The Synthesis and Species 221
systematists in Britain. One can take issue with the claim for Leibniz (as a Great
Chain thinker, the sort of variation he admitted was scalar rather than distributional),
and wonder why de Quetelet has been overlooked as the source of populational think-
ing and instead been called an essentialist.^68 Most oddly, what is missing here and
elsewhere is Karl Popper. Popper had attacked what he called “methodological essen-
tialism” as a malign heritage from Plato,^69 in particular in his Poverty of Historicism,
§10, where he set up nominalism—the doctrine that universal terms are mere labels
attached to sets of things—opposed to “realism,” or “idealism,” which he renames as
essentialism. Popper ascribes to Aristotle the problem this introduces into science:
The school of thinkers whom I propose to call methodological essentialists was
founded by Aristotle, who taught that scientific research must penetrate to the essence
of things in order to explain them. Methodological essentialists are inclined to formu-
late scientific questions in such terms as ‘what is matter?’ or ‘what is force?’ or ‘what
is justice?’ and they believe that a penetrating answer to such questions, revealing
the real or essential meaning of these terms and thereby the real or true nature of the
essences denoted by them, is at least a necessary prerequisite of scientific research,
if not its main task. Methodological nominalists, as opposed to this, would put their
problems in such terms as ‘how does this piece of matter behave?’ or ‘how does it move
in the presence of other bodies?’ For methodological nominalists hold that the task of
science is only to describe how things behave, and suggest that this is to be done by
freely introducing new terms wherever necessary, or by re-defining old terms wherever
necessary while cheerfully neglecting their original meaning. For they regard words
merely as useful instruments of description.^70Popper’s sympathies are clearly with the nominalists. Through Hull’s seminal
essay on essentialism in taxonomy,^71 Popper’s distinction came to be widely accepted
among taxonomists, and Mayr may be influenced either directly or indirectly by
Hull.^72 It is unclear whether he was directly influenced by Popper’s Poverty, but that
work was a cause célèbre in its day, and since Mayr and Popper were both leading
German-speaking academics in the English-speaking world, it would be surprising
if someone as erudite as Mayr had not at least heard of Popper and his ideas.^73 In the
1982 history, Mayr cites Popper only for issues of theory falsification. Why is this?
(^68) See, for instance, Krüger et al. 1990.
(^69) Popper 1957a, 1957b, 1960.
(^70) Popper 1960, 28f.
(^71) Hull 1965a, 1965b.
(^72) Hull (pers. comm.) told the story that, as a graduate student, he delivered the talk on which this paper
was based in front of Popper, and handed it in at the end of semester. Popper took it upon himself to
send the talk to the British Journal for the Philosophy of Science without Hull’s knowledge, and the
author had to ask for it to be returned for revision. Many of his conclusions were not strictly in line
with Popper’s own ideas, but Popper apparently never read the published work, and so Hull never
came under Popper’s withering attack himself.
(^73) Polly Winsor believes, after discussing the issue with various of Mayr’s students and associates, that
Mayr did not know Popper’s work until his attention was drawn to it by Hull’s 1965 paper. Mayr
does cite Popper in his later work [Mayr 1997, 59f] where he explicitly mentions Popper’s attitude to
words and essentialism, but prior to that, his ideas on typology seem to be his own, as Winsor calls it,
“dragon.” According to Winsor, Mayr first used the term “essentialism” as a synonym for “typology”
in 1968 [Winsor 2004].