#1729. glottochronology 再訪[glottochronology][lexicology][family_tree][pidgin][punctuated_equilibrium][speed_of_change]


 「#1128. glottochronology」 ([2012-05-29-1]) で,アメリカの言語学者・人類学者 Swadesh (1909--67) の提唱した言語年代学をみた.その理論的支柱となるのは,"the fundamental everyday vocabulary of any language---as against the specialized or 'cultural' vocabulary---changes at a relatively constant rate" (452) というものだ.Swadesh は複数の言語の調査に基づき,その一定速度は約86%であるとみている.この考え方には様々な批判が提出されているが,仮に受け入れるとしても,なぜ一定速度というものありうるのかという大きな問題が残る.基本語彙が一定の速度で置換されてゆくことに関する原理的説明の問題だ.これについて,Swadesh 自身は,次のように述べている.

Why does the fundamental vocabulary change at a constant rate? . . . . / A language is a highly complex system of symbols serving a vital communicative function in society. The symbols are subject to change by the influence of many circumstances, yet they cannot change too fast without destroying the intelligibility of language. If the factors leading to change are great enough, they will keep the rate of change up to the maximum permitted by the communicative function of language. We have, as it were, a powerful motor kept in check by a speed regulating mechanism. . . . / While it is subject to manifold impulses toward change, language still must maintain a considerable amount of uniformity. If it is to be mutually intelligible among the members of the community, there must be a large element of agreement in its details among the individuals who make up the community. As between the oldest and the youngest generations, there are often difference of vocabulary and usage but these are never so great as to make it impossible for the two groups to understand each other. This is the circumstance which sets a maximum limit on the speed of change in language. / Acquisition of additional vocabulary may proceed at a faster rate than the replacement of old words. Replacement in culture vocabulary usually goes with the introduction of new cultural traits replacing the old ones, a process which at times may be completed in a few generations. Replacement of fundamental vocabulary must be slower because the concepts (e.g., body parts) do not change fundamentally. Change can come about by the introduction of partial synonyms which only rarely, and even then for the most part gradually, expand their area and frequency of usage to the point of replacing the earlier word. (459--60)

 また,glottochronology が前提としているのは,印欧語族の系統樹に示されるような,諸言語の時間的な連続性である.しかし,強度の言語接触の過程としてのピジン化 (pidginisation) の事例などを考慮すると,上記の計算はまるで通用しないだろう.系統樹モデルでうまく扱えるような言語については基礎語彙の置換の一定速度を論じることができるかもしれないが,世代間の断絶を示す言語状況に同じ議論を適用することはできないのではないか.そして,後者の言語状況は,ピジン語の研究や「#1397. 断続平衡モデル」 ([2013-02-22-1]) が示唆するように,人類言語の歴史においては,これまで想定されていたよりもずっと普通のことであった可能性が高い.
 ただし,glottochronology は,系統樹モデルでうまく扱えるような言語に関する限りにおいては,少なくとも記述統計的に興味深い学説でありうると思う.このような限定つきで評価する価値はあるのではないか.

 ・ Swadesh, Morris. "Lexico-Statistic Dating of Prehistoric Ethnic Contacts: With Special Reference to North American Indians and Eskimos." Proceedings of the American Philosophical Society 96 (1952): 452--63.

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