As all the Celtic languages have much the same kinds of mutation under very similar conditions, it must have been present in their
Proto-Celtic ancestor.
Welsh is
P-Celtic whereas
Irish is
Q-Celtic, belonging to the two major branches of the Celtic group, so the mutations antedate the split.
The reasons it arose are phonetic; but the phonetic conditioning is sometimes no longer present in the modern languages.
An example where you can still see it is with Welsh yn 'in'. This causes one of three mutations of Welsh, the nasal mutation. This is similar to Irish eclipsis: a following plosive consonant becomes a nasal. For example
- Caerdydd 'Cardiff' --> yng Nghaerdydd 'in Cardiff'
- Penybont 'Bridgend' --> ym Mhenybont 'in Bridgend'
- Dinbych 'Denbigh' --> yn Ninbych 'in Denbigh'
An example where the
conditioning environment is totally lost is in Irish verbs, where the
imperative and the past tense differ only by mutation:
meas 'think!',
mheas 'thought'.
Both Welsh and Irish have long lists of rules for the different circumstances in which the mutations take place. I think I'll restrict this write-up to a general survey, and perhaps node the details for Welsh later, under soft mutation, nasal mutation, and aspirate mutation. They are not simple at all. You can't even trust analogy. For example, the possessive adjectives of Welsh exhibit all three mutations. With calon 'heart' you get:
- fy nghalon 'my heart'
- dy galon 'thy heart'
- ei galon 'his heart'
- ei chalon 'her heart'
- ein calon 'our heart'
- eich calon 'your heart'
- eu calon 'their heart'
Since
eu is pronounced the same as
ei, it is only the mutation that distinguishes 'his', 'her', and 'their'. With vowels the patterns are different:
- fy arian 'my money'
- dy arian 'thy money'
- ei arian 'his money'
- ei harian 'her money'
- ein harian 'our money'
- eich arian 'your money'
- eu harian 'their money'
Here are all the consonant changes in the
soft mutation:
- calon 'heart' --> ei galon 'his heart'
- telyn 'harp' --> ei delyn 'his harp'
- pensil 'pencil' --> ei bensil 'his pencil'
- gardd 'garden' --> ei ardd 'his garden'
- dant 'tooth' --> ei ddant 'his tooth'
- brawd 'brother' --> ei frawd 'his brother'
- mynydd 'mountain' --> ei fynydd 'his mountain'
- llong 'ship' --> ei long 'his ship'
- rhaff 'rope' --> ei raff 'his rope'
The last two don't take place under all circumstances. The article
y 'the' causes soft mutation of feminine singular nouns, but not those beginning with
ll or
rh.
Here are all the consonant changes in the
nasal mutation:
- fy nghalon 'my heart'
- fy nhelyn 'my harp'
- fy mhensil 'my pencil'
- fy ngardd 'my garden'
- fy nant 'my tooth'
- fy mrawd 'my brother'
Here are all the consonant changes in the
aspirate mutation:
- ei chalon 'her heart'
- ei thelyn 'her harp'
- ei phensil 'her pencil'
- arian 'money' --> ei harian 'her money'