(A) The high points for each species are with fair regularity 10
(b) 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
This is different in three different groups.
First. The group whose food and enemies fluctuate but little. The
only examples of this on our list are the Muskrat and Beaver, more
especially the Muskrat. 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 enemies.
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.