Species

The Development of the Philosophy of Species 285

evolution and against the essentialist approach to species whether or not that was

ever offered in biology, remain. However, in order to explain why species exist, and

what they and other taxonomic kinds are, both unitary causal accounts and epistemic

criteria for judging that the kinds share that unitary causal account are required.

For instance, not all populations form a species. And arguably, not all species are

metapopulations.

The Philosophical Background

The species problem is often discussed in the context of developments in philosophy

generally. For instance, Mill and Venn introduced the notion of natural kinds into

the philosophy of science, and since then philosophers have returned to this as the

basis for species in natural history/biology.^7 With the invention of “essentialism” in

the 1950s in philosophy, natural kinds were identified with essential classes, sets, or

sums.^8

When Hull began his work on essentialism in taxonomy in 1965, he was greatly

influenced by the work of Michael Scriven on explanations and predictions.^9 Under

the dominant theory at the time of explanation, the nomological-deductive (N D)

model, in order for any kind of entity to play a role in an explanation, that kind had

to be subject to laws of regularity, from which, with the appropriate boundary and

initial conditions, one could deduce the state to be explained. Since species were not

subject to laws,^10 species did not play an explanatory role in biology and hence were

not something that fulfilled the appropriate role for a natural kind, in Hull’s view:

On the traditional view, the species category is a class of classes defined in terms

of the properties which particular species possess ... and particular organisms are

individuals ... The relation between organisms, species and the species category is

membership. An organism is a member of its species and each species is a member of

the species category. On the view being urged [by Ghiselin and Hull], both particular

species and the species category must be moved down one category level. Organisms

remain individuals, but they are no longer members of their species. Instead an organ-

ism is part of a more inclusive individual, its species, and the names of both particular

organisms (like Gargantua) and particular species (like Gorilla gorilla) become proper

names. The species concept is no longer a class of classes but merely a class.^11

Considerable ink has been employed to argue whether species are classes or indi-

viduals. The debate continues even today.^12 Ghiselin and Hull argued that species

cannot be sets, because sets cannot be historical; that is, they cannot change over

(^7) I have discussed this history in Wilkins and Ebach 2013.
(^8) Khalidi 2013 offers a nice discussion of these issues.
(^9) Scr iven 1959.
(^10) Under the view that selection occurred at the population level, natural selection was not a law of spe-
cies evolution.
(^11) Hull 1976, 174f.
(^12) Stamos 1998.