The impetuses to morphological change and to language change in general, are admittedly nonlinguistic. Nonetheless, this book argues that their impact points are determined by factors which reside in the morpheme system itself. The book, which is the first comprehensive outline of a historical morphology, aims to detect the panchronic regularities behind such phenomena as the apparently irresistible general process of deflexion or, conversely, the counteracting forces which enabled the persistence over centuries of inflections in some languages. Four complementary and intertwining principles are claimed to be crucial in this process. Two of them, markedness and iconicity relations, are rooted in human condition and communication. The other two, synergetic coherence and morphological distinctiveness, are of a purely linguistic nature and are language-specific. The effects of these determinant forces are demonstrated with examples which, due to the time-depth and the richness of the sources, predominantly come from Indo-European and Germanic languages.