502 LIFE IN THE OCEAN. 



advantage, so that their development will become more rapid; and 

 besides, where there are fewer vegetable organisms animals will be 

 less attracted, and consequently the destruction of them will be less. 



Like the microscopic vegetable organisms the animalcules that live 

 upon them are in general regularly distributed over the face of the 

 waters. This is true especially in the open sea, for inshore the wind, 

 the currents, and the possibility of using a different food often come 

 in to modify the distribution of the greater animals. These modifica- 

 tions are still greater at the spawning season, because the fish then 

 collect together. At this season certain kinds of fishing may become 

 profitable which at other times has to be given up on account of the 

 dispersal of its objects. 



The regularity of the distribution of the little organisms in sea 

 water led Hensen to the notion of applying to the determination of 

 the vegetable production a process analogous to that used for producing 

 eggs. The method of studying plankton quantitatively which Hensen 

 invented and perfected for the purpose mentioned is of great impor- 

 tance for all l)iological investigations on the sea. The fundamental 

 idea of this method is that it is proper to bring back to a single point 

 of view all the observations on the influence of the conditions of the 

 sea and on the mutual relations of marine organisms; on the transfor- 

 mations of matter and the chemical constitution of marine substances; 

 on the quality and quantity of vegetable and animal organisms living 

 in the water. In the first place it is necessary to collect, as far as 

 possible, all the plants which are found under a definite area of the 

 svirface of the water. To do this, Hensen proposes the use of nets 

 with verv narrow meshes, by which a vertical column can be, so to 

 speak, filtered, and make a statistical statement of the organisms 

 so lifted. Unfortunatel}^ the finest nets now obtainable allow the 

 passage of the microscopic vegetations which sometimes appear in 

 important masses in certain regions. Hence it would be necessary 

 to invent for those excessively minute organisms special methods of 

 quantitative analysis. However, Hensen has obtained interesting 

 results. Each drawing of the net represents the total organisms of 

 the plankton down to a size of at least 0.048 mm. that at a given time 

 and place are contained in a vertical column of known dimensions. By 

 reason of the regularity of distribution of the organisms the results 

 can, moreover, be extended to considerable areas — hundreds of square 

 kilometers of open ocean — over which the conditions of life are uni- 

 form. Near the coast and within currents the conditions are different; 

 so that it is proper there to take samples at lesser intervals. Further- 

 more, in order to gain a close acquaintance with the plankton of a 

 bod}' of water, it is necessary to repeat the experiments at the shortest 

 intervals of time possible during at least a year. 



It is not enough to measure the volume of organisms collected. It is 



