98 ANIMAL LIFE AND SOCIAL GROWTH 



the chief causes of climatic variations on the earth 

 are solar variations, lunar variations and other 

 astronomical causes. Of the solar forces among 

 others, we know of variations in sun spots, in 

 sun prominences, in the amount of heat reach- 

 ing the earth's surface, in electro-magnetic and 

 perhaps other phenomena. The lunar forces are 

 known to control tides, and variations in the 

 depth of tides and in upheavals of cold water are 

 supposed to affect the atmospheric pressure and 

 so modify winds, cyclonic storms, rains and 

 temperature. Variations in both solar and lunar 

 activities are probably affected by yet more 

 remote astronomical changes the nature of which 

 has not yet been determined. It is an interesting 

 conclusion to which we are drawn, namely, that 

 in order to understand the organization of an 

 animal community and its fluctuations in num- 

 bers, men must first understand the organization 

 of the universe. 



