Annals of the History and Philosophy of Biology

Spontaneous versus Equivocal Generation in Early
Modern Science
Peter McLaughlin
Abstract
A distinction not often heeded by historians of science between spontaneous generation and so-called
equivocal generation (generatio aequivoca) is essential to an understanding of 17th and 18th century
theories of generation. Equivocal generation was almost universally rejected, but some sort of spontaneous
generation at least of the first life forms whether in one step or in a series of stages is an almost inevitable
part of any materialist system. And thus many scientists in the early modern period believed that spontaneous
generation could be unequivocal. I shall offer some clarification of the distinction using a number of
examples from the 17th and 18th centuries.
As recent scholarship has pointed out, the final demise of spontaneous generation has
been celebrated at least three times in the modern age: once each in the latter 17th, 18th,
and 19th centuries (Mendelsohn 1976, Farley 1977). Each time the size of the organisms
sought for decreased – first insects, then infusoria, then bacteria. And each time the purported
refutation followed upon a revival of the traditional materialist theory of generation:
pangenesis, according to which each part of the body contributes a representative
particle to the germ. Since pangenesis theories tend to view even normal sexual generation
as a sort of spontaneous assembling of preselected particles, they are quite congenial
to ideas of spontaneous generation. Francesco Redi’s Experiments on the Generation of Insects
(1668) has often been taken as the decisive refutation of traditional theories of spontaneous
generation. Reflecting on Redi’s famous experiments on putrefying flesh, John Ray
summed up in 1691:
My Observation and Affirmation is, that there is no such Thing in Nature, as Æquivocal or Spontaneous
Generation, but that all Animals, as well small as great, not excluding the vilest and most
contemptible Insect, are generated by Animal Parents of the same Species with themselves. (Ray
1691, 221)
And in the Lexicon Technicum (1704) of John Harris, a reliable source for Newtonian
and Lockean orthodoxy, the entry for “equivocal generation” maintains that “the
Learned World begins now to be satisfied, that there is nothing like this in Nature.”
However, numerous thinkers in the mid 17th century (as well as in the mid 18th century)
explicitly advocate some form of spontaneous generation; and they also sometimes spe-
Annals of the History and Philosophy of Biology, Vol. 10 (2005): 79-88
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