134 QUANTITATIVE PLANKTON INVESTIGATIONS [PART II 



have used coins as counters, but if the biologist is short of coin any 

 other convenient objects will do — I use lead shot in pill boxes. 

 When the plate is examined the counters in the separate com- 

 partments are then totalled up. 



If the diluted catch measured 100 c.c, and if 1 c.c. of this were 

 examined and 50 copepods were found the total number of cope- 

 pods in the whole is obviously 50 x 100. Now we supposed that 

 there were very many more diatoms in the catch than there were 

 copepods : it may be the case then that in 1 c.c. of the catch, diluted 

 as above, there would be too many diatoms to be easily estimated. 

 If this is the case then 10 c.c. of the first dilution is taken and is 

 mixed with 90 c.c. of water in another flask and 1 c.c. of this second 

 dilution is examined as before. Suppose that this quantity con- 

 tains 500 diatoms : then there were 500 x 100 x 10 in the original 

 catch. 



The enumeration of the separate organisms contained in a 

 catch taken with a Hensen net is thus carried out by methods 

 which are exactly those of the chemist or bacteriologist. The 

 work is laborious but it is not at all difficult or unreliable. No 

 doubt the limits of error are greater than in the case of the 

 counting of bacteria contained in water, milk or air ; and still more 

 so than in the volumetric analyses of the chemists, but a con- 

 sideration of the Hensen methods in greater detail than I have been 

 able to make will shew that it is possible to describe a plankton 

 catch quantitatively with a considerable degree of accuracy. Now 

 we have filtered a certain amount of water from the sea in a 

 certain small area, and we have counted the individual organisms 

 contained in this catch. The question now arises : is this catch a 

 fair sample of the plankton contained in the sea in the area 

 round the spot where the net was fished ? 



Validity of the Hensen methods. Haeckel was the first to 

 doubt the validity of Hensen 's conclusions. In a paper of very 

 great general interest^ he attempted to shew that the problems 

 which Hensen proposed to investigate were not of such a nature as 



^ Plankton- Studien, Jena. This memoir is written in a very attractive manner, 

 but some of the arguments and conclusions are unrehable, and the reader should 

 certainly also read Hensen's answer, "Die Flankton-Expedition u. HaeckeVs Dar- 

 winismm," Kiel u. Leipzig, 1891, before accepting all Haeckel's conclusions. 



