PLANKTON 241 



enormous numbers. Had it not been for the chance observa- 

 tion of the contents of A at half-time, it would naturally have 

 been supposed that, as all the nets agreed in their evidence, 

 the catches were fair samples of what the water contained 

 over at least the area traversed — whereas we now know that 

 the Zoeas were confined to at most the latter half of the 

 traverse and may have been even more restricted. Under 

 these circumstances, an observation made solely in the water 

 traversed during the first seven minutes would have given 

 a very different result from that actually obtained ; or, to 

 put it another way, had two expeditions taken samples that 

 evening at what might well be considered as the same station, 

 but a few hundred yards apart, they might have arrived at 

 very different conclusions as to the constitution of the 

 plankton in that part of the ocean." 



As an example of marked differences in the micro-plankton 

 in small areas, the Norwegian Professor H. H. Gran {Puh. 

 de Circon., No. 62, 1912), finds at two neighbouring stations 

 in the Skagerak two distinctly separated layers of water 

 each with its own characteristic flora. One layer is from 

 the surface to about 20 metres, and the second from about 

 40 metres to 100. There is a boundary layer between the 

 two at about 30 metres. He points out, moreover, that the 

 plankton has a very different character at these two adjacent 

 stations — the Diatoms at the one being what we should 

 expect to find in the southern part of the North Sea, while 

 at the other the Diatom plankton may have come from 

 the north part of the North Sea between Scotland and 

 Norway. His conclusions are : 



" It wiU be apparent already from the few investigations, 

 which have been mentioned here as examples, that an 

 exact quantitative investigation of the plankton at different 

 depths will be able to give interesting information, not only 

 regarding the biological conditions of the species, but also 

 regarding their dependence on the currents. Such an 

 investigation, where the quantity of plankton at certain 



s 



