999 E. A. MINCHIN. 
It is not necessary for me to dwell upon a possible fifth 
stage realised in Piroplasma, where the parasite passes 
through two generations of the invertebrate host. Nothing of 
the sort is known to occur in the case of trypanosomes. 
It is not possible to prove or demonstrate a phylogenetic 
theory. One can only consider how far it suits known facts 
or overcomes difficulties. My theory has only one advantage 
over Léger’s—that of explaining away the difference in the 
invertebrate hosts in different cases. Parasitic flagellates 
are found in the gut of invertebrates as well as vertebrates. 
If special stress be laid on the occurrence of Herpetomonas 
in insects which do not suck blood, such as the house-fly, I 
may refer to Prowazek’s speculations on this form, especially 
his interesting suggestion that the house-fly is descended 
from blood-sucking ancestors, which acquired the Herpeto- 
monas from vertebrate blood, so that H. musce-domestice 
would represent a stranded and persistent larval stage, com- 
parable to, for instance, the axolotl amongst higher animals. 
I have suggested above, and in a former memoir (28), 
the possibility that contaminative infection, the commonest 
of all methods of infection amongst Protozoan parasites 
generally, may occur also in the case of trypanosomes 
infecting vertebrates, basing my suggestion upon the 
encystation observed by me in T. grayi. Encystation has 
not been observed in any other species of trypanosome, and — 
with regard to T’. brucil, the observation of Stuhlmann (41) 
noted above, that the Herpetomonas-forms are found almost 
exclusively in the proboscis of the tsetse, rather indicates, as 
I have already pointed out, that the stage in the life-history 
which tends to become encysted in the case of T. grayi, does 
not do so in the case of T. brucii, which would probably be 
similar, in this respect, to I. gambiense. Moreover, experi- 
mental evidence, so far as it exists, is against the occurrence 
of contaminative infection ; in the case of nagana Bruce (4) 
experimented with food and water from localities where 
the disease is rife, but failed to produce infection with it. 
Bruce also injected feces of flies into susceptible animals, 
