FLUCTUATIONS OF FUR-BEARING ANIMALS 219 
region of northern Alberta, in 1915, signs of the recent 
abundance of rabbits were everywhere. For miles along the 
trails the young growth of poplar had been barked and 
girdled by the hordes of rabbits during the previous winter. 
But hardly a rabbit was to be seen; they had almost com- 
pletely disappeared. 
The cause of the sudden plague which kills off the rab- 
bits when they reach their greatest abundance so that the 
whole countryside is overrun with them is probably of a 
multiple nature. The chief factor is undoubtedly over- 
crowding. This results in an epidemic of various parasitic 
diseases to which rabbits are susceptible, particularly one 
of bacterial origin, which speedily spread throughout the 
rabbit population of the affected territory. 
Constituting as it does the chief food of many of the 
larger fur-bearing animals, such as lynx, fox, and wolf, the 
rabbit is one of the most important factors in determining 
the abundance of these animals. 
Lynx.—The lynx is primarily dependent upon ie rabbit 
as a source of food, although it also devours mice, grouse, 
ducks, stranded ith young deer, or sheep. Its periods of 
abundance, therefore, correspond with those of the rabbit. 
Preble states that the winter of 1903-04 was remarkable 
for the abundance of lynxes throughout the upper Mackenzie 
_ region, this abundance of lynxes being accounted for by the 
enormous numbers of rabbits in the same region at that 
time. Following the last outbreak of rabbits to which I 
have referred, lynxes were very abundant, according to the 
reports that I have received, during the winters of 1914 and 
1915. When the rabbits had disappeared there was a re- 
markable southern migration of lynxes throughout the 
northwest, and, during the winters of 1916 and 1917, they 
were taken in districts in the Prairie Provinces, whither 
they had migrated in search of food in more southerly locali- 
ties than they have been recorded as visiting for many 
