ENVIRONMENT, PAST AND PRESENT 61 



This may involve actual crowding and keen compe- 

 tition for food as in rabbits, or it may not, as in 

 the fisher and marten. Then suddenly there comes 

 an epidemic of diseases resulting in almost complete 

 extermination. Again there comes a slow recovery, 

 the attainment of a peak, another break and so on. 

 Thus in the thirty years that Alberta has been a 

 province and has had game laws, there has been a 

 closed season on grouse in the years 1907, 1917, 

 1927 and 1928. (The closed seasons have followed 

 on the heels of years of maxima.) The same years 

 have seen minima of rabbits and many other ani- 

 mals. The cycles, in fact, with a few exceptions, 

 are synchronous. They appear to bear no direct 

 relation to precipitation; the periodicity does not 

 agree with that of sun-spot changes; yet the fact 

 that many species, unrelated and of dissimilar feed- 

 ing habits, subject to a variety of diseases, come and 

 go together indicates some fundamental underlying 

 influence. There is some reason for believing that 

 annual variations in the ultra-violet radiation of 

 the north may prove to be the key. Reduced radia- 

 tion would mean diminished resistance to disease 

 and one would get exactly the conditions that favor 

 epidemics and universal death but extremely little, 

 unfortunately, is known of radiation in the Cana- 

 dian north and no such ten-year period has been 

 demonstrated. 



