CHAPTER II 



RHYTHMICAL CHANGE IN THE SEA 



THE ocean is regarded as the very type of muta- 

 bility and change, and indeed ordinary observation 

 shows us that its condition is never quite the same 

 from day to day. Many of the changes that occur 

 in it storms and calms, fogs, exceptionally high 

 tides and the like, appear to us to be without order 

 or regularity, that is their causes are so many and 

 so complex that we cannot predict their occurrence. 

 On the other hand there are many changes which 

 are repeated so regularly as to constitute definite 

 rhythms of events. Experience shows that their 

 times can be calculated and that they recur over 

 and over again with the certainty of astronomical 

 phenomena. Such are the tides, the annual waves 

 of temperature, salinity and sunlight ; annual out- 

 bursts of animal and vegetable life ; animal and 

 plant migrations ; spawning periods ; fishery seasons 

 and the like. Of all these periodic changes in the 

 condition of the sea the purely physical ones are 



