LETTER 34. INTERPRETATIONS 229 
The foregoing quotations—which contain, I believe, all 
that Leeuwenhoek ever wrote on such subjects—record a series 
of truly remarkable discoveries; and they prove conclusively 
that he discovered intestinal protozoa and bacteria in man 
and several other animals. He here saw, and recognizably 
described, the flagellate Giardia' and the Spirochaetes and 
other Bacteria in his own faeces. He also, I think, must have 
seen other human intestinal flagellates (possibly Trichomonas 
or Chilomastix), though this is uncertain: but he certainly 
saw bacteria in the excrement of a fowl and in that of a 
pigeon, though curiously enough he failed to find them in the 
dung of cattle and horses. 
Some of the interpretations which have been put upon 
Leeuwenhoek’s words in this connexion must be read to be 
believed. As I have commented upon them elsewhere’ I need 
say no more about them here. I will only note that the 
passages just quoted supply all the evidence there is for the 
statement—frequently met with in the  literature—that 
Leeuwenhoek described the ciliate Balantidium coli, and that 
he himself suffered from dysentery caused by this parasite. 
Beyond all doubt this is a literary fiction lacking all 
foundation. Though an Austrian nobleman® assures us 
that Leeuwenhoek could not have seen what he described, 
and therefore what he saw must have been something totally 
different ; and though a distinguished American medico‘* tells 
us that when Leeuwenhoek says he had weighed 160 lbs. for 
30 years and usually had solid stools, he meant that he had 
suffered from balantidial dysentery since the age of 30: never- 
theless, serious students of protozoology may well rest content 
with the plain and obvious meaning of his own simple words. 
Before we proceed to the next discoveries, it may not be 
amiss to emphasize the novelty of those just recorded. At the 
time when the foregoing observations were made, no protozoa 
or bacteria of any kind were known—except the free-living 
* Of. Dobell (1920). 
* Dobell (1920). 
* Stein (1867), Vol. II, p.321. Although I exposed Stein’s blunders 
some years ago, I note that a recent German writer (Pritze, 1928) still 
accepts them. But as he evidently overlooked my paper, and knows nothing 
of L.’s work, I cannot take his opinions seriously. 
* Strong (1904). 
