50 LIFE IN THE SEA [OH. 



water of the Gulf Stream drift which attains its 

 maximum in the early summer ; the increase in the 

 intensity of the sunlight in the early spring, and the 

 rise of sea temperature in the early summer. They 

 are therefore strictly periodic in their occurrence and 

 if there are fluctuations in the time of appearance of 

 the inflowing Gulf Stream water, or in the rise of sea 

 temperature, or in the change in intensity of sunlight, 

 then the time of appearance of the various maxima of 

 planktonic life may be accelerated or delayed. If 

 there are long-period rhythms in the order of the 

 physical phenomena which we have considered then 

 there ought also to be long-period rhythms in the 

 abundance of life in the sea. This is of course difficult 

 to prove but where we have fishery statistics it is 

 quite evident that there are such periodicities. Only 

 two or three times during the nineteenth century 

 have great shoals of herring appeared off the west 

 coasts of England and Wales, in the estuaries of the 

 Dee and Mersey, and these times no doubt coincided 

 with some unusual combinations of physical conditions. 

 There is a winter herring in the Skagerak, off the 

 coast of Sweden, and this has been recorded since 

 the year 895 and has been found to recur at intervals 

 of about 111 years. 



But the movements and reproductive habits of 

 a marine organism are not entirely dependent on 



