What is ontogenetic development? (child development)

 

Introduction

Take any text topic on human development and then look for whether it provides a definition of’development.’ You will probably find that such a definition is absent or that it is provided in a couple of unenlight-ening sentences. In fact, most of these text topics provide only a cursory definition of the term. The reason is not hard to find: development is one of those terms that we freely use in everyday language and yet when we try to pin it down with a precise definition it assumes an almost evanescent-like quality. As the satirist and evolutionist Samuel Butler (1835-1902) wrote in his Note-Books (1912), published posthumously, “Definitions are a kind of scratching and generally leave a sore place more sore than it was before.” Scratching the surface of the term development exposes a host of seemingly related terms such as differentiation, evolution, growth, and phylogeny. Scratch a bit more and up pops ‘ontogenetic development.’

In what follows, there is no pretense made to distinguish between all these terms, as space limitations do not permit that. The main focus is on comparing ontogenetic development with ontogeny. This brings with it the need to distinguish development from evolution and evolution from phylogeny. Finally, mention will be made of the long-standing pursuit to bring ontogenetic development and biological evolution into a scientifically credible relationship, which is currently leading to the emergence of a new discipline called evolutionary developmental biology.

Ontogeny and development Ontogeny

Like phylogeny, this is a term created by Ernst Haeckel (1839-1919) from combining the Greek word for ‘being’ with that for ‘birth’ or ‘born of.’ Typically, ontogeny is defined as the life history of an individual from the zygote to the mature adult. Thus, it concerns the description of a historical path (i.e., the life cycle) of the ‘common’ individual of a particular species from fertilization to sexual maturity. In the past, it was restricted to the time between conception and birth, with the term ontogenesis being reserved for the history of a particular individual as in, for example, case studies. In either case, ontogeny or ontogenesis, such a history is conveniently broken down into periods, phases, or stages according to some metric of chronological age in order to indicate major age-specific changes and to describe the products of these temporal delineations.

Development

A more general and abstract concept than ontogeny, development has assumed a number of different meanings such that it was treated as being synonymous with the terms differentiation, growth, and evolution. As a concept, particularly prior to the 20th century, it was intended to indicate organized change toward some certain end condition or hypothetical ideal. Thus, like evolution, it was represented as a progressive process of ‘improvement’ applicable to all levels of organization.

The distinction between growth and differentiation, with both serving as synonyms for development, continued to separate the preformationists (development is growth) from the epigeneticists (development is differentiation) throughout the 19th century. However, during the same century, growth started to become something different from development, with the advent of cell theory as formulated by Theodor H. A. Schwann (1810-1887) following Matthias Schleiden (1804-1881). While much of Schwann’s theory proved to be untenable, it led to growth being restricted to quantitative change (viz., increase in cell number by cell division and increase in cell size), and thus continuing compatibility with preformationism. Subsequently termed

Table 1. Examples of quantitative and qualitative regressions during ontogenetic development at different levels of organization.  


Level

Quantitative

Qualitative

Behavioral

Decrease in associated movements

Fetal GMs, rooting, suckling, and some reflexes, imitation, swimming in human newborn

Morphological

Egg-tooth

Physiological

Yolk-sac, placenta

Neuromuscular

Poly- to monoinnervation

Neural

Apoptosis, synapse elimination

Cajal-Retzius cells, axon and dendrite retraction, radial glia, neurons in the dorsal horn of spinal cord

Quantitative regressions involve a decrease in the number of elements (e.g. neurons; synapses). Qualitative regressions consist of replacements of existing structures and behaviors, or their disappearance, once their adaptive functions have been fulfilled. The quantitative change from poly- to monoinnervation occurs with a change from many to just one axon innervating a muscle fiber, which seems to occur both prenatally and during early postnatal life in humans. The egg-tooth is found in birds and crocodiles at the end of their beaks or snouts, respectively. Together with spontaneous and rather stereotyped head movements, it enables the hatchling to be born by breaking open the eggshell. Once it has served this function, it drops off. GMs: general movements of the whole body that are expressed in the healthy fetus and infant with variations in amplitude, speed, and force, and give the impression of being fluent and elegant in performance. Evident at about 10 weeks after conception, they remain in the behavioral repertoire until about 2-3 months after birth. After this age, they are replaced by more discrete movements that have a voluntary-like appearance (e.g., reaching). All told, convincing evidence for qualitative regressions in behavioral development is less easy to come by than at the other levels.

