Sentence:  By an effort of historical sympathy we can cast our minds back into the art of a remote past or an alien present , and enjoy the carvings of cavemen and Japanese colour prints ; but the possibility of this effort is bound up with that development of historical thought which <WSD>is</WSD> the greatest achievement of our civilization in the last two centuries , and it is utterly impossible to people in whom this development has not taken place . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)