CH. VIIl] A CENSUS OF THE SEA 177 



times the stones of a sea beach may be entirely covered with 

 barnacles {Balanus), or periwinkles. But, of course, such instances 

 of great density of population, and also those due to shoals of 

 gregarious fishes like herrings or sprats, are rather exceptional and 

 do not represent the average density of life of an open sea area. 

 Just as in the case of the plankton we find that the density and 

 composition of life depend on, and vary with, the physical conditions ; 

 so also we find that the abundance and nature of both the benthic 

 and nektic fauna are variable, and that the amount of variation 

 is greater the nearer we are to the coast. 



J. F. 12 



