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 is strong 20 th Century <WSD>evidence</WSD> of the effectiveness of Evolution , based on a broad Traditionalism for the creative art of music . 
Meaning of the word marked with <WSD> tags:  your basis for belief or disbelief; knowledge on which to base belief