south of these regions. The cause of the disease and its mode of
conveyance was discovered in 1903. The fly _Glossina palpalis_
which conveys the disease is a biting fly about the size of the common
house fly and lives chiefly in the vicinity of water. When such a fly
bites an individual who has sleeping sickness its bite can convey the
disease to monkeys, on whom the transmission experiments were made.
After biting the fly is infectious for a period of two days. After
this it is harmless, unless it again obtains a supply of living
trypanosomes. There is quite a period in which there are no symptoms
of the disease, although trypanosomes are found in the blood and in
the lymph nodes, and the individual is a source of infection. The
peculiar lethargy which has given the disease its name does not appear
until the nervous system is invaded by the parasites. It is impossible
to compute accurately the numbers of deaths from this disease--in the
region of Victoria Nyanza alone the estimates extend to hundreds of
thousands.

3. In the third mode of insect conveyance the insect does not play a
merely passive rôle, but becomes a part of the disease, itself
undergoing infection, and a period in the life cycle of the organism
takes place within it. In all these cases quite a period of time must
elapse before the insect is capable of transmitting the disease; in
malaria, which is the best type of such a disease, this period is ten
days. Malaria is due to a small protozoan, the _Plasmodium
malariæ_, which was discovered by Lavaran, a French investigator,
in 1882. The organism lives within or on the surface of the red blood
corpuscles. It first appears as a very minute colorless body with
active amoeboid movements, and increases in size, attacks a succession
of corpuscles, and finally attains a size as large as or larger than a
corpuscle. The corpuscles attacked become pale by the destruction of