THE AMERICAN NATURALIST [Vol. XL VII 



"barriers are far less in evidence in the environment of 

 the pelagic fauna than in that of the shore or of the 

 land," and that, while there do exist "limited regions 

 along the margins of great ocean currents" which might 

 afford means of hydrographic isolation, changes in 

 hydrographic conditions such as temperature, density, 

 substances in solution, illumination, etc., are so grad- 

 ual that stratified areas do not exist to any large extent. 

 Furthermore, land faunas are segregated into neigh- 

 boring or remote areas almost entirely with reference to 

 latitude and longitude. With pelagic faunas this is not 

 necessarily, perhaps not usually, the case, for a third 

 dimension — depth — is involved. It therefore follows 

 that closely related pelagic organisms may be coinci- 

 dently distributed as regards latitude and longitude, and 

 still be completely isolated in their vertical distribution. 



This fact signifies that data respecting the isolation of 

 a pelagic fauna will be wholly inadequate unless the ver- 

 tical distribution of the particular species or group 

 under consideration be capable of determination and 

 analysis. In his discussion of "the coincident distribu- 

 tion of related species of pelagic organisms as illustrated 

 by the Chaetognatha " Kofoid ('07), while recognizing 

 the force of this point, has utilized data pertaining al- 

 most exclusively to latitude and longitude. This was 

 consequent upon no lack of appreciation on his part of 

 the real problem involved, but solely to the fact that the 

 necessary data were missing. Moreover, what little has 

 been previously discovered relative to the vertical dis- 

 tribution of this group was based upon observations 

 scattered over such large areas as to make any approach 

 to critical analysis of the problem of isolation almost 

 irmpossible. However, through the efforts of the San 

 Diego Marine Biological Station, a mass of data has been 

 collected which enables an entirely new light to be 

 thrown upon this problem. 



Since 1904 this station has centered its collecting upon 

 an irregular area of about 30 square miles lying be- 



