Sentence:  That he mastered every aspect of his medium according to his own great talents and contemporary judgments , is a good and solid symbol of his people under the tremendous pressures of proclaiming and practising the rigors of a new culture ; and perhaps of even greater significance - his music <WSD>is</WSD> strong 20 th Century evidence of the effectiveness of Evolution , based on a broad Traditionalism for the creative art of music . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)