KINDS OF KINDS: INDIVIDUALITY AND BIOLOGICAL SPECIES

REPRINTED FROM: International Studies in the Philosophy of Science, 3.2 : 119-135
by permission of the Editor.
 

[ ABSTRACT ]

The Nature of the Dispute

The origin of the novel claim about species lies in Darwin's rejection of the Aristotelian world view. Darwin did not, to be sure, invent the idea of evolution; nevertheless, until evolution by random selection had to be taken seriously, the dominant view of species was that they were immutable types, even if their members deviated from the true type. From a philosophical point of view, the real Darwinian revolution was an ontological one. As Mayr (a) has pointed out, it consisted in a shift in explanatory perspective. In the Aristotelian perspective, individuals conformed more or less to types; in the Darwinian, types arise as a result of a process acting on fundamentally variable individuals. An analogous reversal of what needed and what did not need explanation resulted from the Newtonian revolution: continuous motion replaced rest as the "natural state" needing no special causal explanation. Similarly, before Darwin, deviations from type required explanation (Aristotelian science had a branch called teratology, or the science of monstrous births); after Darwin, on the contrary, what requires explanation is that the variety of individuals should form clusters of more or less similar individuals.(1)

But why should we take these clusters of similar individuals — these species — to be themselves individuals?

Whatever may be the explanation for such clusterings, their characteristics, unlike those of Aristotelian species, are susceptible to change. The motivation for the thesis in question, then, is this: Neither sets nor classes can change (Ghiselin (a), (b) (c); Hull (a), (b), Mayr (b)). They therefore seem more suited to map Aristotelian than Darwinian species. A set is defined extensionally: it is no longer the same set if you add or subtract members. A class, if the word is not a synonym for a set, may be defined intensionally, by a predicate. It can't admit of change either. A class is open in the sense that its extension can vary over time, but its defining characteristics can't vary over time. Just as a change in extension annihilates a set (and replaces it with a different set), so a change in intension annihilates the class (replacing it with a different class). Classes are not the kind of things that can change.

Darwin, as we have just seen, introduced a concept of changing species to replace the Aristotelian concept of fixed biological kinds. If, therefore, species continue to be seen as classes, then a most important aspect of the Darwinian revolution becomes incapable of expression:

Classes are precisely the sorts of things that cannot evolve. A sample of lead might be transmuted into a sample of gold, but I do not know what one might mean by the claim that lead as a natural kind has been transmuted into gold. Thus, Ghiselin argues, if species are natural kinds (or classes) they cannot evolve. Conversely, if they evolve, then they cannot be natural kinds (or classes) of the sort that can function in scientific laws. But if species do not belong to the metaphysical category to which they are usually assigned, what sort of thing are they? Ghiselin replies: individuals. (Hull (b) 291)

I shall refer to the decision to view species as individuals as the G-H thesis, for its chief champions, Michael Ghiselin and David Hull. What exactly is entailed by the G-H thesis?

Individuals, in the sense intended, are concrete and they have a history. They may happen to be distinguished from other individuals by their qualitative properties, but if that is so it is merely contingent. We distinguish classes in terms of their qualitative properties: individuals, by contrast, are distinguished essentially by their matter and location, and are therefore individuated indexically.

In this framework, the thesis that species — any specific species — is an individual cashes out as the claim that it is a particular spatiotemporal lineage, where a lineage is defined as "one or a series of demes that share a common history of descent not shared by other demes" (Wiley, 25). This means that a species is to be identified not in terms of its specific characteristics, but in terms of its particular history of descent.(2)   And this is precisely what those who resist this move want to reject. To clarify the contrary intuitions involved, here is a crucial thought-experiment:
 

The Decisive Thought-Experiment

Suppose . . .  beings on [some] distant planet . . .  by some highly improbable evolutionary parallelism are genetically indistinguishable from us (assuming now that genetics provides the underlying trait of Homo Sapiens). Also, it seems to me that members of a species could be created artificially in the laboratory. (Schwartz 302)