Quantitative regressions involve a decrease in the number of elements (e.g. neurons; synapses). Qualitative regressions consist of replacements of existing structures and behaviors, or their disappearance, once their adaptive functions have been fulfilled. The quantitative change from poly- to monoinnervation occurs with a change from many to just one axon innervating a muscle fiber, which seems to occur both prenatally and during early postnatal life in humans. The egg-tooth is found in birds and crocodiles at the end of their beaks or snouts, respectively. Together with spontaneous and rather stereotyped head movements, it enables the hatchling to be born by breaking open the eggshell. Once it has served this function, it drops off. GMs: general movements of the whole body that are expressed in the healthy fetus and infant with variations in amplitude, speed, and force, and give the impression of being fluent and elegant in performance. Evident at about 10 weeks after conception, they remain in the behavioral repertoire until about 2-3 months after birth. After this age, they are replaced by more discrete movements that have a voluntary-like appearance (e.g., reaching). All told, convincing evidence for qualitative regressions in behavioral development is less easy to come by than at the other levels.

Appositional or isocentric growth, it was contrasted with allometric growth (i.e., change in shape) in order to account for qualitative change, largely through the work of Julian Huxley (1887-1975). Treating growth as manifesting both types of change led to a blurring of distinctions between it and development that continues today.

With the rise of systems thinking during the 20th century, further attempts were made to discriminate development from other sorts of change such as growth and metabolism. One such attempt was made by Nagel (1957) who defined the concept of development as involving: “… two essential components. The notion of a system possessing a definite structure and a definite set of pre-capacities; and the notion of a sequential set of changes in the system yielding relatively permanent but novel increments not only to structure, but to its modes of operation as well” (p. 17). The core of Nagel’s definition is that development consists of changing structure-function (‘modes of operation’) relationships at all levels of organization, an issue that goes to the heart of attempts to explain ontogenetic development at the individual level.

Ontogenetic development

When, in 1870, Herbert Spencer (1820-1903) suggested that the development of the individual was analogous to embryonic growth, the way was open to combine ontogeny with development to give ontogenetic development. Once done, it was not long before individual development was divided up into successive, time-demarcated periods, phases, or stages. The result was an even more difficult term to pin down unambiguously. What then do we mean by ‘ontogenetic development’? One definition, capturing those given in some text topics on developmental (psycho-) biology, is the following: “Species-characteristic changes in an individual organism from a relatively simple, but age-adequate, level of organization through a succession of stable states of increasing complexity and organization.”

Defined as such, we are confronted with what is meant by ‘relatively simple,’ ‘organization,’ and ‘stable,’ as well as the previously mentioned term ‘differentiation.’ Moreover, the definition alludes to ontogenetic development being progressive, while at the same time ignoring the possibility of transitional periods between the stable states. Evidence from avian and non-human mammalian species, and to a lesser extent for humans, indicates both quantitative regressions (e.g., cell death) and qualitative regressions (e.g., the replacement of one set of cells by another) as being a normal part of’normal’ development (Table 1). Such evidence forces us to consider ontogenetic development as being both progressive and regressive, and in which there are both quantitative (continuous) and qualitative (discontinuous) changes (Fig. 1). If there is qualitative change (i.e., the emergence of new properties), then there must be transitional periods during which the developing organism undergoes transformation (Fig. 2). Thus, ontogenetic development is typified by progressions and regressions, quantitative and qualitative changes, and instabilities (i.e., transitions) between stable states that become increasingly complex by some criteria. Furthermore, it takes on two forms, one direct and the other indirect or metamorphic (Fig. 3).

 A classification of a variety of developmental functions. Quantitative and continuous changes can reveal linear or exponential functions as well as ones that are asymptotic or comply with a logistic growth function (i.e., there is an initial exponential trajectory that gives way to deceleration and the achievement of a final steady state). Qualitative and discontinuous changes may be manifested in one of two ways. The first consists of a discrete step or sudden jump from one stable state to another, but more complex, state with no intermediary ones. The second, termed a cusp catastrophe, has the same properties but additionally includes a hysteresis cycle, which can be interpreted as a regressive phenomenon. Hysteresis is a strong indication that a developing system is undergoing a transition between two qualitatively different states. With special thanks to Raymond Wimmers for permission to use the plots of the developmental functions.  

