Species, Taxonomy, and Systematics 407it is water. Individuals, unlike kinds, consist of parts that are spatiotemporally
restricted. Consider a paradigmatic individual, a mammalian organism. The
parts of that organism cannot be scattered around the universe at diverse times
if they are parts of a single living organism. Various biological processes, such as
respiration and digestion, require those parts to be causally and spatiotemporally
connected. The parts of a mammal must exist in a particular space-time region.
Generalizing from these examples, the parts of an individual are spatiotemporally
restricted, whereas the members of a kind are spatiotemporally unrestricted.
According to Ghiselin and Hull, membership in a species is spatiotemporally
restricted, thus species are individuals and not natural kinds. What is their argu-
ment that species are spatiotemporally restricted entities? The argument starts
with the assumption that ‘species’ is theoretical term in evolutionary theory, so the
ontological status of species is determined by the role ‘species’ plays in evolution-
ary theory. According to Hull [1978], species are units of evolution in evolutionary
biology, and as such they are individuals. A number of processes can cause a
species to evolve, natural selection is just one such process. Minimally, selection
can cause a species to evolve by changing its gene frequencies from one generation
to the next. More significantly, selection will cause a species to evolve by causing a
rare trait to become prominent after a number of generations. Suppose the second
type of evolution occurs. For such evolution to occur, the selected trait must be
passed down through the generations of a species. Traits are not inherited unless
some causal connection exists between the members of a species. Sex and repro-
duction require that organisms, or their parts (gametes, DNA), come into contact.
Thus, evolution by selection requires the generations of a species to be causally
and spatiotemporally connected. The organisms of a species cannot be scattered
throughout the universe, but must occupy a spatiotemporally restricted region.
Given that species are units of evolution, they are individuals and not kinds.
The species are individuals thesis is the prevalent ontological view of species.
Nevertheless, that view has been contested on a number of grounds. Ruse [1987]
and Ereshefsky [1991] argue that many species are not individuals because they
fail to have the requisite cohesion required of individuals. Many species consist of
asexual organisms. Such organisms are connected through parent-offspring rela-
tions, but no exchange of genetic material exists among the members of a species
in a single generation. Consequently, asexual species are merely historical entities
— their members are connected to a common ancestor, but there is no ongoing
causal connection among contemporaneous members. This is certainly right, but
suggesting that it shows that species are not individuals misses the importance
of the individuality thesis. The important distinction is between being a natural
kind and being an individual. Even as mere historical entities species are spa-
tiotemporally restricted entities and not kinds. The idea that species are defined
by causal connections rather than qualitative similarity is the important claim of
the species are individuals thesis.
Another criticism of the individuality thesis challenges the assertion that all
species are spatiotemporally restricted entities. Kitcher [1984], for example, be-