As we shall see, the example may have to be further refined to delineate some intermediate positions; but it suffices as it stands to delimit the main positions. For Schwartz and Ghiselin are equally categorical in their contrary views. For the champion of the G-H thesis, in spite of the qualitative similarities, neither the doppelgänger on the distant planet, nor the artificially created pseudo-clone, would count as members of the species (Ghiselin, Response 304). But Schwartz says:
 

It seems likely to me that we would grant that these organisms are Homo Sapiens even though they do not share a lineage . . .  with us . . . . [A]rtificial members would not share a lineage with natural creatures, but would belong to the species nevertheless if and only if they had the appropriate genetic code. (Schwartz, ibid.)

Some Skirmishes

That sounds neat, but it is far from conclusive. We can always talk informally about classes changing, while at the metaphysical level we can construe the changes that do take place in terms of a succession of classes resembling one another in certain systematic ways. The (merely apparent) identity through time of a changing class would then be very like personal identity as Hume construes it — as a kind of movie, providing the illusion of change by means of a quick succession of perfectly still images or temporal slices.

Against the G-H thesis, many poor arguments have been adduced. With equal and opposite bravado, for example, Mario Bunge claims flatly that the G-H thesis "renders systematics impossible" (285). Obviously that's not true, although, as we shall see, the G-H thesis is more congenial to some taxonomic schemes (such as phylogenetic systematics) than others (such as pheneticism).(3)   Heise & Starr argue that it is a difficulty with the H-G thesis that "Either one must say that a species is complete . . .  (thereby precluding new members) or one must revise the concept of an individual such that an individual does not have to be complete." (Weise, 289.) But that is a confusion. In a sense, no individual is "complete" until it is dead. (Call no man happy until he is dead, advised Solon.) What is incomplete about the species, on the G-H thesis, is just what is incomplete about the person: not all its parts have yet come into existence.

It might occur to someone that Heise and Starr's point could be reconstructed as follows: a class — the extension of a universal — is potentially infinite, whereas an individual is always finite. But strictly speaking that won't help. For if we take into account the whole history of the universe, they are both equally finite. Gold, for example, is just all the gold there is, was and ever will be: a finite and definite quantity.

Another bad argument is offered by Ruse, who writes:

It seems to me absurd to recognize Michael Ghiselin and Michael Ruse as humans on the basis of their hair, teeth, talk, and so forth, and yet to pretend that these properties do not enter into the ultimate decision about whether we are both members of Homo Sapiens. (Ruse (a),300)

Identification and Reidentification

F [the sortal] gives the general form by which, in the first instance, the point that is at issue may be elucidated. The claim made by D is that, where there is a point at issue, this depends in a systematic way upon the kind of thing that (a) and (b) are . . .
It will be typical of the enemy of D to deny that there can be any better elucidation of identity than the condition of complete community of properties . . .  [A]gainst this . . .  the relation of agreement in every property or relation must flow from identity itself . . .  Roughly,  . . .  what organizes our actual method [of answering a question, for example, about identity between a boy and a man,] is the idea of a particular kind of continuous path in space and time . . . . The kind of continuity in question here is not bare continuity, however. It is the kind of continuity that is brought into consideration by what it is to be a human being. (Wiggins, 48-50)

The Kripke-Putnam Line

There is an irreducibly indexical component in our understanding of natural kinds. What we now identify as water, we first pointed to without knowing much about what it really was. That was a matter for science, which has now told us that water is, essentially but a posteriori, H20. Suppose something were just like water in superficial respects, though at the molecular level it is not H20 but XYZ. XYZ would not be water even if it were called `water' by the inhabitants of a planet otherwise very much like ours. And this, in Kripke's terminology, is because `water' is a rigid designator, designating in every possible world just what it was first used to designate, when it was ostensively defined in our own world.

