Understanding Health By Building Better Bio-Medical Models

Abstract

Complex objects are typically understood, not in terms of their full complexity but via more easy to understand models. To that end, it has been suggested that it is models of the human body that are treated rather than the bodies of patients as they really are. That being the case, better models should lead to better quality treatment. The prevailing textbook model upon which much anatomical and physiological teaching is based is one which sees the body as a ladder of structural levels. This model is inherently reductionist and suffers from what might be called the 'Humpty Dumpty Problem': having dismantled the body for purposes of instruction, it cannot properly reconstruct the individual again. As such, there is currently no theory of the organism per se. To understand the individual organism more fully requires the inclusion of evolutionary thought as applied to the individual rather than to the population. In this respect, an individual's struggle for existence must be considered. To understand that struggle better, the informational value of experiential factors such as feelings of hunger, thirst, pain and illness must be included. A body in need is a body which makes its needs known - it is disturbed; noisy rather than silent. Answering those needs aids individual survival. Such is the case with the experience of illness. Considering the survival needs of individuals allows for a bridge between biological and medical practice and for the building of better bio-medical models via which the individual patient may be better understood and treated.

Current medical thinking is the product of many years of intellectual evolution with influences coming from a number of different sources. As noted elsewhere (See: Trying to Make Sense of Health), medicine has passed through stages comparable with the theological and metaphysical (or abstract) stages proposed by Comte. Now, medicine very much exists within a scientific (or positive) framework. In passing through different phases in its development, medicine has been subject to different modes of thinking. These have coloured the explanations and descriptions of what was being experienced by suffering individuals. Keating and Cambrosio (2003) have proposed that such ways of thinking – that is, medical models – play a key part in all clinical thinking. They go so far as to suggest that '… the object of medicine is not the body per se but, rather, models of the body' (Keating & Cambrosio, 2003). The human body is not necessarily seen as it really is but rather as it appears in terms of the conceptual models that have emerged over time. As a consequence, these models form intellectual frameworks within which one's professional duties are conducted. It is not necessarily the case that the models by which many work are being deliberately or explicitly followed. Rather, a mental image or impression builds up tacitly over time as a result of a number of different influences. It is important to continually strive for the development of the best models possible and not to rely upon some traditional way of thinking merely because it appears to work.

One's model of the human organism can act as a significant guide to how diagnosis, treatment etc. are approached. Furthermore, such models influence how the notions of 'health' and 'disease' are understood. It may even be argued, in keeping with Keating and Cambrosio (2003), that it is models of the body that get treated rather than suffering persons. For example, until the nineteenth century, the prevailing model of the body in Western medicine was based upon the ancient notion of humoralism. How well or unwell one's patient felt was thought to be the product of the way in which four supposed bodily humors – black bile, yellow bile, phlegm and blood – were in proportion to each other. Therapies and treatments of questionable merit were delivered in accordance with how various observations were interpreted in terms of humoral theory. If a patient's ailment was deemed to be related to an excess of the humor blood, for example, this excess was alleviated by the process of blood-letting. The effects of any anaemia that may have resulted from this process seem to have gone largely unnoticed.

1 Thinking More Fully

So far, although largely focused upon what is known of the body's anatomy and physiology, no medical model has drawn fully on biological theory in its broadest sense. There is, for example, no reference to the theory of evolution. This is a significant omission. The theory of evolution is the foundation upon which all biology is now based. It is to biology what the illusive 'Grand Unifying Theory' is to physics. Famously, Theodosius Dobzhansky (1973) stated that 'nothing in biology makes sense, except in the light of evolution'. For any medically useful model to be said to have a rigorously developed biological basis, it too must include something of the evolutionary foundations of biology. Echoing Dobzhansky, Nesse and Williams (1994/5) have suggested that 'nothing in medicine makes sense except in the light of evolution'. Nesse and colleagues have been at pains to emphasise that an understanding of evolutionary theory should form part of medical thinking about the conditions with which it has to deal. However, despite concerted encouragement (Nesse, & Williams, 1997, Nesse, 2001, Nesse & Schiffman, 2003, Nesse et al, 2006), medical schools have been reluctant to adopt the teaching of evolutionary theory within their already crowded curricula.

