The structuralist point of view
One important aspect of language which was neglected by the Neogrammarians is its overall structure. The latter were too concerned with detail. This deficit was later remedied by a number of linguists who were active at the beginning of the 20th century. Because they emphasised the necessity of taking the whole structure of a language into account when offering explanations of language change, they are termed structuralists.
To begin with one can give an example of how structure can explain language change or the lack of it. In English there are two ambi-dental fricatives, a voiced and a voiceless one: /θ, ð/. Their functional load is slight, i.e. there are only a few minimal pairs in which they contrast with one another, e.g. thigh /θai/ - thy /ðai/. However the contrast voiced — voiceless is central to the phonology of English and so a peripheral set of distinctions is retained for reasons of conformity to the overall pattern.
Another point about structure which should be made at this point is that while many languages are definitely related to each other in a genetic sense, they have very different structures. English and German are closely related Germanic languages but the former is largely analytic in its morphology and the latter is still strongly synthetic (i.e. contains a large number of inflections in its grammar). Thus although both languages show a tendency to favour weak verbs over strong verbs (English: dived : dove, German fragte : frug) there has been no tendency in German (as opposed to English) to simultaneously simplify the verb morphology as the remaining morphological structural of German is highly inflectional, cf. the noun system.
A structuralist view of language change can be used to explain series of changes in a language. Take the example of Grimm’s Law again. The Neogrammarians simply described the law. The structuralists considered its origin and its course. Assuming that one sound began to shift, it is obvious that this upsets the structure of the entire sound system. If /p/ shifted to /f/, what happened to the original /f/? If /f/ first shifted to /v/ what happened to the ‘vacuum’ which it left behind? Because one has no way of determining in retrospect what part of the entire sound law occurred first one speaks of a ‘push-pull’ chain which lead to a reshuffling of the sound system of Proto-Germanic. In such an overall view of sound structure, no one element can change without effecting the remaining elements in the system.
Another example of a push-pull chain is the Great Vowel Shift in English. This is a change in the values for long vowels which appears to have begun in the late Middle English period and which was more or less completed sometime after the age of Shakespeare. Basically it consisted of a raising of long vowels by one level, e.g. /o:/ became /u:/ (cf. fool), /e:/ became /i:/ (cf. been). At the same time the two high vowels of Middle English diphthongised as in words like time, house. The matter is one of the entire system of long vowels. While we cannot say what occurred first, one can maintain that either a raising of monophthongs or a diphthongisation of high vowels upset the system (altered the relations in phonological space) and this led to a restructuring of the long vowel pattern which in turn resulted in the unexpected vowel pronunciations for which modern English is well-known.
Incidentally, both instances just discussed do not either expand the entire phonological system or cause it to contract. However, there are many changes which lead to a loss of phonological contrast (system contraction, as in the vocalisation of /x/ in later Middle English) and many which lead to an increase (system expansion, as in the process of Umlaut, e.g. /o/ expands to /o/ and /ø/ in Germanic; this is also known as phonologisation of allophones, a term used by Jakobson).
MERGERS A particular type of sound change is where two sounds are reduced to one. This is technically termed a merger as two segments merge into one. A good example for this is the coalescence of Middle English /a:/ and /ai/ under /ei/ in Modern English, cf. tale and tail which are both /teil/ nowadays; the orthography shows that the original pronunciation was /ta:l/ and /tail/ in the late Middle English period before the Great Vowel Shift set in.
A merger is generally taken to be irreversible because for a later generation it is not possible to recognise that one form may have originally consisted of two. There are cases which seem to contradict this, however. For instance the realisation of boil and bile as /bail/ is a merger which was reversed because it did not occur in all varieties of 18th century English for which it is reported. The pronunciation /bɔɪl/ was re-established for boil because speakers had access to varieties of English in which this still occurred.
PHONOLOGICAL AND SEMANTIC MERGERS In Old English there were two words cwēn ‘queen’ and cwene ‘slut, prostitute’. They continued up to about the time of Shakespeare as two separate words, then with the pronunciations [kwi:n] and [kwe:n] respectively. But sometime afterwards both pronunciations became the same as the /e:/ was raised to /i:/. At the latest by then the derogatory meaning was lost and ‘queen’ was the sole meaning of the merged form.
All the above examples of language change can be considered as internally motivated. There are many changes which are externally motivated, particularly at times when a language is under the influence of a further language. This has been the case in English in at least two main periods in which a Scandinavian and a French influence were to be felt. The nature of the change is dependent on the nature of the contact, thus with the close everyday contact during the Scandinavian period one has many daily lexical items but also a series of morphological borrowings. With French one has mainly lexical borrowings but also the introduction of many extraneous phonemes. Both influences on English can be interpreted from a structural point of view.
The Scandinavian influence on English led to a number of changes in English grammar most notably the importation of foreign forms for the 3rd person plural personal pronouns which began in th- [θ]. From a structural standpoint one can interpret these borrowings as eliminating a structural weakness of late Old English where there was no clear phonological distinction between the singular and plural in the 3rd person.
When considering the French influence on English in the Middle English period one must first of all take note of the fact that in late Old English and early Middle English the loss of inflections (part of the general ‘drift’ in English) had led to former allophonic distinctions of voiced - voiceless fricatives becoming more phonemic-like, e.g. word pairs like teeth : teethe were beginning to emerge. This meant that there was a greater tolerance in the phonological system of English for a voiced — voiceless contrast among fricatives. This fact seems to have furthered the adoption of the voiced fricatives /v, z/ in French words into English (e.g. virtue, zeal), leading to a later phonemic distinction /f/ : /v/ cf. file : vile and /s/ : /z/, cf. seal : zeal. Contrast this situation with other languages: in Swedish, the voiced fricative in journal from French is always voiceless as the Swedish phonological system has no instance of a voiced [s] or [ʃ] and thus no motivation to introduce this extraneous sound with a particular lexical item.