Organismal forms have not always been generated by the highly integrated developmental programmes characteristic of modern multicellular species. Physical forces and other conditional processes played a more prominent role at the earlier stages of evolution, establishing morphological templates that were consolidated by later genetic change. In particular, with the appearance of multicellular aggregates, physical effects relevant to parcels of matter larger than single cells were newly mobilized by gene products (e.g. the ‘developmental-genetic toolkit’ of the animals) that had originally evolved to serve unicellular functions. These mechanisms are responsible for continued generation of morphological novelty, and are ultimately involved in the establishment of the individualized and heritable construction units of morphological evolution known as homologues. Key Concepts:Key Concepts: * Evolution of multicellular organisms began with clusters of individual cells that themselves were products of several billion years of prior evolution. * The genes and gene products that eventually came to coordinate multicellular development originally evolved to serve different, unicellular, functions. * With the emergence of multicellularity, physical effects and processes relevant to viscoelastic, chemically and mechanically active materials came into play, mobilized by the products of ancient genes. * The morphological motifs generated from cell clusters by such ‘mesoscale’ processes are at once predictable and plastic, since the outcomes of physical effects are constrained by the inherent properties of materials and influenced by variable external parameters. * Predictable morphological motifs in aggregates of ancestral animal-type cells include multilayered, hollow, elongated, segmented, appendage-bearing forms, i.e. embryo-like entities. * Morphological phenotypes arising by such epigenetic means will, through processes of stabilizing and canalizing selection, come to depend on programmes of spatiotemporal gene expression. * Since little change in the genomes of single-celled predecessors was needed to generate the panoply of primitive animal forms, radiations of organismal types could have arisen in relatively short periods of time (e.g. the Cambrian explosion). * Even at advanced stages of evolution the conditional nature of epigenetic mechanisms, which include not only the physics of tissue masses but also inductive tissue interactions and influences of the external environment on developmental processes, is retained. * The nonlinear and plastic responses of developmental systems to natural selection and environmental induction will occasionally lead to the crossing of a dynamical threshold, producing morphological novelties: unprecedented constructional elements of the body plan. * Once generated by epigenetic processes, new morphological characters can evolve to become interdependent with other characters (integration), and finally to acquire the status of a homologues of comparative anatomy by becoming organizational elements of the body plan that persist despite changes in the underlying mechanisms that generate them (autonomization).