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1 Classes, Natural Kinds, Sets and Individuals

1 Classes, Natural Kinds, Sets and Individuals

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46



3 The Metaphysics, or Ontology, of Species: Classes, Natural Kinds or Individuals?



species problem”—namely, that species are not classes but logical individuals, a

view that he detailedly elaborated in his 1997 book Metaphysics and the Origin of

Species. In more recent years, species were then, inspired by Ludwig Wittgenstein’s

concept of family resemblance, also characterized as cluster kinds or cluster

classes. The vast majority of biologists dealing with theoretical questions of the

species problem today consider species to be individuals, and the same holds true

for a large number of philosophers of science (although among these one still finds

proponents of the natural kinds/classes view). While appreciating that some of the

cluster kind approaches are appealing, my own view (shared by probably the huge

majority of biologists) is that Ghiselin was right, that his contribution marks a

conceptual breakthrough for the philosophy of biology and evolutionary biology

and that everything that has come after him may modify and improve his insights

but cannot overturn them—for the simple reason that only species as individuals are

compatible with evolution as a historical fact and process.

So, what, in a nutshell, are the conceptual contenders when it comes to the

question what species taxa are? Classes are groups of particular objects and are

defined by essential properties. Essential properties are both necessary and sufficient, meaning that all members of the class exhibit these properties and that all

objects exhibiting these properties are members of the class. These particular

objects are often called elements of the class and are said to instantiate the class

or to be instances of the class. For example, the class of red triangles contains all red

triangles in the universe (and beyond), and every red two-dimensional figure with

three straight sides and three angles is an element of that class. Importantly, classes

are not spatiotemporally restricted: they are independent of time and space! While

elements or instances may come and go, the class remains the same. Even if there is

not a single red triangle in the world, the class of red triangles is still there (but

empty). Classes are eternal and exist at all times and places. Whether such classes

really exist outside our minds is contentious. As briefly mentioned in Sect. 1.5, the

argument over the reality of universals (does “redness” exist, or do only particular

red things exist?) has a long history. The view that universals exist independently of

their instances is usually called realism, while the opposite view that only particulars really exist is called nominalism. However, this dichotomy is perhaps a little

too imprecise as often a further view is distinguished: conceptualism. Under this

trichotomy, nominalism holds that only particulars exist (and universals are merely

words or vocal utterances), conceptualism denotes the view that universals do exist

but are dependent on the human mind (i.e. are concepts in our mind), and realism

grants to universals existence independently of the human mind (Audi 2009, p. 752;

see also p. 169 and 563; Richards 2010, p. 114, Stamos 2003, p. 8f.). For biologists

pondering the ontology of species, it is probably most important to note that

whether or not universals/classes exist in a philosophical sense, any such existence

will be different from the existence of particular historical objects (see below when

individuals are discussed).

Natural Kinds are a specific type of class and in many regards the most important

and most interesting one or, as Hull (1992, p. 183) puts it, “privileged classes”. Like



3.1 Classes, Natural Kinds, Sets and Individuals



47



classes natural kinds have essences (necessary and sufficient properties), but they

are often granted extramental reality which is why they are called “natural”: “To

say that a kind is natural is to say that it corresponds to a grouping that reflects the

structure of the natural world rather than the interests and actions of human beings

[. . .] The existence of these real and independent kinds of things is held to justify

our scientific inferences and practices” (Bird and Tobin 2015). The standard

examples of natural kinds are chemical elements and chemical compounds: any

and every atom and only those that have an atomic number of 79, i.e. that have

79 protons in their nucleus, are instances of the natural kind gold, and any and every

compound particle H2O (and only those) is an instance of water. In that regard,

natural kinds are not different from the class of red triangles or of all cars

manufactured in Germany, but there are intuitive differences as Bird and Tobin

(2015) put it: “Intuitively, to group all instances of the metal zinc together is to

engage in a natural classification, whereas to group together Trajan’s column, the

number two, and Julius Caesar is to classify things in an arbitrary manner. The

classifications ‘cars manufactured in Germany’, ‘culinary vegetables’, ‘altars’ are

not arbitrary, but neither are they natural, since they reflect human interests”.1 What

sets natural kinds like chemical elements apart from classes like cars manufactured

in Germany is that their grouping is based on natural causes or laws and that the

discovery of these natural kinds therefore represents an increase in our knowledge

on the structure of the world. This short depiction is very superficial; the topic goes

much deeper and touches on basic metaphysical questions of philosophy, but these

are the scope of evolutionary biology. What is important to note is that when

species are viewed as classes with essential properties, this does not necessarily

entail that they are unnatural and completely arbitrary groupings.

