106 
Tt'ypanosonia leivisi 
not a stage of the life-cycle of T. lewisi. He does not give any particulars 
about the methods of procuring his fleas and of hatching them out in 
the laboratory. He only states “ the rats of Lincoln, Neb., harbour two 
fleas. They both contain herpetomonad forms...,The flagellates found in 
the fleas are either pure Herpetomonas, pure T. lewisi or a mixture of 
the two.” 
We do not think that he has established the identity of his parasites, 
the only facts in favour of his view, that some of the flagellates of the 
flea’s gut are natural forms (H. pattoni), being (1) his statement that 
he could not find any transitional forms between them and the true 
T. leimsi\ (2) his statement that he found adult Herpetomonad forms 
and cysts which we had not figured in our description of T. leivisi] 
(3) the point that we had a 4^0 infection in our control fleas, which 
were supposed to be “ clean ” in respect to T. lewisi. As a matter of 
fact we did find forms, such as he has described as adult Herpetomonas 
and cysts, in our life-cycle of T. lewisi, and the infection of the control 
fleas might be accounted for by an old infection. 
Now even if the existence of this species (IT. were established, 
this would not prove that the crithidia—and herpetomonas—stages of 
T. lewisi found by us were really natural flagellates. This is shown by 
the study of cultural forms of T. lewisi which quite resemble Herpeto¬ 
monas pattoni (cf. Figs. 35-44 of Swingle’s paper with diagram XII of 
ours). 
His criticism to the effect that o«r control fleas were infected is not 
well founded, and moreover illogical. Even if we consider the question 
from his own point of view we see that only ^ of our control fleas were 
infected with H. pattoni, whereas in his case ^ of the fleas were 
naturally infected. So our fleas harboured only very few natural 
flagellates, whereas the forms found in fleas fed on infected rats were 
quite numerous, which shows that our opportunity of mixing up natural 
flagellates and forms of T. leivisi was very small. Moreover the forms 
we found in the control fleas all belonged to the type of small crithidiae 
considered by Swingle to belong to the life-cycle of T. lewisi. 
So the only conclusion Swingle could have drawn from our control 
experiments was that his own conclusions were possibly wrong. We 
emphasised in our paper that the affinity of these small crithidiae to 
T. lewisi was not quite clear because the crithidiae were found in our 
control fleas. Concerning the other stages found by us (crithidiae and 
herpetomonads, round forms etc.) there cannot be any doubt as to their 
