256 ruivcrsitij of Califoniia Publications in Zoology [Vol. 15 



INTRODUCTION 



The biological survey of the waters of the Pacific adjacent to the 

 coast of Southern California, which was begun by the Marine Bio- 

 logical Association of San Diego in 1901, has been carried on by its 

 successor, the Seripps Institution for Biological Research of the Uni- 

 versity of California, since 1912. The aim has been to obtain as com- 

 prehensive an understanding as possible of the life of the sea, and 

 consequently considerable time and energy have been devoted to the 

 investigations of the hydrographie conditions under which marine 

 organisms live. The hydrographie data thus accumulated, together 

 with all the other field data and a full discussion of the methods of 

 collecting, laboratory analysis, accuracy of result.s, etc., have been 

 published in detail (Michael and McEwen, 1915, 1916). 



However, as Thompson (1904-05, p. 171) says: "We want to 

 learn the general or average hydrographical conditions of temperature, 

 salinity, and current at each place and at each depth, and then to 

 learn the changes or fluctuations that these conditions undergo during 

 the year or longer periods." To realize this ideal obviously demands 

 a very large number of observations taken at small intervals of time 

 and space, and involves the application of methods of averaging and 

 interpolating. Moreover, the results of such observations can not be 

 adequately described or understood without a quantitative investi- 

 gation of their causal relationships. Quantitative investigation brings 

 the facts of observation into closer relationship, as logical consequences 

 of known principles of physics and chemistry, and also indicates the 

 existence of other important and related phenomena, and emphasizes 

 the need of devising means for their direct observation. 



To obtain even a reasonable approximation to the complete knowl- 

 edge of the hydrographie conditions in this region that we desire will 

 require far more numerous and continuous observations than have 

 yet been made. However, it is believed that the data now available 

 warrant a brief account of tlie average hydrographie conditions and 

 their fluctuations. 



