We propose two new concepts, evolutional agent and field. The main purpose of the work is to provide a framework for building software which adapts to changes of requirements autonomously. In open networks, the adaptability to changes of requirements or environments is essential, while there are many free applications and components to serve various requirements. Focusing on this point, we introduce concurrent objects with meta-base architecture, agents, and fields, another kind of object. In our model, adaptive agents, called evolutional agents, adapt to changes by traversing a network and acquiring components as their own functions. Fields are receptacles of software components in networks. Agents evolve into enhanced ones by traveling to fields and acquiring components from the fields. Flage is a framework for building the software architecture based on these concepts.