THE DEFINITIVE HOST 



873 



a protective intermediary host, and such a cycle can be exemplified 

 by the amoeba of dysentery and house-flies. 



This extremely simple carriage is also shown by animals which 

 convey bacteria, but some of these undergo multiplication in the 

 gut of the intermediary host — ^as, for example, the plague bacillus — ■ 

 and may even have their virulence raised — ^as, for example, Eberth's 

 bacillus. Returning again to the animal parasite, this may grow 

 and multiply in this second host, and may invade its tissues and 

 dwell therein for a length of time. This is quite different from the 

 short passage of an amoebic cyst in the flies' intestines. The second 

 host now becomes a true intermediate host, but it is something 

 relatively new interposed in the life-cycle of the parasite, which 

 has not yet adjusted itself to its new host, nor has this host adjusted 

 itself to the parasite; and the result is that the parasite almost 

 invariably causes disease in the intermediate host, which may be a 

 vertebrate or an invertebrate — e.g.: — 



The Definitive Host. 



Parasite. 



Definitive Host. 



Intermediate Host. 



Nature of 

 Parasitism. 



Filaria 

 bancrofti. 



Man but little 

 affected patho- 

 logically. 



Culex and Stego- 

 myia mosquitoes 

 severely affected 

 by infection. 



True parasitism of 

 the vertebrate. 



Plasmodium 

 malavicB. 



Anopheline mos- 

 quitoes unaf- 

 fected patho- 

 logically. 



Man suffers from 

 malarial fever. 



True parasitism of 

 the mosquito. 



It is therefore obvious that these two diseases, from the point 

 of view of evolution, have two quite different origins. The first 

 is originally a parasite of man, and, as Hindle has pointed out, 

 Hanson's original idea of water infection may be the true method, 

 and that ah initio the ' Larvofilaria ' lived in water and pierced the 

 human skin^ as it does to-day on leaving the mosquito, and requires, 

 as Bahr has shown, dampness in order to live while it pierces the 

 skin. The mosquito carrier is therefore a relatively new acquisition, 

 and the mosquito, not having adjusted itself to these conditions, 

 often dies, as Bahr has shown. On the other hand, Filaria bancrofti, 

 barring accidents, causes no symptoms in man, but if there are 

 accidents the disease ensues. 



It is quite otherwise with the malarial parasites, in which the 

 anopheline mosquito is the definitive host and man the intermediate 

 host. Here the mosquito is not affected pathologically, but man 

 suffers from malarial fever. Here the malarial parasite must have 

 been originally a sort of coccidioform parasite of the mid-gut of 



