The significance of biology for the question concerning human nature

I – Introduction: ἄνθρωπον ζητῶ

 

Since Aristotle, humans are considered part of the natural world, φύσει ὄντα.^[1] As the question concerning human essence as human nature became more and more a question about human naturalness (in opposition to nurture, Spirit, technology etc.), and, at the same time, our understanding of the natural world became prerogative of “natural sciences”, philosophers have looked at biology in their search for the human, and what makes them uniquely themselves.^[2] Since the philosophical question about human nature claims to search for an all-inclusive universal answer, it has looked at humans from the perspective of their natural kind, their species: what’s the essence of Homo sapiens? The problem is thus interpreted as that of biological essentialism: because we assume that the essence of humans pertains to the biological, we ask ourselves if our species manifests anything like a biological essence (universal specific properties etc.). In particular, we will look both at the classical view of essences as constitutive intrinsic properties, and at the more recent attempt at conciliating essentialism with the historical and dynamic character of evolutionary species concepts. Given their failure, we will search for a possible answer in the notion of innate nature.

The grounding question we ask is: what kind of answer does biological science give to the philosophical question about human nature? The discussion will be divided in three parts:

-          a review of classical essentialism, with a focus on the notions of universality and normality and a detour through Aristotle;

-          a review of the new essentialism and its shortcomings;

-          a review of the notion of innateness.

 

II – Traditional essentialism

 

The traditional western interpretation of the human as animal rationale puts humans among other living species (as animal), distinguished by them by virtue of their differentia specifica (rationalitas), or their specific essence. This way of looking at human nature is grounded in the traditional link between essence and species. In particular, living species have been the paradigmatic example of something with an essence, from Aristotle to modern proponents of essentialism such as Putnam and Kripke.^[3] There is a good reason for this: not only does the living world seem (at first glance) to be carved into natural joints, but, most importantly, living beings provide a great example of substantiality. An essence explains why, no matter how much it changes, a butterfly never loses its self-identity: it never stops being a butterfly, it never loses its essence (until it dies).

At the same time, however, the advent and expansion of the historical perspective of Darwinian evolutionary biology seems to have marked once and for all the end of species essentialism. Species originate, divide, merge, change, vary, die, so they cannot have eternal and universal essences; their boundaries can be blurred in many ways, both diachronically and synchronically, so they cannot have a clear-cut differentia specifica.

Now, essentialism is hard to die. An essence provides a link between the individual living being and its species – the individuals we study shape our idea of a species (its common characters and variations, its history), which then informs our subsequent study of other individuals (it provides a ground for induction). In other words, it plays both a semantic role (“Homo sapiens” refers to a universal property of the species, that is, a property supposed to belong to all the members of the species – the same way “water” refers to H2O) and a causal/explanatory role (what it refers to is what makes Homo sapiens the way they are).^[4] Essences (or essential properties) can be intrinsic (as per Aristotelian tradition) or relational (as the “new essentialists” propose). Such ambitious role of essences begs the question: does biology really inform us about properties that determine both the species and the individual beings belonging to it at the same time?

In 1986, David Hull proposed an important critique of intrinsic essentialism in biology in his paper On human nature. In particular, he focused on the philosophers’ traditional criteria for a proper human essence: universality and normativity. The philosopher complains: “There must be characteristics which all and only people exhibit, or at least potentially exhibit, or which all normal people exhibit – at least potentially”^[5].

