Actually, reflecting on Rachael's posting, I don't think I'm quite as PC and non-judg(e)mental as I have claimed. I feel in fact feel real pain (thank you for your sympathy, but I'll get over it) when language loses elements of complexity and subtlety, when historically interesting and arguably valuable distinctions (e.g. between British and American usages, between weak and strong verbs) seem to disappear or become eroded. The richer and more ornate a language is, the more things you can do with it.

I always like the analogy between language and the natural world. If a word or a usage dies, the gene pool becomes impoverished. The extinction of a language is as much a tragedy as the extinction of a species.

On the other hand, on the same analogy, I think English should be kept open to enrichment from outside, to robust mongrelisation. New slang, text speech, teenage speech, influences from world English, etc. All great.

What I don't know, and would welcome illumination on, is what it is that determines whether languages become more or less complex - the 'natural history' of language, if you [like] [will]. Does language adapt to its social environment by some sort of process of natural selection, like a living organism? Or is it driven by some sort of inner dynamic towards change? And if so, in what direction? Are there (for example) examples of languages acquiring rather than losing case endings and the subjunctive mood?