1912.] T. gambiense and its Relation to G. palpalis. 537 



It must here be mentioned that all the fly results are obtained in spite 

 of a certain general tendency on the part of the flies to digest their parasites. 

 All the experiments go to show that this negative factor may be taken as 

 being very fairly constant. Any transmitting host whose digestion is rapid 

 tends to show a relatively low percentage of carriers — thus mosquitoes, fleas, 

 and tsetse flies, quite apart from the widely divergent nature of the parasites 

 involved, all produce relatively few carriers. 



Leeches and ticks, for instance, whose digestion is very slow in comparison, 

 give, on the other hand, practically 90 to 100 per cent, of carriers. 



It is advisable to take the same part of the cycle as that selected in the 

 last section for the starting point, namely, immediately after a drop in 

 numbers. Such a period which shows very few trypanosomes and all of the 

 shorter type is an effective period and generally produces about the average 

 number of plus flies. Now this is a result of considerable importance. There 

 is at this time a population of trypanosomes which have just suffered a 

 process of elimination, they have just passed through some set of conditions 

 that has proved fatal to the vast majority. There are two things to 

 consider: (1) the type of trypanosome which has survived, and (2) its 

 condition. The type of trypanosome is very clearly the shorter individuals 

 (see Tables 5, 17, 18), and they must, moreover, be those capable of 

 resisting the particular adverse circumstances to which the majority have 

 succumbed. 



There is therefore at these periods a given type of resistant trypanosome 

 which is, as shown by experiment, capable of infecting flies. It can therefore 

 be concluded that not only are the short types the resistant individuals but 

 also that this type by itself is capable of producing infection in flies. 



The production of resistant strains by the use of drugs and sera has no 

 doubt some relation to these probably temporary states of resistance occurring 

 naturally in the untreated host. And it is not without significance to an 

 understanding of the general biology of trypanosomes that these resistant 

 individuals are also just those capable of carrying on the cycle in the trans- 

 mitting host. This has a bearing too on the interesting discovery of the 

 Commission of 1908, that trypanosomes persisting in antelope are rare in the 

 blood but afford a relatively high percentage of infected flies. The low 

 numbers and the absence of pathological symptoms on the part of the 

 vertebrate imply that the conditions in the latter exercise a high degree of 

 control over the parasite, and, correlated with this, the trypanosomes 

 must in all probability be in a much more active state of resistance to their 

 environment while sojourning in such a host. That this should also be the 

 condition of affairs producing a large number of infected flies is of obvious 