This may not be entirely surprising. The emphasis in modern evolutionary biology is to think largely in terms of groups and populations. Much of the focus has been on how the genetic characteristics of populations change over time with particular attention given to how gene frequencies change. This, perhaps understandably, has diverted attention away from the individual. It is with the process of evolving that evolutionary biology is primarily concerned. Individual organisms are evolved, not evolving, objects; they are steps along the way between the past and the future. This does not fit well with medicine which is focussed primarily on the individual. There is clearly a gap that needs to be bridged. There may yet be an alternative approach. Rather than a detailed technical understanding of evolutionary theory, an appreciation of the fundamental characteristics of biological survival as it relates to the individual as an organism within an evolutionary context may be more appropriate.

2 The 'Ladder of Levels' Model

To understand the notions of health and disease, it is not unreasonable to first try to understand the organism to which these terms are applied – typically using models. One frequently used in the introductory chapters of modern textbooks of anatomy and physiology used by students of clinical and non-clinical subjects alike may be described as the 'ladder of levels' model. It typically takes the form of a list of the structural levels which make up the human body. These range from the atomic and molecular level, via cellular, tissue and organ levels to that of the whole organism (Figure 1).

Figure 1 - The 'Ladder of Level' model.

The linear hierarchy into which the human body is typically divided by standard textbooks of anatomy and physiology.

The precise divisions and their names may sometimes vary but the general principle is widely pervasive.

The arrows are typically included to suggest that the smaller structures combine to form the larger structures.

In textbooks of anatomy and physiology, this model is used to introduce and demarcate what will be discussed in subsequent chapters. Set out by physiological system, each chapter typically focuses on the organs that compose these systems, their structure and what they do. The organism level is usually as far up in the hierarchy that such books are willing to go.

While a 'ladder of levels' is an interesting and, in some cases, useful model, it should be subjected to a more thorough scrutiny than it typically attracts. This is important given its prominence in standard textbooks and the influence it is, therefore, likely to have on understanding not only the organism as a whole but also what constitutes 'health' and 'disease'.

There is also something rigidly linear and one-dimensional about the ladder. Each rung is a supposedly different level of organisation. However, in terms of the size of the things considered, the rungs are not equidistant. Scale is something that is rarely emphasised and a sense of the number of components at each level is largely missing. Also, the rungs of the ladder are not all of the same kind. Most significantly, between organ level and that of the organism, is often a level entitled 'System' – or sometimes 'Organ System' divided in accordance with the way in which the rest of the chapters are divided. The notion of a 'system' is not primarily a structural one. Instead, it is something quite abstract: one based upon notions of commonality of purpose and of co-operation between constituent organs. One is not denying that there are organs arranged in particular ways which allow them to work together. What is at issue, however, is that while the other levels of the ladder represent structural hierarchies, a system is not of the same kind. In the body, the separation of organs into distinct systems is not necessarily obvious. Their association into a system is mainly via their mode of operation. Furthermore, different textbooks are often at variance over how to make these divisions into systems and even over the names the different systems should be given. Often organs do a variety of things that can be said to contribute to more than one system. The kidneys, for example, are best known as organs producing urine but this is really a combination of waste excretion and water balance. In addition, they have key roles to play in acid-base balance and blood pressure regulation amongst other things. Simply belonging to the urinary – that is, 'urine-producing' – system underplays and may even detract from their wider importance. More generally, the nerves and blood vessels which textbooks typically consider within their respective chapters actually form an integral part of the structure and operation of all organs. Overall, as a result of the typical way of describing the body, a sense of bodily integration is easily lost. Indeed, the approach is one of deliberate de-construction.

3 Levels and Qualities

It may be necessary for practical purposes to divide the body in this way but it does not necessarily give the best impression of what it is to be a living entity experiencing, amongst other things, 'health' and 'disease'. To say that an organism is more than the sum of its parts is not wrong. However, at the same time, this is perhaps a somewhat hackneyed adage. It may be better to suggest that the whole is not just more than the sum of its parts but that the whole is qualitatively different to the sum (or any other combination) of its parts.

Instead of trying to understand a phenomenon at the lowest level, it may be more accurate to suggest that certain phenomena belong only to given levels – that is, only to specific rungs on the ladder. This includes that of the organism as a whole. Instead of taking a vertical approach and applying the 'ladder of levels' to the whole organism and separately to each system, one might also look horizontally at each level across all systems. Particular phenomena may only be evident at specific levels. Furthermore, it may be that some things are better understood at just one level. Just as 'the time' is not a property of the structure of a clock but is what is shown on the clock face, so too 'health' and 'disease' may be qualities which belong only to a particular level. What is going on at other levels is, of course, important but it is not necessarily of the same kind as what presents via, say, the organism as a whole.

