Sentence:  Thomas ' findings are , first , `` that the individual items vary greatly and unpredictably in their persistence '' ; but , second , `` that the semantic groups <WSD>are</WSD> surprisingly unvarying in their average persistence '' ( as between M-K and I-E . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)