Figure 1. A classification of a variety of developmental functions. Quantitative and continuous changes can reveal linear or exponential functions as well as ones that are asymptotic or comply with a logistic growth function (i.e., there is an initial exponential trajectory that gives way to deceleration and the achievement of a final steady state). Qualitative and discontinuous changes may be manifested in one of two ways. The first consists of a discrete step or sudden jump from one stable state to another, but more complex, state with no intermediary ones. The second, termed a cusp catastrophe, has the same properties but additionally includes a hysteresis cycle, which can be interpreted as a regressive phenomenon. Hysteresis is a strong indication that a developing system is undergoing a transition between two qualitatively different states. With special thanks to Raymond Wimmers for permission to use the plots of the developmental functions.

A transition in the behavior of a linear system (e.g., a thermostat) is gradual and continuous. For non-linear systems such as living organisms, change can be abrupt and lead to a qualitatively different and more complex state. As illustrated for such systems, that part of the time (t) taken to complete a transition (the transitional period) should be shorter than that spent in the preceding and subsequent states. In the first instance, what one wants to know is how behavior is organized during the period of transition (the transitional process) relative to the preceding and subsequent states. In dynamical systems terminology, this is captured by an order parameter, an example of which might be movement units in studying the development of reaching. The next step would be to identify the event that triggered the transition (the transitional mechanism). Using the same terminology, this is referred to as control parameter, which in the case of reaching could be the degree of postural stability when performing this action.

Figure 2. A transition in the behavior of a linear system (e.g., a thermostat) is gradual and continuous. For non-linear systems such as living organisms, change can be abrupt and lead to a qualitatively different and more complex state. As illustrated for such systems, that part of the time (t) taken to complete a transition (the transitional period) should be shorter than that spent in the preceding and subsequent states. In the first instance, what one wants to know is how behavior is organized during the period of transition (the transitional process) relative to the preceding and subsequent states. In dynamical systems terminology, this is captured by an order parameter, an example of which might be movement units in studying the development of reaching. The next step would be to identify the event that triggered the transition (the transitional mechanism). Using the same terminology, this is referred to as control parameter, which in the case of reaching could be the degree of postural stability when performing this action.

In suggesting metamorphosis as a metaphor for non-metamorphic development, Oppenheim (1982a) makes his point as follows:

Destruction followed by a dramatic reorganization or even the appearance of entirely new features are familiar themes of development in such forms, and the nervous system and behavior are no exceptions. Although I do not wish to offend my colleagues in developmental psychology by claiming that the ontogeny of the nervous system and behavior in ‘higher’ vertebrates is metamorphic in nature, I would argue that even some of the regressions and losses, and other changes that occur during human development are only slightly less dramatic than the changes that amphibians undergo in their transformation from tadpoles into frogs.

Comparing ontogenetic development across phyletic levels in this way brings us to the distinction between phylogeny and evolution.

The differences between direct and indirect forms of ontogenetic development, taken to be two extremes of a continuum of possibilities. Direct development is more or less synonymous with growth. Indirect development, which is the defining feature of metamorphosis, involves radical transformations at different levels of organization, including the behavioral level. It has been suggested that the ontogenetic development of non-metamorphic species such as primates may in fact be better characterized as lying closer to the indirect end of the continuum. In developmental psychology, there is an ongoing debate about whether infants are born with innate cognitive structures for acquiring physical knowledge and thus that subsequent development is analogous to the growth of these structures. Those who oppose this view argue that such structures are emergent properties of the developing cognitive system. Thus, the first view is consonant with the direct form of development and the latter with its indirect counterpart.

• Direct development: newborn or hatchling resembles adult form and mainly undergoes growth to achieve adult-end state.

• Indirect (or metamorphic) development: newborn or hatchling differs markedly from adult in terms of behavioral, morphological, physiological and other traits.

Figure 3. The differences between direct and indirect forms of ontogenetic development, taken to be two extremes of a continuum of possibilities. Direct development is more or less synonymous with growth. Indirect development, which is the defining feature of metamorphosis, involves radical transformations at different levels of organization, including the behavioral level. It has been suggested that the ontogenetic development of non-metamorphic species such as primates may in fact be better characterized as lying closer to the indirect end of the continuum. In developmental psychology, there is an ongoing debate about whether infants are born with innate cognitive structures for acquiring physical knowledge and thus that subsequent development is analogous to the growth of these structures. Those who oppose this view argue that such structures are emergent properties of the developing cognitive system. Thus, the first view is consonant with the direct form of development and the latter with its indirect counterpart.

Phylogeny and evolution Phylogeny

Phylogeny (or phylogenesis) refers to the historical paths taken by evolving groups of animals or plants. More precisely, it is a history made up of the histories of a class of organisms in which every member is the ancestor of some identifiable class of organisms. The key to understanding this more precise definition is identifying what is meant by ‘histories of a class of organisms.’

