445 



the following instances will show: 1. Malaria, untreated, is mostly a chronic 

 disease, often of very long duration, while yellow fever is a very acute 

 disease, running its course, as a rule, within the space of a week. 2. I do 

 not know whether any efforts have been made to ascertain the limit of 

 time that the young malaria embryos may live within the body of the 

 contaminated anopheles after they have been hatched out ; but it is 

 evident that cramped up and crowded together in the tiny body of the 

 insect the embryos of the malaria parasite will not be able to reach their 

 full development, for want of space and of such nourishment as would 

 have been supplied by the human blood. In the body of the stegomyia 

 mosquito, on the contrary, we positively know that the germ of yellow fever 

 continues to live, at least, two months, and, probably, till the natural death 

 of the mosquito host. 3. The anopheles becomes infected by biting a person 

 suffering from malaria at any time when the parasite, in the form of 

 gametes or of resting bodies, happens to be present in the peripheral blood 

 circulation of that person, a condition which may last during several 

 months; while the yellow fever mosquito (Stegomyia fasciata) can only 

 become infected if it chances to bite a yellow fever patient within the 

 first few days of his attack; and when convalescents just recovering from 

 an attack of the disease, are removed to another locality inhabited by non- 

 immunes though there may be an abundance of stegomyia mosquitoes, the 

 infection is not transmitted to their new abode. 



The contrast, however, is more apparent than real, for the paralellism 

 can be easily re-established by simply recognizing that the stegomyia 

 mosquito acts for the yellow fever germ the same part that the human host 

 does for the malaria parasite, and vice versa. 



This recognition it not a mere matter of entomological or biological 

 interest, for it leads to logical deductions which may be of use in directing 

 our efforts to identify the unknown germ of yellow fever, and even if those 

 efforts should prove unavailing, a clue may be obteined for a better 

 comprehension of the nature of this germ as well as of some others which 

 are supposed to be invisible by reason of their extreme minutenses wdiich 

 places them beyond the reach of human vision even with the assistance of 

 our most powerful microscopes. The yellow fever germ being a parasite 

 of a small insect (the stegomyia mosquito) in whose body it must go 

 through all) the phases of development and multiplication by schizogonia, 

 with only a very scant food supply to be obtained from the tissues of its 

 host, must logically be a much smaller protozoon than the malaria parasite. 

 If therefore, it be remembered what difficulties the first explorers had to 

 overcome before the malaria sporozoites were recognised in the salivary 

 glands of the contaminated anopheles it is easy to conceive that the search 

 for a much smaller sporozoite in the blood of the yellow fever patient 

 would be almost a hopeless undertaking. It is possible however that in the 

 body of the contaminated stegomyia some larger resting form, analogous 



