Sentence:  If adjectival meanings show relatively low retentiveness of stems , as I am confident will prove to be the case in most languages of the world , why should our basic lists include 15 per cent of these <WSD>unstable</WSD> forms , but only 8 per cent of animals and plants which replace much more slowly ? 
Meaning of the word marked with <WSD> tags:  lacking stability or fixity or firmness