1994] The Parasitic Protozoa Found in Cancerous Diseases. 315 
half a century, but it is only quite recently that an important 
stage in their life-history has been made out. There are dif- 
ferences of opinion between observers; Mr. Jackson Clarke’s 
amceba-like organisms do not exactly correspond with the var- 
ious forms of parasites described by some other pathologists. 
It is thought possible that the whole life-cycle of the protozoon 
may be passed within its human host; in any case, its exo- 
genous history is not known, and this stage is the one which 
it would be the most useful to discover, since we are, at pres- 
ent, in utter darkness as to the mode in which the contagion 
is conveyed to the host. Cancer is pronounced to be a disease 
in which heredity plays an important part. Does it do so in 
the same way that hereditary predisposition acts in tuberculous 
diseases; not by a direct transmission of the tubercle bacillus, 
but by some mysterious lowering of the vital powers of resist- 
ance? It is hardly possible to imagine that microsporidia, 
hereafter to develop into the protozon of cancer, can remain 
dormant for 50, 60, 70, 80 years. 
The disease [so far as can be ascertained from experiments 
upon animals, themselves liable to cancer] is not directly trans- 
ferable from one host to another. There remains, therefore, 
as a highly probable hypothesis that the exogenous form of 
the protozodn of cancer, like the flagellate monad of malaria 
and the coccidia of the rabbit, must be sought in contaminated 
soil or water. It is because this most important stage of the 
life history of the protozodn of cancer is unknown, that I have 
ventured to present a summary of some of the papers which 
have been appearing for some months in the British Medical 
Jonrnal to the readers of the AMERICAN Naturauist; hoping 
that workers skilled in researches among the Protozoa may 
take up the subject, and may come to the aid of the brilliant 
band of pathologists who have thrown so much light on a most 
difficult problem. 
