220 THE CONDITIONS OF LIFE IN THE SEA [PART III 



the reader that the proportions in which some of these substances 

 occur, nitrogen compounds, silica, and phosphoric acid, for instance, 

 are very minute, and it may well be asked how such an ex- 

 ceedingly attenuated solution can be utilised as a source of food ? 

 It was inconceivable to Murray and Irvine (loc. cit.) that diatoms, 

 for instance, could extract the silica necessary for the formation 

 of their shells from solution in sea water, without the expenditure 

 of a considerable amount of energy. We may take it, however, 

 that the difficulty in believing that such a highly dilute solution 

 could profitably be utilised is connected with a natural tendency 

 to regard the metabolic processes in the lower invertebrata, or 

 protozoa (which processes have not been minutely studied) as 

 more or less similar to those of the warm-blooded animals (which 

 have been studied in detail). In order to obtain its nitrogen 

 food from a medium with a similar concentration to that which 

 is utilised by a diatom a mammal would have to pass an enormous 

 quantity of liquid through its alimentary canal, and one can easily 

 see that the labour of such an absorptive process would be very 

 great. But when we consider that such an organism as a diatom, 

 or a flagellate protozoan, probabl}' absorbs its food over its entire 

 surface, and that relatively to its mass this surface is an enormous 

 one, the apparent difficulty disappears. The smaller an organism 

 is, the less food it requires : but then the smaller it is, the greater 

 (relatively) does the surface, by means of which it absorbs its food 

 solution, become. Putting it in mathematical language, we may 

 say that with diminishing size the surface decreases with the 

 square of the radius, while the mass decreases with the cube of 

 the radius. Assuming that the surface of a single bacterium is 

 only 10/x^ we find that one kilogramme of dry organic substance 

 corresponds to a surface of 62,500 square metres ! (Piitter^). In 

 man, however, one kilo, of dry organic substance corresponds to 

 a, surface of only 0"168 sq. metre. One may conclude, then, that 

 food solutions that are totally inadequate for the satisfactory 

 nutrition of a large animal are sufficiently concentrated to be 

 utilised by an organism of the size of a diatom, just because the 

 means of absorption in the latter are so much more powerful than 

 in the case of the former. 



1 " Stoffhaushalt des Meeres," p. 343. 



