the principle of natural selection was more than empirical, because it had a teleological, "stability over time" component. As an ecologist interested in the behavior and function of large entities over time, Odum sought to give a more general statement of natural selection so that it was equally applicable to large entities as it was to small entities traditionally studied in biology. Odum also wanted to extend the scope and generality of natural selection to include large entities such as the world. This extension relied on the definition of an entity as a combination of properties that have some stability with time. Odum's approach was motivated by Lotka's idea's on the energetics of evolution. Ecosystem simulation Odum used an analog of electrical energy networks to model the energy flow pathways of ecosystems. His analog electrical models had a significant role in the development of his approach to systems and have been recognized as one of the earliest instances of systems ecology. Electron flow in the electrical network represented the flow of material (e.g. carbon) in the ecosystem, charge in a capacitor was analogous to storage of a material, and the model was scaled to the ecosystem of interest by adjusting the size of electrical component. Ecological analog of Ohm's Law In the 1950s Odum introduced his electrical circuit diagrams of ecosystems to the Ecological Society of America. He claimed that energy was driven through ecological systems by an "ecoforce" analogous to the role of voltage in electrical circuits. Odum developed an analogue of Ohm's Law which aimed to be a representation of energy flows through ecosystems. In terms of steady state thermodynamics, Ohm's Law can be considered a special case of a more general flux law, where the flux () "is proportional to the driving thermodynamic force () with conductivity ()", or . Kangas states that Odum concluded that as thermodynamic systems, ecosystems should also obey the force-flux law, and that Ohm's law and passive electrical analog circuits can be used to simulate ecosystems. In this simulation, Odum attempted to derive an ecological analog for electrical voltage. Voltage, or driving force, is related to the biomass in pounds per acre. The analogous concept required is the biomass activity, that is, the thermodynamic thrust, which may be linear. Exactly what this is in nature is still uncertain, as it is a new concept. Such a consideration led Odum to ask two