Sentence:  Those modern scholars who urge that we must keep in mind the fundamental continuity of Aegean development from earliest times - granted <WSD>occasional</WSD> irruptions of peoples and ideas from outside - are correct ; but all too many observers have been misled by this fact into minimizing the degree of change which took place in the early first millennium . 
Meaning of the word marked with <WSD> tags:  occurring or appearing at usually irregular intervals