Another related term that is similar to that of classes is set. Some distinguish

between classes and sets, and some treat these terms synonymously (see Stamos

2003, pp. 20–21). When a distinction is made between the two, classes are usually

defined intensionally, while sets are defined extensionally. An intensional definition

is one based on the specification of the necessary and sufficient (i.e. essential)

properties of a group; an extensional definition is simply giving a list of all members

of a group (the group named above containing Trajan’s column, the number two

and Julius Caesar is an example). For the scope of this book, this distinction is

largely irrelevant, and therefore I will speak of classes and mean by them all groups

defined by essential properties.

The ontological alternative to spatiotemporally unrestricted and/or arbitrary or

artificial entities is to view species as individuals. The term itself is perhaps not the

best choice because intuitively we think of individual organisms when we hear

individual, which makes it somewhat ambiguous. However, individuals may be

more widely defined as spatiotemporally restricted historical entities. The Roman

1



To be precise, this view is only one of the several possibilities and holds that natural kind

classifications are really natural and have extramental reality (naturalism or realism). One can of

course also deny their reality (and, for that matter, the reality of basically everything).



48



3 The Metaphysics, or Ontology, of Species: Classes, Natural Kinds or Individuals?



Empire, the acting company The King’s Men and the Habsburg dynasty are

individuals in this sense—they are historical entities that have a beginning and an

end in time. Once an individual dies or ceases to exist, it can never exist again. A

new empire can rise to power in the Mediterranean with Rome as its capital (and

perhaps even with emperors, a senate and consuls), but it will still be different from

the Roman Empire that our history books tell us about. Unlike classes and natural

kinds that have instances or elements, the relationship between an individual and its

members is that of whole and parts: Roman citizens were not instances of the

Roman Empire but parts thereof, just like Shakespeare was a part of The King’s

Men and emperor Franz Joseph I. of Austria was a part of the Habsburg dynasty. To

make the difference between classes and individuals more visible, Ghiselin (1997,

p. 40) has suggested to reserve the terms inclusion and inclusive for class–element

relationships and to use incorporation and incorporative for whole-part relationships of individuals instead. Thus, Chordata would be a more incorporative (rather

than a more inclusive) taxon than Vertebrata, but although linguistic consistency is

doubtless an advantage, this distinction has never caught on. I will therefore stick to

the less accurate but more common term inclusive.

Being historically contingent entities, individuals do not have essential properties—there is no necessary and sufficient condition to be the Roman Empire. This is

what is really revolutionary about the “species-as-individuals” view—if species are

individuals, they cannot be defined based on properties, let alone essential ones!

Ghiselin (1997, p. 45) neatly sums this up: “what is necessarily true of natural kinds

is true of physical necessity, whereas what is necessarily true of artificial kinds is

true of logical necessity, and what is true of individuals is true only as a matter of

contingent fact, and therefore not necessary at all”.

But not only can species not be defined in terms of properties, they cannot really

be defined at all! Rather, they can only be pointed out ostensively: “this group of

organisms is Homo sapiens”. Although really an act of christening rather than a

definition, this is often called ostensive definition. Names of individuals, including

species, are therefore proper names!2 Indeed it was this insight (that species have

proper names) that made Ghiselin realize that species are individuals, while David

Hull, the most prominent adherent of the individuality thesis among philosophers of

science (e.g. Hull 1976, 1978), arrived there as a consequence of the fact that there

are no laws of nature for individual species (Ghiselin 1997, p. 130). While

Ghiselin’s name is usually associated with the individuality thesis, there are others

who have viewed species and more generally biological taxa as individuals rather

than classes (which Ghiselin readily admits). Wilkins (2009b, p. 173) names

W. Stanley Jevons who, as early as 1873, noted that classes must have definienda

common to all its members but that biological classification is not really a classification in this sense, but rather an arrangement of groups by genealogy which are



2



But see Jensen (2011) who recognizes the similarity of species names to proper names but thinks

that there are also differences, which is why he calls species names “extra-proper names”. I will

not go into further detail here of this largely philosophical discussion.



