5K) Dr. C. Morley Wenyon [March 3, 



zoites of the malaria parasites, if they can get into the salivary glands, 

 will have every chance of being inoculated to the man on whom the in- 

 fected mosquito feeds. This is exactly what happens, and the malarial 

 parasite has, so to speak, availed itstlf of the habit the mosquito has 

 of injecting saliva in order to enter another host. At the present 

 time the fact that some of the sporozoites rind their way to the 

 salivary glands seems to be purely fortuitous, for sporozoites are 

 found in other parts of the mosquito's body also, such as the legs 

 and antennas, and these cannot possibly get into man unless we 

 suppose that the mosquitoes themselves are eaten by man, a very 

 unlikely thing to happen. There is thus a great waste of sporozoites 

 from the point of view of the malarial parasite. It may come about 

 in course of time that a race of malarial parasites will be evolved 

 which produces sporozoites having a special affinity for the salivary 

 glands. It is evident that only those which enter the salivary glands 

 have any chance of further development in man, and if the quality 

 which causes attraction between parasite and salivary gland be 

 hereditarily transmitted to subsequent generations of parasites, a race 

 may be evolved which produces sporozoites with this character more 

 highly developed than at the present time, so that from the point of 

 view of the parasite there will be less waste of sporozoites. When 

 the mosquito has ejected all the sporozoites which have developed 

 from one batch of parasites taken up from the blood, or when any 

 sporozoites remaining in the salivary glands have degenerated and 

 died, the mosquito ceases to be infective. 



Another illustration of the same type of development is seen in 

 the case of the tse-tse fly and the trypanosomes causing sleeping 

 sickness of man, nagana of domestic animals, and other diseases. 

 These blood-inhabiting flagellates may originally have been purely 

 insect parasites, which lived in the intestine, and passed from one 

 insect to another by means of encysted forms which escaped in their 

 dejecta, just as the intestinal parasites of man pass directly from one 

 man to another by encysted forms which escape in the fasces ; or 

 they may have been, as some authorities hold, intestinal parasites of 

 man, or some other vertebrate, which, having invaded the blood 

 stream, have secondarily become adapted to an insect, as apparently 

 occurred in the case of the malarial parasites. Whichever view is 

 correct, and the former seems to me to be the more probable, at the 

 present time the trypanosomes live in the blood of man and animals 

 in Africa, and are handed on from man to man by tse-tse flies. In 

 these flies the trypanosomes undergo an evolution which differs in 

 many respects from that of the malarial parasites in mosquitoes, but 

 resembles them in the ultimate result. The trypanosomes make 

 their way to the salivary glands of the tse-tse flies, and as these 

 insects have the same- irritating habit of injecting saliva when they 

 feed, the trypanosomes are easily inoculated into man. It has been 

 estimated that as many as five thousand trypanosomes can be injected 