It might be argued that the traditional opposition between individuals and classes has been blunted by this reasoning. On the traditional view, individuals are identified by means of token reflexives, whereas classes are individuated in terms of purely qualitative predicates. But on the Kripke-Putnam view, it seems, every natural kind depends in some sense on reidentification of something picked out by an indexical.

This line of argument, however, would be mistaken. Once the reference is fixed by the indexical, the relation between that original specimen (of gold, or water) and a newly discovered specimen hangs entirely on their sharing certain properties. Indeed, this practice is institutionalized in biological taxomony: the individual specimen of a new species is baptized in accordance with precise rules laid down in the International Code of Zoological [or Botanical] Nomenclature; but it is not even required to be typical. (4)  By contrast, the G-H view is that no necessary and sufficient criteria for belonging to a species — or for being a certain individual — could possibly be specified in purely qualitative terms.

The reidentification of individuals is unproblematic in practice; in particular, it is relatively independent of the kind of individual it is — subject to the condition laid down by Wiggins, that we know that kind of thing we are looking for (what sort of conditions govern its successful reidentification). The reidentification of natural kinds, by contrast, is problematic, because they do not, ex hypothesi, have any properties of spatio-temporal continuity. Even where it has originally been picked out by an indexical, therefore, the only criterion of reidentification available is the descriptive one that science specifies as essential to that specific kind.

The nature of any entity (whatever its ontological status) dictates the applicable criteria of its reidentification. How, then, does the nature of a species relate to the peculiar criteria for reidentification of species? Exactly what difference does the G-H thesis make to this question?

At this point, the two sides can be seen to engage in a dialectic. Each accuses the other of incoherence, but each has a logically consistent position, poised in symmetry against the other.

One side (with Schwartz and Ruse) says this: To know what a species really is, find the properties that constitute necessary and sufficient conditions for belonging to that species. In practice, of course, we may identify the bearers of those properties in terms of causal-relational properties of lineages. For those — or so Darwinian theory leads us to expect — explain how things of that kind got to have their characteristic properties in common. These causal-relational properties, therefore, will be good though not infallible predictors of the essential properties definitive of any given species. But the causal-relational properties will provide merely contingent identification of a class essentially defined by essential properties.

The other side (with Hull and Ghiselin) sends back a mirror image of the first. They argue as follows: To know what a species really is, you need to find the lineage to which its members belong. That is, you need to find a group related by certain causal links to some given ancestor. In practice, of course, we may identify the lineages in question in terms of certain properties — similarities and common features — which will be good though not infallible predictors of the relevant causal-relational properties. But those common features will provide merely contingent identification of what is essentially an individual, defined by its causal-relational properties.(5)

Consider yet another way of putting the dispute. Anything we talk about must be identified in some terms or other: "What I am talking about is something that meets the following criterion: x is F." But what counts as x being F? Some properties are defined in terms of particulars or token reflexives, others are wholly general. One side in the dispute (Schwartz and Ruse et al.) insist that F must be a general property; the G-H side insist that it must be a property defined in terms of descent.(6)

But there is room for a third alternative, as this passage from Stebbins will attest:

"[A]ll of us who accept the biological species concept recognize that the individuals and populations that constitute a biological or evolutionary entity that we call a species need to have only one property in common: the actual or potential ability to exchange genes via successful hybridization, and so to cooperate in reproduction that continues the identity of the species." (Stebbins 199)

At this point, a further refinement of the thought-experiment is possible:

Suppose the local and alien beings are actually able to reproduce, and do so. Then what should we say then about them, their progenitors, and their offspring?

Stebbins' criterion of "potential ability" is shown to be satisfied by the "successful hybridization"; so both offspring and progenitors turn out to have been members of the same species all along. For Mayr, on the other hand, it seems that the news about the hybrids must actually change the relationship between the two lineages formerly thought to be separate. Until now, their progenitors genuinely belonged to different species. From now on, however, they are retroactively conspecific.