4 The 'Humpty Dumpty Problem'

While the 'ladder of levels' model can be useful, at the same time it suffers from what might be called the 'Humpty Dumpty Problem'. As presented in textbooks of anatomy and physiology, the 'ladder of levels' model is used as a prelude to the ensuing de-construction of the human body into its component parts. At the end of this process, after having described the various systems and their component organs and processes, there is no form of or attempt at re-construction. Humpty isn't put back together again. The level of the whole organism – the level of the person – is not described. There is perhaps a tacit assumption that somehow the reader knows what an organism is before they even start to read because they are one, living in a world full of others of the same kind. However, this omission of some form of reconstruction may point to a serious shortcoming. The intention of de-constructing the organism is to understand it better. If that understanding cannot be used to make a more meaningful reconstruction, it is questionable whether the aims of that de-construction were fully realised.

5 No Theory of the Organism

What is very much missing from the 'ladder of levels' model is a theory of the organism as a whole entity (Lewis, 2009). Indeed, currently there is nothing in either biology or medicine that might be described as a 'theory of the organism' as such. Yet, it is at this level that each human being primarily exists. It is at this level that human beings interact with each other and with the world in general. Furthermore, it is at this level that 'health' and 'disease' have a bearing upon how life is lived out (Lewis, 2011).

Not only is Humpty not put back together again but it is quite likely that, given the present approach and the lack of a theory of the organism, there is no way of knowing how to even go about doing this. Where all the bits belong is well known as are many of the associated physiological and biochemical processes. However, this is only part of what is needed to understand human beings as whole integrated entities. Without a theory of the whole, a knowledge of the components and merely what they do is insufficient.

6 Information Flows

Importantly, human beings are integrated entities, systems within which there is a constant information flow. The integration of the whole is reliant upon internal communication. Neural and chemical messages have a bearing on the organism beyond their measurable physical properties. Fast acting nerve impulses and slower acting hormonal and chemical signals act to integrate the operation of disparate organs. However, measuring the physical properties displayed by these signals is not the same as understanding the information content they contain or convey. What they bring about in terms of the effect on the organism as a whole is qualitatively different from their physical properties. For example, the cogs inside a clock may move in an understandable way. However, knowing this is not the same as understanding the information content that they convey. A single turn of one cog may signify the passing of a minute while a single turn of another seemingly identical cog may signify the passing of an hour. Indeed, as noted above, what happens inside a clock is quite different to the meaning that is conveyed at the clock face.

Within an organism, much of the information flow occurs below the level of consciousness. While the body is known to be self-regulating, this is understood largely in terms of changes in measurable physical parameters. However, there is another sense in which the organism might be described as being 'self- (or 'auto-)referential'. By this, one is not merely referring to conscious self-awareness. There is a range of internal feedback mechanisms and other responses which result from changes within the body about which one is never conscious. Those that do reach the level of consciousness do so as a variety of different experiences. Importantly, for the organism to remain viable, these frequently provoke behavioural responses which make good any internal physical deficiencies. For example, one becomes aware of the internal physiological state of dehydration via an experience of thirst and is prompted to drink. At an experiential level, this may slake the thirst and may also be associated with the pleasurable experience of taste. At a physiological level, the water balance of the body is restored. It is via the experience of thirst that the body ensures that it has an adequate fluid intake. As has been said in relation to food, '[f]ruit tastes sweet, not nutritious' (Cronin, 1991). What is necessary at a physical level is ensured via experiential surrogates. Feeling ill may be such a surrogate.

7 The Subjective And The Objective

In 1943, the medically trained philosopher, Georges Canguilhem (1904-1995) posed an important medical and philosophical puzzle (Canguilhem, 1966 (trans. 1978)). He describes a man who was going home one day but as a result of, for example, a car accident or murder, died on the way. The precise details are not significant; what is important is what ensued from his sudden death. An autopsy was performed where it was found that the man had a tumour on one of his kidneys. In life, that man knew nothing of this condition; he had experienced no symptoms or ill effects associated with that lesion. Canguilhem asked whether that man should be considered as having had a disease or not. Put another way, should the man have been considered healthy even though there was a symptom-free lesion within.

