1922] on Microscopic Parasites and their Carriers 521 



generally, which leads them in some mysterious manner to special 

 parts of the body which they appear to select by instinct— an instinct 

 which may be nothing more than a chemical peculiarity acquired by 

 a process of natural selection. 



You will readily understand the great importance of the kind <if 

 knowledge which the study of insects in relation to the transmission 

 of disease has given us. The life-history of one of these parasites 

 can be regarded as a circular chain composed of various links. If 

 any one of these links is broken the life of the parasite ceases and 

 the spread of disease is prevented. The protection of infected in- 

 dividuals from the particular invertebrate hosts, the destruction of 

 the invertebrates or the prevention of their breeding, the killing of 

 the parasites in the vertebrate — these and other methods are now in 

 practice in the prevention of malaria, yellow fever, sleeping sickness, 

 and other diseases. Our knowledge of the parasites themselves and 

 of their invertebrate hosts is, however, still defective, and there is 

 much useful work remaining to be done in the realms of Parasitology 

 and Entomology before hygienic measures of this kind can reach 

 perfection. 



[C. M. W.] 



