212 
It is seen that all the results are in conformity with the 
hypothesis that Glossina pal pal is transmits Trypanosoma gambiaut 
mechanically, and that it is probably not able to do so when the space 
between transmitting and infecting feeds much exceeds 48 horns. 
This conclusion is, nevertheless, to our minds a most unsatisfying 
one if we are to regard this Glossina as the chief* or only t carrier of 
T rypanosoma gambiense. 
The experiments of all observers show that it is frequently 
necessary to feed hundreds, almost thousands, of flies oil a susceptible 
animal before it becomes infected ! 
It may be objected that in many of these experiments flies may have been 
unnecessarily fed on the animals after they were infected but before the infection 
was recognised. 'Ihis objection is partially negatived by observations (7, S : see 
page 20) showing that the incubation period of a natural infection by Tryfamncni 
gambiense may be so short as from two to four weeks. 
If the number of successes obtained in such experiments, where 
flies are fed on an infected animal whose blood is swarming with 
parasites, is so small, it scarcely seems possible that mechanical 
transmission by tsetse flies can alone be responsible for the rapi 
spread of sleeping sickness of recent years. From these experiment; 
it seems that, as a rule, a native must be bitten by a comparatively 
laigc number of flies, which have fed not more than 48 hours 
previously on a case of human trypanosomiasis§ before he will become 
infected ; it must be remembered that trypanosomes are usually rather 
rare in the blood of cases of sleeping sickness. 
that Varieties of Glossina other than fal falls may < 
GloisVnn 7 '?J? mbte,,Se must . not be forgotten. It has recently been shown 
Greicr an/ ( 7 ° T ma y transmit trypanosomes other than Tryfanosoma gamin 
domestic animal ’^ ve shown that it will convey the trypanosomes found in vai 
in transmitting r ln v, Uganda ( IO )' In Experiment 212 of this paper we succe 
that ?n Germa g n T pt an .°. so . ma din,or f bon by Glossina falfalis. Koch (9) sugf 
■pallidi-te anri rv' 35 * A |. nca cattI e trypanosomes may be transmitted by Glos 
K^totefc 11 as * which was form 
by G l »torn 7^ ntS transmitte d 7 '. brucei, previously only known to be car 
^ by the^bite^of 1 a^banid'T/l 
than m7n P 7ZTv a ^ a 7 bienS u has as y et been found in nature in no other h 
Commission on%iL c® f h,S was wri «en Report VIII of the Royal Socie 
have found T. gambLse in^iv^ogs^ reCeived ‘ Its authors Mien * 
