The echo is an important symbol in the novel. Mrs. Moore hears the echo on her excursion to the Marabar Caves. While she is insite the caves, the echo seems to turn into a "boum" sound that unnerves her. She has not really wanted to visit the caves, not really wanted to ride on an elephant, she is not in the best of health, and the freaky "boum" sound seems to put her over the edge. After hearing this boum, her character changes. She is no longer optimistic, she becomes apathetic about India and learning about Indian life and culture, so perhaps the echo is symbolic of reality - the reality that cultures really cannot come together, English rule of India is really not good for the Indians. Religion is not the answer to unite all of India (with its Muslims, Hindus and other religions). Mrs. Moore's Christianity is not the answer either. When she realizes all of these things, even on a visceral level, she becomes despondant. So the echo is a type of wake-up call to reality in the novel. This occurs in Part II, Chapter XIV.
The echo, then, is the "muddle of India" that Forester refers to. It is the futility of uniting India, it is the futility of believing that all livings things are or can be united.
After Aziz is acquitted of raping Adela, the Indian crowd begins a type of chant, which echoes throughout the city. They are chanting "missez Moore, mizzez Moore" - believing that somehow, her spirit has come back to exact justice. She is a character who has tried to understand the Indians and who had developed a friendship with Aziz. Aziz recognized that she was respectful of Indian culture (the encounter in the mosque at the beginning), but when she realized that Adela was not going to marry Ronny, she left India and died on the way home to England. The Indians turn her memory into a type of goddess in their chanting echo, so this echo perhaps represents hope that at least on a personal level, Indians and English may be friends - Mrs. Moore and Mr. Fielding, then later, Stella, Mrs. Moore's daughter, who has married Mr. Fielding and returned with him to India.
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