314 
Trypanosoma lewisi 
the two charts. Must it be supposed then, as at first sight it appears, 
that there is no relation between the prevalence of rat fleas in Bombay 
and the occurrence of trypanosomes in the rats ? It cannot be doubted 
that if rat fleas can act as transmitters of T. lewisi the propagation of 
the infection amongst the rats must be aided by a seasonal increase in 
the number of rat fleas. The Plague Commission, for example, have 
shown that the rat flea prevalence in Bombay is associated in a marked 
manner with the prevalence of plague amongst the rats. It is difficult, 
in view of Nuttall’s (1908) recent transmission experiments with rat 
fleas ( Ceratophyllus fasciatus and Ctenophthalmus agyrtes ), to suppose 
that Loemopsylla cheopis is incapable of conveying the infection. 
Moreover certain observations of our own in Bombay strongly suggest 
that the latter insect is able to originate an outbreak of the infection. 
A batch of tame white rats sent out from England were confined 
together in a cage which was deposited on the floor of a room in the 
laboratory; this room was over-run with M. rattus and M. decumanus, 
and needless to say was infested with rat fleas. After a brief interval a 
number of the English rats died and trypanosomes were found in their 
blood ; a few rat fleas were captured on some of these rats. Now the 
construction of the cage was such that rat fleas on the floor of the room 
might readily gain entrance into the cage, whereas transference of lice 
from the wild to the tame rats did not seem at all likely. 
There is one point which deserves attention in a discussion of the 
relation between the prevalence of trypanosomes and of rat fleas. As 
the Plague Commission remark in their report, fleas are most abundantly 
found in the haunts of their hosts owing to the fact that the houses or 
nests of the host are par excellence the breeding places of these insects. 
It may be therefore that owing to the “ concentration ” of fleas in the 
nests and burrows of the rats the opportunities for infection of young 
rats tend as it were to be levelled up. This explanation is not, however, 
wholly adequate, and there remains therefore the supposition that during 
the season of maximum prevalence of trypanosome infections other 
factors in the successful transmission of the infection are operative in 
spite of a diminution in numbers of the transmitting insects and that 
similarly during the off season of trypanosome infections these factors 
are partially in abeyance, so that the influence of an increase in the 
numbers of the transmitting insects is neutralised. 
In connection with the question of the infection of young rats in 
their nests the figures given in the accompanying table (V) were v r orked 
out with the idea that they might establish a relation between the age 
