Explanations of biological pattern formation and developmental regulation in terms of properties of molecular systems may be considered as prototypical examples for an increasing role of theoretical analysis of integrative features not only in developmental biology, but also in other fields. Thus, originally, a strong motivation for the human genome project was to relate biological features to the structure and function of small sets of genes, and ideally to individual genes; however, it is now increasingly realized that many problems require a "systems" approach emphasizing the interplay of large numbers of genes, and the involvement of complex networks of gene regulation. Projects of "transcriptomics" and "proteomics" are being conceived along these lines. Such change in attitudes implies a new emphasis on integrative, systems theoretical approaches. It may be called 'holistic' , if the term is used without irrational overtones, in the general sense of directing attention to integrated features of organs and organisms. In the history of biology, seemingly conflicting reductionist and holistic notions have alternated, with bottom-up as well as top-down approaches eventually contributing to the solutions to basic problems. By now, there is no doubt that biological features and phenomena are rooted in physico-chemical processes of the molecules involved; and yet, systems of components have features that the components themselves do not have, and which cannot be derived from elementary component properties in an algorithmic manner that could be automated. Top-down approaches are necessary as well; integrated systems aspects are becoming more and more relevant not only in developmental biology, but also in brain and behavioral sciences, and in sociobiology (as discussed in Gierer, 1988
). Theoretical biology is expected to be increasingly involved in progress in these fields.