The Development of the Philosophy of Species 313
proper subsets, of cases. Their “identity” lies in the general similarity of the indi-
vidual terms to each other in some metric. FRPs can be cases of (i) general similar-
ity of sufficient but not necessary conditions; (ii) identity by descent but not unity
of defining character; or (iii) practical identity; that is, it is useful to group terms
together. What they are not are classes. All members of a class instantiate a nec-
essary and sufficient set of characters. As Pitcher observes, there are predicates,
like “brother” and “vixen” that are defined by essential conditions like being male,
or being female^89 in addition to having external relations. How to characterize the
distinction between FRPs and these other relational predicates is the subject of a
great deal of literature, but for our purposes here it is enough to note that an FRP is
one that relies on similarity but a general lack of essence. Species can be a family-
resemblance term in three ways:
A. As a Taxon Concept
Notions of species are often distinguished into various categories, and there is noth-
ing much common between them that is not true of any class notion. However, with
fine irony, the categorizations themselves appear to be FRPs, as they also criss-
cross but are not reducible to each other. These may not be inconsistent, but they are
clearly different ways of defining the biological group. Other classifications that are
called species include the morphologically similar group, the karyotypic group (i.e.,
any group that has the same genetic structure and a restricted set of alleles for vari-
ous loci), and the undivided lineage.
B. As a Classification of Organisms
If a taxonomist has three species to classify in relation to each other, the defin-
ing characteristics of each species (i.e., those that are different across all three) are
used in cladistic and morphological taxonomy to establish the similarity relations.^90
The result is mostly just the sort of relationship one finds in Wittgenstein’s games.
Species A and B may share some characters that C lacks, and A and C, and B and
C, may share characters exclusively as well. A complete list of characters may give
numerous set inclusions and intersections that are not consistent across the whole
list of characters. (This is due to several evolutionary cases in the theoretical situa-
tions where an explanation is given in terms of phylogeny: convergence, reticulation,
canalization, and other cases covered by the cladistic term homoplasy.)
C. As a Measure of Conspecificity
The literal case of family resemblance can be extended to cover all members of
inclusion within a species. Individual organisms cover a distribution curve for any
given trait in a sizable census, but location in the curves are not the uniform for all
individuals—that is, an individual at the mean for one or most traits may be on either
tail for any other trait. Use of an FRP to describe an individual in order to justify
(^89) Pitcher 1964, 221n.
(^90) In the common meaning of similarity; a cladist would say the relations are formed solely on the basis
of monophyly, but see Vanderlaan et al. 2013 on the complexities of the notion of monophyly.