This paper examines the evolution of aspectual formations in Insular Celtic. It is argued that the emergence of these formations and their unusual morphosyntactic structure have been determined by internal systemic factors. It is also suggested that the manner in which these formations developed is of importance for our understanding of the processes involved in the emergence of grammatical subsystems in general. It is demonstrated that the Celtic aspectual system evolves in a manner reminiscent of the concept of developmental stratification and, significantly, that it evolves in a remarkably coherent and ordered fashion. The inexorable and structured nature of this evolution would seem to provide evidence for the claim that there may exist within languages a certain teleological impulse; in other words, that rather than being simply random, language change is in some fundamental and meaningful sense goal-directed.