which there may be uniformity of 
conditions, but it ought to be recognised that such regions are not those in 
which the Plankton is of most importance to men. The great fisheries of 
the world, such as those of the North Sea, the cod fishery in Norway, and 
those on the Newfoundland Banks, are not in mid-ocean, but are in areas 
round the continents, where the Plankton is irregular in its distribution. 
As an example of a locality of the second type, showing seasonal, hori- 
zontal, and vertical differences in the distribution of the Plankton, we may 
take the centre of the Irish Sea, off the south end of the Isle of Man. 
Here, as in other localities which have been investigated, the Phyto- 
Plankton is found to increase greatly about the time of the vernal equinox, 
so as to cause a maximum, largely composed of Diatoms, at a period 
ranging from the end of March to some time in May—this year to May 28, 
in the Ivish Sea. Towards the end of this period the eggs of most of the 
edible fishes are hatching as larve. [Statistics and diagrams showing this 
maximum for the last three years were exhibited. } 
This Diatom maximum is followed by an increase in the Copepoda 
(minute crustacea), which lasts for a considerable time during the early 
summer ; and as the fish larvee and the Copepoda increase there is a rapid 
falling off in Diatoms. Less marked maxima of both Diatoms and Cope- 
poda may occur again about the time of the autumnal equinox. These two 
groups—the Diatoms and the Copepoda—are the most important economic 
constituents in the Plankton. A few examples showing their importance 
to man may be given: Man eats the oyster and the American clam, and 
these shell-fish feed upon Diatoms. Man feeds upon the cod, which in its 
turn may feed on the whiting, and that on the sprat, and the sprat on 
Copepoda, while the Copepoda feed upon Peridinians and Diatoms; or 
the-cod may feed upon crabs, which in turn eat ‘ worms,’ and these feed 
upon smaller forms which are nourished by the Diatoms. Or, again, man 
eats the mackerel, which may feed upon young herring, and these upon 
Copepoda, and the Copepoda again upon Diatoms. All such chains of 
food matters from the sea seem to bring one through the Copepoda to the 
Diatoms, which may be regarded as the ultimate ‘ producers’ of food in 
the ocean. Thus our living food from the waters of the globe may be said to 
be the Diatoms and other microscopic organisms as much as the fishes. 
Two years ago, at the Leicester meeting of the British Association, I 
showed that if an intensive study of a small area be made, hauls being 
taken not once a quarter or once a month, but at the rate of ten or twelve 
a day, abundant evidence will be obtained as to: (1) variations in the 
distribution of the organisms, and (2) irregularities in the action of the 
nets. [Hxamples shown.] Great care is necessary in order to ensure that 
hauls intended for comparison are really comparable. Two years’ additional 
work since in the same locality, off the south end of the Isle of Man, has 
only confirmed these results, viz., that the Plankton is liable to be very 
unequally distributed over the depths, the localities, and the dates. One 
net may encounter a swarm of organisms which a neighbouring net escapes, 
and a sample taken on one day may be very different in quantity from a 
sample taken under the same conditions next day. If an observer were 
to take quarterly, or even monthly, samples of the Plankton, he might 
* See Dakin, 7rans. Biol. Soc. Liverpool, xxii., p. 544. 
