METHODS OF PLANKTON RESEARCH. 519 



During the last few years it has become obvious that 

 the catch with the fine meshed bolting silk only gives an 

 incomplete sample of the plankton present in the sea at 

 any given place. Kofoid (8) and Lohmann (11, 13 and 

 18) have both emphasised this error, but it is to the latter 

 that we are indebted for a complete investigation of it 

 and of the means of overcoming any failings in this 

 direction. In an important paper, published in 1902, 

 an account of the comparisons between various methods 

 for catching the smaller plankton organisms was given 

 in detail. The subject has since that time been further 

 investigated, and whilst writing this a detailed and very 

 elaborate account, bringing the plankton work up to date, 

 is going through the press (18). By the kindness of 

 Prof. Lohmann, I have been able to see his tables and 

 read through the proofs of this work. Hensen's method 

 rests on two hypotheses : — (1) That the pelagic organisms 

 in the sea inside a region of like conditions of existence, 

 with regard to time and space, are so equally distributed 

 that by the investigation of relatively small quantities 

 of water, a sufficiently accurate picture of the quantity 

 and quality of the plankton for the whole region can be 

 obtained. (2) That the apparatus used for these investi- 

 gations, namely, the Hensen net, even with its uncon- 

 trollable errors, gave essentially a true estimate of the 

 plankton. The first hypothesis will be discussed later. 

 With regard to the second, there is the possibility of the 

 net failing to catch an important part of the plankton, 

 through small organisms passing through the meshes. 

 Hensen himself in 188T stated (1) that if he allowed the 

 water filtering through the silk net to pass through close 

 silk, filter paper, &c, and investigated the residue, many 

 diatoms, peridinians and silicoflagellates would be found 

 to have passed through the net. He believed, however, 



