EVENING DISCOURSES. 745 
sumers in nature, and the pastures of the sea, as Sir John Murray pointed 
out long ago, are no less real and no less necessary than those of the land. 
Most of the fish which man uses as food spawn in the sea at such a time 
that the young fry are hatched when the spring Diatoms abound, and the 
Phyto-Plankton is followed in summer by the Zoo-Plankton (such as 
Copepoda), upon which the rather larger but still immature food fishes 
subsist. Consequently the cause of the great vernal maximum of Diatoms 
is one of the most practical of world problems, and many investigators have 
dealt with it in recent years. Murray first suggested that the meadows 
of the sea, like the meadows of the land, start to grow in spring simply 
as a result of the longer days and the notable increase in sunlight. Brandt 
has put forward the view that the quantity of Phyto-Plankton in a given 
layer of surface water is in direct relation to the quantity of nutritive 
matters dissolved in that layer. Thus the actual quantity present of the 
substance—carbon, nitrogen, silica, or whatever it may be—that is first 
used up determines the quantity of the Phyto-Plankton. Nathansohn 
in a recent paper? contends that what Brandt supposes never really 
happens; that the Phyto-Plankton never exhausts any food constituent, 
and that it develops just such a rate of reproduction as will com- 
pensate for the destruction to which it is subjected. This destruction 
he holds is due to two causes: currents carrying the Diatoms to un- 
favourable zones or localities, and the animals of the Plankton which feed 
on them. The quantity of Phyto-Plankton present in a sea will then 
depend upon the balancing of the two antagonistic processes—the repro- 
duction of the Diatoms and their destruction. We still require to know 
their rate of reproduction and the amount of the destruction. It has been 
calculated that one of these minute forms, less than the head of a pin, 
dividing into two at its normal rate of five times in the day would at the 
end of a month form a mass of living matter a million times as big as the 
sun. The destruction that keeps such a rate of reproduction in check must 
be equally astonishing. It is claimed that the ‘ Valdivia’ results, and 
observations made since, show that the most abundant Plankton is where 
the surface water is mixed with deeper layers by rising currents.2, Nathan- 
sohn, while finding that the hour of the day has no effect on his results, 
considers that the development of the Phyto-Plankton corresponds closely 
~ with evidence of vertical circulation. Like some other workers, he empha- 
sises the necessity of continuous intensive work in one locality: such work 
might well be carried on both at some point on your great lakes and also on 
your Atlantic coast. The Challenger and other great exploring expeditions 
forty years ago opened up problems of oceanography, but such work from 
vessels passing rapidly from place to place could not solve our present 
problems—the future lies with the naturalists at biological stations working 
continuously in the same locality the year round. 
The problems are most complex, and may vary in different 
localities—for example, there seem to be two kinds of Diatom maxima 
found by Nathansohn in the Mediterranean, one of Chaetoceros 
due to the afflux of water from the coast, and one of Rhizosolenia calcar- 
avis, due to a vertical circulation bringing up deeper layers of water. As 
a local example of the importance of the Diatoms in the Plankton to man, 
let me remind you that they form the main food of your very estimable 
American clam. The figures I now show, and some of the examples 
T am taking, are from the excellent work done on your own coasts in con- 
1 Monaco Bulletin, No. 140. 
2 A decade before, however, Whipple, anda also C. Dwight Marsh, in America, 
had argued that the vernal and autumnal Diatom maxima in deep lakes were due 
to periodic ‘overturning’ of the water due to temperature changes and causing 
diatom spores and a large quantity of organic material to be brought up from 
lower layers to within the influence of sunlight. 
