94 ANIMAL LIFE AND SOCIAL GROAYTH 



less than ten years is constant for that particular 

 region. Similar cycles are found in records of the 

 salmon fisheries of New Brunswick (9.6 year 

 period), in the growth of the giant sequoia trees 

 of California and in a rabbit-destroying disease 

 in Canada. 



This 9 + years cycle has not been much studied 

 in meteorological records. Apparently it is not 

 related to sun spots. It is a little longer than the 

 lunar cycle of 8.85 years and almost half of an- 

 other lunar cycle of 18.6 years. It is interesting 

 to note that the most distinct cycle of rainfall 

 and of the value of farm products in the United 

 States between 1837 and 1930 showed a period 

 of 18.6 years. Parenthetically, during this time 

 there have been six marked financial depressions 

 separated by average periods of 18.4 years. 

 These financial depressions accompany the agri- 

 cultural depressions which, however, they may 

 precede or follow, and it is interesting to note 

 that droughts, financial panics and decreased 

 productivity of American farms have a cycle 

 which is twice the length of that shown by popula- 

 tion fluctuations of rabbits, grouse, foxes, salmon 

 and sequoia trees. 



When we turn from considering the fact that 

 there are cyclic fluctuations in numbers of ani- 

 mals, to attempt to determine the causes of such 



