638 POPULAR SCIENCE MONTHLY. 



" The cases of malaria in the spring and early summer are of 

 the milder, more regularly intermittent varieties (tertian and 

 quartan fever), the severe aestivo-autumnal infections beginning 

 to appear only in the later summer, and reaching their maximum 

 in September." 



Manson has recently suggested that the mosquito is an inter- 

 mediate host for the malarial plasmodium. We have an analogy 

 for this in the part played by the mosquito in withdrawing em- 

 bryo filarise {Filaria sanguinis liominis) from the blood of in- 

 fected individuals and returning them to the stagnant pools fre- 

 quented by the insect. Manson says, in discussing this hypothesis 

 in his Gulstonian Lectures (1896) : 



" We can readily understand how the mosquito-bred Plasmo- 

 dium may be swallowed by a man in water, as so many disease 

 germs are, and we can readily understand how it may be inhaled 

 in dust. Mosquito-haunted pools dry up. The plasmodia in the 

 larvae and those that have been scattered about in the water, find- 

 ing themselves stranded by the drought, and so placed in a con- 

 dition unfavorable for development, pass into a resting stage, just 

 as they do when by quinine or other means man is rendered tem- 

 porarily unsuited for their active life. They may, probably do, 

 become encysted, as so many of the protozoa do in similar circum- 

 stances. The dried sediment of the pool, blown about by winds 

 and currents of air, is inhaled by man, and so the plasmodium 

 may find its way back again to the host from whom its ancestors 

 had, perhaps, started generations back." 



This theory appears plausible, but we find it difiicult to believe 

 that man is essential for the completion of the life cycle of the 

 Plasmodium, for the most concentrated and deadly malarial ema- 

 nations may be given off from marshy places which are far re- 

 moved from the haunts of men. It may be, however, that the 

 mosquito is an essential factor in the development of the plas- 

 modium, and that man, instead of being a necessary intermediate 

 host, only serves occasionally, and in a certain sense accidentally, 

 as such. Perhaps other mammals or birds may serve the same 

 purpose. It has frequently occurred to the writer that the ma- 

 larial Plasmodium, like other amoeboid protozoa, may find its nor- 

 mal habitat, external to the bodies of its insect or animal hosts, 

 upon the stems and leaves of water plants, rather than in the 

 water itself. The fact that malarial fevers do not prevail in the 

 vicinity of swamps when the marsh vegetation is submerged by 

 high water is in favor of this view ; as is also its apparent need 

 of plenty of oxygen, which we infer from its active multiplication 

 in the blood and its parasitic invasion of the red blood-corpuscles. 

 Possibly the mosquito is an intermediate host for the Plas- 

 modium malaricB on a larger scale than Manson suggests. The 



