222 LEEUWENHOEK AND HIS “ LITTLE ANIMALS ”’ 
have here the first recorded observations ever made upon any 
protozoa belonging to the important flagellate family of the 
Trypanosomatidae. But this interpretation, though very 
plausible, is not absolutely certain ; for the details recorded 
are obviously too scanty to warrant dogmatic deductions. 
Nevertheless, it can, I think, be concluded—with complete 
assurance—that Leeuwenhoek observed “ parasitic” protists 
of some sort in the intestine of a horse-fly as early as the 
year 1680. 
We now come to a letter of great historic interest and 
importance—a letter which has rarely been read aright, but 
one which records in no uncertain terms the discovery of the 
intestinal protozoa and bacteria of Man. This letter was 
written in 1681, and in it Leeuwenhoek discusses a variety of 
subjects—such as the structure and falling-out of the hair, 
“blackheads ”’ (comedones), clay, and gout: but the only part 
which here concerns us deals with the discovery of “living 
animalcules in the excrements.” ‘The passages in question 
run as follows:' 
I weigh about 160 pound, and have been of very nigh the 
same weight for some 30 years,” and I have ordinarily of 
a morning a well-formed stool; but now and then hitherto 
I have had a looseness, at intervals of 2, 3, or 4 weeks, 
when I went to stool some 2, 3, or 4 times a day.’ But 
* From Letter 34. 4 November 1681. To R. Hooke. MS.Roy.Soc. 
Published in fullin Brieven, Vol. I (Dutch), and Opera Omnia, Vol. I (Latin). 
The Dutch version first appeared in 1686 (Ontled. en Ontdekk.), the Latin 
[from which the date of the letter was omitted ] in 1687 (Anat. s. Int. Rerwm). 
An English abstract was published by R. Hooke in Phil. Collect. (1682), 
No. 4, p. 93: and another by Derham in the posthumous Phil. Expts. and 
Obss. of Hooke (1726), p. 61. The letter itself was presented to the Society 
at their meeting held on 2 Noy. 1681, and read in English translation at the 
following meeting on Nov. 9 [O.8.]. Cf. Birch, Vol. IV, pp. 99, 101. Part 
of this letter was also translated by Hoole in his Select Works of L.—but 
not the part with which we are now concerned. I have already translated 
and commented upon the paragraphs dealing with intestinal protozoa 
elsewhere (Dobell, 1920). 
* When he wrote this L. was 49 years old. 
* The Latin version of this passage is somewhat ambiguous, and has 
apparently led some readers to suppose that L. frequently suffered from 
diarrhoea lasting for 2-4 weeks at a stretch. It has even, indeed, given rise 
to the belief that he suffered from chronic dysentery for 30 years. His own 
words do not countenance any such conclusions. Cf. Dobell (1920). 
