lO 



DEPTHS OF THE OCEAN 



Aurivillius. 



Pettersson. 



Hensen's work must not be disparaged because his aspirations 

 have been more difficult to reaHse than he at hrst imagined. 

 The difficulties are far from insurmountable, while Hensen 

 himself will be always looked upon as one of the founders of the 

 science of marine physiology. 



In the biology of the sea we have also to consider the 

 geographical distribution of the different species and their 

 dependence upon ocean currents. The Swedish scientists, 

 Cleve and Aurivillius, brought these two questions into special 

 prominence, though no doubt they had been previously con- 

 sidered by others. But with the hydrographical investigations 

 of Otto Pettersson and others the whole subject assumed a 

 new aspect. Thanks to improved methods they succeeded in 

 following the movements of the water-layers, by determining 

 their salinity, temperature, and other hydrographical character- 

 istics ; and from this time forward the plankton was also 

 enlisted as a supplemental means of characterising water- 

 masses of different origin. Cleve with his marvellous power 

 of distinguishing forms was able in a short space of time to 

 determine numbers of species, animals as well as plants, and 

 it is to him we owe the foundation of our knowledge regarding 

 the distribution of plankton-algaj. 



Since the international marine investigations were commenced 



nvestigations. ^bout ten years ago, researches have been carried out in the 



Northern Atlantic, North Sea, and Baltic ; and specialists from 



the different countries of North Europe have gradually extended 



our knowledge, as far as northern species are concerned. 



Simultaneously great improvements have taken place in our 

 methods of studying plankton. Lohmann has made it clear that 

 the catches in the silk nets originally used incompletely repre- 

 sented the flora of the sea, owing to the fact that whole series of 

 the most diminutive organisms slip through the meshes of even 

 the finest straining-cloth. He devised methods for catching them 

 by means of the filter and the centrifuge, and could thus estimate 

 their numbers in a given quantity of sea-water. Coccolitho- 

 phoridse, which the " Challenger " Expedition claimed to have 

 discovered, but which Hensen refused to recognise as self- 

 existent plankton organisms, because he did not capture them 

 himself, were now investigated, and Lohmann was able to 

 declare confidently that they really are algae, furnished with 

 brown pigment granules, the physiological equivalent of 

 chlorophyl, thus confirming the ecrlier discoveries of Sir John 

 Murray, George Murray, Blackman, and Ostenfeld. Lohmann 



International 



Lohmann. 



G. Murray 

 Blackman. 



Ostenfeld. 



