BIOLOGICAL DEVELOPMENT

the progressive changes in size, shape, and function during the life of an organism by which its genetic potentials (genotype) are translated into functioning mature systems (phenotype). Most modern philosophical outlooks would consider that development of some kind or other characterizes all things, in both the physical and biological worlds. Such points of view go back to the very earliest days of philosophy. Among the pre-Socratic philosophers of Greek Ionia, half a millennium before Christ, some, like Heracleitus, believed that all natural things are constantly changing. In contrast, others, of whom Democritus is perhaps the prime example, suggested that the world is made up by the changing combinations of atoms, which themselves remain unaltered, not subject to change or development. The early period of post-Renaissance European science may be regarded as dominated by this latter atomistic view, which reached its fullest development in the period between Newton's laws of physics and Dalton's atomic theory of chemistry in the early 19th century. This outlook was never easily reconciled with the observations of biologists, and in the last hundred years a series of discoveries in the physical sciences have combined to swing opinion back toward the Heracleitan emphasis on the importance of process and development. The atom, which seemed so unalterable to Dalton, has proved to be divisible after all, and to maintain its identity only by processes of interaction between a number of component subatomic particles, which themselves must in certain aspects be regarded as processes rather than matter. Albert Einstein's theory of relativity showed that time and space are united in continuum, which implies that all things are involved in time; that is to say, in development. The philosophers who charted the transition from the nondevelopmental view, for which time was an accidental and inessential element, were Henri Bergson and, in particular, Alfred North Whitehead. Karl Marx and Friedrich Engels, with their insistence on the difference between dialectical and mechanical materialism, may be regarded as other important innovators of this trend, although the generality of their philosophy was somewhat compromised by the political context in which it was placed and the rigidity with which their later followers have interpreted it. Philosophies of the Heracleitan type, which emphasize process and development, provide much more appropriate frameworks for biology than do philosophies of the atomistic kind. Living organisms confront biologists with changes of various kinds, all of which could be regarded as in some sense developmental; however, biologists have found it convenient to distinguish the changes and to use the word development for only one of them. Biological development can be defined as the series of progressive, nonrepetitive changes that occur during the life history of an organism. The kernel of this definition is to contrast development with, on the one hand, the essentially repetitive chemical changes involved in the maintenance of the body, which constitute metabolism, and on the other hand, with the longer term changes, which, while nonrepetitive, involve the sequence of several or many life histories, and which constitute evolution. As with most formal definitions, these distinctions cannot always be applied strictly to the real world. In the viruses, for instance, and even in bacteria, it is difficult to make a distinction between metabolism and development, since the metabolic activity of a virus particle consists of little more than the development of new virus particles. In certain other cases, the distinction between development and evolution becomes blurred: the concept of an individual organism with a definite life history may be very difficult to apply in plants that reproduce by vegetative division, the breaking off of a part that can grow into another complete plant. The possibilities for debate that arise in these special cases, however, do not in any way invalidate the general usefulness of the distinctions as conventionally made in biology. Additional reading A classic work that laid the foundation for the modern interpretation of biological development in terms of gene activities is Thomas H. Morgan, Embryology and Genetics (1934, reprinted 1975). Overviews of biological development include Jonathan Bard, Morphogenesis: The Cellular and Molecular Processes of Developmental Anatomy (1990); Leon W. Browder (ed.), The Cellular Basis of Morphogenesis (1986), a comprehensive synthesis of developmental biology drawing upon knowledge from molecular biology, anatomy, and genetics; Merton Bernfield (ed.), Molecular Basis of Morphogenesis (1993), articles concerned with understanding the molecular mechanisms controlling formation of the basic body plan during development; V.E.A. Russo et al., Development: The Molecular Genetic Approach (1992), organized by model organisms or cell tissue types and describing the diversity and universality in development in terms of molecular genetic basis; and James D. Watson et al., Molecular Biology of the Gene, 4th ed., 2 vol. (1987), comprehensive and authoritative coverage of molecular biology for advanced undergraduate and graduate students. Montgomery Slatkin (ed.), Exploring Evolutionary Biology: Readings from American Scientist (1995), collects 31 essays and articles which cover all areas of evolutionary biology from the interpretation of the oldest fossils to modern discoveries about sex and development.Studies of more specialized topics include Richard R. Ribchester, Molecule, Nerve, and Embryo (1986), a college text discussing developmental neurobiology in terms of advances in molecular biology and observations from high-resolution microscopy; Marit Nilsen-Hamilton (ed.), Growth Factors and Signal Transduction in Development (1994), describing the interactions of growth factors and their receptors and the subsequent signal transduction pathways they activate in directing developmental processes; Michael T. Zavy and Rodney D. Geisert (eds.), Embryonic Mortality in Domestic Species (1994), a comprehensive text which summarizes information on causes and possible therapy for reducing embryonic mortality in domestic farm species; and P.A. Hausen and Metta Riebesell, The Early Development of Xenopus laevis: An Atlas of the Histology (1991), on the key model (frogs) for developmental and embryological studies in vertebrates.