One interpretation derives from Haeckel’s theory of recapitulation, later amended to the Biogenetic Law: phylogeny is a successive build up of adult stages of ontogeny, with descendants adding on a stage to those ‘bequeathed’ them by their ancestors. Accordingly, organisms repeat the adult stages of their ancestors during their own ontogeny. They do so, however, such that previous adult stages appear increasingly earlier during the ontogeny of descendants thereby allowing for the terminal addition of a new stage. Over the years, Haeckel’s brainchild was summarized and handed down with the felicitous phrase ‘ontogeny recapitulates phylogeny.’

Recapitulation theory became discredited when Thomas H. Morgan (1866-1945) showed it to be incompatible with Mendelian genetics. In its place came a diametrically opposed interpretation articulated by Walter Garstang (1868-1949) and Gavin de Beer (1899-1972). Now, ‘histories of a class of organisms’ was interpreted as phylogeny consisting of a succession of complete ontogenies across many generations (Fig. 4). The crucial point about this interpretation is that phylogenetic change occurs through heterochronic alterations in the timing of ontogeny (i.e., by retardation as well as through the acceleration of ontogeny). More specifically, it involves alterations in the timing of somatic growth relative to reproductive maturation (Gould, 1977).

Phylogeny refers to the histories of a class of organisms in which every member is the ancestor of some identifiable class of organisms. These histories can be considered as a successive series of ontogenies that begin with fertilization (•). In this idealized reconstruction, each succeeding ontogeny becomes longer. Furthermore, identifiable stages (-) become proportionally extended with each ensuing ontogeny. Thus, heterochronic alterations in the mechanisms that regulate the process of ontogeny can precipitate phylogenetic change in the form of, for example, speciation.  

Figure 4. Phylogeny refers to the histories of a class of organisms in which every member is the ancestor of some identifiable class of organisms. These histories can be considered as a successive series of ontogenies that begin with fertilization (•). In this idealized reconstruction, each succeeding ontogeny becomes longer. Furthermore, identifiable stages (-) become proportionally extended with each ensuing ontogeny. Thus, heterochronic alterations in the mechanisms that regulate the process of ontogeny can precipitate phylogenetic change in the form of, for example, speciation.

Evolution

When the controversy between supporters of epigenesis and preformationism was in full flow during the 18th century and into the second half of the 19th century, evolution (from the Latin word ‘evolutio’ meaning the unfolding of existing parts) was treated as being synonymous with development. Seemingly introduced by Charles Bonnet (1720-1793) or Albrecht von Haller (1708-1777), both radical preformationists, it was taken to denote any process of change or growth. Once again, it was Spencer who changed things. In his essay the ‘Developmental hypothesis,’ published seven years before Darwin’s Origin of Species (1859), he offered it as a metaphor for organic change, while still retaining the notion of improvement. Although Darwin avoided the term ‘evolution’ in his theory of descent with modification (except as the very last word in the first edition of the Origin), he was, together with the geologist Charles Lyell (1797-1875), instrumental in restricting its scientific usage to biological evolution as distinguished from cultural evolution.

Biological evolution

It is sometimes not fully appreciated that Darwin had two theories of biological evolution: descent with modification and natural selection. In the 20th century, these two master theories spawned a number of associated theories (Fig. 5). His theory of descent with modification, which concerned phylogenetic change or macroevolution (i.e., speciation), led to disputes between proponents of phyletic gradualism and punctuated equilibrium. In contrast, the theory of natural selection, which addresses evolutionary change or microevolution (i.e., continuous small changes in gene frequencies within a population), was united with the theory of population genetics to give rise to the Modern synthesis. In formulating the theory of descent with modification, Darwin accorded ontogenetic development (embryology in his terms) a role in creating phylogenetic change and a chapter in the Origin, although he never spelt out in detail how this might occur. The Modern synthesis, for its part, dispensed with ontogenetic development as being irrelevant to an understanding of evolutionary change, in part because its supporters regarded embryology as still harboring remnants of vitalistic thinking and anti-materialistic doctrines (Mayr, 1982). As a consequence, Darwin’s two master theories have proved to be difficult to integrate. The emergence of evolutionary developmental biology in the last decade is yet another attempt to provide such an integration. Before considering this discipline-in-the making, a few final comments on the distinction between biological evolution and phylogeny are needed.

