1915] 



SETCHELL DISTRIBUTION OP MARINE ALGAE 



297 



arctic regions, where there seems to be no useful purpose 

 served in segregation by assuming two zones rather than 

 one. The second disturbance of zonal areas is through the 

 occurrence of local areas, of greater or less extent, of water 

 of a higher or lower temperature than is normal for the gen- 

 eral 



zone. 



Colder waters occurring among warmer waters 



are found along the west coasts of North and of South 



Ameri 



of northwestern and southwestern Africa, and of 



northeastern Africa. These are due to 



rents 



to up- 



wellings of cold water. Their existence is well substantiated 

 but their cause is still a matter of discussion among ocean- 



ograpb 



When warm waters exist among colder waters 



they occur as "spots" or small areas where the higher tern 



perature is due to comparal 

 general oceanographic condit 



local factors apart from 

 Such disturbances as ud- 



wellings and spots may bring about a puzzling discontinuity 

 in the distribution, very puzzling, indeed, until the immediate 

 cause is discovered. 



Another matter causing seeming disturbance of the limits 

 of temperature zones proposed is the seasonal variation of 

 the temperature of the surface waters. This is variable, but 

 in general may be considered to hold true as follows: The 

 seasonal surface temperature variation as platted for 2° 

 squares is least in the Upper Boreal, Upper Austral and 

 Tropical zones, where it is not over 5°C. in range; is 

 greatest in the Temperate zones where it averages nearly 

 15° C. and may be as great as 27 or 28° C, and is medium in 

 the Subtropical zones and in the Lower Boreal and Lower 

 Austral zones where it approximates 10° C. 



These, then, are the principal features of temperature dis- 

 tribution with which we may be concerned. 



In connection with the empirical establishing of the temper- 

 ature zones previously outlined, I have attempted to arrange 

 each and every species of marine algal benthos thus far 

 described in the zone or zones to which it has been accredited. 

 The work is not as yet by any means completed, but a general 

 view has been obtained for the Rhodophyceae, Phaeophyceae, 

 Chlorophyceae, and Myxophyceae, and the greater part of the 



