NO. 4 ALLEN: PLANKTON DIATOMS 49 



three sections had been taken at exactly the same time, one might 

 think that the differences in abundance of diatoms were due to the 

 northern section being a relatively shallow part of the Gulf from 

 which depths were progressively greater to and through the southern 

 section. Or he might think that differences in abundance were related 

 to increasing distance southward from the influence of the Colorado 

 River drainage. With time differences so prominent as they are in 

 this series it is easy to see that direct comparison of productivity in 

 different sections of the Gulf may lead to wrong conclusions as read- 

 ily as to right conclusions. It is also evident that production in the 

 southern section over the four and one-half weeks' period may be 

 badly represented by catches taken only at the beginning and end. 

 Not a little of the importance of the evidence of this series lies in the 

 fact that it helps to direct attention to the hazards of assuming the 

 constancy of conditions in nature in periods for which direct observa- 

 tions are lacking. Still, it should be understood that rare or discontin- 

 uous observations of natural phenomena are better than none at all, 

 provided they are not used to reach or support conclusions tran- 

 scending the range of their evidence. 



The Northern Section 



In lack of information from a detailed survey of the Gulf of Cali- 

 fornia it is possible to make only roughly suggestive comments on 

 certain features of its topography and their significance. The evi- 

 dence at hand indicates that the northern section in this series is a 

 rather steeply descending trough extending from the region of rela- 

 tively level floor (depth 250 meters or less) above Angel de la Guar- 

 dia Island (or from about Lat. 29° SC) to the twenty-seventh paral- 

 lel of latitude where a considerable unevenness in bottom topography 

 indicates a southern termination of at least part of the trough. In this 

 section there is not much evidence of land drainage of water from 

 the side of the trough and there seems to be no great extent of shoals 

 or marginal lagoons or swamps. Superficially, there seems to be little 

 opportunity for Pacific Ocean waters to exert direct influence on the 

 conditions of organic productivity of the section. Presumably, the 

 direct influence of the Colorado River drainage should be much 

 greater (although probably greatly changed in the future as a result 

 of diversion projects). Apparently, there is no dependable basis upon 



