CH. X] THE CONDITIONS OF LIFE IN THE SEA 207 



tribution and intensity of currents and drifts are not always the 

 same. As these conditions change so do the distribution and 

 abundance of marine life. Probably the total mass of life in the 

 sea remains approximately the same from year to year, but since 

 the total mass of food stuffs in the sea is limited (though very 

 great, of course) it follows that if any change in the life conditions 

 is favourable to the increase in numbers of any one group of 

 organisms it must, of necessity, lead to the decrease in numbers 

 of one or more associated groups. 



The quantitative methods of research that we have been 

 discussing afford one of the means of investigating the conditions 

 of this state of equilibrium, for we are led to enquire, in the first 

 place, into the problem of the absolute abundance of life in the 

 sea. Probably we shall never be able accurately to estimate the 

 density of the benthos and nekton, but it is tolerably certain that 

 the improvement of the methods of quantitative plankton investi- 

 gation will enable us to make very close approximations to the 

 actual density of the organisms comprised in this category ; and, 

 by deduction, to that of some groups of the benthos or nekton. 

 That we should become able to do so is essential to success in the 

 attempt to understand the conditions of equilibrium. Then we 

 must be able to investigate the conditions of nutrition of marine 

 organisms ; and to trace the effect of changes in physical sur- 

 roundings on the nature and intensity of these processes. To 

 attain success in this latter object we must know very exactly 

 what are the food substances that are utilised by the marine 

 animals and plants ; and how and why the nature and abundance 

 of these change from place to place, and from time to time. Even 

 at the present time a good deal of work has been expended on 

 the attempt to answer these questions and I will try to summarise, 

 in this Chapter, the information at our disposal. 



(1) The Nutrition of Marine Organisms. 



In the case of the terrestrial organisms we are able to make 

 a broad distinction between the " producers " and the " consumers." 

 The producers are the plants, and the consumers are the animals. 

 We divide the animals into carnivores and herbivores, and into 



