306 Trypanosoma lewisi 
It is somewhat curious perhaps that notwithstanding the large 
amount of work devoted to the biology of T. lewisi —work which has 
aided materially in guiding researches upon the pathogenic trypano¬ 
somes—very scanty attention appears to have been paid to the question 
of possible seasonal variations in the prevalence of the infection amongst 
rats. Before bringing forward our own observations we may briefly cite 
references to the subject by other writers. 
It is interesting to note that Vandyke Carter (1887) who in 1885 
first observed the occurrence of T. lewisi in Bombay rats (chiefly 
M. decumanus and M. rattus (?)) clearly recognised the possibility of a 
seasonal prevalence of the infection. Thus Carter states in his memoir 
that from his observations—referring presumably to their paucity — 
“ valid inference regarding a possible seasonal prevalence could not be 
made.” Again he writes: “ In the same connection I would here allude 
to the evident narrow place-limitation or endemicity of both rat 
infection and the ‘ surra’ disease with probably also a distinct seasonal 
variation of prevalence.” 
Lingard (1895) examined a large number of rats (M. decumanus ) in 
Bombay during several years and is quoted by Laveran and Mesnil 
in their book on trypanosomes as having found that during the rainy 
season, June to October, 42 °/ 0 of the rats were infected with trypano¬ 
somes, while during the dry season, November to May, only 28°/o were 
infected. These observations of Lingard’s will be referred to at greater 
length below. 
Musgrave and Clegg (1903) remark that in Manila rats (thousands 
examined) T. lewisi has been found in from 20—65 °/ 0 of the individuals 
examined, varying according to the season and to the locality from which 
they were received. Unfortunately these authors content themselves 
with this bare statement—details are absolutely lacking. 
Swingle (1907) recites a few facts from his own experience in 
Nebraska which seem to indicate a seasonal prevalence of the infection. 
“Of the 17 one-fourth grown rats examined in the autumn and winter 
not one was infected with either lice, fleas or trypanosomes, while of 
the seven caught the following spring at the same place all had fleas 
and four were harbouring the parasite.” 
Lastly, Yakimoff (1907) is cited by Mesnil as stating that in an 
investigation of the distribution of the infection amongst the rats in 
St Petersburg the proportion of rats infected was greater in the warm 
than in the cold season. 
The present series of observations was begun on 1st September, 1905, 
