(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 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. Next winter the flood followed the frost and the Rats seemed to have been wiped out. In 1907 1 spent 6 months outdoors in the region and saw only 17 Muskrats the whole time; in 1901 the H. B. Co. exported over 11 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 disaster.
Second. The group that increases till epidemic disease attacks their excessively multiplied hordes. The Snowshoe-Rabbit is the only well-known case today, but there is reason for the belief that once the Beaver were subjected to a similar process. Concerning the Mice and Lemmings, I have not complete data, but they are believed to multiply and suffer in the same, way.
Third. The purely carnivorous, whose existence is dependent on the Rabbits. This includes chiefly the Lynx and Fox, but in less degree all the small carnivores.
In some cases such as the Marten, over-feeding seems as inimical to multiplication as under-feeding, and it will be seen that each year of great increase for this species coincided with a medium year for Rabbits.
But the fundamental and phenomenal case is that of the Rabbits themselves. And in solving this we are confronted by the generally attested facts that when on the increase they have two or three broods each season and 8 to 10 in a brood; when they are decreasing they have but one brood and only 2 or 3 in that. This points to some obscure agency at work; whether it refers simply to the physical vigour of the fact, or to some uncomprehended magnetic or heliological cycle, is utterly unknown.
The practical consideration for the collecting naturalist is this: Beaver, Muskrat, Otter, Fisher, Raccoon, Badger, Wolverine, Wolf, Marten, Fox reached the low ebb in 1904-5. All are on the upgrade; presumably the same applies to the small rodents. Their decacycle will be complete in 1914-15, so that 1910-11 should be the years selected by the next collecting naturalist who would visit the north.
For those who will enter before that there is a reasonable prospect of all these species in fair numbers, except perhaps the Lynx and the Caribou. Evidently the former must be near minimum now (1909) and the latter would be scarce, if it is subject to the rule of the decacycle, though it is not at all proven that such is the case.