In a related vein, it might be suggested that we need not a third kind of logical relation governing reidentification, but a third kind of ontological category besides individuals and kinds. Perhaps, as Mayr (a) and Hull (b) suggest, the logical opposition between particular and kind is too crude for present purposes. We should acknowledge a third category, a special kind of individual with its own kind of criteria of continuity. Hull, in fact, proposes a three-level distinction among logical individuals: the smallest units are Dawkins' "replicators" that "pass on their structure largely unchanged." Ordinary individuals are those "well-integrated, spatiotemporally localized entities capable of only a finite amount of change . . .  of the sort that can have structure and interact as cohesive wholes with their environment." The third sort "need not be as cohesive . . .  [but] they must be just as continuous through time." Those are species, which are properly speaking the entities that evolve: "no entity can simultaneously be selected and evolve, since the characteristics necessary for these two processes are incompatible." (Hull (b), 291)
 

Biological Issues

Arthur Caplan accuses Ghiselin of confusing two separate questions: What is the ontological bedrock? and What is the most useful scheme for purposes of explanation? (Caplan, 285.) But it is far from clear that we can distinguish these. What other point could there be to an ontological classification, after all, than to serve the explanatory and descriptive purposes of science?

On this basis, what we are looking for are the principles according to which it is better "for practical purposes" to look at species in one way rather than the other — to identify a species causally, rather than in some typological way. If you are a strict empiricist, you don't believe in real rather than nominal kinds in any case: the idea is, roughly, that you make up some definition, then see whether anything fits it. At least, from the logical point of view that is what you do; in practice, you would want to sneak a look ahead, as it were, to find out what definitions it might be profitable to frame. The "practical purposes" of science are, of course, theoretical ones. So what are the relevant theoretical purposes here?

Evolving Species

Actually, as I suggested above, that's not a problem: evolution can be viewed as a process in which what is actually in the world evolves. Real things — populations or lineages — change, but species need not. Instead, we can say that the same lineage can go through a number of different species, passing through them as through the nodes of a kind of network in logical space.(7) On this view, then, it could be biopulations that evolve, or ecosystems, or even, as Bunge suggests, the biosphere as a whole. (Bunge 284)

Do we need Species?

These different alternatives will present themselves as more or less plausible depending on your other views about taxonomy. For the phylogenetic taxonomist, there is a perfectly logical way of individuating those extended individuals that constitute a species: we have the same lineage, the same branch of the tree, up to and only up to the point where there is a divergence, or speciation. Species will then be represented by branches, while events of speciation will be represented by nodes on the phylogenetic tree. I have suggested that one could accommodate the view of species as classes by viewing them as patterns through which actual groups of individuals pass. Like geometrical forms, species will not be anything in the real world, though things in the real world — that is, groups of organisms in a lineage — may fit them as models. Even so, I would expect the phylogenetic taxonomist to be more sympathetic to the view of species as individuals.

To a pheneticist, the following view might be more plausible: species are indeed natural kinds, albeit ones the membership of which does not endure. For the pheneticist, as we have seen, distinguishes between species on the basis of the outcome of a calculus of resemblances and differences among characteristics (see Sokal and Sneath).

As for the proponents of Mayr's "biological species concept", which combines some descriptive characteristics with some essentially relational criteria of identification pertaining to reproductive compatibility, they will naturally opt for a modified species-as-individuals view.
 

Philosophical Issues: Temporality and the Unity of Science

The G-H thesis, however, also raises more centrally philosophical issues. I want to conclude by raising four closely related issues: the generality of understanding; the existence of biological laws; the place of temporality in science and biology in particular; and the consequences of the thesis for the unity of science.

Is understanding intrinsically general?