According to Hull, essentialists assume that “some connection exists between universality and innateness”^[6], perhaps because essence precedes any variation by definition, and what is not innate is the result of developmental changes and varying external influences (non intrinsic traits). Allegedly universal traits such as language are thus assumed to be species-specific and therefore grounded in the human genome. Even ignoring the fallacy of deducting specificity from innateness, it is possible to see how the reference to the genotype-phenotype map does not grant us any access to universals. Both phenotypes and genotypes are in fact neither universal nor specific. First, a human might be incapable of language because of lack of the necessary genetic underpinning (phenotypic and/or genetic non-universality). Secondly, even if we tried to reintegrate this human through an appeal to potentiality (“if they had a different genetic make-up…”), according to Hull we could, by the same principle, integrate any other species.^[7] Thirdly, the view of human traits as inherently unique or “human” is dubious:

 

The traits (and genes) which characterize all species save our own vary statistically. For some reason those characters which make us what we truly are happen to be universally distributed among all members of our species (at least potentially among normal human beings) and absent in all other species. I find this coincidence highly suspicious.^[8]

 

Phenotypic and genotypic properties, from the biologist’s point of view, are in a continuum across species: “Zebras and horses look very much alike, but genetically they’re quite different. Human beings and chimpanzees look quite different, but genetically we are almost identical”^[9]. Similar phenotypes can have very different genetic underpinnings, as in the case of analogous traits originated from convergent evolution; while similar genetic make-up can be expressed with great variations, as in the case of homologous traits. Hull’s perspective, in a word, seems to flatten out the relationship of similarity and dissimilarity between species. For example, assuming that we might recognise the same phenotype in organisms of different species, such phenotype would not be intrinsically “zebra” or “horse”. A horse is not a horse because it is made of “horse traits”. Therefore, the presence of one “horse” trait in a zebra does not make the latter more “horse”. Most importantly though, Hull’s view presents a flatter relationship even between individuals of the same species: as it is not the lack of language in a chimpanzee that makes it less human, the lack of language in a human does not make it less human than the rest of their fellow conspecifics – exactly because the appeal to potentiality is not an appeal to some sort of unexpressed human essence.

Still, the ways a human and a chimpanzee lack language still appear to be different - the former seems “supposed” to have it, the latter does not. After all, biologists can be surprised to find that a bee cannot develop its wings, but do not usually wonder why a blue whale is wingless. As we have seen, Hull attributes to the essentialist discourse an understanding of lack as potentiality (e.g. “Organisms that lack a particular trait possess it potentially...”[10]). A brief discussion of Aristotle’s concept of στέρησις can help us illuminate how much the boundaries of classical essentialism can stretch - and why it really fails. In Metaphysics V, Aristotle presents the following examples:

 

We speak of "privation (στέρησις)": (a) In one sense, if a thing does not possess an attribute which is a natural possession, even if the thing itself would not naturally possess it; e.g., we say that a vegetable is "deprived" of eyes. (b) If a thing does not possess an attribute which it or its genus would naturally possess. E.g., a blind man is not "deprived" of sight in the same sense that a mole is; the latter is "deprived" in virtue of its genus, but the former in virtue of himself.^[11]

 

First of all, some traits are natural possessions (τι τῶν πεφυκότων), they are possessed πε-φυκότως, and similarly they are lacked. Here we still cannot interpret the natural possession or lack of traits as being in virtue of a living’s nature (form, essence, be it general or specific): this sense of privation only focuses on the trait, so to say, taken abstractly. A natural possession is an “attribute”, a trait that, formally speaking, can be had by an organism, e.g., eyes, an exoskeleton, a nesting behaviour, photosynthesis. As we ask ourselves only whether the trait itself is instantiated or not, we ignore the essence of the organism supposed to lack it. According to this concept, a mole and a plant are equally eyeless, but not insofar as they are moles or plants, but only because they are eyeless. This first meaning of στέρησις signals the absence of a trait, which is equally possible as its presence (again, we ignore the fact that an eyeless plant, as a plant, can only be eyeless), and thus its formal possibility.

