Even at the most radical estimate there are at least four totally unrelated linguistic stocks represented in the region from southern Alaska to central California.  Nevertheless all, or practically all, the languages of this immense area have some important phonetic features in common.  Chief of these is the presence of a “glottalized” series of stopped consonants of very distinctive formation and of quite unusual acoustic effect.[170] In the northern part of the area all the languages, whether related or not, also possess various voiceless l-sounds and a series of “velar” (back-guttural) stopped consonants which are etymologically distinct from the ordinary k-series.  It is difficult to believe that three such peculiar phonetic features as I have mentioned could have evolved independently in neighboring groups of languages.

[Footnote 168:  Ugro-Finnic and Turkish (Tartar)]

[Footnote 169:  Probably, in Sweet’s terminology, high-back (or, better, between back and “mixed” positions)-narrow-unrounded.  It generally corresponds to an Indo-European long u.]

[Footnote 170:  There seem to be analogous or partly analogous sounds in certain languages of the Caucasus.]

How are we to explain these and hundreds of similar phonetic convergences?  In particular cases we may really be dealing with archaic similarities due to a genetic relationship that it is beyond our present power to demonstrate.  But this interpretation will not get us far.  It must be ruled entirely out of court, for instance, in two of the three European examples I have instanced; both nasalized vowels and the Slavic “yeri” are demonstrably of secondary origin in Indo-European.  However we envisage the process in detail, we cannot avoid the inference that there is a tendency for speech sounds or certain distinctive manners of articulation to spread over a continuous area in somewhat the same way that elements of culture ray out from a geographical center.  We may suppose that individual variations arising at linguistic borderlands—­whether by the unconscious suggestive influence of foreign speech habits or by the actual transfer of foreign sounds into the speech of bilingual individuals—­have gradually been incorporated into the phonetic drift of a language.  So long as its main phonetic concern is the preservation of its sound patterning, not of its sounds as such, there is really no reason why a language may not unconsciously assimilate foreign sounds that have succeeded in worming their way into its gamut of individual variations, provided always that these new variations (or reinforced old variations) are in the direction of the native drift.