CONDITIONS INFLUENCING PREVALENCE 155 



multiplying bladder worm. This, when swallowed with its 

 eaten host, developes, in the flesh -eating animal, once more 

 into the sexually multiplying tape- worm. 



Now, although we are acquainted with a large number 

 of parasitic life histories of this character, we know of no 

 instance in which a parasite with such a history is capable 

 of maintaining the continuity of the species in any other 

 manner, and it will be indeed astonishing if the malarial 

 parasite should prove an exception to what has been 

 hitherto found to be an unvarying law of parasitism; though 

 of course, however intrinsically improbable, it is within the 

 range of biological possibility, that besides finding its way 

 to its inteimediate host along with the venom of the Mos- 

 quito, the parasite may also be capable of assuming the 

 form of a resting spore, or some kindred reproductive 

 mechanism, and being in this way conveyed to water, food, 

 &c., through the agency of the living or dead Mosquito. 

 There is not, however, a single recorded fact in the natural 

 history of the disease that suggests the probability of such 

 an occurrence, and practically speaking, everyone possessing 

 any special knowledge of helminthology will be convinced 

 that either the idea that the Mosquito is the alternative host 

 of the malarial parasite is a huge mistake ; or it is, under 

 natural circumstances, the one and only method of infection. 

 There is in reality no tenable middle position. 



Most of the apparent exceptions depend on the fact that 

 like most other two-host life-history parasites, the host 

 carrying the asexual phase of the malarial parasite may do 

 so for years without any perceptible inconvenience. A 

 bladder worm may have to lie imbedded in the tissues of 

 an ox for years before the animal is turned into beef and 

 devoured by a man. 



Then its opportunity has come and it developes into a 

 tape-worm each sexually mature, j^^'og^ottis of which is a 

 complete, hermaphrodite, sexually mature animal. 



So with the malarial parasite. An infected person may 

 have no visible symptoms, but lurking in his tissues are the 

 parasites ready to start again on their course of asexual 

 nmltiplication should any accident bring the resisting power 

 of the host sufficiently low. 



