192 AGGREGATION, COMPETITION AND CYCLES 



relation to rainfall and sunspots (1909), and the last two decades have 

 witnessed a steadily increasing attention to the problems in this field. 

 The use of tree rings in Sequoia has extended the available record 

 from that of less than 200 years for the longest tables of rainfall, to 

 3000 years (Douglass, 1919, 1928, 1936; Huntington, 1914, 1925; 

 Antevs, 1925), and the varve layers promise to increase this more 

 than threefold (Antevs, 1922, 1928). Clements has dealt with the 

 sunspot cycle in connection with rainfall and drought and the effect 

 upon ecological processes (1916, 1920, 1921a, 1929), while Elton has 

 devoted much attention to the relation between sunspots and animal 

 numbers (1924). Meteorological contributions, notably such as those 

 of Walker, Clayton, Clough, and Brooks, have been numerous (cf. 

 Clements and Chaney, 1936). In the general field of cycles, the 

 classical endeavor is that of Briickner (1891) ; though this was not 

 primarily concerned W'ith the sunspot cycle, the view is now widely 

 accepted that the Briickner cycle of about 35 years is its triple. 



It is now generally recognized by students of the subject that 

 the sunspot cycle is reflected in cycles of magnetic phenomena, of 

 pressure, temperature, and rainfall, but that these may differ for 

 major regions of the globe at any particular time. In central and 

 western United States, the average temperature is slightly higher at 

 the sunspot minimum than at the maximum, while the present evidence 

 favors the view that rainfall tends to be deficient at both minimum 

 and maximum and to be above the normal in the median years be- 

 tween the extremes. An apparent outcome of this is the division of 

 the sunspot cycle into two half-cycles of 5-6 years each. In addition 

 to the double cycle already mentioned and the triple or Briickner 

 cycle, there are others of greater length; these are mostly multiples 

 of the simple cycle of 10-12 years, but have little relevance to the 

 present purpose. 



Cycles in Mammal Populations. The best-known cycles among 

 mammals are those of the European lemming and of the varying hare 

 or snowshoe rabbit of North America and its major predators. These 

 also serve to illustrate the two major types, namely, a short cycle of 

 4-5 years and a longer one of about 10 years, which appear to cor- 

 respond more or less closely with the half-cycle and simple cycle of 

 sunspots, respectively. As the case of the lemming shows, the length 

 of the short cycle may be highly variable. Though the maximum 

 in Norway has usually occurred at intervals of 3 or 4 years, migration 

 was absent in 1898 altogether, and it has occasionally recurred in 2 or 

 5 years. In southern Sweden, the time has varied from 3 to 6 years, 

 while the maximum itself has twice persisted for a period of 3 years. 



