4 
parts of the country. Rabbits, hares, and field mice frequently cause great 
damage to fruit trees and young forest trees. Coyotes, wolves, and moun- 
tain lions take a large toll of sheep, cattle, and horses, and large sums have 
been expended for bounties in combating them. House mice and rats are 
well-known destructive pests, and have been known to carry the germs of 
diseases to man and the domestic animals. 
In addition to the obvious reactions on human enterprise of the species 
commonly classed as vermin, there are other relationships that are not 
so well known. Though it is commonly known to farmers, trappers, fur 
dealers, and naturalists that there are wide fluctuations in the numbers of 
various species of wild mammals and birds from time to time, the extent 
of such variations and their causes are not so well understood (Elton, 1924, 
Anderson, 1928, 1929, 1942). Many plagues of various species of field 
mice or voles have been recorded from North America, Europe, and Asia. 
In Arctic America lemmings and mice regularly reach the peak of abund- 
ance about every 4 years and this is followed by a rapid wiping out of 
nearly all the small rodents by disease. Arctic foxes and snowy owls 
increase in proportion to the lemmings and mice and fall away very soon 
after the depression in rodent life. The common snowshoe rabbit or varying 
hare has a very noticeable cycle of abundance, which reaches the peak about 
every 10 years. Muskrats and grouse show a similar but less-marked cycle, 
and various animals that prey on the others, such as lynx, red fox, mink, 
marten, etc., have parallel periods of abundance and scarcity. The small 
rodents have some local effect in settled regions, mainly in injury to agricul- 
ture, but in the wilderness their fluctuations have a tremendous effect upon 
the fur trade. As Elliot Coues once wrote: “They have one obvious part 
to play, that of turning grass into flesh, in order that carnivorous Goths and 
Vandals may subsist also, and in their turn proclaim, ‘All flesh is grass’ ” 
(Jordan, 1929, p. 397). 
The inter-relations between these different forms of life present still 
deeper and more obscure problems, which unfortunately have not received 
the attention they deserve. Pathologists have devoted most of their time 
to the study of diseases that affect mankind directly, and have discovered 
the relation of mosquitoes to yellow fever and malaria, of certain of the 
trypanosomes to sleeping sickness, of hook worms and other biological 
agents to other pathological conditions, and of rats and ground squirrels to 
the spread of bubonic plague and other diseases. It is also known that 
certain species of snails are secondary hosts to species of flukes that live 
in the bodies of various kinds of wild and domestic mammals, and that 
many kinds of parasitic worms and their allies infest the internal organs 
of other animals, but only a small number of their life histories have 
been thoroughly worked out. The most lowly species of mouse or shrew 
may be hosts to various internal and external parasites, or bear trypano- 
somes in their blood that may work havoc on themselves and on other 
species that feed on them or on the same range. 
The problems of wild animal life are so varied and interlocking that 
the co-operative studies of systematic zoologists, parasitologists, patholo- 
gists, veterinarians, and ecologists, extending over a period of years, are 
essential to the solving of some of the important questions. Only the barest 
beginnings have been made in the scientific study of most of these questions 
as they apply to a few species. The key to some of these problems is 
