as most dangerous districts for human beings to live in. Even
in civilized communities the ravages of the disease have, under
conditions most conducive to malaria, been fearful, so that only most
urgent requirements of mining, manufacturing, or similar material
processes have prevented the obliteration of entire communities.

The cause of the heavy death roll resulting from a bold defiance of the
reputation of these localities--a defiance bravely adopted by hardy
pioneers, by agents of trading companies, and by representatives of
governments--has been, up to the last ten years, assigned to the
water-laden condition of low-lying ground. Swamps and stagnant pools,
moisture-laden air, and a hot climate have been universally considered
to be the cause of the fever, and the transmission of the disease has
been supposed to be due to the passage through the moist air of the
germs of the disease, although the exact form and behavior of these
germs was unknown. Certain specifics have been proved by experience to
have some value. For instance, it has been found that planting a row of
trees between the house and a pool from which malaria might come has
been of aid in warding off the disease. In a number of cases a thick row
of eucalyptus trees, so associated in the popular mind with this purpose
that they are known as the malaria tree, have been planted as a tight
hedge with apparently very useful results. Drainage or filling up the
low lands has always been found to reduce the prevalence of the disease.

Many years ago the use of quinine in large doses was found to be a
specific, and the writer well remembers, on the occasion of his visit to
a malarial region, buying quinine at the grocery store by the ounce in
the same way that one would buy spices or tea, the dose being a
teaspoonful. Why quinine should prevent the daily or periodical chills
characteristic of the disease was not known, or why a row of eucalyptus
trees interfered with the development of the disease was not known, and
people generally were content to rest with the knowledge of these facts
only.

_Mosquitoes and malaria._

[Illustration: FIG. 78.--Resting positions for ordinary mosquito (left)
and malarial mosquito (right).]

In the year 1900, however, English scientists, working in the Roman
Campagna, demonstrated conclusively that which had been vaguely
suggested before, namely, that the cause of malaria is a parasite
composed of little more than an unform