According to its second meaning, στέρησις, on the contrary, is seen from the perspective of the essence of the lacking organism. For Aristotle, an individual kiwi would be flightless κατὰ τὸ γένος, according to its species[12], but not flightless in itself. On the other hand, an individual human is blind καθ᾽αὑτό, in virtue of themselves.^[13] How are we to understand this difference? In both cases the essence of the organism is not indifferent. It is the essence of the kiwi that determines that the kiwi is not lacking flight because of itself (for example, because of its contingent development, because of illness or violence[14]), but because it belongs to a taxon of flightless birds. Similarly, it is the essence of a human that determines that their blindness, or lack of sight, is, so to say, something missing, not determined by the human species, but caused by the human themselves. Both the flightless kiwi and the blind human are lacking a trait for what they concretely are, but in different senses: the kiwi is flightless because it is a kiwi; on the contrary, as humans do have sight, a human is blind because they are able to miss it. According to this sense of στέρησις, the way a human can have the trait they lack is different from the way a trait can be present or absent. Because of their essence, a human has the concrete possibility of having a trait, and therefore, of missing it.

In an essentialist context, a flightless kiwi is a normal kiwi, but perhaps an atypical bird. Is a blind human normal? To the extent that sight is not a specifically human character, that is, it is not assumed to be the species-defining differentia specifica, its lack is not said to deprive a human of what makes them so – a lack like blindness is taken as uncontroversial.

However, if something like language is specific and essential to humans, blindness and lack of language might be two different kinds of privation. According to Aristotle, what makes humans human “has no other cause. Man has many causes: "animal," "twofooted," etc.; but nevertheless, man is in virtue of himself man (καθ᾽αὑτὸν ἄνθρωπος ὁ ἄνθρωπός ἐστι)”.^[15] “Animal”, “twofooted” are here the “causes” that, so to say, make up a species and determine a certain being to be the way they are κατὰ τὸ γένος (we could say that a kiwi has many causes: “bird”, “flightless” etc.). Similarly, even the lack of essential traits as such is possible only in virtue of the humanity of the human themselves. A human can be concretely “without language” only because they are human in the first place, even if we assumed that language is what makes humans human. (Notice how, despite the stark contrast with Hull’s assumption behind his talk of horses and zebras, that the organism has no essence, the conclusion is the same: traits do not make horses more “horse” nor humans more “human”.)

The argument is clearly circular - and here’s the strength and the weakness of essentialism:  human essence is so empty it can include everyone – even the supposedly inhuman; at the same time, it grants us no knowledge of universally shared specific essential traits.

Hull says: “On [...] rare occasions babies are born with little in the way of a cerebrum. If there is a significant sense in which they nevertheless retain the potentiality for language use, it eludes me”^[16]. According to Hull, such a condition would constitute a case for abnormality in essentialism: “Organisms that lack a particular trait actually possess it potentially or else are abnormal for not possessing it”^[17]. Now we understand where the problem resides. In this lack, in fact, the essentialist sees neither a formal possibility (a chimp could have a different DNA^[18], not because it is a chimp, but because we consider the trait abstractly, as we have seen in the first concept of στέρησις) nor just a concrete possibility (as in the case of a man that can concretely be affected by the accident of blindness because of violence, illness etc.^[19]), but the contingency of the human individual as such: de individuo non est scientia, because no matter how atypical and lacking, it remains, a priori, human.

Essence proper, in its circularity, is so inclusive that it neutralises variations. We can now see how it does not do the crucial evolutionary sense of diversity justice. The real problem of the normality/abnormality discourse, from the point of view of the evolutionary critique of essentialism, is not (just) its supposed exclusionary character, but its abstractness. As Hull repeatedly states, variations, ab-normalities and so on constitute biological species.^[20]

A possible solution to fill the emptiness of the essence is to look at statistical norms – normal developmental pathways^[21], or perhaps those traits which happen “to be prevalent and important for the moment”^[22], adaptations, functions etc.^[23] When we look at normality from the point of view of evolutionary biology, however, we do not find anything that resembles a norm or an archetype – and it’s not just a matter of extension. What is the statistical norm (e.g. an adaptation) in a species, ours included, is always contextual, both in space (for example, it depends on the environment in which organisms develop as on the organisms’ developmental plasticity^[24]) and in time (“all alleles which we now possess were once more than just rare: they were unique”^[25]). Even if, for a moment in the species’ evolutionary history, a trait is de facto universal, it never depends only on, say, humans themselves (strongly contradicting Aristotle’s aforementioned claim).

