CHAPTER VIIl. 



A CENSUS OF THE SEA. 



I HAVE discussed the question of the distribution of the 

 plankton before quoting any general statements regarding the 

 abundance of life in the sea. Obviously the value of these 

 statements will depend on whether or not a sample collection made 

 with a quantitative net in a certain part of the sea represents also 

 the contents of the sea a mile or two further off. This is just the 

 difficulty that is encountered in all investigations that depend on 

 sample observations. Weather forecasts depend on the determina- 

 tions of the positions and movements of atmospheric pressure 

 disturbances, and these latter are based on the readings of a small 

 number of barometers, scattered over a wide area. So also the 

 calculation of the average annual rainfall of a county is made 

 from the readings of a few rain-gauges situated here and there 

 over the district, and the amount of rain at those places where 

 there are no instruments may vary, more or less, from that 

 recorded at the observing stations. Our conclusions regarding the 

 geographical distribution of animals on the surface of the earth 

 are based on the results of sample collections, and the latter are 

 necessarily few in number, and are often imperfect. There is 

 always a certain degree of uncertainty in results which are based 

 on the study of samples. It must be assumed that the samples 

 represent the conditions that obtain at other intermediate places, 

 and this is not necessarily the case. 



The validity of all conclusions as to the general abundance of 

 microscopic life in the sea depends on the truth of the postulate, 

 that wherever in the sea the physical conditions are uniform, there 

 also the composition and abundance of the plankton is uniform. 



