Sentence:  [W]e must remember what ruthless and utter destruction our own species has wrought, not only upon animals, such as the vanished bison and the dodo, but upon its own inferior races. The Tasmanians, in spite of their human likeness, were entirely swept out of existence in a war of extermination waged by European immigrants, in the space of fifty years. Are we such <WSD>apostles</WSD> of mercy as to complain if the Martians warred in the same spirit? 
Meaning of the word marked with <WSD> tags:  A pioneer or early advocate of a particular cause, prophet of a belief.