On Wed, 6 Sep 2017 06:26:55 +1100, A Walker Scott <[log in to unmask]> wrote:

>Neither zoo nor vine is native. Zoo is a Greek borrowing and vine was
>borrowed from Old French. Thus is a function word, which was covered in one
>ofvthe messages in this thread, probably the one you snipped here.

Indeed.  There was perhaps too much meaning packed away in my word "regularly":

>On Tuesday, September 5, 2017, Matthew George <[log in to unmask]> wrote:
>> On Thu, Aug 31, 2017 at 12:02 PM, Alex Fink <[log in to unmask]
>> <javascript:;>> wrote:
>> > English never had such clusters, nor any opportunity to make them.  This
>> > is because English doesn't have an inherited voicing distinction in
>> > fricatives; all initial fricatives are regularly voiceless.

By "regular" I meant 'which arise from regular sound change'.  _Zoo_ and _vine_ don't, they're not native at all.  

The change in _thus_ is irregular, in that its conditioning is not just phonological; it only occurs in function words.  Even if this change had started out as a regular change conditioned by the prosodic weakness of words like _the_, it seems to have undergone analogical extension to yield _thither_ and _therefore_ and so on.  (That this didn't yield /Dr Dl/ clusters is in some sense totally accident.  I can imagine a nearby parallel universe where e.g. English did like German and ended up with "thrin" /DrIn/ instead of "therein".)

I see that my statement was sloppy in a way you didn't point out.  English does have an inherited voicing contrast in *noninitial* fricatives.  Old English had only one series of fricatives, but its rule -- voiceless next to word boundary or another voiceless sound, voiced elsewhere -- was disrupted from allophonicity in two regular ways.  One was loss of final vowels, which is what caused the contrast in "house" noun /haus/ vs. verb /hauz/; the former was OE _hūs_ whereas the latter bore verbal suffixes.  The second was cluster simplification, which is how we got e.g. the (near-minimal) "whistle" < OE _hwistlian_ vs. "weasel" < OE _wesle_.  

It's impossible to say such things for sure, but probably this inherited contrast in non-initial positions helped make English as receptive as it was to loans and irregular changes establishing the contrast in initial position as well.