There are many websites about this topic, Most say he never really said them, BUT, its so well written you could imagine a native american saying those things. It really speaks to how the native americans lived and thought. It was a really well written speech, reguardless who actually did it, It sounds better than a Chief did it though...
"Without the beasts, Man would die from great lonliness of the spirit, for what happens to the beasts...so happens to man." (Cheif Seattle??)
Plus there are many versions of the speech, some are very different than others, I have read about 3 versions so far...