— 52 — 



samples we still need special supplementary investigations. To determine this 

 it would be necessary to know the rate of the reproduction of the organisms, 

 which bring us back to the point already referred to, viz : that investigations 

 as to the history of development of the individual plankton organisms are 

 necessary, since it would be impossible to commence the study of the rate of 

 reproduction of the individual species without first knowing its different stages 

 of development. The study of the history of development, as well as that 

 of the rate of reproduction, must however be regarded as a difficult matter, 

 requiring cultures of the organisms in question. Such have been made, indeed, 

 of late years, but only in a few cases, and this work is yet only at its earliest 

 commencement. It has proved possible to follow the history of development 

 of some few forms, but the experimental technique has not yet been suf- 

 ficiently mastered to permit of any statement as to the rapidity with which 

 development takes place under natural conditions in the sea. It is also necessary 

 to take into consideration the fact that the rapidity of development varies 

 with the season of the year. 



In connection with this problem, experiments have also been made, 

 chiefly by Germany and Holland, as to the quantity of inorganic substances 

 contained in sea water, and necessary for the thriving of the plant organisms 

 of the sea. Most of these substances are doubtless always present in sufficient 

 quantity ; there are however some (nitrogenous, phosphoric, and silicate com- 

 pounds), which are present in so small quantities as to make it a question 

 whether their scarcity at times may not have a retarding effect upon the 

 growth of the plant organisms. And as the plant organisms of the sea form 

 the principal basis of the nutrition of its animal organisms, this is a point of 

 fundamental importance for the production of matter in the sea. The investi- 

 gations which have been carried out hitherto are, however, not so exhaustive 

 as to permit of the question being regarded as definitely solved. 



4. Plankton as an aid to hydrographical work. 



Mention was made in the introduction of the fact that the plankton or- 

 ganisms, following as they do the ocean currents, can be of importance in a pu- 

 rely hydrographical regard, as an aid to determination of the currents' direction. 

 This has also been shown by the investigations hitherto made. Organisms of 

 Atlantic origin are carried with the Atlantic water northward round Scotland 

 into the North Sea, even into the Skagerak, or go in a more north-easterly di- 

 rection from the Faroe-Shetland Channel over towards the coast of Norway. 

 Organisms from the southern part of the North Sea follow the current northward 

 along the western side of the Jutland peninsula, and turn into the Kattegat, 

 appearing in the lower water layer, the current being here overlaid by the less 



