2 68 FOUNDERS OF OCEANOGRAPHY 



that a small tow-net will capture hundreds of millions of 

 individuals in a few minutes. And these mjnriads of micro- 

 scopic organisms, so abundant as to colour the water, after 

 persisting for a few weeks, may disappear as suddenly as 

 they came — which is another problem for the oceanographer. 

 Then later in the summer follow the swarms of Copepoda and 

 many other kinds of minute animals, and these again may 

 give place in the autumn to the second maximum of Diatoms, 

 or in some years of the Dinoflagellates, such as Ceratium and 

 Peridinium—all of which requires explanation. 



I have already referred to some of the theories which 

 have been advanced to account for these more or less periodic 

 changes in the plankton, such as Liebig's " law of the 

 minimum," which limits the reproduction of an organism 

 by the amount of that substance necessary for existence 

 which is present in least quantity — it may be nitrogen, or 

 silicon, or phosphorus. According to Raben, for example, 

 it is the accumulation of silicic acid in the sea -water during 

 winter that determines the great increase of Diatoms in 

 spring, and again in autumn, after a further accumulation. 

 Some writers have considered these variations in the 

 plankton to be caused largely by changes in temperature, 

 supplemented, according to Ostwald, by the resulting 

 changes in the viscosity of the water ; but, as I have 

 indicated above, my opinion is that those investigators 

 are more probably correct who attribute the spring develop- 

 ment of phyto-plankton to the increasing power of the 

 sunhght and its value in photosynthesis, the process by which 

 green plants (including Diatoms) obtain the necessary supply 

 of carbon from the carbon dioxide in the sea-water. 



As was pointed out by Edward Forbes just seventy years 

 ago, the seas around the British Islands (his " Celtic 

 Province ") are the meeting-ground of northern (" Boreal ") 

 and southern (" Lusitanian ") faunas — " The Celtic Province 

 is the neutral ground of the European seas ; it is the field 

 upon which the creatures of the north and those of the 



