140 HEREDITARY CHARACTERS 



creased in virulence. The ordinary course of the disease 

 is as follows. First there is a certain incubation period, that 

 is, a period between the inoculation with the trypanosomes 

 and their appearance in the blood. Then a short period 

 during which the trypanosomes are more or less numerous. 

 They then disappear again, and after a period they reappear. 

 These relapses continue at varying periods until the animal 

 dies. For instance, in rats inoculated with Trypanosoma 

 gambiense, the ordinary course of the disease without treat- 

 ment is from two and a half to four and a half weeks, often 

 with three relapses and terminating in death. The probable 

 explanation of these relapses is that they form the crises 

 in a struggle between the trypanosomes and the animal in 

 which they are living. At one time the reaction of the 

 organism is sufficient to almost exterminate the trypano- 

 somes. The few trypanosomes that survive, however, are 

 the most resistant, and these give rise to a fresh generation 

 with a higher mean of resistance than the preceding. Some 

 individuals will vary towards a greater, some towards less 

 resistance than their parents. Those that vary towards less 

 will be killed by the environment. A further reaction takes 

 place, and the same thing happens again. The trypano- 

 somes almost invariably win in the end. If this explanation 

 is correct, we ought to find that the strain of trypanosomes 

 successfully inoculated into highly resistant animals is sub- 

 sequently virulent to animals that are less resistant. The 

 baboon is highly resistant to T. gambiense, and the following 

 experiment shows how the strain of trypanosomes passed 

 through it gained in virulence. 1 The baboon was inoculated 

 on 21st August. Parasites were first seen on 1st October, 

 disappeared, and reappeared on 4th October ; death occurred 

 on 10th October. At death no trypanosomes were found in 

 the blood, but a rabbit was inoculated with it. Twenty-three 

 days afterwards parasites were found in the rabbit, and other 

 animals were inoculated from it. The ordinary period of 



1 Thomas and Breinl, Liverpool School of Tropical Medicine, Memoir XVI. 

 October 1905. 



