774 



DEPTHS OF THE OCEAN 



the open ocean on the whole. In all or most of these samples 

 the numbers of individuals have been counted after the return 

 of the expedition, — a laborious and painstaking piece of work, 



i23J 



Fig. 566. — Volumes of "Plankton" in the Atlantic and in the North Sea, 



ACCORDING TO THE INVESTIGATIONS OF THE " HOLSATIA " IN 1885 AND THE 



" National "^IN 1889. (AfterjHensen, from Steuer.) 



which has added greatly to our knowledge of marine biology. 

 In Chapter IX. I have had occasion to refer to many important 

 facts for which we are indebted to these expeditions, but I 



