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 unstable forms , but only 8 per cent of animals and plants which <WSD>replace</WSD> much more slowly ? 
Meaning of the word marked with <WSD> tags:  substitute a person or thing for (another that is broken or inefficient or lost or no longer working or yielding what is expected)