Linguistic science is still based on the formulation that phonetic change is regular and invariable, provided the conditions in which the sounds stand are the same. The moderate chorus of dissent in recent years has come largely from the dialect-geographers, who have found variations which to them indicate the invalidity of the formulation; but in reality their findings demonstrate rather that the differences of environmental conditions are unexpectedly complex in character: dialectal areas are unexpectedly small, social dialects are found to exist, dialect mixtures are omnipresent, especially where the movement of population is free.