GENERAL BIOLOGY 783 



oceans, seem to indicate that the oceans are not quite so poor 

 as Hensen's results would imply. But the nature, reproduction, 

 and vertical distribution of the " plankton " differ entirely in the 

 warm part of the ocean and in boreal waters. The only thing 

 we can do at present is to compare these two classes of 

 conditions, and to compare the groups of phenomena regarding 

 adaptation found in the ocean. 



Generally speaking, 1 think our experience justifies the 

 opinion that the scientific investigation of an ocean must 

 commence with observations of a qualitative kind. A chemist, 

 intent upon the investigation of a complex chemical compound, 

 sets to work in the same way, first endeavouring to make out 

 the nature of the single components of the compound, and in 

 many cases he will find it practicable to make preliminary, 

 merely relative, estimates as to the quantity of each component 

 present before entering into an absolute quantitative analysis. 



Hensen himself has shown how to make a definite 

 selection in the case of the complex " plankton "-problem by 

 taking up for quantitative investigation the occurrence of one 

 single organism, viz. the pelagic egg of the plaice. In this Pelagic 

 case, of course, an infinitely more clearly defined and sharply '^^^■«^g§^- 

 limited problem presented itself, and Hensen endeavoured to 

 solve it for certain areas of the North Sea and the Baltic, 

 developing the very interesting idea that the number of 

 spawning plaice might be arrived at by studying the number 

 of pelagic eggs within a restricted area, and ascertaining the 

 number of eggs spawned by the average female plaice. While 

 studying the cod eggs of the Norwegian Sea I have very often 

 had occasion to consider the same problem, but I have never 

 ventured to attempt its solution. Even in this case I con- 

 sidered it necessary, first of all, to make qualitative investiga- 

 tions, commencing with a detailed study of the areas where the 

 eggs of each species occur. 



The Norwegian waters are peculiar in varying greatly in 

 depth : in the course of a few miles one may find depths 

 ranging from a few to a couple of hundred fathoms ; they 

 are very instructive although, compared with the North Sea or 

 the Baltic, they exhibit extreme conditions. 



Another point to be considered is the fact that eggs, as 

 soon as spawned, are carried away by currents, the distance 

 which they travel depending on various local conditions. The 

 influence of these currents must, therefore, be ascertained, as 

 the eggs cannot be considered as stationary. 



