744 EVENING DISCOURSES. 
(2) Irregularities introduced by the interactions of the different 
organisms. 
(3) More or less periodic abnormalities in either time or abundance 
caused by the physical changes in the sea, which may be grouped together 
as ‘weather.’ [Illustrated by diagrams. ] 
These are all obvious factors in the problem, and the constitution of the 
Plankton from time to time throughout the year must be due to their inter- 
action. The difficulty is to disengage them from one another, so as to deter- 
mine the action of each separately. 
Amongst the physical conditions coming under the third heading, the 
temperature of the sea is usually given a very prominent place. There is 
only time to allude here to one aspect of this matter. 
It is often said that tropical and sub-tropical seas are relatively poor 
in Plankton, while the colder Polar regions are rich. In fishing Plankton 
continuously across the Atlantic it is easy from the collections alone to 
tell when the ship passes from the warmer Gulf Stream area into the colder 
Labrador current. This is the reverse of what we find on land, where 
luxuriant vegetation and abundance of animal life are characteristic of the 
tropics in contrast to the bare and comparatively lifeless condition of the 
Arctic regions. Brandt has made the ingenious suggestion that the explana- 
tion of this phenomenon is that the higher temperaure in tropical seas 
favours the action of denitrifying bacteria, which therefore flourish to such 
an extent in tropical waters as seriously to diminish the supply of nitrogen 
food and so limit the production of Plankton. Loeb,’ on the other hand, has 
recently revived the view of Murray, that the low temperature in Arctic 
waters so reduces the rate of all metabolic processes, and increases the length 
of life, that we have in the more abundant Plankton of the colder waters 
several generations living on side by side, whereas in the Tropics with more 
rapid metabolism they would have died and disappeared. The temperature 
of the sea-water, however, appears to have little or no effect in determining 
the great vernal maximum of Phyto-Plankton. 
Considering the facts of photosynthesis, there is much to be said in 
favour of the view that the development and possibly also the larger 
movements of the Plankton are influenced by the amount of sunlight, quite 
apart from any temperature effect. 
Bullen * showed the correlation in 1903-07 between the mackerel catches 
in May and the amount of Copepod Plankton in the same sea. The food of 
these Copepoda has been shown by Dakin to be largely Phyto-Plankton; and 
Allen has lately * correlated the average mackerel catch per boat in May with 
the hours of sunshine in the previous quarter of the year [curves shown], 
thus establishing the following connection between the food of man and the 
weather: Mackerel—Copepoda-—Diatoms—Sunshine. One more example 
of the influence of light may be given. Kofoid has shown that the Plankton 
of the Illinois River has certain twenty-nine-day pulses, which are appa- 
rently related to the lunar phases, the Plankton maxima lagging about six 
days behind the times of full moon. The light from the sun is said to be 
618,000 times as bright as that from the full moon; but the amount of solar 
energy derived from the moon is sufficient, we are told, appreciably to affect 
photosynthesis in the Phyto-Plankton. The effectiveness of the moon in this 
photosynthesis to that of the sun is said to be as two to nine, and, if that is so, 
Kofoid is probablyjustified in his contention that at the time of full moon 
the additional light available has a marked effect upon the development of 
the Phyto-Plankton. 
As on land, so in the sea, all animals ultimately depend upon plants 
for their food. The plants are the producers and the animals the con- 
* Darwin and Modern Science (Cambridge, 1909), p. 247. 
2? M. B. A. Journ., viii. -269. ’ Tbid., vii. 394. 