3.1 Classes, Natural Kinds, Sets and Individuals



49



individuals in the light of evolution. To Stamos (2003, p. 186) I owe another early

source of the notion that species are individuals: the very young Julian Huxley

mentions “the species-individuality of which we are the parts” (Huxley 1912,

p. 24), and he also seems to think that in the light of evolution, species must have

individuality. Willi Hennig (1966, p. 81, drawing on the philosopher Nicolai

Hartmann) was also very clear about this when he said that biological taxa are

not timeless abstractions but that “there can be no doubt that all the supra-individual

categories, from the species to the highest category rank, have individuality and

reality. They are all [. . .] segments of the temporal stream of successive ‘interbreeding populations’. As such they have a beginning and an end in time”. Similar,

implicit or explicit, references to the individuality of biological taxa can be found

throughout the evolutionary and phylogenetic literature.3 The reason for this is

quite simple: species as timeless abstractions (classes or natural kinds) are hard to

reconcile with the historical process of evolution. How could the physical and

historical process of evolution produce eternal and timeless entities?! To accept

this would make biological classification an artificial enterprise dealing with mere

intellectual abstractions rather than real groups: “For species to evolve, it is

metaphysically necessary for them to be individuals, and an ‘evolutionary’ species

definition that treated them as if they were sets [classes] would be a contradiction in

terms, or an oxymoron at the very best” (Ghiselin 1997, p. 113).4

It is very important to realize that the “class/kind vs individual” debate refers to

the ontological status of species taxa! It asks what kind of thing a species taxon like

Homo sapiens or a tiger is. The issue of what the species category is (i.e. the group

of all species taxa) is an entirely different question, and except for species category

nominalists (who deny that something like the species rank exists), everyone agrees

that the species category is a class with all species taxa as its members or elements.

What exactly defines this class—what its necessary and sufficient properties are—is

the vexed issue of which species concept is the best, but everyone who favours a

certain species concept has already agreed that the species category is a class.

The individuality thesis does not stop at the species level. It includes all higher

taxa as well inasmuch as they correctly represent history, i.e. phylogeny. Monophyletic supraspecific taxa are also individuals: they originate in the form of their

stem species, and at some point, they become extinct (or still exist today). Just like

species, higher monophyletic taxa cannot be defined by properties either. They can

only be pointed out in the Tree of Life, and nested parts of this tree that fulfil the

conditions of monophyly are given a name (ostensive definition), which is a proper

name. The properties that are unique to this taxon (its autapomorphies) must not be

3

For the German-speaking world, Rieppel (2011) traces the view of species as individuals back to

pre-evolutionary times when the German Naturphilosophie as developed by Friedrich Schelling

assigned individuality to species due to their passing through time (spatiotemporal restrictedness).

4

Stamos (2003, p. 287, footnote 4) presents an interesting quote from Bertrand Russell in regard to

Darwin’s Origin of Species: “The doctrine of natural kinds [. . .] was suddenly swept away forever

out of the biological world” (Russell 1914, p. 22). Russell, however, seems to have held different

views on the ontology of species at different times and was overall not very interested in the topic.



50



3 The Metaphysics, or Ontology, of Species: Classes, Natural Kinds or Individuals?



confused with its definition—they only serve to discover, not to define the

monophylum!



3.2



Whatever else Species Might Be, They Must also Be

Individuals



As mentioned before, the term “individual”, looking back on the debate, may not

have been the best choice. I would agree with Ghiselin that it is an appropriate term

for an entity that is spatiotemporally bounded, but there are stricter definitions of

individuals, stipulating, for example, that in addition to spatiotemporal restriction,

an entity is only an individual if its parts are interconnected. In the case of species,

this interconnection would be some kind of cohesion among the single conspecific

organisms. Usually species are compared to organisms, the prime examples of

individuals, and the discussion is about whether species show the same kind and

degree of integration and cohesion as single organisms (e.g. through gene flow or

exposure to similar selection regimes) and, if they don’t, whether they should be

called individuals. Accordingly, there have been futile arguments over the use of

the term rather than the important issues that stand behind it—quite apart from the

fact that even the autonomy and concreteness of organisms is a matter of degree

with sometimes vague boundaries (think of slime moulds or colonial organisms).

One has to be careful here that one does not just have a purely terminological

debate. Wiley (1980, 1981, p. 74f.) has made an interesting (but terminological)

point by introducing the term “historical entity” or “historical group”. Of course

every individual is a historical entity, too, but Wiley wants to make a distinction

between two different kinds of historical entities: those that show cohesion among

its parts and partake (at least potentially) in natural processes, and those that don’t.

