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192 LEEUWENHOEK AND HIS “ LITTLE ANIMALS”’ 
conceive to exist in the secret parts of Nature: and from 
this appeareth also, that all we have yet discovered is but 
a trifle, in comparison of what still lies hid in the great 
treasury of Nature; and how small must be those particles 
of water’ which, to all appearance, pass many at a time 
through such tiny vessels. 
I hope that with this I have satisfied your require- 
ments.” 
I have given the foregoing letter in full for several reasons. 
In the first place, it serves to illustrate Leeuwenhoek’s fond- 
ness for simple mathematical deductions ; secondly, it shows 
very clearly that, in the case of his “smallest animalcules”’, he 
was dealing with bacteria; and thirdly, it shows the success 
and failure of the application of his “uniformitarian” principles 
to microscopic creatures. In his estimate of the size and 
numbers of the bacteria present in a minute drop of water, he 
was not mistaken: but when he proceeded to show, by simple 
arithmetic, the magnitude of the blood-vessels in such 
organisms, he went ludicrously astray. His education and 
his century both failed him. His greatness and his littleness 
are here revealed simultaneously—to those endowed with the 
accumulated knowledge of the next 250 years. Yet even at 
the present day Leeuwenhoek’s mistaken conclusion may 
serve as a warning to the biologist with statistical tendencies. 
It is surely worthy of remembrance, even now, that the most 
flawless mathematical calculations may sometimes be wholly 
erroneous. 
Leeuwenhoek’s next observations on ‘‘animalcules” are 
contained in a further letter to Robert Hooke. After 
recording some observations of no* present interest, the letter 
ends thus: * 
a 
" j.e., the molecules of which water itself is composed. 
> Referring to Const. Huygens’s request that he should put down on 
paper his calculations concerning the magnitude of the parts of the “little 
animals”’ (vide supra). 
° A few words which are perhaps not irrelevant to the present subject 
will be referred to later (see p. 207 infra). 
* Letter 30. 5 April 1680. To R. Hooke. MS.Roy.Soc. Published in 
Brieven, Vol. I (Dutch), and Opera Omnia, Vol.I (Latin): also an English 
abstract in Phil. Trans. (1693), Vol. XVII, No. 196, p. 593; but this does 
not include the passage here quoted. 