There are probably two kinds of people in the world: one class cannot understand an individual without relying on some property or properties, . . .  the other class . . .  think that they do understand what it is to recognize an individual without relying on some property or properties . . . . Ghiselin is a member of the second class, I am a member of the first. (Heise 290)

This assumption is deep, though I confess that what I mean by that is mainly that I can't think how to defend it. But I can say this much, at least, about the intuitive considerations that underlie it. There is no objection to saying that in physics we are talking about individuals — some actual stuff, some actual things. And I suppose it is logically possible that one might acquire direct and intuitive understanding of a particular thing without relating it to any kind of thing. Still, insofar as our aim is scientific understanding, we want to understand why just this happens and not something else; and if it were to happen differently elsewhere to something of just this kind we would require an explanation. The causal power of a mere change of place-time might do the trick, but then that causal power would have to be expressed in terms of a universal law governing the effects of changes in space-time, or of different locations. If we offer an explanatory law, we mean it to apply to stuff that shares with the stuff at hand a crucial set of causal powers. For that kind of stuff is defined in terms of those causal powers (with the implicit proviso: "so long as we have got them right.") To increase one's scientific understanding is to grasp increasingly basic causal powers in virtue of which things behave as they do.

The present view of understanding as timelessly general is not impugned by modern science's increased rehabilitation of irreversible processes and absolute time.(8)   In some sense, to be sure, the laws of science themselves are time-bound — on the other side of the Big Bang, or the black hole from which it sprang, the laws we now know may not hold. But we shall want an explanation of why the laws of one time, just so far into the history of the universe, are different from those of another. And that explanation will have to be general. In this way, the generalizing thrust of understanding demands that some timeless meta-level laws be found to explain the coming into being of the laws that govern our physics. Such laws would refer to what Evan Fales has called "fundamental entities" the nature of which is bound up with their compliance to natural laws, as opposed to "derived entities" the behaviour of which is explained in terms of the former. And it may be, as he has claimed, that the number of such truly law-bound fundamental entities is very small, even in physics (Fales, 70ff). Still, everything else that we can be said truly to understand would, on this picture of science, be understood in terms of those fundamental laws and entities.

A corollary for the somewhat fashionable view of the disunity of science is this: any remaining disunity in science simply defines something we still don't understand.

Are there Biological Laws?

The claim entailed by the G-H thesis is not, of course, that biological organisms belonging to species don't behave as they do in virtue of certain law-governed causal powers. Rather, it is that the causal powers in virtue of which they behave as they do are not ones they have in virtue of being members of a certain species, any more than a stone has the causal powers it has in virtue of being a pretty medium sized polished pink stone with dark vein-like markings. That's not to say that the properties of species, or of a given stone, are beyond scientific understanding. Some of their properties, to be sure, may be brought into certain explanations; but they won't have the privileged scientific form, which allows us to say, implicitly: "This is what being a thing of that kind involves."

Now recall the stand I have taken against Caplan's objection: there isn't anything more to ontology than what proves explanatory at the most basic level. In conjunction with that stand, the G-H thesis does have the consequence that either species cannot be the fundamental ontological entities of biology, or there can be no understanding of the fundamental biological entities.

All this does not altogether preclude the existence of biological laws. If there are any, however, they govern not members of species as such, but the processes that lie behind biological stability and change. The best putative example of such a law is the Hardy-Weinberg law. This is often compared to Newton's first law of motion, because it tells us, in effect, what is to count as staying the same: if not everything in the distribution of genes remains the same, and the changes cannot be imputed merely to random fluctuations, the Hardy-Weinberg law licenses the inference that some "force" (selective pressure) has come to make it change. Other laws might govern the rate of mutation; or the boundary conditions that have to be satisfied if we are to get group selection, or speciation, and so forth. But those laws have nothing directly to say about the properties of those relatively arbitrary spatio-temporal slices of life we call `species'.

