20 Professor Svmmers on 



to the intercommunication between tracts of that vast country 

 which in former times were absolutely isolated from each other, 

 and hence this dread disease had spread from the Congo to 

 Uganda. In reference to the second factor — how disease germs 

 or parasites reached our bodies — there was ample evidence to 

 show that these might be in some cases swallowed with food or 

 drink, or inhaled, or they might enter by direct contact, as in the 

 case of blood-poisoning, but of late they were beginning to see 

 that insects could be the vehicles for carrying these causes of 

 disease. Their domestic fly might convey the poison of typhoid 

 and implant it on food, such as milk and sugar, by which it was 

 in turn conveyed to the human system. And in the case of a 

 disease which in autumn caused a terrible mortality among 

 infants and young children— diarrhoea — the fly was probably one 

 of the factors in carrying the poison-germ from its breeding- 

 ground, especially in stables. Again, the mosquito conveyed the 

 malarial and yellow fever organisms, and it was by a thorough 

 crusade against these insects that the Americans were able now 

 to cut the Panama Canal, which Dr. Lesseps failed to accomplish 

 owing largely to the death-roll among his workers from malaria 

 and yellow fever. At the present time the mortality among those 

 working in ihis tropical district between North and South America 

 was as low, if not lower, than it was in Belfast. It was also 

 known that the flea played a part with the rat in carrying the 

 plague contagion. The latest great discovery was that an insect 

 called the tsetse fly was the vehicle by which the parasitic 

 trypanosoma which caused the sleeping sickness was carried 

 from person to person. 



Professor Symmers, who was received with applause, then 

 proceeded with his lecture. About seventy years ago, he said, a 

 French physician discovered in the body of fishes a certain 

 microscopic animal which up to that time was unknown to 

 science. Within two years of that time the same or a similar 

 parasite was pointed out in the blood of frogs in quite a number 

 of places in Europe. Lewis found rats in the neighbourhood of 