In one sense, this is a story about how the man feels. He feels fine. If asked how he was, he would have said that he was well. He was able to go about his everyday life unimpeded until it was tragically cut short. In another sense, this story is both a corollary to the work of Morgagni and a presaging of the modern problem of defining 'health' and 'disease'. Although the man feels well, he has within him a serious lesion. How should he be classified in a modern scientifically medical sense?

With modern Western medicine, there are many precautionary procedures which are now performed routinely. Various screenings are performed, not because one is in obvious need of medical assistance but because it is deemed wise to assess the physical state of certain groups of people for some reason. In the past, how one felt was the main and sometimes only reason for seeking assistance from others. If one felt a need within oneself which could not be met by retiring to one's bed for a period of time, then one called upon someone else for remedial help. The man in Canguilhem's puzzle may well have had his lesion spotted had he gone for one of today's routine screenings or check-ups. However, these were not available when Canguilhem posed the question. Many decades old, his question remains pertinent. Importantly, it demonstrates that there are two quite distinct aspects of the individual which need to be considered together. One is the lived-experience of the individual. The other is the physical nature of that individual. Where once disease was literally 'dis-ease' associated simply with personal experience (Lewis, 2011), medical advances have realigned the disease focus (post-Morgagni) primarily onto the physical constitution of the patient and onto the search for a lesion, in particular.

8 Feeling Alive

How one feels – one's experience of oneself – is so 'second nature' that its importance, indeed its centrality to staying alive, is easily overlooked. In order to stay alive and to ensure a comfortable quality of life, a number of things must be managed simultaneously. One must not be too hot or too cold. One must not be too thirsty or too hungry. One's bowels and bladder should not become uncomfortably full. One must not be too tired or try to do what one's body – via the aches and pains experienced – is, in effect, telling one not to do. Each of these is an experience for which there is an underlying physical basis. The body's biochemical reactions have an optimal temperature range. This coincides with a range which is correspondingly experienced as 'comfortable'. The body also has an optimal state of hydration. Dehydration coincides with a state which is correspondingly experienced as thirst. A flow of substances derived from a sufficient provision of food is necessary for the maintenance of bodily structure and the provision of energy. Experiences of hunger and intestinal emptiness help ensure that a flow of these substances is maintained. One must not go beyond the daily limits that the body sets for itself. Thus, in order for the body to continue to operate within safe limits, a range of different phenomena are experienced when those limits begin to be exceeded. These, in turn, elicit different behavioural responses from the individual such as eating, drinking etc. Here, resting may also be viewed as a behaviour.

9 Feeling Pain

The phenomena mentioned above are typical everyday experiences. To these may be added pain. One naturally avoids being in too much pain or discomfort. Yet, even though pain and discomfort are not uncommon, they are not typically included with the other ordinary experiences mentioned above. Nevertheless, pain provides important signals; surely as important as the likes of hunger and thirst. Indeed, to be unable to feel pain is to risk one's whole ability to survive (Melzack & Wall, 1988).

Without medical help, the life expectancy of those born with congenital absence of pain is much shorter than that enjoyed by those who can experience pain. Even with medical help, the effects can be quite marked. It is easy to bite the inside of one's mouth during eating. Lips are frequently bitten and the tip of the tongue not uncommonly bitten off. Small cuts and grazes go unnoticed and particles damaging to the eye go without response. Thus, pain and discomfort should be seen as having important biological roles and not just as something unpleasant (Melzack & Wall, 1988).

If the benefits of something as marked as pain can be easily overlooked, the benefits of being able to experience seemingly trivial discomforts and irritations are missed almost completely. For example, one frequently shifts one's body position in response to feelings that are barely noticeable. Indeed, this even occurs during sleep. Not to do so can prove highly damaging and has the potential to prove fatal. To remain constantly in the same position results in joint and muscle stiffness. If severe, this could progressively impede one's ability to move. Failure to move often enough can also lead to decubitus ulcers. In extreme cases, these can become infected leading to septicaemia and even death.

