290 Species
I do not consider that these are either essences or that they form kinds, despite the
extensive defense of that view LaPorte presents in terms of naming groups and what
their necessary referents are thereafter. In contrast, I want to make the following
claims and defend them:
- Species and other evolutionary taxa do not form natural kinds; they form
natural groups. - Natural kinds in biology are model-dependent and are neither inductively
projectable nor informative about anything not contained in the model[s] in
which they occur. - Natural groups are both inductively projectable and informative.
- While natural kinds are not historical or concrete objects, but are timeless
abstractions, natural groups are concrete and historical. - Natural kinds are not natural in the sense that they have objective existence
independently of cognition; natural groups are natural in that way.
These are broad and contentious claims, so I shall attempt to make them plausible
and provide support for them. To achieve this, I shall rely on historical practice of
biologists, and in particular of recent taxonomy.
Taxa and Kind Terms
A taxon is any class of objects in a classification in modern biology, and species are
taxa. The term has been shrouded in ambiguity since it was coined, due exactly to
the problem we confront now. It was unclear whether a taxon was a kind that was
defined or whether it was a concrete group. In biological systematics, there have been
three main ways of conceiving of classes since evolutionary theory became dominant
in biology, and in particular since the end of the World War II. These three are, in
historical order: classes are grades, classes are clades, and classes are clouds. We use
the word “class” in this case simply because we use the term “classification”—it is not
meant to imply any conclusions about the nature of classifications. Sometimes in logic
a class is understood to be an intensionally defined grouping. This is not something
we must accept in biology from the outset. In fact, a “universal” term, which is how
classes are often described when discussing kinds in biology, is under the Aristotelian
system merely any term for which there are two or more particulars under its rubric.
Terms in the biology debate, and to a lesser extent in the broader context of phi-
losophy of language and essentialism in science, tend to be part of the problem. We
are so used to seeing kind terms as intensional classes that it has become part of
the Received View that prior to some threshold point, usually the publication of the
Origin of Species in 1859, we all thought that taxa in biology were definitional and
essential kinds. This is historically untrue, as we have seen, but in any event, we must
take care not to import these assumptions into the topic with our terms.
A grade can be defined as a defined kind. It is some state that may be acquired or
achieved or entered in virtue of acquiring, achieving, or arriving at some set of jointly
sufficient and severally necessary properties (JSSNPs), which form the essence of
the class. Such a conception has several implications I shall attempt to bring out.