In this paper, we propose new concepts, Evolutional Agents and Field Oriented Programming. The main purpose of the work is to provide a framework for building software which adapts to changes of requirements autonomously. Such adaptability is essential in open networks. Meanwhile in networks, many free applications exist and have a great potential for software reuse. We focus on the point and incorporated concurrent object oriented model with recta-architecture and a concept called field. In our model, evolutional agents are autonomous objects with meta-architecture which adapts to changes by acquiring components scattered over networks as their own functions. Fields are receptacles of software components in networks. Agents evolve into adaptive ones by moving among fields and acquiring components from the fields. Flage language is a framework for describing such a software architecture. Moreover, formal semantics of agent evolution in Flage is proposed because it is necessary to rigorous verification of specifications and programs in software development in practical situations of industrial fields. The semantics of Flage is based on an algebraic framework to enable automatic verification procedures.