Biology and Temporality

[on the G-H thesis] species are historical entities. They come into existence, live, and die. But this means that these very important biological items are essentially different from what one might think are comparable physicochemical items, like gold or water . . . . This all seems to me to be the thin end of a very unfortunate wedge — one that many have tried to drive firmly between physics and chemistry on the one hand and biology on the other . . . . Down that road lies vitalism. (Ruse (a) 300)

One answer is given by Beckner: biology involves historical concepts, where a historical concept is defined as

Any concept, C, that is so defined that it is logically impossible to apply it correctly to an object, S, unless S has a specified history. Thus, "hybrid" is historical because no organism can (logically) be a hybrid unless the history of its generation meets certain specifications. Such concepts as "mutant," "homology," hepertely," "convert," "graduate," "retired," and "veteran" are also historical. (Beckner, 314)

It has sometimes been suggested that there is a mode of explanation different from the strictly scientific, which is the purely narrative (see Brooks). Certainly, being told that A followed B commonly generates a feeling of understanding. If there were such thing as a purely historical science, it would have to be one in which such narratives constituted all the understanding afforded by that particular discipline. But narration, too, seems to me to presuppose the more general sense of understanding which underlies the requirements of science. For my feeling of understanding will only be generated by a narrative if I have some general conception of the kinds of events that succeeded one another. In the pure sense, then, I don't think there can be anything that is both a science and purely historical.

What then are Hull, Ghiselin and Ruse arguing about?

Actually there are two theses here, which must be carefully distinguished, as they have opposite consequences for the relation of biology to the rest of science. One thesis is that species are historical entities: that, clearly, is indeed at the core of the H-G thesis. The other is that biology is therefore an intrinsically historical science. And that, as Alex Rosenberg has pointed out, does not follow. On the contrary, argues Rosenberg, it is precisely because species are historical that biology isn't:

Biology is no more historical than astronomy, but, like astronomers of the Milky Way, biologists perforce pursue the intricacies of particular objects (species) in greater detail and with greater contingent limitations on generalization than microparticle physicists. (Rosenberg 299)

Biology and the Unity of Science

From all this, though, there is something we can learn about the differences between different sciences. Some sciences — biology in particular — can be simply interested in the particular, as well as in the laws in virtue of which they behave as they do; but they are less interested in finding laws that describe them as such. Others, like the most fundamental areas of physics, are not really interested in the particulars that form the pretext of their study, but only in the laws that account for the behaviour of those particulars.

So perhaps we should revise the claim that science cares only about the general. If so, it would not be because some individuals are such that real laws don't apply to them. It would be, rather, because they are such that the applicable laws are too complicated. We persist in studying those objects, particular as they are, in the hope of obtaining understanding, general as that must be. And we do so, after all, because those objects are so intrinsically interesting. That is a dichotomy which the unity of science can easily survive: on the one hand, there are those science in which the most captivating thrill lies in the prospect of rock-bottom understanding. In that context, we care little about any individual objects. On the other hand, there are those sciences in which we are captivated above all by individual objects. In those sciences, among which are biology and psychology, we think the study worth while without having to be committed to the ontological primacy of their privileged objects. (9)
 
 

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REFERENCES

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Caplan, Arthur L. "Pick your poison: Historicism, essentialism, and emergentism in the definition of species", The Behavioral and Brain Sciences (1981) 4:285-6

Dawkins, Richard. The Selfish Gene. London: Oxford University Press 1976.

Duncan, Thomas, and Tod F. Stuessy, eds., Cladistics: Perspectives on the Reconstruction of Evolutionary History. New York: Columbia University Press, 1984.

Estabrook, George F. "Phylogenetic Trees and Character-State Trees", in Duncan and Stuessy.

Fales, Evan. "Natural Kinds and Freaks of Nature", Philosophy of Science (1982) 49:67-90.

Ghiselin, Michael T. (a). "On Psychologism in the logic of taxonomic controversies", Systematic Zoology 15:207-215 (1966).

Ghiselin, Michael T. (b). "Categories, Life, and Thinking", The Behavioral and Brain Sciences 4(1981):269-283 (1981).

