LEXICAL DIFFUSION. We have amassed a sizable amount of evidence drawn from the history of Chinese, English, and Swedish, as well as from language acquisition, to substantiate the claim that a phonological change propagates itself gradually across the lexicon, from morpheme to morpheme. In partial answer to the second question, concerning the actuation of phonological processes, we propose that the principal determinants of sound change are to be sought in the inherent constraints of the physiological and perceptual apparatus of the language user. To illustrate the point, we have investigated the attrition of the consonantal endings in a large number of Chinese dialects, and have found significant parallels in the development of well-known Indo-European languages. We argue that the cross-linguistically prevalent pattern and schedule of the related processes are traceable to their PHONETIC ACTUATION. Experimental evidence, both perceptual and physiological, is produced in support.*