 A functional account of normality as essence is no less fallacious. What’s the normal, real function of a hand? What is the function of sex? Hull’s implicit reference to the notion of functional cooptation shows us how every variation in functioning is equally abnormal and normal.^[26]

 

From the perspective of commonsense biology, human non-reproductives such as old maids and priests may be biologically abnormal, but from the perspective of professional biology, they need not be.^[27]

 

To sum it up, given the immense variability of humans, in the quest for human nature, it is not useful to stretch essences to absolute inclusivity (paradoxically making them extremely rigid: Aristotle’s living species are eternal). “Nor does it help to switch from traditional essences to statistically characterized essences.”^[28] The first strategy eventually fails the explicative/causal role of essences, while the second, despite being explicative for some humans, fails their universal semantic role.

 

III – New essentialism

 

In light of the impasses of traditional essentialism, a new essentialism has been proposed in the last 20 years.^[29] This view hopes to conciliate the discourse about essences with the importance of history and variations for evolutionary biology. I will present it briefly through Samir Okasha’s proposal for a relational view of essences in his 2002 paper Darwinian metaphysics: species and the question of essentialism.

Okasha shows how Darwinism does not just discard intrinsic essences, but also a specific conception of biological traits:

 

modern biology offers no grounds whatever for supposing that intra-specific variation is confined to some particular set of “accidental” traits, leaving an invariant shared essence. On the contrary, Darwinism leads us to expect variation with respect to all organismic traits, morphological, physiological, behavioural and genetic.^[30]

 

If there are no core-traits, variations are not merely indifferent accidents, and the living is therefore not an essentially-defined ὑποκείμενον, but the product of a causal history of variations and of a context-dependent development. No Homo sapiens or Balænoptera musculus has an intrinsic human or whale core that makes them so.

How are humans and whales part of their own species then? In Okasha’s words, “on the most popular accounts of the species concept found in contemporary evolutionary biology, organisms are assigned to species on the basis of relational properties”^[31]. For example, organisms are part of the same species because they’re part of a “group of interbreeding natural populations that are reproductively isolated from other such groups”^[32] (Mayr’s biological concept), or of the same ecological niche (they exploit the same resources and habitats, according to van Valen’s ecological concept^[33]), or because they instantiate “particular chunks of the genealogical nexus, bounded by speciation events and extinction events”^[34] (phylogenetic concept). None of the properties emphasised by these concepts are intrinsic: “Two molecule-for-molecule identical organisms could in principle be members of different species, on all of these species concepts”^[35]. Incidentally, it is important to notice that different species concepts usually are used in different theoretical and scientific contexts, and thus give us potentially conflicting descriptions of the world.

This last paradoxical example highlights a problematic character of relational essences, that is, their indifference towards an organism's actual properties. Taken alone and in itself, a whale is not really a whale. Here Okasha reasons that if we replace essential intrinsic properties “with whatever relational property we think determines species membership – we do sever the semantic and causal/explanatory roles”^[36]. Sure, Balænoptera musculus will still refer to a group of individuals relatively well determined in time and space – but no species concept (biological, phylogenetic, ecological etc.) can exhaust why whales have the traits that they have – or that can or could have. As the causes will be found in different fields (for example, an adaptation can be explained by natural selection, but its variations are also a matter of developmental plasticity), species-defining relational properties are too restrictive (“Morphology is indicative of that ability [interbreeding], but not the causal outcome of it”^[37]) and potentially in reciprocal contradiction. A whale, insofar as it is a Balænoptera musculus, is never just itself, but an extended network of historical and contingent non-specific causes. For this reason, Okasha (following Laporte, 1997) reasons that “the property in virtue of which any given organism belongs to its species is a property the organism could have lacked”^[38], “properties that, at least in some cases, seem clearly accidental rather than essential to the organisms which possess them”^[39]. The interbreeding and the ecological concepts rest on contingent events of reproductive or ecological isolation that might just not occur, and the phylogenetic concept still rests on the former non-temporal ones to define what a speciation is.

