and malarial
neuralgia, are extremely rare.

The exact cause of fever-and-ague and other malarial diseases is unknown,
but it is demonstrated that, whatever the cause is, it is originated under
a combination of circumstances, one of which is undue moisture in the
soil. It is not necessary that land should be absolutely marshy to produce
the miasm, for this often arises on cold, springy uplands which are quite
free from deposits of muck. Thus far, the attention of scientific
investigators, given to the consideration of the origin of malarial
diseases, has failed to discover any well established facts concerning it;
but there have been developed certain theories, which seem to be sustained
by such knowledge as exists on the subject.

Dr. Bartlett, in his work on the Fevers of the United States, says:--"The
essential, efficient, producing cause of periodical fever,--the poison
whose action on the system gives rise to the disease,--is a substance or
agent which has received the names of _malaria_, or _marsh miasm_. The
nature and composition of this poison are wholly unknown to us. Like most
other analogous agents, like the contagious principle of small-pox and of
typhus, and like the epidemic poison of scarletina and cholera, they are
too subtle to be recognized by any of our senses, they are too fugitive to
be caught by any of our contrivances.

"As always happens in such cases and under similar circumstances, in the
absence of positive knowledge, we have been abundantly supplied with
conjecture and speculation; what observation has failed to discover,
hypothesis has endeavored and professed to supply. It is quite unnecessary
even to enumerate the different substances to which malaria has been
referred. Amongst them are all of the chemical products and compounds
possible in wet and marshy localities; moisture alone; the products of
animal and vegetable decomposition; and invisible living organisms. * * *
* Inscrutable, however, as the intimate nature of the substances or agents
may be, there are some few of its laws and relations which are very well
ascertained. One of these consists in its connection with low, or wet, or
marshy localities. This connection is not invariable and exclusive, that
is, there are marshy localities which are not malarious, and there are
malarious localities which are not marshy; but there is no doubt whatever
that it generally exists."

In a report to the United States Sanitary Commission, D