158 A CENSUS OF THE SEA [PART II 



If this is not the case, if the plankton is distributed throughout 

 the sea in a haphazard or fortuitous manner, then we cannot make 

 any general statements about its abundance, and the investigation 

 of marine microscopic life, apart, of course, from the collection of 

 material for speciographic or morphological study, is futile, a pro- 

 ceeding " without rhyme or reason." But I do not think that the 

 student of marine biology will readily come to this conclusion. It 

 is true that there are multitudinous factors in operation which 

 cause the plankton near the shore to be highly irregular in its 

 distribution ; and it is also true that little reliance can be placed 

 on the collections made by an ordinary surface tow-net, as repre- 

 senting the general contents of the sea in microscopic life. And 

 we know also that the quantitative plankton net is a somewhat 

 imperfect instrument. But apart from a comparatively narrow 

 margin of coastal waters, the physical condition of the sea may be 

 the same over wide areas, and when we make hauls with a vertical 

 plankton net in such offshore waters we find that the composition 

 and abundance of microscopic life is usuall}^ pretty much the same 

 over areas of sea many square miles in extents There is, in fact, 

 a certain order in the distribution of plankton, just as there is a 

 certain degree of constancy and regularity in the occurrence and 

 density of the bottom living animals, or the migratory fishes. If 

 we find that the nature and amount of the catches made by the 

 quantitative nets are, roughly speaking, similar over wide areas of 

 sea, then it is quite a valid proceeding to make statements of the 

 average numbers of the various organisms which are present in 

 the sea at certain times in the year, and per unit volume of 

 water. But the reader must understand such results are only 

 approximate ones, and that errors arise not only from the imper- 

 fection of the methods, but also from the lack of uniformity in the 

 distribution of the plankton. 



There is a certain mass of life always present in the sea (1) in 

 the form of microscopic and macroscopic organisms drifting 

 passively with the movements of the water, this is the plankton ; 

 (2) in the form of actively swimming fishes and other animals 

 (nekton) ; and (3) as bottom living plants and animals (benthos). 

 It is (theoretically) possible to estimate the absolute number of 



^ See Appendix. 