To begin with, evolution in the biological sense is a theory proposing a number of mechanisms (e.g., natural selection, mutations, genetic drift) that can be made to account for micro- and macroevolutionary changes. Unlike the study of phylogeny as pursued by paleontologists, evolutionary theory is ahistorical and concentrates on the determinants that bring about these changes. Thus, there is a distinction to be made between the reconstruction of a phylogenetic history and the mechanisms of events that can explain the processes implicated in that history. Put another way, the study of phylogeny involves the description of a succession of products while evolutionary theory addresses the processes and mechanisms underlying such successive products. In this sense, the distinction between phylogeny and evolution parallels that between ontogeny and development (i.e., ontogenetic development is not a function of time, but rather a system of processes and related mechanisms that take place over time).

A summary of some of the many adjunct theories derived from Darwin's master theories of descent with modification and natural selection. The Modern synthesis arose from an integration of the theories of natural selection and population genetics during the first half of the 20th century, chiefly but not only, through the work of  A. Fisher (1890-1962), Sewall Wright (1889-1988), and Theodosious Dobzhansky (1900-1975). In turn, the synthesis gave rise to a number of adjunct theories. The theories of punctuated equilibrium and molecular evolution are difficult to classify exclusively: the former because it incorporates r- and A-selection theory and the latter in that they attempt to address phylogenetic descent. Punctuated equilibrium, more than the other theories, tries to take account of the nexus between ontogeny and phylogeny. More specifically, it rests on the assumption that alterations in the timing of ontogenetic development can lead to phylogenetic changes.

Figure 5. A summary of some of the many adjunct theories derived from Darwin’s master theories of descent with modification and natural selection. The Modern synthesis arose from an integration of the theories of natural selection and population genetics during the first half of the 20th century, chiefly but not only, through the work of  A. Fisher (1890-1962), Sewall Wright (1889-1988), and Theodosious Dobzhansky (1900-1975). In turn, the synthesis gave rise to a number of adjunct theories. The theories of punctuated equilibrium and molecular evolution are difficult to classify exclusively: the former because it incorporates r- and A-selection theory and the latter in that they attempt to address phylogenetic descent. Punctuated equilibrium, more than the other theories, tries to take account of the nexus between ontogeny and phylogeny. More specifically, it rests on the assumption that alterations in the timing of ontogenetic development can lead to phylogenetic changes.

To round off the comparisons, it was claimed in the past that the basic difference between ontogenetic development and biological evolution was that the former relies on deterministic processes and the latter on stochastic processes. Now, however, both are regarded as being based on determinism (i.e., ‘necessity1) and on (constrained) stochasticity (i.e., ‘chance’). With this distinction in mind, we can turn to evolutionary developmental biology.

Evolutionary developmental biology

Haeckel’s recapitulation theory had the effect ofdriving a wedge between developmental and evolutionary biology for many years thereafter. Nevertheless, individuals such as Richard Goldschmidt (1878-1958), with his ‘hopeful’ monsters arising as a consequence of small changes in the timing of embryonic development, and Conrad H. Waddington (1905-1975), with his diachronic biology and its associated concept of epigenetics, made valiant efforts to overcome the neglect of ontogenetic development in the Modern synthesis. What they lacked was the present day array of techniques in molecular biology that would have allowed them to test their ideas more fully. In recent years, there has been a renewal of interest in forging closer links between developmental and evolutionary biology with the arrival of what promises to be a new synthesis, namely, evolutionary developmental biology (or evo-devo for short).

In ontogenetic development, epigenetics serves to mediate the connections between genotype and phenotype (top). Such an intermediary agent is replaced by selection in the Modern synthesis, which acts on the variation created by mutations (middle). Until recently, and most notably with Edelman's theory of neuronal group selection, the concept of Darwinian selection has not been ascribed a prominent role in the study of ontogenetic development. Evolutionary developmental biology attempts to go beyond the Modern synthesis in accounting for the role of epigenetics in biological evolution as well as for selection processes acting on ontogenetic development at any stage (bottom). The solid arrows indicate events within a generation and the dashed ones those that take place between generations.

Figure 6. In ontogenetic development, epigenetics serves to mediate the connections between genotype and phenotype (top). Such an intermediary agent is replaced by selection in the Modern synthesis, which acts on the variation created by mutations (middle). Until recently, and most notably with Edelman’s theory of neuronal group selection, the concept of Darwinian selection has not been ascribed a prominent role in the study of ontogenetic development. Evolutionary developmental biology attempts to go beyond the Modern synthesis in accounting for the role of epigenetics in biological evolution as well as for selection processes acting on ontogenetic development at any stage (bottom). The solid arrows indicate events within a generation and the dashed ones those that take place between generations.

