CHAPTER Ix 
THE PERIODIC FLUCTUATIONS OF OUR FUR-BEARING 
ANIMALS 
(CHARTS I TO V) 
TuHrovuGHoutT the animal world we find that not only is 
the abundance of the higher predatory animals dependent 
upon the abundance of the lower forms upon which they 
prey, or which may indirectly affect their food supply, but 
this dependence may result in remarkable periodic fluctua- 
tions in the numbers of the predatory forms. In the insect 
world this phenomenon of rise and fall in abundance is well 
known; a striking example occurred in 1915, when, owing 
to the unusual prevalence of aphids, or plant-lice, through- 
out Canada, wasps were extraordinarily abundant every- 
where in the same region, the probable explanation being 
that the wasps increased abnormally owing to the presence 
of an unusual amount of food in the form of ‘‘honey-dew,”’ 
which is excreted by the plant lice. Similarly, the abun- 
dance of certain species of our food fishes is affected by the 
amount of food in-the shape of small crustacea, or small 
fishes, in the sea, such food being variable in quantity. 
For instance, Bullen* showed that, in the years 1903-1907, 
there appeared to be a direct correlation between the num- 
ber of mackerel taken during May and the abundance of 
the small copepod crustacea. 
Darwin’s illustration of the relation of cats to the abun- 
dance of clover-seed is well known, but may be recalled. 
Red clover depends on the visits of bumble-bees for its 
fertilization. The abundance of bumble-bees in any dis- 
trict depends very largely on the number of field-mice which 
* Jour. Marine Biol. Assn., vol. VIII, p. 394, 1909. 
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