152 Crithidia, Herpetomonas, Trypanosoma 
in several sheep-keds taken from the same sheep, the conclusion was 
irresistibly suggested to my mind that this parasite could be nothing 
else than the trj^panomonad form of a Trypanosome in the sheep. At 
Miss Robertson’s suggestion, she and I instituted a searching examina¬ 
tion of the blood of that particular sheep, with the result that I found 
the Trypanosome whose presence was suspected. 
Having regard to all the known facts, the only reasonable and logical 
view to take is that these parasites in blood-sucking Insects are phases 
of some Trypanosome, especially where the Insects feed on animals known 
to harbour a Trypanosome {e.g. rat-fleas, mole-fleas, sheep-keds, etc.). 
This is the view of commonsense. So far as jjroof goes, this view has 
already been conclusively proved by the work of Minchin and Thomson, 
in the case of the parasites of the rat-flea; and sooner or later it will 
be proved in other cases. Most assuredly, however, the onus probandi 
lies rather on those who hold the opposite view ; and, so far as I am 
aware, neither Miss Porter nor anyone else has attempted to prove that 
any of these “ Grithidiae” in blood-sucking Insects are true independent 
forms. 
Secondly, the above view represents not only my personal opinion. 
It is also the opinion of the great majority of Protozoological workers, of 
whom I need only refer to Prof. Minchin, whose name is generally 
recognized as that of one of the foremost authorities on Trypanosomes 
and allied parasites. Miss Porter mentions Swingle as being opposed 
to this view. Swingle is certainly not now opposed to it; he is at any 
rate quite open-minded, and indeed if anything in favour of inj^ view. 
In a recent paper on the transmission of T. lewisi by rat-fleas^ etc., 
he says on p. 131, with reference to Strickland’s account of Crithidia 
ctenophthalmi: “It should be noted that these fleas all came from rats 
infected with Trypanosomes, which fact strongly suggests that his 
Crithidiae were really transformed Trypanosomes.” Again, in a foot¬ 
note to p. 141, with reference to my discovery of the Trypanosome in 
the sheep, he says: “ While there is considerable evidence favouring 
his conclusion, it seems to me there is still a possibility that C. melo- 
•phagia is a true Insect Flagellate, which has never been successfully 
introduced into the sheep’s blood.” “ Considerable evidence ” in favour, 
it will be noted, and only a “possibility” against. Lastly, I am sure 
Dr Swingle will not object to my quoting a letter which I received 
lately from him: “ I received your excellent article on Avian Haemo- 
protozoa soon after I had sent my work to the printer. It is interesting 
' Journal Inf. Diseases, viii. 1911, p. 125. 