Okasha’s proposal of relational essences as merely semantic thus remains empty: to say that the only essential character of a human is to belong to the contingent causal network of unessential (reproductive, ecological etc.) properties we call “Homo sapiens” is to say that there is nothing essential about a human but its own unessentiality. As Marc Ereshefsky put it: “relational essentialism does not resurrect biological essentialism because it is not essentialism”^[40].

 

IV – Innateness

 

If the philosopher still wants to look at biology in their search for human nature, then clearly, they cannot answer their question in terms of an all-encompassing class-defining differentia specifica. However, an answer could still be put in terms of innateness.

It is uncontroversially true of all and (perhaps) only humans that they are born from humans. While, as we have seen, this relational property is not enough of an essence, it might still indicate where human nature can be found. The question about human nature is, so to say, reinterpreted as the question about the natural human: issues of extension and explanation (semantic and explanatory role of essence) are discarded, since the essence does not individuate a natural kind (which at best exists in the context of a theory^[41]) but a human’s ownmost natural possession. One human’s nature is seen as what they are “born – or conceived – with”, what is inherited and remains a necessary and determining bedrock for further development. For this reason, what we are asking here is not the (empirical) question about what is innate and what is not, but whether we can find anything like an innateness at all, let alone an innate human nature.

Philosophers in search of human nature might find the notion of innateness particularly appealing, for a number of reasons. Innate traits are seen as our natural passively received possessions, as opposed to our (perhaps cultural) actively acquired traits, and thus they sustain the idea of a human nature (and of nature in general) as something existing behind our backs. Consequently, we might interpret innateness both in a proximate and in an ultimate sense (à la Ernst Mayr^[42]): our nature resides in what is inherited, as an innate set of necessary properties acting as a bedrock for further contingent development (already in Aristotle, because “man is born from man”, the μορφή of humans is itself φύσις^[43]); our nature is the product of evolution.

In biology, on the contrary, the notion of innateness is controversial. But while this might be more evident at the level of phenotypic development (where the boundary between innate and acquired is more blurred), it is the gene the best candidate for instantiating innate traits. We will see how they do not do the job – neither as genes-for, nor in themselves.

The first issue concerns the ontological status of an innate trait. To say that we are born (or conceived) with some form of inherited natural possession is to assume that, from the first moment of our ontogenesis, we already possess specific traits potentially: we inherit such potential traits in the form of hereditary material, and we actualise them during development. This is why, for example, in the modern-synthetic interpretation of genes, such material has both an epistemic primacy over the organism (genes are what causes the organism – in the form of “coding for”) and an ontological one (the genes themselves are actualized in the organism, as it survives and reproduces – in the form of “expression”).

Now, the informations composing the developed trait are, so to say, dispersed in a network of interrelated causes – genes, epigenes, environment, experiences etc. “The idea that genes ‘carry information’ about phenotypes in a special sense which distinguishes them from other causes is [...] a highly contested idea”^[44] in contemporary biology. For example, in temperature-dependent sex determination, “environmental signals carry information about sex in the same unproblematic [...] sense as the SRY gene”^[45]. Genes alone, abstracted from this developmental network, are ineffective (“All the genes can code for, if they code for anything, is the primary structure (amino acid sequence) of a protein”^[46]). Most importantly though, it is hard to see how they are something belonging to the organism at all, if the living organism as such has no relationship with them whatsoever. In truth, the same holds for any other unit of transmission. For example, the environment minus its effects on the organism simply is not the environment that determines the development of the same organism. Similarly, something merely present in one’s body is far from being a cause for the organism, let alone its nature. What this shows us is that, in a sense, there is nothing behind our backs, at least in the sense of (proximate) innate natural possessions waiting to be developed. Everything is always, so to say, in front of the developing organism.

