How did these feminine mutations originate in Welsh?

Question

It is known that the celtic languages have mutations, for instance:

Welsh:

*transcription depicts North Welsh dialects

• normal form: Cymru [ˈkəmrɨ̞] (Wales);

• soft mutation: Gymru [ˈɡəmrɨ̞] (ex.: Gwelais i Gymru "I saw Wales");

• nasal mutation Nghymru [ˈŋ̊əmrɨ̞] (ex.: Yng Nghmru "in Wales");

• and aspirated mutation: Chymru [ˈχəmrɨ̞] (ex.: Brasil a Chymru "Brazil and Wales").

The system development is fascinating, however, something intrigues me: why do only feminine nouns (at least in welsh, except the ones beginning in <ll> or <rh>) undergo soft mutation after the article? What has triggered it?

(Ex.: cath (cat) [kaːθ] and y gath [ɪˈɡaːθ](the cat)

Because as far as I am concerned, it is more usual to indo-european languages to show gender at the end of words. I thought this gender classification could be a later development but it wouldn't explain why some words starting with <ll> and <rh> were left as feminine.

Edit: it just occured to me that it could be due to the "original" feminine article (I mean its etymon).

Answer 1

Long ago many words ended in sounds which were for some reason lost. It was those now lost sounds that triggered different kinds of assimilation and other consonant changes in the words that followed them.

E.g.: Welsh bach [baχ] 'little', but merch fach [merχ vaχ] 'little girl': in Proto-Celtic 'girl' was *merkā, so the [b] of bach got between 2 vowels and changed to [v]. Now the *ā disappeared and we cannot see any obvious reason for the mutation. It still can be seen in yng Nghymru, where both consonants undergo assimilation: [n] in yn becomes velar [ŋ] under the influence of the velar [k] in Cymru while at the same time the [k] in Cymru becomes nasal [ŋ̊] under the influence of the nasal [n] in yn.

It's all about the history of the language, in the past some words sounded differently and those old forms triggered the sound changes which still remain although the sounds that triggered the changes are now mostly lost and forgotten.