The starting point for evo-devo is credited to the Dahlem Workshop (1981) on evolution and development (Bonner, 1982). At that time, there were major advances in molecular biology such as recombinant DNA technologies that enabled cross-species comparisons of developmental mechanisms at the molecular level. In addition, a distinction had been made between developmental regulator genes and structural genes, starting with the Francois Jacob – Jacques Monod (1910-1976) operon model (1961). Whereas the Modern synthesis, or more correctly population genetics, assumed that ontogenetic development was stable and resistant to change, and therefore irrelevant for understanding evolutionary change, evo-devo treats it as a major agent of such change.

What are the defining features of evo-devo? They can be summarized as follows:

1. Genes alone can explain neither development nor evolution.

2. Developmental processes (i.e., epigenetics) link genotype to phenotype (Hall in Sarkar & Robert, 2003). Due to the stochastic nature of such processes, there is no one-to-one relationship between genotype and phenotype.

3. Developmental mechanisms evolve.

4. Developmental constraints act on particular kinds of phenotypic variation and thus restrict the availability of evolutionary pathways. According to Gilbert (2003), these consist of physical constraints (e.g., elasticity and strength of tissues), morphogenetic constraints (e.g., there are only a limited number of ways a vertebrate limb can be formed), and phyletic constraints (e.g., due to the genetics of a species’ development). In these respects, ontogenetic development exerts deterministic influences on biological evolution.

5. Evolutionary biology should not persist in trying to explain adaptation, but instead should try to account for evolvability (i.e., the potential for evolution). Stated otherwise, this means accounting for the possibility of complex adaptations via transformations in ontogenetic development. And finally, the key feature of evo-devo:

6. Most evolutionary changes are initiated during ontogenetic development. The implication here seems to be that alterations in the actions of regulator genes rather than structural genes give rise to macroevolutionary changes.

If all of the above signal a new synthesis, how then does it differ from the Modern synthesis? Figure 6 attempts to encapsulate the main differences.

Evo-devo is one of at least three current initiatives to integrate ontogenetic development with biological evolution in a testable and unifying theory. Another is developmental evolutionary biology (abbreviated to devo-evo) and a third is dynamical systems theory (DST). At the present time, there is a lack of clarity as to the essential differences between them. Both devo-evo and DST have been criticized for underplaying the roles of genes in evolution, while at the same time emphasizing those for developmental constraints (Gilbert in Sarkar & Robert, 2003). For example, DST, as represented in  Goodwin’s topic How the Leopard Changed its Spots (1994), accords explanatory equality to all levels of organization, and thus does not assign instructive or at least permissive roles to genes. Such differences in emphasis between scientists engaged in a common cause are perhaps a hallmark of the first stages in forming a new discipline. If this is achieved, then we will have a foundation for promoting new insights into ontogenetic development that Waddington and his contemporaries could only have dreamed about.

Conclusions

The main thrust of this entry has been to capture the phenomenological features of ontogenetic development that distinguish it from other terms such as evolution, ontogeny, and phylogeny. Furthermore, evolution was contrasted with phylogeny in order to prepare the ground for an introduction to evolutionary developmental biology with its promise of unifying the developmental and evolutionary sciences. To quote Samuel Butler again, it is to be hoped that we have not left “… a sore place more sore than it was before.”

With regard to ontogenetic development, two related points can be emphasized. Firstly, we still need a theory of developmental transitions that is sufficiently detailed to guide us toward teasing out the processes and mechanisms involved in specific instances. Secondly, if the primary aim of studying ontogenetic development is to describe and explain change within individuals over time, then we also require a better understanding of the functional significance of the considerable variability that typifies intra-individual change. If such variability both increases and decreases over time, what does this mean? Does, for example, increasing variability herald the onset of a developmental transition and a decrease its offset? Most grand theories of development have either ignored or paid insufficient attention to such issues.

Finally, a comment on the new arrival evolutionary developmental biology. It has resulted in reuniting ontogenetic development with biological evolution through the aegis of molecular biology. While appearing to hold great promise for understanding the causal relationships between genotype and phenotype both within and between generations, it remains to be seen what impact it will have on the practice of studying child development. As the saying goes, “In theory, there is no difference between theory and practice, but in practice there is a great deal of difference.” Hopefully, this will not be the case if the theoretical implications of evolutionary developmental biology become more widely appreciated amongst those of us who study child development.

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