The experience of those with leprosy further demonstrates the value of being self-aware (or auto-aware) in this way. Nerve damage in leprosy leads to loss of peripheral sensation. What are, to the unaffected person, minor cuts and grazes typically go unnoticed. This can prove highly damaging to the extent that the loss of fingers and toes typical of leprosy is not as a direct product of the infection but of the inadvertent damage resulting from the loss of peripheral sensation and the secondary infections which set in leading to auto-amputation.

Pain and discomfort are not valued for their potential benefits. Instead, perhaps unsurprisingly, they are simply dis-valued for their unpleasantness and avoided accordingly. Indeed, when in pain or discomfort, what is known intellectually about these experiences may be of little or no consolation, given the distress being felt.

The ability to experience pain and discomfort is perhaps a key physiological feature of being an organism. This is exemplified by the fact that, like any other physiological mechanism, it can sometimes operate in a way that is counterproductive for the individual concerned. Pain can become chronic and can, as a result, become the object of an individual's suffering, in and of itself. It can even continue to be experienced without apparent cause.

10 Feeling Ill

Other important, dis-valued experiences are those feelings associated with 'illness'. Illness is easier to define than either 'health' or 'disease' (Marinker, 1975). The word 'illness' now means what the word 'disease', when it literally meant 'dis-ease', used to mean. Thus, illness is a particular sense of unease. It is possible to describe illness as a subjective, personal experience; a feeling of being unwell. As such, illness is a characteristic of human life. Everybody has felt ill at some point or another. When somebody says that they are feeling ill, all can equate with that experience indirectly. However, being able to describe with precision what a given bout of illness – or of being 'unwell' – is like can prove very difficult. Associated with illness is a variety of feelings such as malaise, nausea, tiredness, headache etc. Illness is not one specific thing or a particular set of named phenomena. Instead, it is a variable constellation of negatively valued inner experiences grouped under a single term.

The experience of illness is important in that it provokes remedial responses. Just as phenomena like hunger and thirst elicit important behavioural responses which ensure the survival of the individual, so too does the experience of illness. For what one considers as minor illness, all that is usually required is rest and warmth. Brought on by minor infections, for example, this allows the immune system greater call on the body's otherwise shared resources in combating that infection. Body temperature may be raised, quickening biochemical processes and providing a less hospitable environment for the infective agent. Simply going to bed may be enough until the feeling of illness passes. On other occasions, one may instinctively feel that one's illness is more serious than one can manage on one's own or that it will not pass off. It is then that the help of others may be sought.

11 The Silence of the Body

Many of the experiences one has of one's body are subtle. Furthermore, one typically responds to them before they become too severe. For example, one does not wait until one's hunger or thirst become overtly distressing before deciding to eat or drink. Such responses ensure that the operation of one's body continues virtually unnoticed. Indeed, for much of the time that one is going about one's daily business, one is largely unaware of one's own body and what is going on internally. In effect, the body only draws attention to itself when a need must be met. The French surgeon René Leriche (1879-1955) even went so far as to consider health as 'life lived in the silence of the organs' (in Canguilhem, 1966 (trans 1978)). This was, in effect, how life appeared to the man in Canguilhem's problem. There were no 'noises' requiring his attention. As a result, he went about his daily life without distraction.

Strictly speaking, one might argue that Leriche was defining health as an absence of an experience of illness and that he was only focusing upon experiential aspects. Here there is little or no direct reference to the inner workings of the body. From this, it would reasonably follow that feeling 'well' is not so much having specific positively valued feelings but having a silent body: having no negatively valued feelings of illness, pain or discomfort.

Thus, one may discern two distinct dimensions: the 'lived' and the modern (scientifically) medical. The 'lived' dimension is that of the individual who responds to the presence or otherwise of subjective experiences. The modern medical viewpoint gives particular attention to the physical condition of the patient seeking objectively discernible phenomena. It is important to make this distinction. It enables quite separate aspects of being alive to be recognised: that there are distinct physical and experiential dimensions to life. For a fuller understanding of 'health' and 'disease', both dimensions should be taken into account.

12 The Struggle For Life

Darwin recognised something that is now easily overlooked by those living in the relative comfort enjoyed in the developed world - that life is a struggle. The latter part of the subtitle of the book which launched the modern understanding of evolution – 'On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life' – spells this out (Darwin, 1859). Darwin here, and throughout the book, draws attention to the fact that only some are 'preserved'; many perish. Where there are those who perish, there is also suffering. Nowhere is this more vivid than in the phrase 'survival of the fittest'. Darwin did not coin this phrase. Neither is it to be found in the earliest editions of 'On The Origin Of Species'. It began to appear from the fifth edition (1869) after Herbert Spencer (1820-1903) first used the expression in his 'Principles of Biology' (1864). Now the phrase has become inextricably linked with the notion of evolution.

