1915] 



SETCHELL DISTRIBUTION OF MARINE ALGAE 



299 



Commenting on the above, it may be surmised that the 

 percentage in one zone is high on account of many new or 

 little known species which have been collected only once, while 

 the percentage of species occurring in two successive zones 

 of different temperature is low because of our incomplete 

 knowledge. Concerning the species credited to three zones, 

 the percentage is small but perhaps not much lower than will 

 be found on final careful revision. Here seasonal occurrence 



and "spot" distribution will undoubtedly be found to be con- 

 cerned in the overlapping, as it will be also in the case of 

 overlapping in two zones. Concerning the occurrence in four 

 or five successive zones of different temperatures the percent- 

 age although small will, with very little doubt, be decidedly 

 decreased or even entirely erased when the doubtful cases 

 are investigated and cleared up. There may be a fraction 

 of one per cent still left, however, and if there is, I doubt 

 not that some fairly simple physiological explanation of their 

 toleration of such an extreme range of temperature will be 

 found. The disturbances in the uniformity of regular in- 

 crease or decrease in the temperature of surface waters, as 

 referred to latitude, have already been mentioned as due to 

 cold upwellings and spot variation according to local physical 

 peculiarities. These disturb, of course, the zonal distribution. 



Where such intrusive areas of colder or warmer water are 



extensive, the distribution in those areas must be considered 

 in connection with the nearest zone of similar temperature. 

 Spot distribution also, may be so referred but only in general 

 considerations of distribution. Otherwise it must be con- 

 sidered specially. 



The disturbance of regular zonal distribution which must 

 have special consideration from the zonal point of view is 

 that which arises from seasonal variation in the surface tem- 

 perature accompanied by seasonal occurrence of a certain ele- 

 ment of the flora in some district or province of a region of 

 the particular zone. 



Seasonable amplitude varying on an average from about 

 5°C. to 15° C. in extent, as I have mentioned before, is found in 

 the various temperature zones. Seasonal duration, or, at 