Ghiselin, Michael T. (c). "Species Concepts, Individuality, and Objectivity", Biology & Philosophy (1987) 2:127-143.

Ghiselin, Michael T. Response. "Taxa, life and thinking", The Behavioral and Brain Sciences (1981) 4:303-310.

Heise, H. and Starr, M.P. "Nomenifers: Are they christened or classified?" Systematic Zoology 17:458-67; quoted in Heise.

Heise, Helen. "Universals, particulars, and paradigms", Biology & Philosophy, (1987) 2:289-290.

Hull, David (a) "Are Species Really Individuals?" Systematic Zoology 25:174-191 (1976).

Hull, David (b). "Metaphysics and Common Usage" (Comments on Ghiselin (b) The Behavioral and Brain Sciences 4(1981):290-291.

Hull, David (c) "Cladistic Theory", in Duncan and Stuessy.

Kitcher, Philip. "Ghostly Whispers", Biology & Philosophy (1987) 2:184-192.

Kornet, Diedel J., Biologische Taxa als Natuurlijke Soorten. Filosofische Reeks 22. Fac. Wijsb. University of Amsterdam. Delft: Eburon 1987.

Kripke, Saul. "Naming and Necessity", in D. Davidson and G. Harman, eds., Semantics of Natural Language. Dordrecht: D. Reidel 1972.

Mayr, Ernst (a). "Typological versus Population Thinking", in Evolution and the diversity of Life. Cambridge, MA: Harvard University Press 1975. Reprinted in Sober.

Mayr, Ernst (b). "The Ontological Status of Species: Scientific Progress and Philosophical Terminology", Biology & Philosophy (1987) 2:145-166.

Mayr, Ernst (c). "Species Concepts and Their Applications", in Sober.

Mayr, Ernst (d). The Growth of Biological Thought. Cambridge, MA: Harvard University Press 1982.

Prigogine, Ilya and Isabelle Stengers. La Nouvelle Alliance: Métamorphose de la Science. Paris: Gallimard 1979.

Putnam, H. "The Meaning of 'Meaning'", in Mind, Language, and Reality: Philosophical Papers, vol.2. Cambridge: Cambridge University Press 1975.

Reed, Edward. "The Demise of Mental Representations", Biology & Philosophy (1987) 2:297-298.

Rosenberg, Alex. "Typologies: Obstacles and opportunities in scientific change", The Behavioral and Brain Sciences (1981) 4:298-299.

Ruse, Michael (a). "Species as Individuals: logical, biological, and philosophical problems", The Behavioral and Brain Sciences (1981) 4:299-300.

Ruse, Michael (b). "Biological Species: Natural Kinds, Individuals, or What?" British Journal for the Philosophy of Science (1987) 38:225-242.

Salthe, Stanley, "The world represented as a hierarchy of nature may not require `species'", Biology & Philosophy (1987) 2:300-301.

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Sober, Elliott, ed. Conceptual Issues in Evolutionary Biology: An Anthology. Cambridge, MA: MIT Press-Bradford Books 1984.

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Stebbins, G. Ledyard. "Species Concepts: Semantics and Actual Situations", Biology & Philosophy (1987) 2:198-203.

Wiggins, David. Sameness and Substance. Oxford: Basil Blackwell 1980.

Wiley, E.O. Phylogenetics: The Theory and Practice of Phylogenetic Systematics. New York, Chichester, Brisbane, Toronto: John Wiley and Sons 1981.
 

