116 SECTIONAL “ADDRESSES. 
and ,ge familiar to every student of zoology or of medicine. At the 
tims, of our last meeting in Liverpool, insects were suspected of acting 
as transmitters of certain pathogenic organisms to man, but these cases 
were few and in no single instance had the life-cycle of the organism 
been worked out and the mode of its transmission from insect to man 
ascertained. The late Sir Patrick Manson, working in Amoy, had shown 
(1878) that the larvee of Filaria bancrofti undergo growth and metamor- 
phosis in mosquitoes, but the mode of transference of the metamorphosed 
larvee was not determined until 1900. Nearly two years after our last 
meeting here the part played by the mosquito as host and transmitter of 
the parasite of malaria was made known by Ross. In addition to these 
two cases at least eight important examples can now be cited of arthro- 
pods proved to act as carriers of pathogenic organisms to man—e.g. 
Stegomyia—yellow fever, Phlebotomus—sandfly fever, tsetse-flies— 
sleeping sickness, Conorhinus—South American trypanosomiasis 
(Chagas’ Disease), Chrysops—Filaria (Loa) loa, the flea Xenopsylla 
cheopis—plague, the body-louse—trench fever, relapsing fever and 
typhus, and the tick Ornithodorus—African relapsing fever. In select- 
ing examples for brief consideration I propose to deal very shortly with 
malaria, although it is the most important of the insect-carried diseases, 
because the essential relations between the Anopheles mosquito and the 
parasite are known to everyone here. There still remain lacunz in our 
knowledge of the malarial organisms. Ross and Thomson (1910), 
working in this city, showed that asexual forms of the parasite tend to 
persist in small numbers between relapses, and suggested that infection 
is maintained by these asexual stages. Such explanation elucidates 
those cases in which relapses occur after short intervals, but the recur- 
rence of the attacks of fever after long intervals can only be explained 
by assuming that the parasites lie dormant in the body—and we know 
neither in what part of the body nor in what stage or condition they 
persist. Nevertheless, the cardinal points about the organism are estab- 
lished, and preventive measures and methods of attack based on a 
knowledge of the habits and bionomics of Anopheles have been fruitful 
in beneficial results in many parts of the world. 
If we desire an illustration of the vast difference to human well-being 
between knowing and not knowing how a disease-germ is transmitted 
to man, we may turn to the case of yellow fever. When this pestilence 
came from the unknown, and no one knew how to check it, its appear- 
ance in a community gave rise to extreme despair and in many cases 
was the signal for wholesale migration of those inhabitants who could 
leave the place. But with the discovery that Stegomyia was the trans- 
mitting agent all this was changed. The municipality or district took 
steps to organise its preventive defences against a now tangible enemy, 
and the successful issue of these efforts, with the consequent great saving 
of life and reduction of human suffering in the Southern United States, 
in Panama, in Havana and in other places, is common knowledge. 
It is a striking fact that during 1922 Central America, the West Indies, 
and all but one country of South America were free from yellow fever, 
which has ravaged these regions for nearly two centuries. The cam- 
paign against Stegomyia is resulting, as a recent Rockefeller report 