The survival Spencer had in mind was that of the individual. This is important to remember. Writing in 1864, his was not the emphasis on populations or gene frequencies that has come to dominate modern evolutionary biology. Extending this idea, it is noticeable that even among those who do survive, there are some who fare better than others. Even among those who are surviving, there are those who will, from time to time, seek help from others – not least the medical profession. In this, they are, in effect, seeking help with various aspects of ensuring their survival and enhancing their biological fitness. It is this focus upon the individual from an evolutionary perspective that may well help forge a greater integration of biological and medical thinking (Lewis, 2009). Here, the theoretical framework that allows biology to make sense (Dobzhansky, 1973) and medicine's focus on the individual meet (Lewis, 2011). For an improved and truly biomedical model, the notion of individual survival – the personal 'struggle for life' – is central.

13 Conclusion

Currently, the survival perspective is a direct but poorly explored bridge between biology and medicine. As described earlier, it is how the individual feels that is of particular importance in how life is lived out. One's experience of one's own body – whether the organs are silent or 'noisy' – determines how one goes about one's daily life. In a lived context, therefore, one's experience of oneself is perhaps more important than biological science tends to realise. Traditionally, biological science has not given much emphasis to the inner feelings of the individual. These are, after all, inaccessible to direct investigation. Yet, experiences such as illness and the behavioural changes that ensue from this have a direct bearing upon individual survival chances. At the same time, feelings of illness, pain and discomfort form the starting point for medical involvement. The help offered by the medical and allied professions is essentially help with individual survival. In so doing, the focus is directed largely on the physical state of the body. However, when seeking to build better biomedical models, it is not only the physical state of individuals that matters. It may well be that the notion of survival, with its physical and experiential dimensions, is what bridges biology and medicine better and how the notion of health may come to be better understood.

Acknowledgements

I would like to thank Annette Lewis for her help in the preparation of the manuscript of this chapter.

References

Canguilhem, G (1966 (trans 1978)) On the Normal and the Pathological (C Fawcett, Trans.). Dordrecht: D Reidel Publishing Company.

Cronin, H (1991) The Ant and the Peacock. Cambridge: Cambridge University Press.

Darwin, CR (1859) On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. London: John Murray. http://darwin-online.org.uk/contents.html#origin

Dobzhansky, T (1973) Nothing in biology makes sense except in the light of evolution. American Biology Teacher, 35: 125-129.

Keating, P, & Cambrosio, A (2003) Biomedical Platforms - Realigning the Normal and the Pathological in Late-Twentieth-Century Medicine. Cambridge Mass.: MIT Press.

Lewis, S (2009) Seeking a new biomedical model. How evolutionary biology may contribute. Journal of Evaluation in Clinical Practice, 15(4): 745-748.

Lewis, S (2011) Conceptual Models of the Human Organism: Towards a New Biomedical Understanding of the Individual. In M Komorowska & S Olsztynska-Janus (eds.), Biomedical Engineering, Trends, Research and Technologies. Vienna: Intech.

Marinker, M (1975) Why make people patients? Journal of Medical Ethics, 1: 81-84.

Melzack, R, & Wall, P (1988) The Challenge of Pain (2nd ed.). Harmondsworth: Penguin Books.

Nesse, R (2001) Medicine's missing basic science. The New Physician(December): 8-10.

Nesse, R, & Schiffman, J (2003) Evolutionary Biology in the Medical School Curriculum. Bioscience, 53(6): 585-587.

Nesse, R, Stearns, S, & Omenn, G (2006) Medicine Needs Evolution. Science, 311: 1071-1073.

Nesse, R, & Williams, G (1994) Why We Get Sick - The new science of Darwinian medicine. New York: Times Books.

Nesse, R, & Williams, G (1995) Evolution and Healing - The new science of Darwinian medicine. London: Weidenfeld and Nicolson.

Nesse, R, & Williams, G (1997) Evolutionary biology in the medical curriculum - what every physician should know. Bioscience, 47: 664-666.

Spencer, H (1864) Principles of Biology. London: Williams and Norgate.