Actually, foxes have been bred into something like a dog, using the same selection methods hypothesized to have bred wolves to dogs. I suppose all furry four-legged mammals are the same kind, if religion requires it.
Thats the Russian experiement isn't it?
Wolves and dogs can be interbread. I don't remember about this fox thing.
I suspect that isolated populations may take a considerable period of time before breeding becomes completely impossible.
The subject of mules relates here.