estimations of 

 organisms in 

 the ocean. 



7/2 DEPTHS OF THE OCEAN chap. 



A first attempt in this direction was made by Sir John 

 Murray during the cruise of the "Challenger," by calculating 

 the amount of calcium carbonate in the form of living organisms 

 per square mile of the ocean by lOO fathoms in depth. 

 Quantitative No one has devotcd more time and thought to this problem 



than V. Hensen, who has been indefatigable in his endeavours 

 to devise methods for an absolute determination of the quantities 

 of organisms contained in the ocean, his avowed intention 

 being to ascertain the quantities of "primitive food for 

 marine animals." ' From theoretical considerations he con- 

 cluded that the primitive food of marine animals necessarily 

 consisted of the microscopic plants living in the surface waters 

 of the ocean, and that the effect of currents would be to 

 distribute these minute plants quite regularly and uniformly. 

 He held the idea that a hoop-net hauled vertically from bottom 

 to surface would filter a column of water with a diameter very 

 nearly corresponding to the diameter of the net, and that in 

 this way it was possible to calculate the catch per square metre 

 of surface. The volume of the catch might be measured, 

 and the number of individuals belonging to all the species 

 might be counted. Definite figures might thus be obtained 

 representing the abundance of each species per square metre 

 of surface, and the area of the water being known, the 

 aggregate quantities might be calculated. In order to count 

 all the micro - organisms he invented a method based on 

 the principle employed in physiology for the purpose of 

 counting blood corpuscles, viz. to dilute a sample of known 

 volume with a known volume of liquid in which the organisms 

 become evenly distributed. With a specially devised instru- 

 ment a small sample (say i c.c.) is taken out and its contents 

 counted. 



This method has added greatly to the practical working of 

 biological ocean research, and will undoubtedly increase in 

 importance in future. Like all other means of research it 

 must be employed with judgment, and the special nature of the 

 investigations must decide whether it may be applied and at 

 what stage with advantage. The application of the method has 

 led to much discussion, the enthusiastic advocates of the 

 method considering it imperative that it should be used in all 

 "truly scientific" investigations on the micro-organisms of the 

 ocean, while its opponents have looked upon it as a means of 



' V. Hensen, " Uber die Bestimmung des Planktons," F. Bericht der Coiitniission zitr wiss. 

 Untersuclnmg der detitschen Meere in Kiel, 1887. 



