104 Mosquitoes 



known to us, indicate that the organism is among the small- 

 est of living beings, and belongs to that group which is 

 rapidly becoming more than hypothetical — the ultra- 

 microscopic forms. The small size may be a result of 

 rapid multiplication, and it is not improbable that, after 

 the incubation period, larger forms will be recognised in 

 the digestive tract of the mosquito and in the salivary 

 glands. 



A single genus of protozoa is known at the present 

 time that fulfils all the conditions of the yellow-fever or- 

 ganisms ; amongst its species are some that are at times 

 ultra-microscopic, that have a characteristic change of 

 hosts from warm-blooded forms to mosquitoes, and that 

 are characterised by remarkable virulence. This is the 

 genus Spirocluzta, and in it alone at the present time do 

 we find the type that satisfies all the conditions known of 

 the organism of yellow fever." 



Dr. Carroll also puts the argument for the theory 

 of a protozoan strongly : 



" It seems quite rational to exclude it from the bac- 

 teria, because : (i) It has never been cultivated nor 

 stained by any of our known methods. (2) The work 

 of Marchoux, Silimbeni, and Simond has shown that the 

 blood of a patient after its withdrawal loses its power to 

 infect within two days, if kept exposed to the air, and 

 within five days if air be excluded. (3) The disease has 

 been shown to be absolutely non-contagious in regions 

 where Stegomyia fasciata{calopus) is not present,/, e. in Pe- 

 tropolis, nearRioJaneiro. (4) We know no bacteria that 

 live in the tissues of animals as the yellow-fever organism 

 does in the mosquito, for months, as a harmless parasite. 

 The logical conclusion, therefore, would seem to be that 

 the parasite of yellow fever belongs to the animal kingdom, 



