vi PREFACE 



of research, and new methods of investigation. Hensen's primary 

 object in devising methods and apparatus by means of which it 

 might be possible to estimate the actual numbers of the microscopic 

 animals and plants inhabiting parts of the sea, seems to have been 

 the calculation of the numbers of certain species of edible fishes 

 in the North Sea and Baltic. But whenever it became possible to 

 estimate (even roughly) the numbers of plants and animals, of 

 microscopic dimensions, in restricted sea-areas, a host of interesting 

 questions immediately suggested themselves ; and these would not 

 have arisen — indeed did not arise— apart from the stimulus of the 

 application of methods of quantitative research to marine biological 

 investigation. Thus we had Hensen's discussions of the pro- 

 ductivity of the sea, and his comparison of this with the fruitful- 

 ness of the land; and Brandt's investigation of the chemical 

 composition of the plankton, with all the interesting questions and 

 speculations which were suggested by the results of these two 

 series of researches. Then whenever the plankton became regarded 

 as the " pastures of the sea " the question of the distribution of 

 the sources of nutriment of these marine pastures inevitably 

 suggested itself; and new and accurate determinations of the 

 proportions and distribution of the ultimate inorganic food-stuffs 

 of marine organisms were instituted. And when it was found 

 by means of quantitative plankton investigations that the mass of 

 life in different parts of the oceans of the world varied greatly per 

 unit volume of sea, it was seen that the cause of this variation 

 could reside only in the concomitant variation of the proportions of 

 the ultimate inorganic food-stuffs. But why do the latter vary ? 

 Brandt suggested bacterial action, and very soon the investigation 

 of the nitrogen-bacteria of the sea commenced — with the most 

 fruitful results — and a further insight was gained into the question 

 of the circulation of nitrogen in nature. Other causes were then 

 suggested, and it has become probable that the circulation of food- 

 stuffs in the sea is to be closely associated with the distribution of 

 oceanic streams and currents. 