NOTES:

^{1.  In fact, although the long title of Darwin's book famously contains the phrase, The Origin of Species, Darwin destroyed the Aristotelian concept of species but did little to replace it. Several approaches have been suggested to make modern sense of species, but none is without difficulties. On this, see the papers on the species concept in Sober.}

^{2.  For a formal characterization of the relevant relation of descent, see Kitcher. I leave it open whether the G-H thesis entails that all higher taxa are also individuals, and also whether higher taxa are in fact individuals on independent grounds. (Diedel Kornet impressed upon me the importance of this qualification.) It is worth observing, however, that even if, as I believe, the G-H thesis does have that consequence, it does not follow that the category of taxa comprising species is not itself a natural kind (see Mayr (b)). Further, it might still be true that the category of species is a privileged taxon in being the only one that is a natural kind. This may partly account for the opinion, often expressed by biologists, that the species taxon is more "real" than other taxa.}

^{3.  Phylogenetic systematics, sometime called "cladism" but now split into further clades of its own (see Hull (c)) is based on the following assumptions: "species have somewhat more biological reality than other taxa; each species has evolved from a single immediately ancestral species; and each species differs from this immediate ancestor in some properties that change during this evolution." (Estabrook, p. 136.) Pheneticists, by contrast, base taxonomy entirely on observable resemblances and differences, arguing that "[We] cannot make use of phylogeny for classification, since in the vast majority of cases phylogenies are unknown." (Sokal and Sneath, p. 21.) The phylogenetics of taxonomical theories are almost as intricate as the taxa they describe. For more details, see Wiley, Sober, Mayr (c).}

^{4.  This point was brought to my attention by Diedel Kornet. For further discussion, see Kornet.}

^{5.  The symmetry just described has led Evan Fales to argue that "the reconstrual of species as historical individuals does not eliminate essentialist enigmas . . . . [E]ach of the questions about natural kinds is now simply reborn as a question about individual essences" (84-85). His argument for this is that "although Hull rejects the existence of monadic essential properties, he appears to accept relational ones; specifically, the causal relations between a species and its genetic origins" (85). This seems to me right, insofar as "essentialist enigmas" concern the possibility of giving "real" or "nominal" necessary and sufficient conditions for the reidentification of something. But for present purposes that is not what matters. What concerns us here is not whether individuals have essences, but precisely how individual essences are different from kinds.}

^{6.  Edward Reed puts the point this way: "Members belong to categories, Ghiselin has shown, because the members participate in certain objective relations and processes. Michael Ghiselin and his brother resemble each other because they were born of the same parents (not because they resemble some third thing, "a Ghiselin"). We are all specimens of Homo Sapiens because we are all in reproductive competition, not because we share properties. In fact, whatever features we do share are products of evolutionary processes. It is not being a rational animal (or whatever) that makes one a human; rather it is because one is a human (that is, a sector of the evolutionary nexus Homo Sapiens), that one may exhibit rationality and animalness." (Reed, 297)}

^{7.  That story might have disadvantages again, in that change at a point of bifurcation will be treated differently from change in a species without bifurcation. Thus Mayr complains the "the cladists simply combine all inferred descendants of a given species into a 'monophyletic' taxon, even if they are as different as birds and crocodiles." (Mayr (d) 228. Diedel Kornet drew my attention to this passage.)}

^{8.  "Après  plus de trois siècles, la physique a retrouvé le thème de la multiplicité des temps . . . . L'histoire, que ce soit celle d'un être vivant ou d'une société, ne pourra plus jamais être réduite à la simplicité monotone d'un temps unique, que ce temps monnaie une invariance, ou qu'il trace les chemins d'un progrès ou d'une dégradation." (Prigogine and Stengers, 274-5.)}

^{9.  Diedel Kornet generously shared her ideas with me at at the 1988 Dubrovnik Seminars, and this paper has greatly benefited from her comments, criticisms, and suggestions. For her own prior discussion, which I understand anticipates some aspects of the present paper, see her monograph (Kornet), which I am unfortunately not able to read because it is in Dutch. I am also grateful for other valuable comments and discussion from members of the seminar, particularly Brian Baigrie and Yelena Mamchur. At an earlier stage, Sergio Sismondo helped me to clarify my thoughts on the subject of species.}
 

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