110 THE ARCTIC PRAIRIES 



(6) In the different species these are not exactly 

 coincident. 



(c) To explain the variations we must seek not 

 the reason for the increase — that is normal — but for 

 the destructive agency that ended the increase. 



This is different in three different groups. 



First. The group whose food and enemies fluctu- 

 ate but little. The only examples of this on our list 

 are the Muskrat and Beaver, more especially the Musk- 

 rat. Its destruction seems to be due to a sudden great 

 rise of the water after the ice has formed, so that the 

 Rats are drowned; or to a dry season followed by 

 severe frost, freezing most ponds to the bottom, so 

 that the Rats are imprisoned and starve to death, or 

 are forced out to cross the country in winter, and so 

 are brought within the power of innumerable enem'es. 



How tremendously this operates may be judged 

 by these facts. In 1900 along the Mackenzie I was 

 assured one could shoot 20 Muskrats in an hour after 

 sundown. Next winter the flood followed the frost 

 and the Rats seemed to have been wiped out. In 

 1907 I spent 6 months outdoors in the region and saw 

 only 17 Muskrats the whole time; in 1901 the H. B. Co. 

 exported over IJ millions; in 1907, 407,472. The fact 

 that they totalled as high was due, no doubt, to their 

 abundance in eastern regions not affected by the dis- 

 aster.' 



Second. The group that increases till epidemic 

 disease attacks their excessively multiplied hordes. 

 The Snowshoe-Rabbit is the only well-known case to- 

 day, but there is reason for the belief that once the 

 Beaver were subjected to a similar process. Concern- 