Still, the crucial notion of adaptation seems to imply that something is, in fact, not only inherited, but it is so in a relatively and functionally unchanging way (fitness is heritable^[47]), and thus constitutes a good candidate for innateness. While the fitness of the parent does not immediately imply the fitness of the offspring, it is the same fitness that is supposed to be rebuilt by the organism’s network of genetic, epigenetic, environmental, and even behavioural and social causes. In this sense, we might find innateness in the adaptations we receive from the millions of years of evolution. And yet, the attribution of adaptation to something preceding us – in other words, the interpretation of adaptation as something passively received and contextually re-enacted (with uncertain results) – is problematic. Adaptation (as the production of adaptive traits) might be considered as an in-development process – surely allowed by inherited causes (genes, or an environmental niche), but not in itself innate. This holds for potentially all living beings. According to geneticist Eva Jablonka, this is shown, for example, by the notions of canalisation (the buffering of developmental pathways from environmental and genetic variations) and adaptive plasticity (the ability of genes of developing different phenotypes according to the environment)[48]: “Biochemical, neural and behavioural exploration can generate several alternative routes that all end in the same dynamically stable state. [...] Canalization, therefore, necessarily implies plasticity. Adaptive plasticity, on the other hand, requires that some processes are canalized”^[49] – the same adaptation can be reconstructed differently in different generations, and “The response to new conditions can be reversible or irreversible, adaptive or nonadaptive, active or passive, continuous or discontinuous”^[50].

Jablonka has recently emphasised how this sense of adaptation is particularly evident in humans:

 

At the conceptual level, acknowledging the existence of epigenetic inheritance renders the classical nature-nurture dichotomy obsolete, because it means that heredity (‘nature’) can be developmentally constructed (‘nurtured’). The ‘biosocial’ entails reciprocal interactions between biological and sociological factors, showing, in this case, both how social processes impact epigenetic ones and how epigenetic effects impact the social for both the directly induced generation and for descendent generations^[51].

 

As the keyword “biosocial” is potentially extendable to the whole hominid evolution thanks to the historical study of the culture/epigenetics interactions^[52], it is hard to see how the search for the “innate” can grant us any access to something like a natural possession, existing before human actions, overdetermining them. During their development as living beings, humans both give shape to what they transmit and reconstruct what they have inherited, potentially putting it into question. This is what Jablonka means by epigenetic inheritance as extended plasticity: as “developmentally induced epigenetic changes can be heritable for few or many generations, [...] plasticity can therefore have a temporal (hereditary) dimension” blurring the distinction “between ontogenetic plasticity and heredity”^[53]. The notion of a (remote) innate nature fails here too: the very constitution of heritage is a transgenerational active process, as the human organism receives nothing passively.

 

V – Conclusions

 

Do humans have a nature at all – distinguishing them from other living beings, or individuating a natural and more fundamental side in humans? Biology seems to answer negatively. As evolutionary biology undermines the idea of nature both as essence and as cause, humans appear, like any other species, as contingent and self-determining beings. They have no essence, and no nature overdetermines them. Far from delegitimizing it, this rebuke forces us to rethink the question itself. Searching for an answer in another field (dismissing the biological in favour of the Spirit, of the technological, the religious etc.) only ignores the problem.  Why do we ask about human nature, and why do we look at biology as its appropriate place? What does this lack of nature – our evolutionary and developmental contingency – say about our questioning? Perhaps, if the essence and cause of the living is nothing like a nature, we will have to restructure the relationship between nature and life from scratch.

 

References

 

Aristotle, Metaphysics.

Aristotle, Physics.

M. Ereshefsky, 2010, What's Wrong with the New Biological Essentialism, in Philosophy of Science 77, 5, pp. 674-85.

M Ereshefsky, Species, 2010, on https://plato.stanford.edu/archives/spr2010/entries/species/.

