1912.] Trypanosoma gambiense in Glossina palpalis. 247 



infections. Thus, one of the cages fed ou one of these monkeys showed an 

 infected fly in which the infection had not yet reached forward beyond the 

 mid-gut on the 22nd day, and another on the 56th day, in which the 

 salivary glands were not yet infected. There was no possible chance of a 

 " pick up " infection in either case. A stray fly, showing a very backward 

 infection, is generally due to having allowed a cage to stay too long on the 

 test animal, that is until after it has produced an infection. A new cycle 

 may then be started in a fly which had escaped on the previous occasion. 

 This point is, I may mention in passing, another argument in favour of the 

 failure of the trypanosomes to establish themselves in the fly being due 

 rather to the flagellates than to any absolute inhibiting quality or condition 

 in the recalcitrant glossina. 



Monkey 199 illustrates some particularly important points in regard to the 

 cycle of T. gambiense as a whole. It has been shown, by many experiments 

 -carried out at Mpuniu, that infected buck produce a high percentage of plus 

 flies, but that monkeys infected by means of these flies give in turn only the 

 usual low percentage characteristic of cycles started from monkeys. 



The important features are — 



1. The long period during which the trypanosomes had been in the buck, 

 namely, 15 months. 



2. The infection of 199 by direct injection of the blood from the 

 bush-buck. 



3. The large percentage of infective flies yielded by 199 when infective. 



4. The loss of this last character when the strain is transmitted by flies to 

 other clean monkeys. 



Xow, the result of a great deal of biological work during recent years has 

 been to establish the general idea that the function of conjugation or 

 nuclear fusion is not reproduction, but the preservation of the characters of 

 the species as a whole, and the neutralisation of the undue tendency to 

 variation produced by unchecked individual multiplication. In the case of 

 trypanosomes, the individuals run through a relatively very large number of 

 generations in the vertebrate, and in consequence, as is well known, are 

 capable of developing very well marked strains, which might almost be 

 termed varieties. The function of the fly, as is obvious from these experi- 

 ments, is to sift out these variations of the individual strains, and to produce 

 a fairly even type. There is at present no sound evidence of conjugation in 

 any trypanosome life-cycle so far worked out, and the question must be left 

 unprejudiced. It is a very plausible suggestion that the great and 

 undoubtedly stimulating change of environment that occurs in the 

 alternation of hosts has gradually led to the suppression, and finally 



