president's address. 13 



phosphoi'us. According to Eaben it is the accumulation of silicio acid 

 in the sea-water that determines the great increase of Diatoms in spring 

 and again in autumn. Some writers have considered these variations 

 in the plankton to be caused largely by changes in temperature supple- 

 mented, according to Ostwald, by the resulting changes in the viscosity 

 of the water; but Murray and others are more probably coiTect in 

 attributing the spring development of phyto-plankton to the increasing 

 power of the sunlight and its value in photosynthesis. 



Let us take next the fa/ct — if it be a fact — that the genial warm 

 waters of the tropics support a less abundant plankton than the cold 

 polar seas. The statement has been made and supported by some 

 investigators and disputed by others, both on a certain amount of evi- 

 dence. This is possibly a case like some other scientific controversies 

 where both sides are partly in the right, or right under certain con- 

 ditions. At any rate there are marked exceptions to the generalisation. 

 The Gennan Plankton Expedition in 1889 showed in its results that 

 much larger hauls of plankton per unit volume of water were obtained 

 in the temperate North and South Atlantic than in the tropics between, 

 and that the warm Sargasso Sea had a remarkably scanty microflora. 

 Other investigators have since reported more or less similar results. 

 Lohmann found the Mediterranean plankton to be less abundant than 

 that of the Baltic, gatherings brought back from tropical seas are fre- 

 quently very scanty, and enormous hauls on the other hand have been 

 recoi'ded from Arctic and Antarctic seas. There is no doubt about the 

 large gatherings obtained in northern waters. I have myself in a 

 few minutes' haul of a small horizontal net in the North of Norway 

 collected a mass of the large Copepod Calanus finmarchicus sufficient 

 to be cooked and eaten like potted shrimps by half a dozen of the 

 yacht's company, and I have obtained similar large hauls in the cold 

 Labrador current near Newfoundland. On the other hand, Kofoid and 

 Alexander Agassiz have recorded large hauls of plankton in the Humboldt 

 current off the west coast of America, and during the Challenger 

 Expedition some of the largest quantities of plankton were found in the 

 equatorial Pacific. Moreover, it is common knowledge that on occa- 

 sions vast swarms of some planktonic organism may be seen in tropical 

 waters. The yellow alga Trichodesmium, which is said to have given 

 its name to the Eed Sea and has been familiarly known as ' sea-sawdust ' 

 since the days of Cook's first voyage,' may cover the entire surface over 

 considerable areas of the Indian and South Atlantic Oceans ; and some 

 pelagic animals such as Salpae, Medusae and Ctenophores are also 

 commonly present in abundance in the tropics. Then, again, American 



^ See Journal of Sir Joseph Banks. This and other swarms were also 

 jioticed by Darwin during the voyage of the Beagle. 



I 