A. Borghini, E. Casetta, 2012, Quel che resta dei generi naturali, in Rivista di estetica 49, pp. 247-271-

P. Griffiths, S. Linquist, 2021, The Distinction Between Innate and Acquired Characteristics, on https://plato.stanford.edu/entries/innate-acquired/.

D. Hull, On Human Nature, 1986, in PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association 1986, 2, pp. 3-13.

L. Illetterati, Declinazioni del concetto di natura tra ontologia e storia, available on https://www.youtube.com/user/AccademiaIISF.

E. Jablonka, 2016, Cultural epigenetics, in The Sociological Review Monographs 64, 1, pp. 42–60.

R. Lewontin, 1970, The units of selection, in Annual Review of Ecology and Systematics 1, pp. 1-18.

E. Mayr, 1961, Cause and effect in biology, in Science 134, 3489, pp. 1501-1506.

S. Okasha, 2002, Darwinian metaphysics: Species and the question of essentialism, in Synthese 131, pp. 191-213.

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^[1] See Aristotle, Physics, 192b.

^[2] L. Illetterati, Declinazioni del concetto di natura tra ontologia e storia, Paesaggi contemporanei.

^[3] See Okasha, 2002, pp. 191-192.

^[4] See for example ivi, p. 203.

^[5] Hull, 1986, p. 4.

^[6] Ivi, p. 5.

^[7] Ibidem.

^[8] Ivi, p. 6.

^[9] Ivi, p. 7.

[10] Ibidem.

^[11] Aristotle, Metaphysics, 1022 b 22-27, available on http://www.perseus.tufts.edu/hopper/; my emphasis.

[12] In this modern use, we might translate γένος as species or a higher taxon. For example, the Apteryx genus (understood essentialistically) is flightless, so an individual Apteryx australis would be flightless according to the essence of its genus.

^[13] See ivi, 1022 a 25-30.

[14] Cfr. ivi, 1022 b 30-33.

^[15] Ivi, 1022 a 30-35.

^[16] Hull, 1986, p. 7.

^[17] Ibidem.

^[18] Ivi, p. 5.

^[19] Metaphysics 1022 b 30-35; 1046 a 35-36.

^[20] Hull, 1896, p. 10.

^[21] Ivi, p. 8.

^[22] Ivi, p. 9.

^[23] Ibidem.

^[24] Ivi, p. 8.

^[25] Ivi, p. 9.

^[26] “An organ evolved to perform a function might be commandeered to perform another”, ibidem.

^[27] Ivi, p. 10.

^[28] Ivi, p. 11.

^[29] See M. Ereshefsky, Species, on https://plato.stanford.edu/archives/spr2010/entries/species/, 2010.

^[30] Okasha, 2002, p. 197.

^[31] Ivi, p. 199.

^[32] Mayr, 1968, quoted in ivi, p. 200.

^[33] Ibidem.

^[34] Ibidem.

^[35] Ivi, p. 201.

^[36] Ivi, p. 203.

^[37] Ivi, p. 204.

^[38] Ivi, p. 205.

^[39] Ivi, p. 206.

^[40] Ereshefsky, 2010, p. 683.

^[41] See for example A. Borghini, E. Casetta, 2012: “i generi naturali sono convenzioni fissate su proprietà [...] inserite in una rete causale appropriata [...] cioè che ruota intorno a un centro definito dalla nostra miglior teoria”, pp. 267-268.

^[42] Mayr, 1961.

^[43] Aristotle, Phys., 193b.

^[44] P. Griffiths, S. Linquist, The Distinction Between Innate and Acquired Characteristics, on https://plato.stanford.edu/entries/innate-acquired/, 2021.

^[45] Ibidem.

^[46] Godfrey-Smith, 1999, p. 328, in ivi.

^[47] Lewontin, 1970.

[48] Jablonka, 2016, p. 43

^[49] Ivi, p. 45.

^[50] Ibidem.

^[51] Ivi, p. 55.

^[52] Ivi, p. 53.

^[53] Ivi, p. 55.