coast of Africa, or to Asiatic seaport cities. On the other hand, if the
disease could be transmitted by infected clothing, bedding, etc., there
seems no good reason why it should not have been carried to these
distant localities long ago.

The restriction as regards altitude, however, probably depends upon the
fact that the mosquito which serves as an intermediate host is a coast
species, which does not live in elevated regions. It is a
well-established fact that yellow fever has never prevailed in the City
of Mexico, although the city has constant and unrestricted intercourse
with the infected seaport, Vera Cruz. Persons who have been exposed in
Vera Cruz during the epidemic season frequently fall sick after their
arrival in the City of Mexico, but they do not communicate the disease
to those in attendance upon them or to others in the vicinity. Evidently
some factor essential for the propagation of the disease is absent,
although we have the sick man, his clothing and bedding, and the
insanitary local conditions which have been supposed to constitute an
essential factor. I am not aware that any observations have been made
with reference to the presence or absence of _Culex fasciatus_ in high
altitudes, but the inference that it is not to be found in such
localities as the City of Mexico seems justified by the established
facts already referred to.

As pointed out by Hirsch, "the disease stops short at many points in the
West Indies where the climate is still in the highest degree tropical."
In the Antilles it has rarely appeared at a height of more than seven
hundred feet. In the United States the most elevated locality in which
the disease has prevailed as an epidemic is Chattanooga, Tennessee,
which is seven hundred and forty-five feet above sea level.

It will be remembered that the malarial fevers are contracted as a
result of inoculation by mosquitoes of the genus _Anopheles_, and that
the malarial parasite has been demonstrated not only in the blood of
those suffering from malarial infection, but also in the stomach and
salivary glands of the mosquito. If the yellow fever parasite resembled
that of the malarial fevers, it would no doubt have been discovered long
ago. But, as a matter of fact, this parasite, which we now know is
present in the blood of those sick with the disease, has thus far eluded
all researches. Possibly it is ultramicroscopic. However this may be, it
is not the only infectious disease ge