Summary In the early years of the nineteenth century, the English chemist John Dalton (1766–1844) developed his atomic theory, a set of theoretical commitments describing the nature of atoms and the rules guiding their interactions and combinations. In this paper, I examine a set of conceptual and illustrative tools used by Dalton in developing his theory as well as in presenting it to the public in printed form as well as in his many public lectures. These tools—the concept of ‘atmosphere’, (...) the pile of shot analogy, and Dalton's system of chemical notation—served not just to guide Dalton's own thinking and to make his theories clear to his various audiences, but also to bind these theories together into a coherent system, presented in its definitive form in the three volumes of A New System of Chemical Philosophy (1808, 1810, and 1827). Despite these links, Dalton's contemporaries tended to pick and choose which of his theories to accept; his system of notation failed to be adopted in part because it embodied the whole of his system indivisibly. (shrink)

Modularity has become a central and remarkably useful concept in evolutionary developmental biology, offering an explanation of how independent, interacting units make possible developmental events and evolutionary changes. These modules exist at several different levels of organization, from genes to signal transduction pathways to cell populations. Cell populations, which are multicellular modules, provide an opportunity both to clarify our notion of modularity and to reexamine such central concepts as cell-to-cell communication. Rosine Chandebois’s work on “cell sociology” is reframed in the (...) language of modularity in order to show how groups of cells maintain their collective identity during development and how signaling information from the group’s environment is received by the constituent cells. In a “test case” for our account of multicellular modules, we consider the vertebrate neural crest, a migratory cell population made up of several subpopulations. The neural crest proves yet again to be a complex case, showing the importance of taking into account both developmental and evolutionary transformations of multicellular modules. (shrink)