230 LEEUWENHOEK AND HIs “ LITTLE ANIMALS ”’ 
forms described by Leeuwenhoek himself a few years earlier. 
No animal or “‘animalcule” smaller than a worm was known 
to live inside the body of man: and the existence of hordes of 
micro-organisms within the bodies of healthy animals was as 
wholly unsuspected as it was unheard-of. 
It is worthy of note, moreover, that neither here nor else- 
where in his writings does Leeuwenhoek associate entozoic 
protozoa or bacteria with the causation of disease. He found 
them in normal hosts, and it probably never occurred to him 
that any of them might have pathogenic properties. Having 
no medical education, and no preconceived notions regarding 
““animalcules,” he recorded his findings simply and objectively, 
and it was left to others to elaborate his great discovery into 
the vast present-day corpus of medical protozoology and 
bacteriology. In a sense, therefore, he missed the great 
practical implications of his revelation." But it must not be 
forgotten that the micro-organisms which he studied were all, 
in all probability, harmless; and consequently he deserves 
every credit for not speculating in excess of his facts. If 
every worker on the same subjects during the next 250 years 
had possessed an equally conservative and scientific spirit, a 
great deal of unnecessary confusion in our knowledge of 
“microbes ”’ might have been avoided. 
No further observations on entozoic protozoa were recorded 
until 1683. But in this year Leeuwenhoek wrote another 
highly interesting letter, in which he described many novel 
observations on frogs and other animals. He here accurately 
described, and discussed, the frog’s spermatozoa and _ blood- 
corpuscles—for the first time; and in the course of his 
description he interpolated an account of various “ animalcules ” 
which he had discovered incidentally. Some of these animal- 
cules were undoubtedly protozoa, but they are so involved 
* Contemporary medical workers, however, were not slow to seize upon 
the pathological possibilities of L.’s discoveries. For example we find, 
as early as 1683, that “the ingenious Fred. Slare M.D. and ¥.R.8.”, 
commenting upon a murren” in Switzerland which carried off many 
cattle, says: ‘I wish Mr. Leewenhoeck had been present at the dissections 
of these infected Animals, I am perswaded He would have discovered some 
strange Insect or other in them.”—Slare [alias Slear] (1647 ?-1727) was 
a physician and chemist. He qualified at Oxford in 1680, and became a 
Fellow of the Royal Society in the same year—just after L. himself. 