The first Wiley calls individuals (and he includes species here), the second he calls

historical entities or groups. His definition thus subdivides further what is called

individual by others (e.g. Ghiselin or Hull), and Wiley (1981, footnote on p. 75)

admits that this distinction is primarily interesting from a philosophical rather than

from a biological perspective. While, according to Wiley, species are individuals in

the narrow sense, higher monophyletic taxa are not because, unlike species, they

lack cohesion and do not partake in natural processes (Wiley 1980, 1981, p. 75).

This distinction, I think, is correct, but whether it grants a new term (and one that is

rather vague) is a different matter. I will use the term “individual” in a general

sense, i.e. as a term for historical (¼ spatiotemporally restricted) entities. Everything that has a beginning and an end in space and time is an individual under this

definition, an individual sensu lato, as it were. What is important is that both

individuals sensu lato and individuals sensu stricto are clearly very different from

their ontological alternative: classes or natural kinds. If biologists or philosophers

deny individual status to species, it must therefore always be asked whether this is

because of a stricter definition of the term individual or because they consider



3.2 Whatever else Species Might Be, They Must also Be Individuals



51



species to be classes/natural kinds. The former is primarily an issue of nomenclature, while only the latter is a true ontological or metaphysical issue. I have chosen

to use the sensu lato definition of individual not because I think it is a good

application of the term (although I do think it is) but mainly for historical reasons:

it is the conception of individuals held by Michael Ghiselin who is the father of

(at least the explicit) treatment of species as individuals (Ghiselin 1966, 1969,

1974a5). Therefore, I think that it is historically consistent to use and evaluate the

term in its original contextual meaning. Whether Wiley is correct in classifying

species as individuals sensu stricto and whether higher monophyletic taxa are really

different from species in that regard is a valid and interesting question, but it is one

that is asked already within the paradigm of species as spatiotemporally bounded

entities, which precludes their being viewed as natural kinds. Or as Ghiselin (1997,

p. 59) puts it: “cohesion or the lack of it does not seem to imply a deep metaphysical

cut [. . .] The difference between historical entities and cohesive individuals may be

important, but it is far less profound than that between historical entities and

classes”. This view is shared by Baum (1998, p. 643) who treats entities with a

common history that lack causal interactions as “simply one type of individual

(broad sense)”. Ghiselin (1997, p. 52) concludes that cohesiveness is sufficient but

not necessary for individuality, and it is also sometimes temporary in organisms,

giving slime moulds (“social amoebae”) as an example (p. 55). This example is

revealing because it shows that the term organism is by no means as clear-cut as

many philosophers, who mainly think of large vertebrates when they think of

organisms, would like it to be (Richards 2010, p. 163f.). Accordingly, cohesiveness

is not included in Ghiselin’s list of “six criteria by virtue of which individuals may

be recognized and individuality may be defined: 1. non-instantiability[6], 2. spatiotemporal restriction, 3. concreteness, 4. not functioning in laws, 5. lack of defining

properties, and 6. ontological autonomy” (ibidem, p. 49). Mishler and Brandon

(1987) give a slightly different but similar list: spatial boundedness, temporal

boundedness, integration and cohesion. The difference between the latter two is

that cohesion (in their terminology) implies that an entity “behaves as a whole with

respect to some process”, whereas integration refers to “active interaction among

parts of an entity. In other words, does the presence or activity of one part of an

entity matter to another part?” (p. 400).

I will not discuss all these criteria in detail. This has been done in many of the

publications cited here, and it is beyond the scope of the book. It is also beyond

what is of immediate relevance and benefit for practicing biologists. Instead, I will



5



Ghiselin (1997, pp. 14ff.) gives a short summary of how he came to think of species as

individuals, that his first publication with the individuality thesis was the one from 1966, that he

elaborated on it in his 1969 book, but that his 1974a paper really triggered the general discussion.

6

Non-instantiability refers to the fact that there are no instances of individuals as there are

instances of classes: while all concrete circles are instances of the abstract class of circles

(by meeting the essential condition(s) of class membership), individuals do not have such

instances or elements. Instead, they have parts, and just like my left arm is a part of me and not

an instance or element, Greece and France are parts of the European Union, not their instances.



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