II] RHYTHMICAL CHANGE IN THE SEA 37 



springs of the ' even ' years than it is in the springs 

 of the ' odd ' years. There is also a period of about 

 twelve years and we have reasons for believing 

 that this is to be associated with the twelve-yearly 

 period of sunspot change. The years when there are 

 the greatest numbers of sunspots are also the years 

 when the solar radiation is most intense, for the 

 sunspots are the indication of increased activity in 

 the sun. Both the temperature and salinity of the 

 sea are subject to these long-period changes. 



All these rhythmic changes, the tides, sea tem- 

 perature and salinity, and intensity of sunlight are 

 of cosmic origin, and we can trace them back to 

 certain fundamental causes the rotation and revolu- 

 tion of the sun, and the intensity of solar radiation. 

 In the case of the tides we can predict the rhythmic 

 change with great accuracy ; we can also predict the 

 annual temperature and salinity waves, but not the 

 exact dates of the maxima and minima ; and we can 

 predict with accuracy the times of sunrise and sunset, 

 and the daily declination of the sun, but not the 

 amount of cloud-covered sky. We know that the 

 sun is a variable star, but we do not know the exact 

 period of its variability, nor the range of variation 

 of its radiation. 



All organic changes, that is, the periodic changes 

 in the habits of marine organisms, must ultimately 

 depend on these cosmic changes, but the inter- 



